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 associated 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 regulations.

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 4×6 (m)j, each containing two sporozoites.

Cyclospora is an emerging pathogen in the last few years, due in part to several 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.


Cyclospora infects the small intestine, causing an illness characterized by prolonged 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 endemic 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 diarrheal illness.


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 distribution, 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 sporulates after 7 to 12 days at ambient temperature. Infections are seasonal, correlating 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 outbreak in 1996, berry growers and exporters in Guatemala voluntarily introduced control measures to improve water quality and sanitary conditions on farms. The use of the Hazard Analysis and Critical Control Point (HACCP) system 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 notoriety 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, Trachi-pleistophora, Vittaforma, and Micro-sporidium.


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-infected 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 protracted, debilitating illness in HIV patients, associated with increased morbidity and mortality (Sobottka 1998).

Laboratory Diagnosis

Microsporidiosis is likely to be under-reported due to inherent difficulties in diagnosis.


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 regulation 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.