NationalLibraryOfScotland/medical-history-of-british-india huggingface dataset:
111
The year 1894 we have already commented upon. The rainfall over the
affected district was phenomenal. As we have shown the epidemic was not a
one.
V.-CORRELATION OF FIGURES REPRESENTING FEVER MORTALITY WITH THOSE
FOR RAINFALL AND PRICES.
The close connection between these three series of figures has led me to work
out the values with a view to ascertaining the amount of correlation between each
series.
The figures representing fever are the number of deaths in October and Novem-
ber of each year taking the unit as 10,000. Those for rainfall are the averages in
inches of the rainfall at 24 stations in the plains. For prices have been taken the
number of lbs. at which wheat is selling to the rupee deducted from 60, this being to
convert the high rates into the higher figures instead of these being represented as
they would otherwise be by the lower figures.
A fourth value is also used, namely the coefficient already mentioned in
Chapter XI, as obtained by dividing each year's rainfall average by the average
for the previous year.
The actual values used are given in an Appendix.
Working out the coefficients of correlation between the fever series and each
of the three other series we get :—
1. Fever with Rainfall
—
Coefficient of correlation
.
.
.
.
.
.
.
.67
Three times probable error
.
.
.
.
.
.
.
.168
2. Fever with Prices—
Coefficient of correlation
.
.
.
.
.
.
.
.61
Three times probable error
.
.
.
.
.
.
.
.201
3. Fever with coefficient obtained by dividing each year's average
rainfall by that of the previous year—
Coefficient of correlation
.
.
.
.
.
.
.
.74
Three times probable error
. .
.
.
.
.
.
.144
Correlating fever with various combinations of the other values, it was found
that the correlation coefficient could be increased thus :—
4. Fever with Rainfall × Prices—
Coefficient of correlation
.
.
.
.
.
. .
. 80
Three times probable error .
.
.
.
.
.
.
.114
5. Fever with Rainfall × Prices × Coefficient—
Coefficient of correlation
.
.
. .
.
.
.
.83
Three times probable error
.
.
.
.
.
.
.
.099
112
6 . F e v e r w i t h P r i c e s × C o e f f i c i e n t —
Coefficient of correlation
.
.
. .
.
.
.
.80
Three times probable error .
.
.
. .
. .
.114
The means and standard deviations for each series of values are given beneath
each column in the Appendix.
Plotting out the correlations graphically we get the results shown in Charts 6
and 7.
The following abstractions from these are very significant:—
In the 20 years in which rainfall was below the mean (16.6) the
average mortality was
.
.
.
. .
.
.
81,000
In the 10 years in which rainfall was between 16.6 and 20 the
average mortality was
.
.
.
.
.
.
.
104,000
In the 9 years in which rainfall was between 20 and 30 the aver-
age mortality was
.
.
.
.
.
.
.
.
177,000
In regard to Rainfall × Prices
In the 25 years below the mean (40.2) the average mortality was
83,000
In the 10 years between 40.2 and 60 the average mortality was .
121,000
In the 5 years over 60 the average mortality was
.
.
.
217,000
In Chart 6 whilst the epidemic years are seen to follow the correlation line, a
very interesting group of non-epidemic years characterised by heavy rainfall are
seen markedly diverging from this. The group illustrates very clearly the fact
that heaviness of the rainfall is not the only factor at work. It is significant that
all these years are years of low prices.
VI.—GENERAL DETERMINING CAUSES OF EPIDEMICS.
There seems very little doubt that the two factors, rainfall and scarcity, are the
determining causes of the epidemic malaria seen in the Punjab. Broadly speak-
ing until plague appeared malaria must have been the main agent which brought
to a head in actual mortality the effects produced by the great economic stresses.
Just as in famines malaria cannot act until nature is about to bring them to an end,
so there can be little doubt that the effects of scarcity are to a large extent held
over until the appearance of the first heavy monsoon. Then though the effect of
the rain is to reap a harvest of deaths the period of stress is brought to an end.
The enormous proportion of deaths among infants and the aged in the epi-
demic mortality has already been referred to. The result of the almost cyclical
influences we have indicated upon the population is a curious one and is expressed
in the following abstract from the Census of India, which, though it refers to the
effects of famine, applies equally to those of epidemic malaria:
—
"All sections of the population however, are not equally affected; the very
old and the very young suffer most, while those in the prime of life
sustain only a comparatively small diminution in the numbers . .
113
. . Consequently at the close of a famine the population consists
of an unsusally small proportion of children and old persons and a large
proportion of persons in the prime of life, i.e., at the reproductive ages.
For some years therefore in the absence of any fresh calamity the growth
of the population is very rapid . . . . . This more rapid growth con-
tinues for some years, but then as the persons who at the time of the
famine were in their prime pass into old age and their place is taken by
the generation born shortly before the famine withits numbers greatly
reduced by the mortality which then occurred, the birth rate falls not
only below that of years following the famine but also below the
average, and the pendulum continues its swing backwards and forwards
between periods of high and low birth rate, but its oscillations gradually
become fainter until they cease from natural causes to be apparent or,
as more often happens, until some fresh calamity obliterates them."
That at the back of such colossal manifestations as the epidemic of 1908 there
should be profound and not easily averted natural influences at work is scarcely to
be wondered. at. When we approach the question of prophylaxis, we shall do well
to bear in mind the magnitude of the influences against which we are pitting our-
selves and not be led in a foolish vein to reduce the remedy of the whole matter to
trifling proportions.
115
PART IV.
EXPERIMENTAL WORK ON THE MECHANISM OF FULMINANT
MALARIA.
CHAPTER XIV.
The Nature of Intense Malaria.
In all that has gone before we assume that malaria has, during the epidemic
period, been present in a more intense form than usual. But we have not defined
intensity nor considered what such a term implies.
Spleen Rate.
The results of recording the size of 3,134 spleens in children in one or other of
the five classes we have hitherto used in recording spleen rate are as follows :—
58
278
1,035
713
1,032
Omitting the last class or those that were negative and recording graphically
in percentages we get the result shown in Chart 8 and the following values :—
2.7
13.2
49.2
34.8
Whether we take the rates as ascertained in the examination of 1,256 children
in Amritsar or the figures for 1,695 children examined elsewhere in the Punjab, we
get a very similar curve as shown by the two series of figures :—
Amritsar
.
.
.
.
.
.
2.4
11.7
48.3
37.4
Elsewhere
.
.
.
.
.
.
2.9
14.1
49.8
33.0
If we take the rate for Hakiman Katra, where the spleen rate works out at 91.6
per cent., we get the variation shown in the figure, and if we work out the rates for
an area in the central part of the town we get a variation in an opposite direction,
also shown in the figure.
Grouping together those observations which showed an approximately similar
spleen rate, we get the following associated variations in the number of spleens in
the different classes :—
Amritsar.
Spleen rate 90 per cent. or over
.
.
.
.
5.1
16.2
46.8
31.7
„ „ 80 „ under 90 per cent.
.
.
1.7
13.7
53.3
31.1
„ „ 7 0 „ „ 8 0 „
.
.
.
1.3
9.1
54.9
34.5
„ „ 50 „ „ 70 „
.
.
.
.7
9.2
48.2
4 1 . 8
„ „ 3 0 „ „ 5 0 „
.
.
.
2.0
20.4
77.7
„ „ under 30 „
. .
.
31.2
68.8
Q 2
116
Similarly treating the figures for rural area we get the results as follows :—
Various rural areas.
Spleen rate 100 per cent. .
.
.
.
.
. 10.4
29.5
44.8
16.4
„ „ 80 „ under 100 per cent.
.
. 3.1
15.5
54.8
26.4
„ „ 60 „
„ 80 „ .
.
. 2.1
12.2
54.8
30.8
„ „ 40 „
„ 60 „ .
.
.
.7
3.5
53.2
42.5
„ „
under 40 .. .
.
. 1.5
3.0
40.9
54.5
Some approximation of the curves above given is so general that exceptions
have only been found when one was dealing with a mixture of two communities.
In this case the values at once warned one that conditions were unusual.
With an increase in the number of persons showing evidence of malaria infec-
tion there is always under the conditions in the Punjab an increased intensity as
shown by the size of the organ.
Parasite Rate.
If in a similar manner we record the parasite finds in 599 blood examinations
of children giving as values the number of parasites found in 100 microscope fields,
we get the result shown in Chart 9 and numerically recorded below :
—
Parasite finds in 599 children
Under 1 per 100 fields, i.e., negative to standard employed 373
Under 5 per 100 field
. .
.
.
.
.
. 92 40 per cent.
Over 5 but under 10 per 100 fields
. .
.
. .
. 48 25 „
„ 10 „ „ 20 „ ,, „ .
.
.
.
.
.
. 39
1 7 „
„ 2 0 „ „ 5 0 „ „ „ .
.
.
.
.
. . 32 14 „
„ 50 „ „ 100 „ „ „ . .
.
.
.
. . 10
4 „
„ 100 „ „ 200 „ „ „ .
.
.
.
.
. . 3
1 „
„ 200 „ „ 500 „ „ „ . . .
.
.
.
. 1 . 4 „
„ 500 „ „ 1,000 „ „ „ .
.
.
.
.
. . 1
.4 „
Comparing this curve with that shown by the distribution of parasite values
found in the blood of children at Palwal, we see a progression towards the higher
values in a very regular way.
We may suspect that intensity of malaria is partly at any rate dependent upon
the numerical prevalence of parasites.
Relation of Spleen Rate and Parasite Rate to Epidemic Conditions.
We have seen that the spleen rate of Amritsar and Delhi and elsewhere in the
Punjab is normally so high as to approach 100 per cent. Parasite infection is also
found in a large proportion of the children. These conditions are not in themselves
necessarily associated with the excessive mortality of epidemic times. But from
what we have said in the last section it would be possible nevertheless that the
epidemics were associated with an increase of the numerically higher infections.
117
In figures already given in the section upon Amritsar it will be seen that the
autumn increase in spleens and parasites was of this nature, and that the condition
in February 1909 was as regards parasites and spleen rate more intense than even at
the height of the fever season of 1909. The high parasite rate at Palwal shortly
after the epidemic is also remarkable and is probably not normal. A remarkable
feature in regard to spleen rate and parasite rate demonstrated by Charts 10 and 11
is the relative stability of the curves which are not apparently easily displaced to
any degree.
118
CHAPTER XV.
Experiments upon the Quantitative Factor in the Transmission of
Proteosoma to Sparrows by the Bites of Culex.
When commenting upon the possibilities in connection with the causation of
malarial epidemics Colonel Leslie, I. M. S., suggested to me that the effect of the
number of bites by infected anopheles might be found to cause differences in the
resulting infection and in the incubation period. I had already thought over the
considerations outlined in the first chapter of this section and I saw much to favour
the view that " intensity " in malaria might be a quantitative matter and that it
might depend upon the number of infected anopheles by which the members of a
community were being bitten.
Among other facts I called to mind the severe infections that one often sees
following even a single night's exposure under circumstances in which one has
every reason for believing that infected anopheles abound and the comparative
mildness of attacks seen in those who live under conditions in which infected ano-
pheles are comparatively infrequent. That intensity of an attack of malaria might
be the result of dosage of infection would also explain the peculiar fact that a person
immune to a certain degree of malaria frequently succumbs if he be exposed to a
greater intensity of the same disease.
If true this conception would also go far to explaining the occurrence of epide-
mic conditions and the apparently increased virulence or intensity of the disease
at these times.
The difficulties of experimenting on the human subject are great and I deter-
mined therefore to obtain such data as I could from observations upon Proteosoma
in sparrows, the material for which is unlimited.
The experiments recorded below were carried out by feeding bred Culex fati-
gans upon infected birds and later noting the effect of bites of these upon healthy
birds. The sparrows when brought to the laboratory were at once examined and
infected ones placed aside. Those found free from infection were, prior to being
utilised for experiment, quarantined for a period of at least twelve days. Before
being used their blood was again carefully examined for proteosoma. During the
period of quarantine they were carefully protected from the bites of culex in a
series of large mosquito nets, only two mosquitoes (neither of which contained
sporozoites in the glands) being found throughout the course of the experiments to
have gained admission to these birds.
In the experiments only mosquitoes which in the morning were seen to contain
blood were noted as having fed. Birds which had been enclosed with mosquitoes
119
which had apparently not fed were liberated as it was not possible to be sure they
had not been bitten.
After the experiment as before the birds were most carefully protected from
stray mosquitoes.
I—EXPERIMENTS TO SHOW THE EFFECT OF THE BITE OF A SINGLE INFECTED
MOSQUITO.
Experiment 1.
21st May 1909. —Batch 1 mosquitoes fed on Sparrow 17 in whose blood were a fair num-
ber of proteosoma.
24th May 1909. —A mosquito dissected showed fairly numerous zygotes present.
28th May 1909. —Sparrow 32 fed on by a single mosquito of this batch.
. .
2nd June 1909 and 3rd June 1909. —Examined daily. Blood negative.
4th June 1909.—2 parasites per 100 fields.
5th June 1909.—2 parasites per 100 fields.
6th June 1909.—Blood negative.
8th June 1909.— 1 parasite per 100 fields.
Incubation period—7 days.
Infection—very mild.
Experiment 2.
22nd May 1909.—Batch 2 mosquitoes fed on Sparrow 19 in whose blood were numerous
proteosoma.
26th May 1909—1 mosquito dissected showed numerous zygotes (in places as many
as 5 per field) in the gut.
30th May 1909.—Sparrow 52 fed on by a single mosquito of this batch. Mosquito dis-
sected showed sporozoites.
31st May 1909 to 4th June 1909. —Examined daily. Blood negative.
5th June 1909. —A single proteosoma parasite seen after long search.
6th June 1909.—2 parasites per 100 fields.
7th June 1909.—2 parasites per 100 fields.
8th June 1909.—Negative.
11th June 1909—Negative.
Incubation period—6 days.
Infection—mild.
Experiment 3.
1st June 1909.—Sparrow 47 fed on by a single mosquito of Batch 2. A mosquito dis-
sected on this day showed numerous sporozoites in the glands.
2nd June 1909 to 7th June 1909. —Examined daily. Blood negative.
8th June 1909.—10 parasites in 100 fields.
9th June 1909.—55 parasites in 100 fields.
10th June 1909.—100 parasites in 100 fields. Numerous segmenting forms. Bird quiet.
Incubation period—7 days.
Infection—medium.
120
Experiment 4.
31st May 1909.—Batch 5 mosquitoes fed on Sparrow 19.
3rd June 1909. —A mosquito dissected showed a moderate number of zygotes.
10th June 1909.—Sparrow 158 fed on by a single mosquito from this batch. A mosquito
dissected showed not very numerous sporozoites in the glands.
11th June 1909 to 16th June 1909. —Examined daily. Blood negative.
17th June 1909.—5 parasites per 100 fields.
18th June 1909.— 2 parasites per 100 fields.
20th June 1909. —2 parasites per 100 fields.
22nd June 1909.—Negative.
24th June 1909. —Negative.
Incubation period—7 days.
Infection—mild and evanescent.
Experiment 5.
8th June 1909. —Sparrow 156 bitten by a single mosquito of Batch 5. This mosquito
dissected showed a fair number of sporozoites in the glands.
12th June 1909 to 26th June 1909.—Examined daily. Blood negative.
Result—negative.
Experiment 6.
10th June 1909.—Sparrow 159 fed on by one mosquito of Batch 5. Mosquito dissected
showed scanty sporozoites.
15th June 1909 to 26th June 1909.—Examined daily. Blood negative.
Result—negative.
Experiment 7.
11th June 1909.—Sparrow 169 fed on by 2 mosquitoes of Batch 5. These mosquitoes
dissected showed in both cases scanty sporozoites in the glands.
15th June 1909 to 19th June 1909.—E xamined daily. Blood negative.
20th June 1909.— 1 parasite per 100 fields.
21st June 1909.—8 parasites per 100 fields.
22nd June 1909.—28 parasites per 100 fields.
23rd June 1909.—2 parasites per 100 fields.
25th June 1909. —Negative.
28th June 1909.— 2 parasites per 100 fields.
Incubation period—9 days.
Infection—mild.
Experiment 8.
30th May 1909.—Batch 7 mosquitoes fed on Sparrow 23, whose blood contained numerous
proteosoma.
6th June 1909. —A mosquito dissected showed numerous sporozoites in glands.
7th June 1909. —Sparrow 110 fed on by one mosquito of above batch.
12th June 1909 and 13th June 1909.—Examined daily. Blood negative.
14th June 1909.—16 parasites per 100 fields.
121
16th June 1909.—80 parasites per 100 fields.
16th June 1909.—240 parasites per 100 fields.
20th June 1909.– 370 parasites per 100 fields.
22nd June 1909.—Bird dead. Proteosoma in heart's blood, 400 per 100 fields segmenting
forms.
Incubation period—7 days.
Infection—mortal.
Experiment 9.
18th June 1909.—Batch 12 mosquitoes fed on Sparrow 179 whose blood contained
numerous proteosoma.
27th June 1909. —Sparrow 184 fed on by a single mosquito from Batch 12. Dissected
showed abundant sporozoites in glands.
3rd June 1909.—Blood negative.
4th June 1909.—5 parasites per 100 fields.
5th June 1909.—25 parasites per 100 fields.
Bird accidently killed.
Incubation period—7 days.
Infection —
II.—EXPERIMENTS TO SHOW THE EFFECT OF A NUMBER OF BITES BY SCANTILY INFECTED
MOSQUITOES.
Experiment 10.
28th May 1909.—Batch 3 mosquitoes fed on Sparrow 31.
31st May 1909.—Mosquitoes found to have only a few zygotes. Every mosquito ex-
amined however was infected.
31st May1909. —Sparrow 54 bitten by 6 mosquitoes of Batch 3.
1st June 1909 to 8th June 1909. —Examined daily. Blood negative.
9th June 1909.—4 parasites per 100 fields.
10th June 1909.— 2 parasites per 100 fields.
11th June 1909.— 2 parasites per 100 fields.
Incubation period—9 days.
Infection—mild.
Experiment 11.
2 2nd June 1909.—Sparrow 57 fed on by 12 mosquitoes of Batch 3.
3rd June to 7th June 1909.—Examined daily. Blood negative.
8th June 1909.— 16 parasites per 100 fields.
9th June 1909.—8 parasites per 100 fields.
10th June 1909.—8 parasites per 100 fields.
14th June 1909.— 4 parasites per 100 fields.
17th June 1909.—Negative.
Incubation period—6 days.
Infection—mild. R
122
Experiment 12.
4th June 1909.—Sparrow 58 fed on by 5 mosquitoes of Batch 3.
5th June 1909.— Sparrow 58 fed on by another 5 mosquitoes of Batch 3.
6th June 1909.—Sparrow 58 fed on by another mosquito of Batch 3.
7th June 1909.—Sparrow 58 fed on by two mosquitoes of Batch 3.
Two mosquitoes were dissected showing in both cases very scanty sporozoites.
8th June and 9th June 1909.— Examined daily. Blood negative.
10th June 1909.—2 parasites per 100 fields.
11th June to 14th June 1909.— Examined daily. Blood negative.
15th June 1909.—2 parasites per 100 fields.
Incubation period—6 days.
Infection—extremely mild.
Experiment 13.
10th June 1909.—Batch 9 mosquitoes fed on Sparrow 108.
18th June 1909.— Sparrow 177 fed on by 8 mosquitoes of above batch. Dissected, five
showed very scanty sporozoites and three were negative.
22nd June to 28th June 1909.— Examined daily. Blood negative.
Result—negative.
I I I . —EXPERI MENTS TO SHOW THE EFFECT OF A NUMBER OF HEAVI LY I NFECTED MOSQUI TOES.
Experiment 14.
30th May 1909.— Sparrow 43 bitten by 10 mosquitoes of Batch 2.
31st May 1909.—Sparrow 43 bitten by 5 more of same batch.
31st May 1909.—Mosquito dissected after feeding on sparrow, glands swollen with sporo-
zoites.
31st May to 3rd June 1909.—Examined daily. Blood negative.
4th June 1909.—30 parasites per 100 fields.
5th June 1909.—170 parasites per 100 fields.
6th June 1909.—340 parasites per 100 fields.
7th June 1909.—290 parasites per 100 fields.
Bird ill, blood very anæmic.
9th June 1909.—170 parasites per 100 fields.
Bird not so ill.
25th June 1909.—600 parasites per 100 fields.
Bird moribund. Flagellating forms present.
Incubation period—5 days.
Infection—mortal.
Experiment 15.
14th June 1909.—Batch 11 mosquitoes fed on Sparrow 43.
21st June 1909.—Sparrow 180 fed on by 8 mosquitoes of Batch 11.
Mosquitoes dissected showed 2 with abundant sporozoites.
4 with fairly abundant sporozoites.
2 with scanty sporozoites.
123
26th June 1909.— 8 parasites per 100 fields.
27th June 1909.— 40 parasites per 100 fields.
28th June 1909.—125 parasites per 100 fields.
29th Jane 1909.—480 parasites per 100 fields ; bird seedy.
2nd July 1909.—Bird died.
Incubation period—5 days.
Infection—mortal.
Experiment 16.
13th June 1909.—Batch 10 mosquitoes fed on Sparrow 43 which was a very heavily
infected experimental bird (vide Exp. 14).
22nd June 1909.—Sparrow 45 fed on by 6 mosquitoes of Batch 10.
( 1 ) mosquito with enormous numbers of sporozoites in glands.
These dissected showed ( 2 ) very abundant sporozoites.
( 3 ) abundant sporozoites.
26th June 1909.—2 parasites per 100 fields.
27th June 1909.—105 parasites per 100 fields.
28th June 1909.—320 parasites per 100 fields.
29th June 1909.—130 parasites per 100 fields.
30th June 1909.— 140 parasites per 100 fields.
3rd July 1909.—1,220 parasites per 100 fields.
7th July 1909.—Bird died.
Incubation period—4 days.
Infection—mortal.
Experiment 17.
14th June 1909.—Sparrow 173 fed on by 3 mosquitoes of Batch 14.
These dissected showed
(1) abundant sporozoites in glands (Series A).
(2) scanty sporozoites in glands (Series A).
(3) scanty sporozoites in glands (Series A).
15th June to 21st June 1909.—Examined daily. Blood negative.
22nd June 1900.— 2 parasites per 100 fields.
23rd June 1909.— 20 parasites per 100 fields.
24th June 1909.—2 parasites per 100 fields.
25th June 1909.— 2 parasites per 100 fields.
Bird recovered.
Incubation period—8 days.
Infection—mild.
Experiment 18.
18th June 1909.—Batch 13 mosquitoes fed on Sparrow 23 whose blood showed numerous
proteosoma.
27th June 1909.—Sparrow 181 fed on by 3 mosquitoes from Batch 13.
These dissected showed
(1) abundant sporozoites in glands.
(2) scanty sporozoites in glands.
30th June and 1st July 1909.—Examined daily. Blood negative.
R 2
124
3rd July 1909. —1 parasite per 100 fields.
4th July 1909.—12 parasites per 100 fields.
5th July 1909.— 88 parasites per 100 fields.
6th July 1909.—64 parasites per 100 fields.
7th July 1909.— 75 parasites per 100 fields.
Bird recovered.
Incubation period—6 days.
Infection—medium.
IV.—EXPERIMENTS TO SHOW EFFECTS OF BITES BY INFECTED MOSQUITOES UPON BIRDS
ALREADY INFECTED.
Experiment 19.
30th May 1909.—Sparrow 39 fed on by 4 mosquitoes of Batch 2.
2nd June 1909. —5 parasites per 100 fields.
3rd June 1909.— 1 parasite per 100 fields.
4th June 1909.—1 parasite per 100 fields.
5th June 1909. —Negative.
8th June 1909.—40 parasities per 100 fields. Bird seedy and anæmic.
9th June 1909.—32 parasites per 100 fields.
10th June 1909.—8 parasites per 100 fields.
14th June 1909.—12 parasites per 100 fields.
Bird recovered.
Incubation period—(?)
Infection—probably superadded to original mild infection which had probably been
overlooked.
Experiment 20.
20th June 1909.—Sparrow 177 scanty proteosoma.
25th June 1909.—Sparrow 177 proteosoma 2 per 100 fields.
26th June 1909. —Sparrow 177 fed on by 5 mosquitoes of Batch 13.
28th June 1909.— 3 parasites per 100 fields.
30th June 1909.— 5 parasites per 100 fields.
3rd July 1909.— 34 parasites per 100 fields.
4th July 1909.—28 parasites per 100 fields.
5th July 1909.—100 parasites per 100 fields.
Bird escaped.
Incubation period—(?)
Infection—superadded.
In the course of these experiments a number of points have presented them-
selves. When commencing the experiments I had thought only of the " number "
of infected mosquitoes concerned. But it is obvious that there is another matter
to be considered.
A mosquito (culex) fed on a sparrow whose blood contains only a few gametes
of proteosoma, shows in the mid-gut only a few zygotes, and in mosquitoes so fed