Synopses

Coccidioidomycosis: A Reemerging Infectious Disease

Theo N. Kirkland, M.D., and Joshua Fierer, M.D.
Departments of Pathology and Medicine, University of California, San Diego
School of Medicine
Department of Veterans Affairs Medical Center, San Diego, California, USA
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          Coccidioides immitis, the primary pathogenic fungus
          that causes coccidioidomycosis, is most commonly found
          in the deserts of the southwestern United States and
          Central and South America. During the early 1990s, the
          incidence of coccidioidomycosis in California increased
          dramatically. Even though most infections are
          subclinical or self-limited, the outbreak is estimated
          to have cost more than $66 million in direct medical
          expenses and time lost from work in Kern County,
          California, alone. In addition to the financial loss,
          this pathogen causes serious and life-threatening
          disseminated infections, especially among the
          immunosuppressed, including AIDS patients. This article
          discusses factors that may be responsible for the
          increased incidence of coccidioidomycosis (e.g.,
          climatic and demographic changes and the clinical
          problems of coccidioidomycosis in the
          immunocompromised) and new approaches to therapy and
          prevention.

Emerging infectious diseases have been defined as infections that have
newly existed in a population or have existed but are rapidly increasing in
incidence or geographic range (1). In what sense is coccidioidomycosis an
emerging infectious disease? Coccidioidomycosis is not a new disease; it
was first recognized and reported slightly more than 100 years ago by a
medical student in Argentina (2). In fact, coccidioidomycosis has affected
inhabitants of the desert Southwest for thousands of years (3). However, in
the past several years, the number of cases of coccidioidomycosis has
increased dramatically, and the clinical symptoms of this illness have
changed in patients with acquired immunodefficiency syndrome (AIDS). In
this article, we explore some of the reasons for the increased incidence of
coccidioidomycosis, review the new clinical data, and discuss current
approaches to therapy and prevention.

Etiology

Coccidioidomycosis is caused by Coccidioides immitis, a dimorphic fungus
that grows as a mold in the soil. The mold forms arthroconidia within the
hypha, a type of conidia formation known as enteroarthric development
(Figure 1) [Figures not available in ASCII] (4). C. immitis is the only 
species within the primary pathogenic fungi that has this type of conidia development. Alternate conidia undergo autolysis, leaving empty spaces between viable
arthroconidia. The arthroconidia are released into the atmosphere when the
wind ruptures the hypha. C. immitis infects humans and animals almost
exclusively by the respiratory route (5). Once inhaled, the arthroconidia
cluster in the lungs and undergo a dramatic morphologic change. The round
cells, which develop into spherules, undergo repeated internal divisions
until they are filled with hundreds to thousands of offspring, termed
endospores. This process occurs over 48 to 72 hours (6). When the spherule
ruptures, each released endospore has the capacity to develop into a mature
spherule.

Epidemiology

C. immitis is primarily found in desert soil. It is present in highest
numbers in the San Joaquin Valley in California, southern Arizona, southern
New Mexico, west Texas, and the desert areas of northern Mexico (Figure 2)
 [Figures not available in ASCII].
The organism is also found in scattered foci in coastal southern
California, southern Nevada, and Utah (7) and is endemic in a few areas in
Central and South America, especially in Venezuela (7). C. immitis is
distributed unevenly in the soil and seems to be concentrated around animal
burrows and ancient Indian burial sites (8,9); it is usually found 4 to 12
inches below the surface of the soil (7).

Since C. immitis infects humans by the respiratory route, exposure to dust
is one critical factor determining the risk for infection (10).
Coccidioidomycosis is not spread from person to person, except in
extraordinary circumstances. Coccidioidomycosis probably had its most
profound effect on the population of the United States during World War II
when several training airfields were built in the San Joaquin Valley. The
rate of new infections in military personnel was 8% to 25% per year (10).
Coccidioidomycosis was the most common cause of hospitalization at many
airbases in the Southwest. Though the death rate was very low, many
soldiers were sick for weeks to months, and their training was completely
disrupted. At least in part because of efforts to minimize dust, the
infection rate declined as the war went on (10).

The incidence of coccidioidomycosis varies with the season; it is highest
in late summer and early fall when the soil is dry and the crops are
harvested (10). If it rains at this time of the year (which is unusual in
southern California), disease incidence declines as the amount of dust
decreases. Dust storms are frequently followed by outbreaks of
coccidioidomycosis. One particularly severe dust storm in 1977 carried dust
from the San Joaquin Valley up to the San Francisco Bay area and resulted
in hundreds of cases of nonendemic coccidioidomycosis in areas north of the
San Joaquin Valley (11). More recently, an earthquake centered in
Northridge, California, was associated with 170 cases of acute
coccidioidomycosis in Ventura County, which normally has a low incidence of
this disease. The airborne dust associated with landslides triggered by the
earthquake was implicated in the increase in the number of cases (12).

Occupational or recreational exposure to dust is also an important
consideration. Agricultural workers, construction workers, or others (such
as archeologists) who dig in the soil in the disease-endemic area are at
increased risk for the disease (13,14). During World War II, C. E. Smith,
one of the most perceptive and influential epidemiologists to study
coccidioidomycosis, recommended dust control as a primary measure to reduce
risk for exposure (10). However, because the desert is inherently dusty,
many cases of coccidioidomycosis are acquired just by driving through the
disease-endemic area.

Clinical Illness

C. immitis is transmitted by the respiratory route. Smith et al., in a
prospective study of cases of coccidioidomycosis acquired during World War
II by soldiers at three San Joaquin Valley airbases, skin-tested the airmen
periodically and questioned them about illnesses in the interval. They
found that most infections (60%) were asymptomatic and resolved
spontaneously; 15% were not severe enough to require medical care, and 25%
were clinically important and required a substantial amount of time off
work (15). In symptomatic patients, the pulmonary illness ranges from a
self-limited flulike illness to pneumonia (16). Approximately 5% of primary
infections result in erythema nodosum or erythema marginatum with
associated noninfectious arthritis; most of those patients have a
self-limited infection (17). Particularly in persons with diabetes,
multiple thin-walled chronic cavities tend to develop as a residual effect
of pulmonary coccidioidomycosis (18). Unlike in tuberculosis, in
coccidioidomycosis, dissemination almost always becomes evident within a
few weeks of the primary pneumonia, although in cases of limited
dissemination it may not become clinically evident until months later
(15,19). Cocidioidomycosis can disseminate and cause miliary disease, bone
and joint infection, skin disease, soft tissue abscesses, and meningitis
(15,16). These extrapulmonary complications are uncommon (<5% of
infections).

The risk for disseminated coccidioidomycosis is much higher among some
ethnic groups, particularly African-Americans and Filipinos. In these
ethnic groups, the risk for disseminated coccidioidomycosis is tenfold that
of the general population (5,20). Presumably, a gene (or genes) that
increases susceptibility to infection is more prevalent in these ethnic
groups than in the general population. Such a resistance gene has been
identified in mice (21-23), but not yet in humans. The mechanism by which
the resistance genes affect the course of the disease in mice is not clear.
Pregnant women and the immunosuppressed are also at high risk for
developing disseminated disease (Figure 3) [Figures not available in 
ASCII] (24). One study demonstrated that the growth rate of spherules was 
influenced by human sex hormones, which may partially account for the 
increased risk of disseminated disease in pregnancy (25). Pregnancy 
also redirects the immune response toward humeral (TH2) immunity and away 
from delayed hypersensitivity (TH1) (26), which may influence resolution of
coccidioidomycosis. Generalized suppression of cell mediated immunity also 
increases the risk of disseminated disease (27). Coccidioidomycosis is 
particularly severe in patients with organ transplants or AIDS.

Though disseminated coccidioidomycosis is uncommon, and symptomatic
coccidioidal pneumonia usually resolves without therapy, many of these
patients are very ill for weeks to months. Galgiani reported that a group
of college students in Tucson who had coccidioidomycosis required an
average of six clinic visits before the disease resolved (16). Therefore,
this can be an expensive illness in terms of medical costs and time lost
from work or school, even when the infection resolves spontaneously.

Coccidioidomycosis Epidemic in California

Kern County, in the San Joaquin Valley, California, is one of the most
highly coccidioidomycosis-endemic regions. The number of new cases of
coccidioidomycosis in the area has varied widely from year to year; a low
incidence of coccidioidomycosis from 1987 to 1990 (<500 reported cases a
year in Kern County), was followed by a high incidence from 1991 to 1994
(28-30). The number of reported cases, which were identified by serologic
testing at the Kern County Health Department (the reference serology
laboratory for the county), probably represent approximately 10% of the
total number of infected persons in that county (Figure 4) [Figures not 
available in ASCII] (28). The medical costs for infected persons in Kern 
County are estimated at $66 million (29). In 1992, 4,500 new cases were 
reported to the California State Department of Health Services (30), most 
from Kern County; the number of new cases also increased in almost all 
counties in central and southern California (30). The increase in reported 
cases in California in 199192 was dramatic but certainly an underestimate 
of the magnitude of the problem (31).

The epidemic seems to be waning, for reasons that are not clear, but the
marked increase in incidence from the 1980s to 1991 through 1993 is
indisputable. What factors may account for this increase? One major
consideration is the weather. C. E. Smith observed years ago that the
number of cases of coccidioidomycosis was higher in the summer after a
rainy winter than after a dry winter (10). In March 1991, a 5-year drought
in California ended with a heavy rainfall. Rainfall was also heavy in the
winters of 1992 and 1993. Though the relationship between the weather and
the density of C. immitis in the soil may never be understood in detail,
the following scenario seems plausible. During drought years, the number of
organisms competing with C. immitis decreases. C. immitis does not thrive,
but it remains viable though dormant. After heavy rain, the arthrocondia
germinate and multiply to a higher density than usual because of the lack
of competing organisms. Once the soil dries in the late summer and fall,
the arthroconidia become airborne and potentially infectious (29).

Another reason for the sudden increase in disease incidence might have been
the number of susceptible persons in the disease-endemic area. The number
may have been the result of both increased migration of susceptible persons
and decreased immunity in the indigenous population. Immunity comes from
prior infection and is manifest as a positive coccidioidin skin test. In
almost all cases, coccidioidomycosis confers lifelong immunity. As a result
of years of low incidence, the number of nonimmune persons may have
increased, as evidenced by the decrease in prevalence of positive
coccidioidin skin tests among local high school students. In 1939, 50% to
60% of high school students in the San Joaquin Valley had positive skin
tests (17), but in the 1980s only 3% to 5% of high school students had
positive skin tests (T. Larwood, pers. comm.). Given the historical data,
this estimate seems low, but another study also found a low prevalence. In
1985, workers in Tucson estimated that 30% of a random sample of persons in
a Hispanic neighborhood had positive skin tests (32). In addition to the
drought, irrigation of fields, the increasing amount of land under
cultivation, and a decrease in indoor dust due to the widespread use of air
conditioning may also have played a role in the relatively low incidence of
infections in the 1980s.

Coccidioidomycosis in the Immunosuppressed

C. immitis is a primary pathogen that can cause disease in immunologically
healthy persons. In the population as a whole, fewer than 5% of infected
persons have persistent pulmonary infection or extrapulmonary dissemination
of the disease (16). The incidence of clinically significant disease in
immunosuppressed patients is much higher. In one study symptomatic
coccidioidomycosis developed in 18 (7%) of 260 renal transplant patients in
Arizona over a 10-year period, primarily in the first year after
transplantation (33). This rate was substantially higher than the rate of
infection in patients who were undergoing hemodialysis. Approximately 12
(67%) of infections in the patients with renal transplants were
disseminated; the remainder were confined to the lung. Of patients with
disseminated disease, 10 (83%) died, despite intensive therapy with
amphotericin B. In another study from Tucson, all confirmed cases of
coccidioidomycosis during a 4-year period were reviewed. The dissemination
rate was 8 (73%) of 11 of patients who were receiving immunosuppressive
therapies, compared with only 15 (14%) of 110 healthy controls (34). As
more patients in the disease-endemic area receive liver, lung, and heart
transplants, this problem will increase.

Pregnant women, especially those in the third trimester, are at high risk
for developing disseminated coccidioidomycosis if they become infected
(24). In the first and second trimesters, the risk is much lower. The
reason is not entirely clear, but two factors may play a role: 1) the high
sex hormone levels found in late pregnancy enhance the growth of C. immitis
in vitro (25), and 2) the shift in the T-cell immune response late in
pregnancy toward TH2 cytokines (26) interferes with resolving the
infection. In experimental animals, pregnancy increases the severity of
leishmaniasis, another infection hat is controlled by a TH1 T-cell response
(35).

Coccidioidomycosis in AIDS patients is also very likely to be
life-threatening. The first cases of coccidioidomycosis described in AIDS
patients were atypical, with a reticulonodular chest x-ray pattern,
positive blood cultures, and infection of multiple organs (36). As we have
gained more experience with coccidioidomycosis in HIV-infected persons, we
have learned that the clinical spectrum is broader than originally
reported. Fish and his colleagues collected data from 77 AIDS patients with
coccidioidomycosis who were treated by physicians in Arizona and California
(37). They grouped the patients according to their clinical symptoms
(Figure 5) [Figures not available in ASCII].

Although the largest group of patients had diffuse pulmonary infiltrates, a
significant fraction had focal pulmonary disease, meningitis, or other
extrapulmonary disease. Six patients had only a positive serologic test
with no other evidence of infection. Excluding the patients who had only a
positive serologic test, 81% of the patients in this series had a positive
serologic test for coccidioidomycosis, either for IgM or IgG antibodies.
However, only 69% of patients with diffuse pulmonary disease had a positive
serologic test, and the death rate in this group was also the highest
(70%). In all clinical groups, death was correlated with the number of
circulating CD4 T-cells at the time of diagnosis. For clinicians, however,
the most important message from this study is that coccidioidomycosis is
not a uniformly fatal complication in patients with AIDS, and that many
forms of this disease, including meningitis, respond to therapy. Patients
with <200 CD4 T-cells/l are more likely to have severe, disseminated
infections.

A more recent prospective study of 170 HIV-infected persons in an area of
Arizona where coccidioidomycosis is endemic showed a cumulative incidence
of coccidioidomycosis of 25% over 41 months (38). The most important risk
factors were the level of CD4 T-cells and the diagnosis of AIDS (as opposed
to HIV infection). HIV-infected patients with AIDS or <250 CD4 T-cells/l
were 8 to 35 times more likely to get coccidioidomycosis. History of
coccidioidomycosis, a history of a positive skin test for
coccidioidomycosis, or a prolonged stay in the disease-endemic area were
not associated with an increased risk for infection. These data suggest
that most cases were primary infections in severely immunosuppressed
patients. Since patients with AIDS were not more likely to be exposed to
the spores of C. immitis and all patients were seen prospectively at
4-month intervals and tested for antibody to C. immitis, severe
immunosuppression appeared to increase their risk for infection, as well as
disease. As in the retrospective study reviewed above, the clinical
symptoms varied widely, ranging from mild to extremely severe. Only one
patient had antibody titers to C. immitis by complement fixation test
without any other evidence of disease.

Treatment

Various drugs are now available for treating coccidioidomycosis. In
addition to amphotericin B, which must be given intravenously and is
considerably toxic, triazole compounds have been found to be active agents
for treating most manifestations of coccidioidomycosis. Fluconazole, in an
uncontrolled study, was reported to be effective primary therapy for
coccidioidal meningitis; since untreated coccidioidal meningitis is
uniformly fatal, robust conclusions could be drawn from this trial (39).
Forty-seven consecutive patients were treated with 400 mg/day of
fluconazole; during the first 6 months of therapy, 33 (70%) of the patients
responded to therapy. (A response was defined as a 40% reduction in a
score, on the basis of clinical measurements and cerebrospinal fluid
findings.) Two patients who did not respond to therapy died of
coccidioidomycosis; both were HIV-positive. Because of previous experience
with high relapse rates when azole therapy is stopped, the authors
recommended lifelong treatment with fluconazole. In a small study, four of
five patients treated for meningitis with itraconazole as sole therapy
responded favorably (40). A recent article emphasized the high relapse rate
after azole therapy is stopped (41). The alternative treatment to the
azoles is amphotericin B. If amphotericin B is used to treat meningitis,
however, it must be given intrathecally as well as intravenously, and this
greatly increases the risk for a toxic reaction to that drug.

Clearly, fluconazole and itraconazole can be used to treat patients with
nonmeningeal coccidioidomycosis (42-44). Whether one of these drugs is
superior to the other, or how either one compares to amphotericin B is not
known. It seems prudent to treat extremely ill patients with amphotericin
B, at least until their clinical situation stabilizes, although no
published studies support that point of view. However, few (if any)
patients with the acute miliary form of coccidioidomycosis have been
included in any of the reported studies of any of the azole drugs. New
agents that are more active against coccidioidomycosis are still sorely
needed.

Prevention

Simple environmental measures, such as planting grass or paving roads in
highly populated areas, decrease the amount of airborne dust and lower the
risk for coccidioidomycosis (10). These measures do not necessarily
eradicate C. immitis from the soil but lower the risk for airborne
dispersion of the organism. At present, no practical method exists for
eliminating C. immitis from the soil.

Vaccine Development

An alternative approach is to vaccinate persons at risk. A vaccine is
feasible because natural infection almost always confers lifelong immunity
from reinfection. Furthermore, good animal models exist to test vaccine
candidates (21). Finally, genetically susceptible mice can be successfully
immunized, which suggests that the genetically susceptible human population
would also benefit from vaccination (21).

One vaccine that has been tested is a killed spherule vaccine developed by
Pappagianis and Levine. It protected mice and other animals from
experimental infection with C. immitis (45). Between 1980 and 1985, a
double-blinded human study compared results of a formalin-killed spherule
vaccine with results obtained from a placebo. In this study, which involved
almost 3,000 people, only a minority of the vaccinated persons had positive
skin test results to C. immitis. Although the incidence of
coccidioidomycosis was low while this study was conducted, no difference
was found in the number of cases of coccidioidomycosis or the severity of
the disease in the vaccinated group compared with that for the
placebo-receiving control group (46). One explanation for the
ineffectiveness of this vaccine may be that relatively small numbers of
killed organisms could be injected into human without unacceptable local
side effects of pain and swelling. Nevertheless, the vaccine trial made it
clear that immunization with tolerable numbers of whole killed-spherules
does not provide immunoprotection against coccidioidomycosis in humans.

Since the cell wall of C. immitis is made up primarily of nonprotein
macromolecules, it contains a large amount of material presumably
nonantigenic for T lymphocytes. Therefore, the whole organism is not the
ideal vaccine candidate. Ideally, one would like to vaccinate patients
selectively only with antigens that stimulate a protective T-cell-mediated
immune response. These antigens have been difficult to identify, and a
consensus on what they are does not exist. Various approaches have been
used to obtain antigenic proteins. In one, a lysate of arthroconidia
(coccidioidin) or spherules (spherulin) was made (47). Alkali treatment has
also been used to extract antigens from arthroconidia and spherules (48).
Another approach has been to use C. immitis antigens obtained without
extraction or autolysis. The advantage of this method is that one should
obtain reproducible preparations of intact proteins. Cole and co-workers
(49) found that when the outer conidial wall was removed from
arthroconidia, the organism released various proteins (called the soluble
conidial wall fraction). This mixture of proteins was extraordinarily
effective in stimulating the proliferation of C. immitis-immune T cells in
mice. Another antigenic mixture is a membranous material consisting
primarily of proteins and lipids that the spherule phase of the organism
spontaneously releases (the spherule outer wall). This spherule wall
fraction has been shown to be an active antigen in T-cell-mediated immune
responses in mice (50).

All of these mixtures are heterogeneous and difficult to fractionate
biochemically. This is probably due, at least in part, to differences in
glycosylation, which makes physically separating the proteins difficult. To
resolve this problem, Galgiani and his colleagues deglycosylated the
proteins from a toluene spherule lysate by using hydrogen fluoride (51,52).
Although this treatment does remove all sugars, it is extraordinarily harsh
and yields less than 10% of the initial protein, with most of the protein
forming an insoluble precipitate. Nevertheless, the resulting product
reacts with reference antiserum to C. immitis in immunoelectrophoresis.
This antigen also stimulated a proliferative T-cell response in patient
lymphocytes but not in those of the control group (noninfected donors).

Another way to attack the problem of generating pure C. immitis antigens is
to use molecular biologic techniques. The advantage to this approach is
that once antigens are molecularly cloned, and the protein is expressed, an
essentially unlimited source of completely defined antigen is available.
Therefore, one would not have to repeatedly grow C. immitis, extract the
antigen, and purify it from a complex mixture. In addition, with the
molecular approach, antigens could be delivered as part of a living vaccine
system, should that be required to effectively immunize people against
coccidioidomycosis. We believe that systematically identifying and
evaluating C. immitis T-cell reactive antigens in experimental animals is a
rational approach to the ultimate development of a vaccine. Our
laboratories, in collaboration with Garry Cole, have used a murine T-cell
line that is specific for soluble conidial wall fraction antigens to
identify one cloned fragment of a C. immitis protein (53). Recently,
genomic DNA clones coding for this protein have been identified and
sequenced. Significant homology exists between this C. immitis antigen and
the human enzyme 4 hydroxyphenylpyruvate dioxygenase (54). This protein has
been expressed in bacteria and was found to elicit T-lymphocyte
proliferative responses in mice immune to C. immitis. We are testing its
efficacy as an experimental vaccine.

With the exception of alkali extracted spherules (55) and whole killed
spherules (45), none of the T-cell reactive antigens have been shown to be
immunoprotective in experimental models. However, it is reasonable to
expect that some antigen, or mixture of antigens, will be found that can
confer protective immunity in experimental animals. Molecular strategies
are available to accomplish this task and are an important area of future
research. Once a vaccine has been successfully tested in animals, another
human vaccine trial would be feasible.

Dr. Kirkland is associate professor of pathology and medicine; Dr. Fierer
is professor of medicine and pathology and head of the Division of
Infectious Diseases, University of California, San Diego School of
Medicine. Drs. Kirkland and Fierer have worked together for the past 15
years. Currently, they are focusing on the genetic determinants for
resistance to infection and on identifying candidates for a
coccidioidomycosis vaccine.

Acknowledgments

We are grateful to Dr. Ron Talbot, Director of the Kern County Public
Health Laboratory, and Dr. Tom Larwood for sharing unpublished data with
us. The experimental work done in our laboratories has been supported by
National Institutes for Health grants AI19149 and AI37232 and by the
Research Service of the Department of Veterans Affairs.

Address for correspondence: Theo N. Kirkland, M.D., Department of Pathology
and Medicine, University of California, San Diego School of Medicine,
Department of Veterans Affairs Medical Center (111-F) 3350 La Jolla Village
Dr., San Diego, CA 92161, USA; fax: 619-552-4398; e-mail:
tkirkland@ucsd.edu.

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