Do Insects Transmit AIDS?

September 1987

NTIS order #PB88-143177
DO INSECTS

TRANSMIT AIDS?

by

Lawrence Miike

Health Program

Office of Technology Assessment

United States Congress

Washington, D.C. 20510-8025

September 1987

A Staff Paper

in OTA’s Series on

AIDS-Related Issues

The views expressed in this Staff Paper do not

necessarily represent those of the Technology

Assessment Board, the Technology Assessment

Advisory Council, or their individual members.

For sale by the Superintendent of Documenta, U.S. Government Printing Oflice

Washington, DC 20402
TABLE OF CONTENTS

Page

SUMMARY . . .. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 111

INTRODUCTION . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1

IS INSECT TRANSMISSION POSSIBLE?● . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2

Mechanical Transmission of Other Viruses. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3

HIV Survival in Insects. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5

How Probable is Insect Transmission? . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9

Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15

IS THE EPIDEMIOLOGY OF HIV INFECTIONS CONSISTENT

WITH INSECT TRANSMISSION?● . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17

Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 21

WHAT FURTHER INFORMATION IS NEEDED?● . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 24

Biology of HIV . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 24

Insect Studies . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 25

Epidemiological Studies . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 28

APPENDIX A: Probability of Insect Transmission . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 31

APPENDIX B: Workshop Participants . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 35

REFERENCES . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 

. . 

. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . ......OOO . . . . . . . . . . . . . . . . . 37
Do Insects Transmit AIDS?

SUMMARY

The AIDS virus, or HIV (Human Immunodeficiency Virus), can be transmitted

through sharing of contaminated needles by intravenous drug users. Can HIV infections

therefore be transmitted by bloodsucking insects, such as biting flies, mosquitoes, and bedbugs?

The conditions necessary for successful transmission of HIV through insect bites, and

the probabilities of their occurring, rule out the possibility of insect transmission of HIV

infection as a significant factor in the way AIDS is spread. If insect transmission is occurring

at all, each case would be a rare and unusual event.

There are theoretically two mechanisms through which biting insects might transmit

HIV infections. First, when bloodsucking insects feed on HIV-infected persons, the virus might

be ingested with blood, reproduce and multiply in the insect, migrate to the insect’s salivary

glands, and be injected into uninfected persons on whom the insect subsequently fed. This is

known as biological transmission and is the mode through which such diseases as malaria are

transmitted.

The second possible mechanism through which bloodsucking insects might transmit

HIV is known as mechanical transmission. The insect might begin to feed on an HIV-infected

person and be interrupted for one reason or another. Instead of returning to its original host,

the insect might move on to another, uninfected person to complete its meal. The insect might

then transfer part of the fresh blood remaining on its mouthparts from its previous feeding

attempt to the uninfected person, and might also regurgitate some of its previous bloodmeal.

Experiments in which insect cells were cultured with HIV and in which insects were

artificially fed with high concentrations of HIV-infected blood have shown that the AIDS virus

does not multiply in insects. If transmission of HIV infection by bloodsucking insects is

occurring, it would have to be through mechanical transmission during interrupted feeding.

The probability of HIV transmission from an insect bite would be calculated by

multiplying (not adding, because each event’s probability is independent of each other) the

following factors: 1) how frequently interrupted feeding occurs, 2) the probability that the

insect had bitten an HIV-infected person prior to biting an uninfected person, and 3) the

probability that the insect bite contained enough HIV to transmit infection.

The frequency of interrupted feeding depends on the type of insect; in general, the

larger the insect and the more painful the bite--such as with horse flies--the greater the

probability that interrupted feeding will occur. Other bites, such as from mosquitoes and

bedbugs, are usually unnoticed and therefore usually uninterrupted. With others, such as ticks,

if their feeding is interrupted, the probability of quickly transferring to another person is

extremely low.

In mechanical transmission, the maximum amount of HIV that insects would be able

to transfer would be the amount of virus in the blood they had ingested prior to biting an

uninfected person. Experience with viruses actually transmitted in this manner has shown that

the amount of blood that might be transferred is limited to the amount of blood on the insect’s

mouthparts (on the order of 1/100,000 of a milliliter of blood). An uninfected person would

also have to be bitten within an hour of the insect’s biting an infected person; and both infected

and uninfected persons would have to be in close proximity to each other (a few hundred feet

for mosquitoes and biting flies, in the same household for bedbugs), or else the insect will not

have an opportunity to transfer to another person if its feeding was interrupted.

. . .

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Most HIV-infected persons (70 to 80 percent) do not have detectable levels of

infectious virus in their blood. Those that do have measurable HIV have very low levels, much

below the levels that are needed for insect transmission of other viral diseases. Only rarely does

an HIV-infected person have a blood virus level that might contain enough infectious HIV for

insect transmission.

The question of insect transmission as an issue has persisted only in the case of Belle

Glade, Florida, one small community in the United States. Of 76 AIDS cases diagnosed in

western Palm Beach County (which includes Belle Glade) between 1982 and 1985, 13 (17

percent) had no identifiable risk factors, but 10 of the 13 had died before epidemiological

investigations could be completed. Seventeen additional cases were labeled as having known

risk factors on the basis of being born in Haiti. If these 17 cases are added to the 13 with no

official risk factors, then 39 percent (30/76) had no known risk factors. However, it does not

necessarily follow that insect transmission must be the cause. Of 736 persons who were tested

for HIV infection in Belle Glade, only 26 were positive (3.5 percent prevalence). Eighty-eight

percent were ages 18 to 49. The percent of positive adults decreased with age, and no positive

results were found in 121 children ages 2 to 10, nor in 94 adults over 60 years of age. There

was no clustering of HIV-infected persons within households, except for two pairs of sexual

partners.

These findings are consistent with sexual transmission. More importantly, they do

not support the hypothesis that insect transmission of HIV infections is occurring in Belle

Glade, because of the absence of infection in both the younger and older age groups. The

absence of infection in persons over age 60 is also consistent with recent introduction of HIV.

Although epidemics of HIV infection are occurring in parts of Africa, similar patterns have

been found there in areas endemic for HIV infections.

There is one preliminary finding from French researchers who collected various

species of insects from areas in Africa with high rates of HIV infection, and who found HIVlike nucleic acid sequences in the DNA of some of these insects. There was no evidence that

these insects could transmit infections. The HIV-like nucleic acid sequences may be some other

virus in the DNA of these insects (which is known to occur) or may be artifacts resulting from

the identification methods used. These findings need to be confirmed by other laboratories and

in further field studies of insects in areas endemic for HIV infections. If these preliminary

findings are confirmed, insects will be proven capable of becoming infected with HIV, but not

proven to be capable of transmitting HIV infections. However, the potential of insects to act as

reservoirs for future HIV infections would then need to be investigated further.

Public concerns that insect transmission of HIV infections might be occurring could

be addressed through studies of selected human populations. Surveillance activities could be

carried out in areas where conditions most conducive for insect transmission occur, such as in

densely populated urban areas in the tropics and subtropics, with high percentages of HIV

infections in the exposed population. Such studies might look for insect transmission within

households (through bedbugs and mosquitoes), in small neighborhoods (through mosquitoes and

biting flies), and among populations heavily exposed to biting flies, such as migrant farm

workers.

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INTRODUCTION

Infections of HIV (Human Immunodeficiency Virus), the virus that causes AIDS, are

spreading rapidly among intravenous drug users, and there have been rare occasions in which

health care workers became infected after being accidentally stuck with contaminated needles.

If infection can occur by these routes, is it possible for the AIDS virus to be transmitted from

an infected to an uninfected person by biting insects? What evidence is there for actual

transmission by insects? If insect transmission is possible, how serious is the threat, and what

research is being conducted and could be conducted that could answer these questions?

Inquiries into whether insect transmission of HIV could occur fall into two general

categories. First, field and laboratory studies of insects can determine: 1) whether their

feeding/biting habits are compatible with their possible role as vectors of HIV transmission, 2)

whether these insects are capable of drawing up and transmitting enough virus to cause

infection, and 3) whether the microbiology of the virus in the insect is conducive to

transmission. Second, epidemiological studies can be conducted among populations who live in

environmental conditions favorable to insect transmission, to see if there are HIV infection

patterns: 1 ) that cannot be explained by established risk factors (such as sexual transmission or

intravenous drug use), and 2) that are consistent with transmission through insect vectors.

The insects of primary interest as possible vectors in the spread of HIV infections are

biting flies, mosquitoes, and bedbugs. Other possible insect vectors include lice and fleas.

In this Office of Technology Assessment (OTA) Staff Paper: 1) the evidence for the

possibility of insect transmission of HIV infection is summarized, and 2) areas for further

investigation are identified.

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IS INSECT TRANSMISSION POSSIBLE?

Transmission of some types of infectious diseases through bites of bloodsucking

insects is well known to occur when the disease agent is able to reproduce and multiply in

insect vector itself and concentrates in the insect’s salivary glands. Under these conditions

the

of

biological transmission, the agent not only is subsequently available in much greater quantities

than the amount originally taken up by the insect, but also is concentrated in fluids (saliva,

which in the insect contains anti-clotting substances to make feeding easier) that

injected into the next person or animal that the insect bites.

There is no evidence that the AIDS virus multiplies in insects. Nor is

will be

HIV known to

naturally infect other animals besides humans, so the “reservoir” of infection that insects might

dip into to transfer HIV to uninfected humans is limited to humans infected with HIV. (The

chimpanzee is so far the only animal that can be infected experimentally, although it will not

develop the AIDS disease, so a search is still being conducted to find an appropriate animal

model for AIDS.)

Mechanical transmission theoretically could occur when the biting insect is

interrupted while feeding on an infected person and shortly thereafter completes its feeding on

an uninfected person. The maximum amount of virus that could theoretically be transmitted

would be the amount of virus previously ingested, but in all likelihood the amount would be

limited to the quantity of virus left on the insect’s mouthparts. For example, the common

bedbug imbibes approximately l/100th of a milliliter (1 x 10-2 ml) of blood in a full bloodmeal,

and approximately 3/1 ,000th of a milliliter (0.3 x 10-2 ml) in a partial, interrupted bloodmeal

(44). However, the amount of blood on a bedbug’s mouthparts is estimated to be only

7/100,000 of a milliliter (0.007 x 10-2 ml) (56). Comparable levels (0.001 x 10-2 ml) of residual

blood on the mouthparts of horse flies have been quantified (24). If 10 percent of the residual

blood on the mouthparts of horse flies are deposited when biting the second person or animal,

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Do Insects Transmit AIDS?

the range of potential deposition would be between 10-6 and 1O-s ml (0.0001 x 10-2 to 0.001 x

10

-2 ml). These estimates are well correlated with transmission experiments among animals (37).

The insect could also regurgitate some of its previous bloodmeal when feeding on its next

victim, but the extent to which this occurs is not known and probably depends on the feeding

habits of each type of insect.

Whether infection will occur from insect bites by mechanical transmission depends

primarily on the amount of virus injected into an uninfected person. The injected dose depends

on the following factors: 1 ) the amount of virus ingested by the insect and present on the

insect’s mouthparts after feeding on an infected person, 2) the number of bites from infected

insects that an uninfected individual incurs, 3) the time interval between biting an infected

person and a subsequent uninfected person (the amount of virus remaining in the insect

decreases over time), and 4) the amount of virus remaining in and on the insect that will

actually be injected into the uninfected person. The amount of virus ingested depends on the

concentration of

infected person.

that bit infected

within the range

virus in the blood and the amount of blood withdrawn by the insect from an

The number of bites from infected insects depends on the number of insects

persons (which depends on the prevalence of HIV-infections in the population

of the insect), but were interrupted in their feeding and moved on to bite

uninfected persons. And as noted above, not all of the remaining virus is expected to be

injected into a subsequent victim. Only a part of the blood that is on the insect’s mouthparts

(and the amount in regurgitated blood, if regurgitation occurs) is usually injected.

Mechanical Transmission of Other Viruses

Mechanical transmission, if it occurs, would be expected to play a relatively minor

role in transmission, as compared to the recognized routes of HIV transmission, and so would be

difficult to detect in natural conditions (i.e., through field and epidemiological studies).

Through field experiments, mechanical transmission has been shown to occur with a few animal

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diseases, such as equine infectious anemia virus (EIAV) in horses and bovine leukemia virus

(BLV) in cattle, both transmitted by horse flies. There is evidence that several other viral

diseases of humans and/or domestic animals might be transmitted mechanically. In the case of

Rift Valley fever, a hamster model has been used to

experimentally. Dengue fever virus has been shown

mosquitoes in human volunteer studies. In addition,

demonstrate mechanical transmission

to be mechanically transmitted by

hepatitis B virus (HBV) has been

mechanically transmitted to rabbits and guinea pigs by bedbugs that have fed on HBV-infected

blood.

With experimental transmission of EIAV, when horse flies ingested a partial

bloodmeal containing approximately 1 million infectious doses per milliliter (108/ml) from an

infected horse and the bloodmeal was completed on an uninfected horse, transmission of

infection could occur: 1 ) with one horse fly, if the uninfected horse was bitten immediately to

complete the bloodmeal (33), or 2) with 25 horse flies, if bitten within 30 minutes (34).

However, when mosquitoes were used instead of horse flies and the same concentration of

virus-containing blood was ingested by the insects, transmission of EIAV infection did not

occur with up to 200 mosquitoes (80).

With experimental transmission of BLV, when blood containing more than 1,000

infectious doses per milliliter (10305/ml) was ingested by horse flies, infection was transferred

from infected cattle when uninfected

(24). While the virus level in BLV is

lymphocytes (the type of white blood

goats and sheep were bitten immediately 50 or more times

relatively low, BLV causes an increase in the number of

cell to which the T-cells in humans belong). Although

the proportion of infected cells remains low, the absolute number of infected lymphocytes

increases greatly.

A hamster animal model has been developed for insect transmission of Rift Valley

fever, a disease of humans.

(108/ml) was ingested from

When blood containing 100 million infectious doses per milliliter

one animal by tsetse flies and the bloodmeal completed on an

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Do Insects Transmit AIDS?

uninfected animal

When mosquitoes

within 1 hour, 70 percent

instead of tsetse flies were

of the individual transfers resulted in infection.

used under similar conditions, 15 percent of the

individual transfers resulted in infection when one strain of Aedes aegvp ~ti was used as the

vector (36), and 75 percent (3 of 4) of individual transfers resulted in infection when another

strain of Aedes aegvpti was used (42).

Finally, there is

situations that could result

the common bedbug, HBV

evidence that some insects can harbor the hepatitis B virus (HBV) in

in human infection. In artificial feeding and transmission studies of

survived in the bugs for at least 7.5 weeks but was undetectable

after 18 weeks. In these experiments, infection was successfully transferred to rabbits and

guinea pigs on which HBV-infected bugs had recently fed (40). HBV is also excreted in the

bedbug’s feces (41). In a non-experimental study, one group of investigators isolated the

hepatitis B virus in mosquitoes and bedbugs they had collected in areas with high rates of

hepatitis B infection in the human population (81,82).

HIV Survival in Insects

Experiments with HIV in insects have centered on the following questions. With cell

cultures (in vitro experiments): 1 ) can HIV bind to insect cells, and 2) can HIV replicate

(multiply) in insect cells? With live insects (in viv o experiments): 1 ) how long can HIV survive

in the insect, 2) does the virus multiply in the intact insect, and 3) can insects transmit HIV

through interrupted feeding? From field studies: is there evidence for HIV infection in insects

collected in areas endemic for HIV infections?

In humans, HIV has an affinity for T-helper cells (a type of white blood cell) and

binds to the surface of the cell at the site of the “CD4” molecule that characterizes a specific

subset of T-helper cells. Experiments have shown that, despite the lack of the CD4 molecule,

HIV can bind to some types of insect cells, such as cells from some species of mosquitoes and

fruit flies (3,1 1).

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Further experiments have shown that HIV does not replicate in these cells. HIV can

be integrated into the insect cell’s genome--i.e., in the proviral or DNA form of HIV, which is

an RNA virus in its free form--a condition that is necessary for replication. However, no

replication of HIV occurs; i.e., no “free” virus is present. This lack of replication in insects

appears to be due to ineffectiveness of the part of HIV (the “long terminal repeat” sequences, or

“LTR”) that allows the virus to take control of its host cell’s genes in order to promote

replication in human T-helper cells (72) and is probably due to an undetermined intracellular

mechanism that is specific to insect cells (1 1). Fruit fly, tick, moth, and mosquito cells have

been examined, and none have shown replication of HIV (1 1,65,72).

The finding that HIV could integrate into the genome of some insect cells in cell

culture experiments led Chermann and his colleagues at the Pasteur Institute to inquire into “the

possibility of finding, in endemic zones, insects which would carry genomic sequences

homologous to those of the HIV virus” (1 1). They therefore collected bloodsucking and nonbloodsucking insects from areas endemic for AIDS in central Africa, as well as insects from the

Paris area (for control purposes) (3,10,1 1,12).

In African insects, HIV-like nucleic acid sequences were found in the DNA of some

mosquitoes, tsetse flies, bedbugs, and ticks, but also in cockroaches and antlions (antlions eat

other insects, primarily ants). Termites, crickets, wasps, and dragonflies were negative. All

insects from the Paris area were negative. The positive results were found in urban and

suburban areas, but not in rural areas, reflecting the distribution of HIV infections in the

African areas from which the insects had been collected.

HIV-like nucleic acid sequences were found in both male and female mosquitoes

(male mosquitoes do not take blood meals), suggesting that reproductive cells had been infected

in prior generations (i.e., transovarian transmission). However, as with the laboratory

experiments, there was no evidence of free virus.

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Do Insects Transmit AIDS?

These findings are puzzling for a number of reasons. First, the HIV genome

appeared to be present in all cells of positive insects, although the method used could not

specifically identify which and how many cells contained the HIV-like nucleic acid sequences.

HIV is able to enter only a few specialized human cells, so a possible explanation is that the

findings are artifacts resulting from the assay methods used.

Second, HIV was found in the genome of cockroaches and antlions, insects that would

not be expected to be positive. Cockroaches could ostensibly have eaten HIV contaminated

materials, but why antlions?

Third, 30 percent of mosquitoes (male as well as female) in areas endemic for HIV

infection apparently contained HIV-like nucleic acid sequences, a rate that is higher than the

prevalence of HIV infection in the human population of the collection area. This infection rate

among mosquitoes is high even for severe epidemics of known insect-transmitted infectious

diseases. On the other hand, variable prevalence rates for hepatitis B virus (HBV) infection in

mosquitoes have been found. In high prevalence areas of Africa, one study found only 12 of

1,658 mosquitoes infected with HBV (81), but other studies have found that 131 of 247 (25) and

10 of 42 mosquitoes (6) contained HBV. In the case of wild-caught bedbugs in South Africa,

HBV infection rates of 157 and 138 per 1,000 bedbugs have been recorded (41).

The existence of HIV-like nucleic acid sequences in the DNA of insects in areas

endemic for HIV infection is consistent with the laboratory experiments that have shown that

such integration of HIV is possible. However, the validity of these findings must be confirmed,

because the positive results may be due to non-specific reactions with the assay reagents used,

contamination of the specimens, or reactions with other RNA viruses that can be found

integrated in insects’ DNA (4). Insects and other species are known to sometimes contain pieces

of DNA that are homologous with viruses. Thus, one explanation of the presence of HIV-like

nucleic acid sequences in these insects may be that other retroviruses had been integrated into

the insects in previous generations.

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Taken

colleagues at the

contaminated by

together with the lack of replication of HIV in insects, Chermann and his

Pasteur Institute emphasize that their findings “suggestt that insects might be

infected human material and thus could be carriers of HIV genes but not~

vectors as clearly evidenced by previous epidemiological studies” (their emphasis) (12). If some

insects are natural reservoirs for the viral genome, further research would be needed to

determine how HIV replication is suppressed in insect cells, and to assess the possibilities for

activation of HIV replication in these insects.

Live mosquitoes and bedbugs have also been fed blood with high concentrations of

HIV or have had HIV-contaminated blood injected into their thoraxes (44,49,50,51,55,65,69).

At these high concentrations, HIV persists in mosquitoes for a few hours up to 48 hours

(50,51,55,69), and in bedbugs, for 1 to 4 hours (43) up to 72 hours (55).

Survival time may be related to the amount of virus initially ingested. A feeding

dose of between 10,000 to 100,000 tissue culture infectious units per ml (104-106/ml) resulted in

no survival in mosquitoes but 1 to 4 hours survival in bedbugs (39,43,44). Forty-eight hour

survival in mosquitoes was accomplished with a feeding dose of approximately 1 million tissue

culture infectious units per ml (106/ml) (51). Bedbugs fed on approximately 10 million tissue

culture infectious units per ml (107/ml) retained detectable virus for 72 hours (55).

Survival time of HIV in insects after feeding thus appears to depend on the type of

insect and on the concentration of HIV in the bloodmeal. There is also a suggestion that

survival may depend on the virus. When HIV-2 (a variant of HIV found in parts of Africa)

(13) was used instead of HIV (which would be HIV-l), survival for several hours was found in

mosquitoes with a feeding dose of 10,000 tissue culture infectious units per ml (104/ml) (69), a

dose that resulted in no survival when HIV-1 was used (see above).

These experiments demonstrated that, while virus was detectable for varying periods

after feeding, it eventually disappeared. Moreover, the concentration did not increase,

indicating that no virus replication occurred. Similar results--i. e., no virus replication--have

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Do Insects Transmit AIDS?

been found for HTLV-I (66), a virus that is closely

“HTLV-111” or “LAV” prior to its present name).

Experiments have also been conducted to

related to HIV (HIV was identified as either

see whether insects could transmit HIV

from infected blood to uninfected blood through a membrane during interrupted feeding.

Mosquitoes fed on blood infected with 1 million tissue culture infectious units of HIV per ml

(l OG

/ml) did not transfer virus, even though HIV survived in the mosquitoes for up to 48 hours

(51). Control experiments had shown that as few as 100 infected lymphocytes could transmit

the virus, suggesting that very little blood was being transferred by the mosquitoes. No

transmission occurred with mosquitoes or bedbugs with 104

-105 infectious units per ml (recall

that in this experiment, no virus was recovered in the feeding mosquitoes, but virus was

recovered in bedbugs for 1 to 4 hours post-feeding) (39,43).

Thus, the feeding experiments show that detectable levels of HIV are present in

mosquitoes and bedbugs for a short period of time and are primarily related to the

concentration of HIV in the blood meal. However, transfer of infection has not taken place

despite the presence of detectable levels of HIV in these insects. These findings tend to

confirm the hypothesis that only a very small amount of the ingested blood is transferred by

insects in interrupted feeding. Furthermore, the longer the time interval between feedings, the

less virus will be available for transfer. It should also be noted that the feeding experiments

generally involved HIV concentrations much greater than would be achieved under natural

conditions of feeding on HIV-infected humans.

How Probable Is Insect Transmission?

Among the major factors affecting the probability of insect transmission of HIV

infection are: 1 ) the prevalence of HIV-infections in the population within the range of the

suspect insects, 2) the frequency of interrupted feeding among these insects, 3) the time interval

between feeding on an infected person and feeding on an uninfected person, 4) the

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concentration of HIV in infected blood, and 5) the amount of HIV-containing blood that is

injected (and perhaps regurgitated) into the uninfected person.

In HIV infections, the amount of virus in blood is often very low or not detectable.

The pattern that seems to be emerging is as follows. HIV is found in blood soon after infection

but usually disappears from blood as antibodies are produced (1,26). The virus then reappears

in blood prior to the onset of developing clinical disease (29), with HIV appearing in the blood

as much as 12 months (46) to 32 months (59) prior to the development of clinical disease.

These observations are supported by recent evidence from studies of the wives/female sexual

partners of hemophiliacs with HIV infections, in which the level of infectivity of the

hemophiliacs increased with loss of their immune functions (28). The presence of HIV without

the development of antibodies can also occur (17), especially in infants and young children, in

whom perhaps as many as 10 percent may lack antibodies but are infected with HIV (7).

However, at any point in time, there will be many antibody positive, HIV-infected

persons who will have no detectable virus in their blood. For example, various investigators

have been able to detect HIV in the serum of antibody positive persons in only 1 of 13 persons

(46), 6 of 30 persons (59), 40 of 198 persons (16), and 20 of 78 persons (54), or in

approximately 20 percent of HIV antibody-positive persons. When lymphocytes from peripheral

blood were cultured, 59 of 284 (77) and 18 of 39 (54) HIV antibody-positive persons had

detectable HIV. In the latter study, one-third, or 6 of the 18 persons with positive

lymphocytes, also had detectable HIV in their serum (54). Levy estimates that HIV can be

cultured from the serum of about 30 percent of HIV antibody positive persons, and that HIV is

present in about 1 of every 50,000 (5x10 -4) white blood cells (lymphocytes constitute between

20 and 30 percent of white blood cells), or in about 1 of every 10,000 circulating lymphocytes

(10-4), ranging between 1 of every 1,000 to 1 million (10-3 to 10-6) circulating lymphocytes (47).

Piot and Schofield estimate that only 1 in 1 million (10-6) circulating lymphocytes are likely to

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Do Insects Transmit AIDS?

be infected with HIV (63). Gallo and his colleagues estimate that less than 1 in 10,000 (10-4 to

10

-5) circulating lymphocytes are infected (32,35).

Furthermore, the availability of drugs to treat clinical AIDS may significantly alter

the presence of virus in the blood of HIV-infected persons, especially if anti-HIV drugs are

used to treat viremia and not just those who have already progressed to clinical disease. The

first available drug, AZT (now known as zidovudine, or RetrovirR

), has been shown to

significantly decrease the level of HIV in blood (9,19).

The amount of HIV circulating in blood is also very small compared to the amounts

of virus in other diseases in which mechanical transmission through insect vectors has been

shown in field or laboratory experiments. Serum contains less than 10 tissue culture infectious

doses per milliliter; when infectious serum is diluted more than 10 times, evidence of HIV

activity usually cannot be obtained (56). Others have found that only undiluted serum (not a

tenfold dilution) yielded infectious virus, although in one study, 1 of 78 persons with antibodies

to HIV had 25,000 (2.5 x 104

) tissue culture infectious doses per milliliter (54).

In contrast, the amount of blood on a horse fly’s mouthparts is only on the order of

10

-5 ml; if a 10 percent deposition of this bloodmeal residue is taken as a minimum, the range

of potential deposition would be between 10-6 and 10-5 ml (23). This range of potential

deposition correlates well with actual transmission trials of EIAV and BLV (37). In the human

HIV situation, such a deposition of blood would have a low probability of containing even one

tissue culture infectious dose of HIV.

If a person is bitten by a bloodsucking insect, what are the chances that infection

with HIV will occur? The probability of infection will be determined by numerous factors, all

of which must converge through a chain of events that makes insect transmission of HIV

infection extremely improbable. If it does occur, each case of insect transmission would be a

rare and unusual event.

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The following chain of events

person, be interrupted for one reason or

person within 1 hour; 2) the first partial

and the insect must complete its feeding

must take place: 1 ) the insect must start to feed on one

another, and then complete its feeding on another

feeding must take place on an HIV-infected person,

on an uninfected person; and 3) the blood that the

insect transfers must contain enough HIV to infect the second person. The probability of the

occurrence of this chain of events is not just the sum of the probability of each of these events;

instead, each event’s probability is independent and therefore must be multiplied by the

probability of the others. For example, suppose that each event’s probability was 1 in 10. Then

the probability of the entire chain of events would be 1 in 1,000. One

interrupted, but only 1 in 10 of those interrupted would have bitten an

and then bitten an uninfected person, and only 1 in 10 of those insects

enough HIV to infect the second person.

How often does interrupted feeding take place in the normal

in 10 bites would be

HIV-infected person

would have transferred

feeding behavior of

bloodsucking insects? In general, the larger the insect and the more painful the bite, the greater

the probability that their feeding will be interrupted. Horse flies and other large biting flies

would therefore be among the insects whose initial feeding attempts would

interrupted. Other bites, such as from mosquitoes and bedbugs, are mostly

Interrupted feeding does not usually occur with bedbugs, and when it does

most often be

noticed afterwards.

occur, they usually

return to the same individual. Mosquitoes are usually relatively rapid feeders, often being

three-fourths to fully fed before the person being attacked is aware of them (8). Still others,

such as ticks, virtually never have the chance to engage in interrupted feeding. Thus, on the

basis of interrupted feeding alone, mechanical transmission of HIV infections by ticks can be

ruled out.

If interrupted feeding does take place, what are the chances that the insect had first

fed on an HIV-infected person and then completed its feeding on an uninfected person? The

probability will be determined principally by the following factors: 1) the probability that an

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Do Insects Transmit AIDS?

insect that has been interrupted in its feeding will move on to another person to complete its

feeding rather than attempting to bite the first person again, 2) the distance between the HIVinfected person and the uninfected person, and 3) the prevalence of HIV-infected persons in

the population within the feeding range of the insect (a few hundred feet for flying insects, in

the same household for bedbugs).

How often will an insect that has had its feeding interrupted move to another person

to complete its bloodmeal? Experiments conducted with horse flies resulted in the following

findings. Four horses were placed in a square of 9 meters to a side and allowed to be bitten by

750 horse flies of eight species. Only those flies that transferred from the original horse to

another were captured. There appeared to be a correlation between horse fly size and transfers;

i.e., the larger the horse fly, the greater the chances of the fly transferring from its original host

to another horse. Only about 2 percent of the time did the horse flies transfer to another horse

instead of refeeding on the original host horse (20,37). Thus, even when interrupted, horse

flies, which are among the insects most prone to engage in interrupted feeding, seldom do.

These experiments were also conducted in a very small area, and the probability that the horse

flies would have transferred between animals when their feeding was interrupted would

decrease rapidly as the distance between horses increased.

How would the prevalence of HIV infections in the population within the feeding

range of bloodsucking insects affect the probability of transfers between HIV-infected and

uninfected persons? If 1 percent of the population was HIV-infected, and if interrupted

feeding actually occurred, the chances of an insect first feeding on an HIV-infected person and

then completing its feeding on an uninfected person would be 0.0099 (0.01 x 0.99), or

approximately 1 in 100. If we assume a2 percent probability that the insect will actually

transfer between hosts (i.e., that infected and uninfected persons are within a few yards of each

other), the probability increases to O.000198 (0.0099x0.02), or about 1 in 5,000. Using similar

assumptions but varying the prevalence of HIV infections, the probability when the prevalence

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of HIV infections is 5 percent would be 0.00095, or approximately 1 in 1,000; for a prevalence

of 10 percent HIV infections, the probability would be 0.0018, or approximately 1 in 500; if 50

percent of the population is already infected with HIV, the probability would be 0.005, or 1 in

200. (See appendix A.) These probabilities are based on experiments with horse flies, with

horses in close quarters. The probabilities would decrease significantly as the distance between

infected and uninfected persons increased. Furthermore, the risks should be considerably lower

with other insects, such as mosquitoes and bedbugs, for which interrupted feeding occurs much

less frequently.

If all of the prior events occurred, what are the probabilities that enough HIVinfected blood will be transmitted to cause infection? Recall that, on average, only 20 to 30

percent of HIV-infected persons will have detectable levels of HIV in their blood, and almost

all of those with detectable levels will have very small amounts of HIV in their blood.

Researchers have been working for several years to develop a simple predictive model

for mechanical transmission of other pathogenic agents. While use of currently available

information results in a wide range of probabilities, the chances of mechanical transmission

occurring under defined circumstances are predictable. The amount of blood transferred from

an infected person to an uninfected person either contains an infectious dose or it doesn’t.

When estimates are made to predict how many insects bites it would take to reach a probability

of one for transmission, it means that at least one insect would be expected to transfer

infection.

Two examples of probability estimates are presented in appendix A for transfer of

infection by bloodsucking insects that have recently fed on an HIV-infected person and that

still contain some HIV-infected blood. While the probability estimates are low, they vary quite

widely. For mosquitoes, the probability ranges from 1 in 33,000 bites for mosquitoes (based on

the amount of tissue culture infectious units of HIV in the blood of an HIV-infected person,

the amount of blood transferred, and comparisons with the rate of infection from needlestick

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injuries) to 1 in

fever virus).

Thus,

Do Insects Transmit AIDS?

10 million bites (based on mechanical transmission experiments with Rift Valley

when the probabilities for each of the necessary chain of events--interrupted

feeding, feeding on an HIV-infected person before feeding on an uninfected person, and

transfer of sufficient blood to infect the second person with HIV--are multiplied by each other,

the resulting probability of insect transmission will be extremely remote.

Finally, the risk of HIV infections through needle-stick injuries may provide some

perspective on the probabilities of transmission through bloodsucking insects. HIV infections

through needle-stick injuries occur approximately 0.3 percent (0.003)of the time (48,56,75). In

contrast, hepatitis B virus (HBV) infections through needle sticks occur 6 to 30 percent of the

time (15), with an average transmission rate of 12 percent (30,75,76). There is also one reported

case of a needle-stick injury from a needle that had been used on an AIDS patient who was also

infected with HBV. The person who was accidentally stuck became infected with HBV, but not

with HIV (27). The volume of blood that would be inoculated with a needle-stick injury (with

a 25 gauge needle) has been estimated at one-thousandth of a milliliter (10-3 ml) (57), or about

100 times the amount on a mosquito’s proboscis and 14 times greater than in the mouthparts of

a bedbug (56).

Conclusion

Experiments with mechanical transmission of other viral diseases have shown that,

under the right conditions, transmission through insect vectors can occur. Experiments designed

to answer the question of whether HIV can survive in bloodsucking insects long enough to be

transmitted if interrupted feeding occurs have shown that it is theoretically possible. However,

based on the conditions necessary for successful transmission of other viral diseases, and on the

biology of HIV infections in humans, the probability of insect transmission of HIV is extremely

low.

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A chain of unlikely events must occur if mechanical transmission will be at all

possible, and the probability that a bloodsucking insect’s bite will transmit HIV infection will be

determined by multiplying the probabilities of each of these events.

The probability of mechanical transmission by insects would be increased if the

following conditions were met: 1) a high percentage of HIV-infected persons in the population

within the range of bloodsucking insects, 2) continuous, high virus concentrations in the blood

of HIV-infected persons, and 3) very high concentrations of bloodsucking insects, so that the

exposed population is literally bitten hundreds or even thousands of times by insects that have

bitten HIV carriers and then bitten susceptible individuals within a short period of time (an

hour or less).

Some environmental situations may exist in which persons will suffer literally

hundreds of bites from bloodsucking insects, but it is highly improbable that most of these

insects will have bitten an HIV carrier immediately before. This situation may change as the

prevalence of HIV infection increases, but the majority of the exposed population will have to

be already infected before insect bites can become even a plausible route of transmission of any

consequence to the remaining uninfected population. Even if the prevalence of infection

increases significantly, there is no indication that the current pattern of low or undetectable

virus concentrations in the blood of most HIV-infected persons will change. If there is a

change, hopefully it will be toward even lower levels of HIV in the blood, as more drugs to

treat AIDS and HIV infections are developed.

These conclusions are based on the available evidence, and there undoubtedly will not

be unanimous agreement as to whether this evidence is sufficient or all-inclusive. For example,

other infections may enhance a person’s susceptibility to being infected with HIV (64).

However, even under conditions where HIV infection may be enhanced, there is no reason to

believe that insect transmission would be preferentially enhanced over other modes of

transmission. People with other infections may be more vulnerable to HIV infection than

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Do Insects Transmit AIDS?

persons without these other infections, when all are exposed to the same route of transmission.

In other words, different natural histories of HIV infection may occur in a given population

that is exposed to HIV infection in the same manner. The point, however, is that further

investigations on the possibilities and probabilities of insect transmission of HIV may be needed

to confirm (or refute) the conclusions that can be made on the available evidence. What these

investigations might consist of will be addressed later in this Staff Paper.

However, it is agreed that further investigations are needed of the tentative findings

of Chermann and his colleagues at the Pasteur Institute that some insects in areas endemic for

HIV infection contain HIV-like nucleic acid sequences in their DNA. These findings need to

be confirmed by other researchers, especially because some of the tentative findings--such as

the apparent presence of the HIV genome in antlions-- cannot be reconciled with what is known

about HIV. If confirmed, the most important issue from the standpoint of insects as possible

transmitters of HIV is to investigate the possibility of changes in the intracellular mechanisms of

these insects that might make biological transmission-- replication and expression of HIV--

possible.

IS THE EPIDEMIOLOGY OF HIV INFECTIONS

CONSISTENT WITH INSECT TRANSMISSION?

Two questions can be directed at the epidemiological information: 1) are there HIV

infection patterns that cannot be explained by established risk factors (such as sexual

transmission or intravenous drug use), and 2) are these HIV infection patterns consistent with

transmission through insect vectors?

The major routes of transmission in the United States are clearly related to sexual

practices and intravenous administration (i.e., intravenous drug use, blood-clotting factors for

hemophiliacs, and blood transfusions), and in infants, through perinatal transmission from HIV-

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Office of Technology Assessment

infected mothers. The conclusions that can be reasonably drawn from related insect-borne viral

diseases and the biology of HIV infections support these findings and make it clear that insect

transmission, if occurring, is not a significant cause of HIV infections.

Proponents for insect vectors as a major cause of HIV infection in parts of the

United States have alleged that 22 percent of AIDS cases in Florida, 33 percent in Miami, and

50 percent in the Belle Glade community of south Florida have no identifiable risk factors (79).

On the other hand, the Centers for Disease Control (CDC) have reported that only 4.8

percent of adult cases of AIDS in Florida had no reported risk factors. Of the 76 adult cases of

AIDS (3 pediatric patients were born to HIV-infected mothers) reported in western Palm Beach

County, Florida (including Belle Glade), between July 1982 through September 15, 1986, 13 (17

percent; 11 men and 2 women) had no reported risk factors; but 10 had died before

epidemiological investigations could be completed. Twenty-seven of the risk factor cases were

attributed to heterosexual contact. However, only 10 had heterosexual contact with a person

with AIDS or at increased risk for AIDS; the other 17 were categorized as heterosexual contacts

solely on the basis of being born in Haiti, where heterosexual transmission is believed to play a

major role (67).

If the 17 persons born in Haiti were assigned to the “no risk factor” group, a much

larger percentage (39 percent vs. 17 percent) of AIDS cases in western Palm Beach County

would have no identifiable risk factors. However, can insect transmission account for these

cases?

Proponents of insect transmission state: “In Belle Glade, the absence of antibody to

HIV in 134 children tested has been used as an argument against insect transmission... children

have far less exposure to insects than adults (they go to bed early, go to school, and do not

work in the fields). By virtue of their tender age children have had less time to be exposed to

agents found in a very small percentage of mosquitoes” (78). No evidence is available to

support this hypothesis.

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Do Insects Transmit AIDS?

Moreover, CDC has reported a broader picture of HIV infection prevalence rates for

Belle Glade residents. Of the first 736 persons for whom data entry had been completed,

antibodies to HIV were found in: 1) none of 121 children ages 2-10 years; 2) 14 (8.9 percent) of

157 persons ages 18-29; 3) 7 (4.4 percent) of 160 persons ages 30-39; 4) 2 (1.8 percent) of 113

persons ages 40-49; 5) 3 (3.2 percent) of 91 persons ages 50-59; and 6) none of 94 persons over

60 years of age. Eighty-eight percent of HIV antibody-positive adults were ages 18-49 years;

90 percent of adult AIDS patients reported in the United States are in that same age group (67).

More importantly, in Belle Glade the percent of adults positive fo rHIV antibody decreased

with age, and none of 94 persons over age 60 were positive. These findings are not consistent

with insect transmission, because if insects were responsible for causing HIV infections in this

population, why are there no HIV infections in people under 18 years of age or over the age of

60? Furthermore, the rationale stated above for absence of antibodies to HIV in children does

not explain the decreasing prevalence of HIV antibodies with age and the absence of antibodies

in those over age 60.

The most striking difference between HIV infections and AIDS in Americans and

Europeans versus Africans (and Caribbean) is the ratio of male to female cases. In Africa the

male-female ratio is 1:1, while the American and European ratios are 13:1.

The American and European ratios are explained by male homosexual contact as

being a major mode of transmission of HIV, and the number of women began to rise only after

they had sexual contact or shared intravenous drugs with HIV-infected men. The African (and

Caribbean) ratio has suggested heterosexual transmission of HIV and exposure to contaminated

needles for medical purposes, transfusion with unscreened infected blood, cultural rites such as

scarification (5,62), and affliction with a wide variety of diseases that result in immunologic

changes which would make a person more susceptible to HIV infection (64).

HIV infections in Africa have been associated with sexually active age groups (52,53),

and female prostitutes (45,61) and their clients (14). For example, in a study of 1,078 subjects

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in Lusaka, Zambia, the prevalence of infection was low in subjects less than 20 years of age

and more than 60 years of age. The peak prevalence occurred in men between the ages of 30-

35, and in women, between the ages of 20-25. The infection rate between men and women was

not significantly different. None of 12 subjects under the age of 15, and none of 21 subjects

above the age of 70, were infected (52). These types of findings have been used to support the

hypothesis of heterosexual transmission, and the lack of HIV infection among the older

population has been interpreted as reflecting recent introduction of HIV infection. Even if

these findings are not considered persuasive for heterosexual transmission, they nevertheless do

not support the hypothesis that insect transmission is the operative cause, because of the lack of

HIV infections in the younger and older age groups. Insect transmission would also require a

large reservoir of infected persons in order to transmit HIV to uninfected persons, and a large

reservoir of infected persons would not exist if HIV were recently introduced.

In an evaluation of 2,464 Haitians in 1985-86 and 191 Haitians for whom blood had

previously been collected during a 1977-79 dengue fever outbreak, it was concluded that HIV

infections were recently introduced into Haiti and were more prevalent

lower socioeconomic groups (60). The finding of HIV infections being

in urban areas and in

more prevalent in lower

socioeconomic groups in urban areas of a tropical country might be taken as a proxy for

crowded, unsanitary conditions conducive to high concentrations of biting insects, and hence,

supportive of insect transmission as a possible cause for HIV infection. But recent introduction

of HIV into Haiti would again argue against insect transmission because of the lack of a large

reservoir of infected persons. Furthermore, in Africa, higher education levels (in contrast to the

Haiti findings of a correlation with lower socioeconomic standing) have been correlated with

HIV infection (52,61,62). One explanation for the

highly educated persons have greater opportunities

and social interactions, and thus exposure to HIV.

African pattern of infection is that more

than more poorly educated persons for travel

This could also explain the higher prevalence

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Do Insects Transmit AIDS?

of HIV

and the

even in

infections in urban versus rural areas that have been found in

United States.

These findings do not reflect insect transmission as a major

tropical countries,

might be caused by insect

HIV-infected persons and

In one study of

but they do not rule out the possibility that

transmission. One such possibility arises in

Africa (53), Haiti (60),

factor in HIV infection,

some HIV infections

households containing

extremely large numbers of bloodsucking insects, such as bedbugs.

45 Florida households containing persons with clinical AIDS and/or

HIV infections, none of 90 uninfected children and none of 29 household contacts became

infected. Uninfected persons who did become infected became so through sexual contact (18).

In contrast, in an African household (Rwanda), at the time of testing for HIV infection, the

mother, father, and three sons ages 6 years, 5 years, and 18 months, were all HIV antibody

positive (38). The mother and all three sons had clinical symptoms, while the father did not.

The investigators hypothesized that bloodsucking insects, among other causes, might have

accounted for the infections among all three brothers, because the children’s symptoms started

developing several months to years afterbirth. However, in children with AIDS reported to the

Centers for Disease Control from 1982 to 1985, the estimated incubation period for AIDS in

children has increased each surveillance year, with the longest incubation exceeding 7 years (68).

Moreover, the mother developed clinical signs of AIDS 3 months after the birth of the first

child, which means that she was likely to have been already infected (but without clinical

disease) during the pregnancy. Thus, it is more likely that perinatal transmission was the cause

of HIV infections in all three children.

Conclusion

The major routes of transmission of HIV infection in the United States (and Europe)

are firmly established; i.e., sexual practices of homosexual (and bisexual) men and sharing of

contaminated intravenous drug paraphernalia. Methods to inactivate the virus in blood clotting

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Office of Technology Assessment

factors (for hemophiliacs), and exclusion of high risk persons and use of the HIV antibody test

in blood and plasma donations have reduced infection through therapeutic use of blood and

blood products. Perinatal HIV infections in infants from infected mothers and spread from

HIV-infected males to females are increasing. In tropical countries (e.g., in Africa and the

Caribbean), heterosexual transmission seems to be the dominant mode.

The nature of epidemiological studies is to look for associations--i. e., risk factors that

can be correlated with HIV infections and with progression in HIV-infected persons to the

clinical disease, AIDS. These types of studies cannot be expected to identify risk factors in all

cases because of statistical limitations. Moreover, eliciting a possible cause of infection from

HIV-infected persons is difficult in some cases because the greatest known risk factors are sex

and drug practices that many persons morally object to and/or that are illegal.

Uncertainty about the cause of HIV infections in a proportion of the cases, however,

is clearly not proof that another transmission route, such as insect transmission, is involved. A

plausible causal explanation must exist. In surveys of various populations, wherever in the

world these have been conducted, infected persons have been concentrated in the sexually active

age groups. Doubts may exist that heterosexual transmission is the explanation for the equal

prevalence rates of HIV infection in men and women in Africa (and the Caribbean), but there

is no reason to conclude that insect transmission must therefore be operating.

While the data from insect studies indicate insect transmission of HIV infection as

extremely improbable, situations may exist in which some insect transmission might occur. If

so, insect transmission is likely to occur in limited areas with unusual environmental

characteristics. For example, in field studies with equine infectious anemia virus (EIAV),

transmission through horse flies can be significantly reduced by keeping infected animals more

than 200 yards away from uninfected animals (21). Conditions conducive to insect transmission

would be: 1 ) extremely crowded living conditions, 2) a very high biting insect population, with

persons routinely suffering hundreds to thousands of bites (dependent on the vector potential of

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Do Insects Transmit AIDS?

the biting insects; i.e., the types of insects), and 3) a high rate of HIV infection among the

persons within the range of the insects. Even if these conditions were satisfied, the laboratory

evidence shows that: 1 ) at any given time, most HIV-infected persons do not have infectious

virus in their blood, and 2) when infectious virus is present, it is at a much lower concentration

than the levels that have been necessary for transmission with other insect-borne viral diseases.

Environments meeting the criteria for potentially sustaining insect transmission could

be investigated in a more purposeful manner to determine if insect transmission in fact occurs.

If insect transmission can occur, then it would be in very localized neighborhoods (through

flying, biting insects) that satisfy the three conditions identified above, or in households

containing HIV-infected persons (e.g., through bedbugs or mosquitoes). Thus, the place to look

for evidence of insect transmission is in densely populated neighborhoods in proximity to large

numbers of suspect vectors, in households in densely populated urban areas in the tropics with a

high prevalence of HIV infections, and among migrant farm workers exposed to large numbers

of biting flies.

In sum, there is no evidence that insect transmission causes HIV infections in

temperate zones or even in tropical climates. This conclusion is consistent with the HIV insect

experiments and with experience with other insect-borne viral diseases. Insects as an unusual

cause of HIV transmission in specific situations such as those described above are a possibility,

but are still improbable. These situations, however, could be investigated in a more purposeful

manner, as described in the next section.

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WHAT FURTHER INFORMATION IS NEEDED?

Biology of HIV

Insect studies indicate that the amount of infective HIV in the blood of HIV-infected

persons is inadequate to support insect transmission. Data on HIV blood levels depend on the

sensitivity of the assay system used to measure HIV in blood. If the test systems in use detect

only a fraction of the true concentration of infectious virus particles, conclusions regarding the

quantity of virus ingested by a vector will have to be revised. On the other hand, cell culture

techniques currently in use to measure infectivity may not measure all the HIV that is present,

such as viral particles bound up in antigen-antibody complexes in the serum, but they likely

measure all infectious(unbound) particles (47} Thus, the tissue culture techniques may just as

likely be identifying HIV at some blood levels that may not be clinically infectious, because the

body’s defenses may successfully sequester and eliminate some HIV before a threshold viral load

is reached that would overwhelm the body’s defenses. So even if insects do transmit some HIV,

the amount transferred may not be enough to result in infection.

Nevertheless, better assays need to redeveloped in order to measure more accurately:

1) the amount of HIV and its various forms in blood, and 2) the infectivity of the various forms

of the virus that is found in blood--i.e., intact virus, partial virus particles, free virus/virus

particles in the serum or liquid portion of blood, and intracellular virus (virus in lymphocytes).

Second, fluctuations in the concentration of virus in the blood of HIV-infected persons over

time need to be more clearly understood. From current research, we know that virus is usually

present in blood after initial infection but before a substantial amount of antibody is produced,

then disappears and appears again as clinical disease develops. Third, variations in the infective

potential of HIV need to be examined. Persons with HIV-2 infection may have higher

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Do Insects Transmit AIDS?

concentrations in their blood than persons with HIV-1 infections. One feeding experiment has

also shown that HIV-2 is recoverable in mosquitoes for a short time at bloodmeal concentrations

that do not result in recoverable virus when HIV-1 has been used, and it is known that HIV-1

(and probably HIV-2) can frequently change its envelope (the outside surface of the virus), the

part of the virus that is crucial to infection of cells.

Insect Studies

HIV is recoverable in some species of mosquitoes and bedbugs for a few hours to 3

days only after feeding on blood that has been artificially infected with very high

concentrations of virus. Different species often have different feeding patterns, behavior, and

physiology. Some species regurgitate blood, others seldom or never do. Not only do different

species of mosquitoes often vary in behavior, so may subspecies and different geographic and

genetic strains of a species (31,73). Because of possible variations in the biological relationship

between HIV and different species and strains of mosquitoes and bedbugs, similar feeding

experiments could be conducted on more types of mosquitoes and bedbugs, not only to confirm

the previous findings regarding survival time,

biological transmission (i.e., whether HIV can

of the recent finding that HIV is harbored by

but also to check further for the possibility of

replicate and multiplying these insects). Because

the Langerhans cells of the epidermis, studies

could be conducted as to whether HIV could be picked up from these cells while the insect

probes for a blood capillary (8). Other insects that could also be tested include lice, fleas, horse

flies, and sand flies.

When mosquitoes and bedbugs are artificially fed on highly infected blood,

interrupted in their feeding, and allowed to complete feeding on uncontaminated blood, no

transfer of HIV has been found. This implies that transfer of infection between living sources

does not occur, but the only animal model for HIV infection that is available is the chimpanzee

(which can become infected but does not develop disease). Since chimpanzees are in very short

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supply and are already inadequate to meet the needs of higher priority HIV-related research,

new animal models need to be found. Existing, relevant models might include monkeys and

STLV (the monkey counterpart of the human HIV virus).

A unified model for human retrovirus mechanical transmission could be established

using EIAV and BLV infections, which range in measurable titers between 1 and 108 infectious

particles per milliliter of blood. The potential for a variety of insect species to act as vectors

within a range of retrovirus concentrations in ingested blood could then be evaluated (22).

It is known that Rift Valley fever virus (RVFV) ingestion by mosquitoes and

subsequent transmission can be enhanced if mosquitoes are infected with another organism

(microfiliariae) (74). In these experiments, 64 percent of mosquitoes without the other organism

took up detectable levels of RVFV, with 5 percent of refeeding mosquitoes transmitting the

virus. These numbers were increased to 88 percent and 31 percent, respectively, when infected

with microfiliariae. The presence of this other organism, however, only enhances the ability of

mosquitoes to transmit RVFV and does not turn mosquitoes from ineffective to effective

transmitters of RVFV. Furthermore, the way in which microfiliariae are thought to enhance

transmission of RVFV is by puncturing the midgut wall, thereby allowing RVFV to infect the

salivary glands, which is reflected in the observation that injected virus becomes more rapidly

transmissible than does ingested virus. No HIV has been recovered in mosquitoes that have

been directly injected with HIV-contaminated blood (see earlier discussion on “HIV Survival in

Insects”).

HIV

HIV

The

The effect on transmission of concurrent feeding of mosquitoes and bedbugs with

and hepatitis B virus (HBV) could also be investigated.

The preliminary findings of French researchers that field insects in areas endemic for

infection contain HIV-like nucleic acid sequences in their DNA need to be confirmed.

finding of HIV-like nucleic acid sequences in both male and female mosquitoes (male

mosquitoes do not take blood meals) reflects the possibility of transovarian transmission; i.e.,

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Do Insects Transmit AIDS?

integration into DNA such that it is inherited by subsequent generations. This is known to

occur with other viruses (2,7 1). However, the French findings may represent artifacts or other

endogenous insect nucleic acid sequences related to HIV but unrelated to active HIV infection

of these insects. Insects found to be positive by Chermann and his colleagues could be provided

to other groups to see if the findings are reproducible on the same materials. Bloodsucking and

other types of insects could be collected in neighborhoods at high-risk for HIV infections, and a

search made for free virus (RNA form) as well as integrated virus (DNA form).

The reasons for HIV binding to insect cells in the absence of the CD4 molecule (the

marker and entry point of HIV in human cells) also need to be determined, as well as the

mechanism in insect cells that inhibits the ability of the integrated form of HIV to take over

cell production processes in order to replicate and multiply. Researchers will be confirming and

pursuing these findings (if confirmed) in the search for new methods to inhibit HIV replication,

which could theoretically help in the search for new drugs against HIV. But these preliminary

findings should also be investigated for the potential of insects to act as reservoirs and future

sources of

cells were

have been

HIV infection.

[n the experiments showing that HIV could be integrated into insect cells, cultured

placed in direct contact with HIV and incubated in the laboratory. No experiments

performed in which insects have been fed or injected with HIV-infected blood, and

their offspring and subsequent generations examined for HIV integration into their DNA.

Laboratory studies using insects that are fed HIV-infected blood or in whom HIV was injected

intrathoracically, such as the types of studies conducted to determine HIV survivability in

mosquitoes and bedbugs, could also be

Electron microscopy studies could also

form after artificial feeding.

used to look for HIV integration into these insects’ DNA.

be undertaken to

Parallel investigations could also be conducted

the United States for EIAV and BLV. Insects associated

look for virus in virion or lymphocyte

among domestic animal populations in

with infected and virus-free herds

-27 -
Office of Technology Assessment

could be screened for incorporated retrovirus genomes. Laboratory studies using insects fed

directly on infected animals or insects injected intrathoracically

reconducted.

Epidemiological Studies

with purified virus could also

The epidemiological patterns of infection in Africa are especially difficult to decipher

at this time, and there is widespread agreement that prospective studies are needed in different

population groups to examine both transmission factors and those factors that make people who

are HIV-infected more prone to develop clinical disease (Quinn etal., 1987). However, even

though transmission factors are not well understood, the information that is available is not

supportive of insect transmission as a major factor in infection, even in Africa. Prospective

studies of the kinds needed to identify methods of transmission could also be broadened to

address the possibility of insect transmission.

If insect transmission is occurring, then the logical place to conduct these

epidemiological studies is in neighborhoods and households with high prevalence rates for HIV

infection and with superabundant concentrations of bloodsucking insects. Household studies of

the kinds conducted in the United States to examine the issue of whether HIV infections can be

transmitted through casual contact (18) can be easily adapted to look for insect transmission

under the conditions just described. Although more difficult to design,

could be similarly conducted.

Epidemiological studies with intervention strategies built into

conducted. Insect control programs could be instituted in appropriately

neighborhood studies

them could also be

identified

neighborhoods, and the incidence and cumulative prevalence of HIV infections compared with

similar neighborhoods without insect control programs. Various intervention strategies could

-28 -
Do Insects Transmit AIDS?

also be compared; i.e., strategies designed to decrease sexual contact and/or intravenous drug use

could be instituted in some neighborhoods, while insect control programs could be instituted in

other neighborhoods.

-29 -
Do Insects Transmit AIDS?

Appendix A:

PROBABILITY OF INSECT TRANSMISSION

I. PROBABILITY OF MOSQUITO TRANSMISSION OF HIV, BASED ON

HAMSTER ANIMAL MODEL FOR RIFT VALLEY FEVER VIRUS (RVFV):

It is known that inoculation of less than 1 infectious unit of RVFV will infect hamsters 100

percent of the time.

In mechanical transmission experiments, when 108 units of virus per ml of blood was present in

the first animal and the blood meal completed on a susceptible animal within 1 hour, actual

RVFV transmission rates to hamsters by mosquitoes were approximately 15 percent.

Therefore, if 1/109 milliliter (10-9, or 0.000,000,001 ml) of blood had been inoculated, 100

percent of the hamsters would have become infected.

Thus, mosquitoes must have inoculated less than 10-9 ml, or approximately 10-10 ml of blood

during an interrupted feeding.

If we assume that an HIV-infected person has an HIV concentration of 103 units per ml of

blood (which is about 100 times the concentration usually found in an HIV-infected person

without symptoms of disease), the probability of a mosquito that began feeding on an HIVinfected person and completed its feeding on a susceptible person inoculating one unit of virus

would be 103/1010, or 10-7, or 1 in 10 million.

(Provided by Char!es Bai!ey, Chief, Department of Arboviral Entomology, U.S. Armv Medical

Research Institute, Ft. Detrick, Maryland)

II. PROBABILITY OF BEDBUG TRANSMISSION OF HIV, BASED ON

AMOUNT OF BLOOD ON THE MOUTHPARTS OF A BEDBUG:

Volume of blood on bedbug mouthparts = 7 x 10-5 ml

Number of lymphocytes on bedbug mouthparts:

= (number of lymphocytes in blood) x (vol. in bedbug mouthparts)

= (2.5 x 106 lymphs/ml) x (7 x 10-5 ml)

= 175 lymphocytes/bug

(Continued...)

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Office of Technology Assessment

Concentration of virus in HIV-infected person’s blood:

serum = usually less than 10 tissue culture infectious doses per ml

lymphocytes = usually about 1 in 1 million (10-6)

= (2.5 x 106 lymphs/ml) x (1 x 10-6 infected lymphocytes)

= 2.5 tissue culture infectious doses per ml

Total tissue culture infectious dose per bedbug bite:

From serum: 1 to 10

From lymphocytes: 2.5

= (3.5 to 12.5 tissue culture infectious doses/ml) x (7 x 10-5 ml)

= (24.5 to 87.5) x 10

-4 tissue culture infectious doses per bite

# of bites required so that at least one bite will transfer 1 tissue culture infectious dose:

= 1/((24.5 to 87.5) X 1 0

-4)

= 4,082 to 1,143

Therefore, at each bite, there is a 1/1,143 to 1/4,082 chance that 1 tissue culture infectious dose

of HIV will be transmitted.

These estimates are based on the assumptions that serum usually contains less than 10 tissue culture infectious doses per milliliter of HIV (54,56) and that only about 1 in 1 million (10-6)

lymphocytes are infected (56,63). However, an occasional HIV-infected person has been shown

to contain up to 25,000 tissue culture infectious units of HIV per ml (54), and lymphocyte

infection has been estimated by others to range from 1 in 1,000 to 1 in 1 million (47), and

averaging less than 1 in 10,000 (10-4to 10-5). On the other hand, only 20 to 30 percent of

HIV-antibody positive persons have recoverable virus in their serum at any one time (see text).

The volume of blood injected through an accidental needle-stick injury has been estimated at

0.001 ml (10-3 ml) (57), and the rate of HIV infection through needle-stick injuries has been

estimated at 0.3 percent (1/333)(48,56,75). The volume of blood injected through a needlestick injury is about 14 times greater than the amount of blood on the mouthparts of a bedbug

and 100 times greater than on a mosquito’s proboscis (56). Therefore, the risk of infection

following a single mosquito transfer would be 0.003 percent (1/33,000), and the risk following a

bedbug transfer would be 0.02 percent (1/4,667). The bedbug estimate is similar to the

estimates derived above from the amount of blood on a bedbug’s mouthparts, when the

assumptions are that serum contains less than 10 tissue culture infectious doses of HIV per ml,

and that only about 1 in 1 million lymphocytes are infected.

(Modified from estimates provided by Thomas Monath, Director, Division of Vector-Borne Viral

Diseases, Centers for Disease Control, Ft. Collins, Colorado)

-32 -
III.

Do Insects Transmit AIDS?

PROBABILITY OF A HORSE FLY TRANSFERRING FROM AN

HIV-INFECTED HOST TO AN UNINFECTED HOST DURING

INTERRUPTED FEEDING:

When horse flies are interrupted in feeding, they will usually return to the original animal to

continue feeding rather than move to another animal. Even when the animals are close

together, horse flies move to another animal only about 2 percent of the time (20,37). If we

assume that horse flies will move to another host 2 percent of the time if interrupted in their

feeding and that horse flies and HIV-infected humans are randomly distributed, what are the

probabilities of an uninfected person being bitten by a horse fly that had been feeding on an

HIV-infected person when the horse fly was interrupted in its feeding? The probabilities

would vary with the prevalence of HIV infection in the target population.

In the first example, the prevalence of HIV infection is 1 percent. If a horse fly first bit an

HIV-infected person, the probability of its then completing feeding on an uninfected person

would be: 0.01 x 0.99 x 0.02 = 0.000198, or about 1 in 5,000. The probabilities associated with

different prevalence rates of HIV infection are also presented.

1 Percent Prevalence of HIV Infection:

Bite #1 Bite #2

1% HIV positive 1°/o HIV + = (0.01 x 0.01 x 0.02) = 0.000002

99% HIV - = (0.01 X 0.99 X 0.02) = 0.000198

99% HIV negative 1% HIV + = (0.99 X 0.01 X 0.02) = 0.000198

99% HIV - = (0.99 X 0.99 X 0.02) = 0.0196

5 Percent Prevalence of HIV Infection:

Bite #l Bite #2

5% HIV positive 5% HIV + = (0.05 x 0.05 x 0.02) = 0.00005

95% HIV - = (0.05 x 0.95 x 0.02) = 0.00095

95% HIV negative 50/0 HIV + = (0.95 x 0.05 x 0.02) = 0.00095

95% HIV - = (0.95 X 0,95 X 0.02) = 0.018

(Continued...)

-33 -
Office of Technology Assessment

10 Percent Prevalence of HIV Infection:

Bite #l Bite #2

10% HIV positive 10% HIV + = (0.1 x 0.1 x 0.02) = 0.0002

90% HIV - = (0.1 X 0.9 X 0.02) = 0.0018

90% HIV negative 10% HIV + = (0.9 X 0.1 X 0.02) = 0.0018

90% HIV - = (0.9 X 0.9 X 0.02) = 0.016

50 Percent Prevalence of HIV Infection:

Bite #l Bite #2

50% HIV positive 50% HIV + = (0.5 X 0.5 X 0.02) = 0.005

50% HIV - = (0.5 x 0.5 x 0.02) = 0.005

50% HIV negative 50% HIV + = (0.5 X 0.5 X 0.02) = 0.005

50% HIV - = (0.5 x 0.5 x 0.02) = 0.005

These calculations show that even if the prevalence of HIV infection was 50 percent, if horse

flies are interrupted in their feeding, only 5/1,000 of the time would the horse fly have bitten

an infected person first and an uninfected person second. The chance of transmitting HIV

would be much less, because only 20 to 30 percent of those infected would have HIV in their

blood. Furthermore, most bites would involve an insufficient amount of blood to transmit an

infectious dose of HIV when in fact the virus was present in the blood of the first person

bitten. These probabilities are also only for interrupted feeding and do not account for the

times, which would probably be in the (great) majority of cases, that insects did not bite two

persons in quick succession.

-34 -
Do Insects Transmit AIDS?

Appendix B:

Participants of July 8, 1987, OTA Workshop on:

Can the AIDS Virus be Transmitted by Insects?

Charles Bailey

Chief, Dept. of Arboviral Entomology

U.S. Army Medical Research Institute

Ft. Detrick, Maryland

William Blattner

Chief, Family Studies Program

National Cancer Institute

Bethesda, Maryland

Baruch Blumberg

Vice President for Population Oncology

Fox-Chase Cancer Center

Philadelphia, Pennsylvania

Donald Burke

Chief, Dept. of Virology

Walter Reed Army Institute of Research

Washington, D.C.

Kenneth Castro

Medical Epidemiologist

AIDS Program

Centers for Disease Control —

Atlanta, Georgia

Jean Claude Chermann

Head of Unit

Biology of Retroviruses

Pasteur Institute

Paris, France

Lane Foil/Charles Issel

Depts. of Entomology and

Veterinary Science

Louisiana State University

Baton Rouge, Louisiana

Robert Gwadz

Scott Halstead

Associate Director

Health Sciences Division

Rockefeller Foundation

New York, New York

Peter Jupp

Medical Entomologist

National Institute for Virology

Johannesburg, South Africa

Caroline MacLeod/Mark Whiteside

Co-Directors

Institute of Tropical Medicine

North Miami Beach, Florida

Philip Markham

Director of Virus Operations

Bionetics Research, Inc.

Rockville, Maryland

Thomas Monath

Director

Division of Vector-Borne Viral Disc

Centers for Disease Control

Ft. Collins, Colorado

William Reeves, Jr.

Director

Gorgas Memorial Laboratory

Panama City, Panama

Leon Rosen

of Director, Arbovirus Program

Pacific Biomedical Research Center

University of Hawaii-Manoa

Honolulu, Hawaii

Gregory Tignor

Laboratory of Parasitic Diseases Deputy Director

National Institute of Allergy Yale Arbovirus Research Unit

and Infectious Diseases Yale School of Medicine

Bethesda, Maryland New Haven, Connecticut

-35 -
1.

2.

3.

4.

5.

6.

7.

8.

9.

10.

11.

12.

13.

Do Insects Transmit AIDS?

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Office of Technology Assessment

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