Microsporidia and Cyclospora: Epidemiology
and Assessment of Risk from the Environment.
THE ISSUE
The goal of this paper is to critically analyze the
human health risk associat-
ed with waterborne exposure to two relatively newly recognized protozoan
agents: Cyclospora and Microsporidia, so that an approach to developing
appropriate water testing and purification guidelines can be facilitated.
An overview of each of these parasites is presented, along with
a discussion of
the health risk that these organisms pose to humans through exposure
in our
water supply. The goal is to identify gaps in our current understanding
to
assist the development of appropriate and reasonable water quality
regula-
tions.
RESEARCH STRATEGY
An analysis of the human health risk associated with waterborne
exposure to
Cyclospora and Microsporidia was conducted by Patricia Mota, M.D.
and
Stephen Edberg, Ph.D., A.B.M.M. (Yale University School of Medicine).
Cyclospora: History/Life Cycle/Biology
The first reported association of Cyclospora with human disease
appeared in
the literature in 1979 in Papua, New Guinea. Previously thought
to be blue-
green algae, CLBs (cyanobacterium-like or coccidia-like bodies),
or large
species of Cryptosporidium, the observation that these organisms
sporulated
and contained typical sporocysts and sporozoites confirmed their
coccidian
nature.
In 1986, coccidian-like oocysts 8 to 10 um were identified in the
stools of
patients with diarrhea. The organism was identified and named Cyclospora
cayetanensis by Ortega, Oilman, and Sterling.
The entire life cycle of Cyclospora can be completed within human
hosts, and
this organism does not replicate outside human hosts. The unsporulated
oocysts are excreted outside the body, and in order to become infectious,
the
oocyst must first sporulate in the environment. The excreted oocyte
requires 7
to 13 days at 25-32°C to sporulate to two ovoid sporocysts [(each
4x6 (m)j,
each containing two sporozoites.
Cyclospora is an emerging pathogen in the last few years, due in
part to sev-
eral well-publicized outbreaks of diarrheal illness that highlight
the impact of
a global food market. Among the many gaps in our current understanding
are
unknown factors in the life cycle and host susceptibility.
Clinical
Cyclospora infects the small intestine, causing an illness characterized
by pro-
longed watery diarrhea, fatigue (sometimes profound), and anorexia
in
humans. The incubation period from infection to onset of symptoms
averages
one week (Colley 1996). The stools of infected patients are typically
watery,
without blood or white blood cells, and may be explosive. Immunity
is not
protective, so that re-infection can occur. Cyclospora infects
patients of all
ages, both immunocompromised and immunocompetent. Travelers to
endem-
ic areas are at an increased risk of infection.
Laboratory Diagnosis
The gold standard for diagnosis of Cyclosporiasis is microscopic
identification
of oocysts, which is best performed by a skilled and experienced
microscopist.
Cyclospora oocysts can easily be confused with Cryptosporidium
parvum,
another coccidian parasite which is often sought as an etiologic
agent of diar-
rheal illness.
Epidemiology
Both children and adults are infected with Cyclospora, with
higher attack rates
for ages greater than 18 months (Gumbo 1977). The host range
for infection
with Cyclospora has not been definitively characterized.
The entire life cycle
can be completed in the human intestine. There is apparently
a worldwide dis-
tribution, including regions of endemicity, e.g., Nepal,
Haiti, and Peru. Thus,
there is increased risk to U.S. citizens through travel to
endemic areas.
The oocyst is the environmentally resistant form of the organism
which sporu-
lates after 7 to 12 days at ambient temperature. Infections
are seasonal, cor-
relating with rainy season (spring/summer) intemperate zones;
in the U.S. most
cases have occurred April through August. Intriguing possibilities
exist for
link of prevalence of Cyclospora with the Guatemalan "mal de
mayo," a spring
diarrheal illness.
About 50 outbreaks have been associated with Guatemalan raspberries
in the
last few years, especially during the spring of 1996 and
1997. After the out-
break in 1996, berry growers and exporters in Guatemala
voluntarily intro-
duced control measures to improve water quality and sanitary
conditions on
farms. The use of the Hazard Analysis and Critical Control
Point (HACCP) sys-
tem has been voluntarily introduced in conjunction with
the U.S. Food and
Drug Administration and the Centers for Disease Control
and Prevention.
Microsporidia: Introduction/Life Cycle/Biology
Microsporidia are ubiquitous spore-forming protozoan parasites
of the phylum
Microspora that infect much of the animal kingdom.
They first gained noto-
riety as the etiologic agent of disease in silkworms
during the last century.
Over 100 genera and almost 1,000 species of microsporidia
have been
described (Weber 1994). Taxonomic relationships are
based on a combination
of analysis of ultrastructural features and knowledge
of the life cycle, as well
as analysis of 16S rRNA genes. Genera associated
with human disease
include: Encephalitozoon, Enterocytozoon, Nosema,
Pleistophora, Trach-
ipleistophora, Vittaforma, and Microsporidium.
Clinical
After some initial debate about pathogenicity, microsporidia
are now widely
accepted as true enteric pathogens, causing disease
in the majority of infected
patients. Microsporidiosis is strongly associated
with diarrhea in HIV-infect-
ed patients, so that the diarrheal symptoms are
not due to immunodeficiency.
Infection with microsporidia is becoming increasingly
recognized in HIV
patients. The symptoms of intestinal microsporidiosis
are similar to those of
isosporiasis and cryptosporidiosis, although
milder. Microsporidiosis is a pro-
tracted, debilitating illness in HIV patients,
associated with increased morbid-
ity and mortality (Sobottka 1998).
Laboratory Diagnosis
Microsporidiosis is likely to be under-reported
due to inherent difficulties in
diagnosis.
Epidemiology
Although much has been learned of the biology of microsporidia,
relatively little is known compared to what is needed in order
to establish
a reasoned and full assessment of the risk
to human health.
Because E. bieneusi (one of the species
of Micro sporidium) is most likely
to be
associated with gastrointestinal illness,
transmission is postulated to occur
by
the ingestion of spores. Spores are
shed in fecal material, so that ingestion
will occur through fecally-contaminated
food, water, etc. The small size of
the
organisms poses additional challenges
for their detection and removal from
water sources. It is important to note
that there are many nonpathogenic
species ubiquitous in the environment
which may result in false-positive
detection assay results.
MAJOR FINDINGS AND SIGNIFICANCE
A critical review of Aeromonas hydrophila infections, and its potential
for reg-
ulation indicates:
- Cyclospora is primarily a food-associated parasite, is found
in surface
water when the organic content is high, and should not be found in
subsurface water.
- Microsporidia primarily affects AIDS patients. It can be transmitted
by
water. It is oxidant sensitive, and of small size.
Mota, P., and Edberg,
S.C., Microsporidia and Cyclospora: Epidemiology and Assessment
of Risk from the Environment. Critical Reviews in Microbiology.
26(2); 2000.
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