Cryptosporidium Parvum: Transmission and Infection

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Cryptosporidium parvum: Transmission and Infection

Cryptosporidium parvum is a protozoan intestinal parasite causing a short-term enteric illness in individuals with functioning immune systems, and can cause a potentially fatal infection in immunosuppressed individuals. Because of C. parvum's resistance to many of the procedures used to process drinking water and food, and the parasite's extremely high fecundity, the potential for a large scale outbreak is very high. In fact, C. parvum was responsible for an outbreak in Milwaukee in 1993 when an estimated 403,000 people became ill. This was the largest waterborne outbreak of disease in United States history. This paper will cover some aspects of C. parvum's life cycle, human sickness caused by the parasite, routes of transmission, and practices of control.

There are six different species of Cryptosporidium, C. parvum being the only species which infects mammals (Gutierrez, 1990). Oocysts, which are ingested by the mammal host, each contain four sporozoites. Upon excystment in the small intestine, the sporozoites infect an intestinal epithelial cell by becoming attached to the base of the microvilli. The sexual stages follow, where zygotes and eventually oocysts are formed. But C. parvum also has an "auto-infecting" asexual stage in which thin walled oocysts are produced to cause infection farther along in the intestine (Donnelly & Stentiford,1997). Approximately 20% of the oocysts produced will have these thin walls, leaving 80% of the oocysts to be excreted out of the host and into the environment, where they will be infective immediately. Thirty oocysts are enough to cause infection, and one infected person can excrete over a billion oocysts in one day (Graczyk et al., 2000).

Symptoms of Cryptosporidiosis in immunocompetent individuals include watery diarrhea (up to 3 liters a day), cramps, weight and appetite loss, nausea, vomiting and malaise (Gutierrez, 1990). Symptoms begin 3 to 5 days after the initial infection, and can last up to 2 weeks. Several relapses may occur due to the auto infecting mechanism of the parasite, but an otherwise healthy individual will rarely experience any more than 21 days of symptoms. Oocysts, however, may continue to be shed in the host's feces for up to 2 months (Gutierrez, 1990).

Because there is no known treatment for cryptosporidiosis, symptoms in immunocompromised individuals may be much more severe. The infective dose may be as little as one oocyst, and severe diarrhea can occur, causing the individual to pass up to 20 liters of fluid in one 24 hour period (Donnelly & Stentiford, 1997). This inevitably leads to death.

Transmission of the parasite can occur in several different ways. Direct transmission can occur by handling infected animal or human feces. One quarter of reported direct transmission infections occurred by direct contact with feces, while the rest were reported to have happened by person to person contact (Donnelly & Stentiford, 1997).

Person to person transmission can occur through poor hygiene habits or by handling human waste. Daycares and nursing homes are at a high risk for person to person transmission because of the high risk of handling infected feces. Family outbreaks are common, as are outbreaks among children at nurseries (Donnelly & Stentiford, 1997).

Indirect transmission by the water or foodborne route is one of the most common ways C. parvum is spread. Because of the oocyst's resistance to chlorination, several outbreaks have been caused by waterborne transmission. In one study (Carpenter et al.), oocysts were removed from the feces of an experimentally infected calf, cleaned of fecal matter, and placed into different amounts of chlorinated water at different temperatures. Although this experiment had been performed before, this was the first time that simulated recreational water was used. To simulate recreational water, the pH was balanced, CaCl2 was added, as was organic material. The organic material was meant to simulate organic matter which might be found in a swimming pool, such as hair and feces. Feces is known to cause a negative effect on chlorine disablement of C. parvum oocysts. This is beneficial to the parasite because passage of the oocysts occurs in the feces. The experiment found that oocysts maintained for three days at 200 C in 2 parts per million chlorine were still infective to laboratory mice. These conditions were meant to simulate recreational swimming pool conditions. This means that if an infected person were to release oocysts in a swimming pool, other swimmers could become infected with the disease for up to three days.

Contamination of water reserves is also an important factor in controlling potential outbreaks (Graczyk et al., 2000).Often lakes, rivers, streams and ponds surrounding livestock fields are contaminated with the parasite, which can cause contamination of a drinking water reserve. Testing for Cryptosporidium in a water reserve on a regular basis is not feasible due to cost, so an outbreak would very likely occur before the problem would be known. Furthermore, some of the methods used to purify drinking water are not useful in controlling Cryptosporidium parvum, so contamination of the source water would result in an outbreak. Several different factors would contribute to such contamination.

An experiment was performed in Lancaster County, Pennsylvania to test different factors contributing to contamination (Graczyk et al., 2000). Digital maps of a particular floodplain were made showing all locations of cattle farms relative to the locations of water routes to and reserves of water (lakes, ponds, creeks and rivers). At 64% of the farmyards located within the floodplain, cattle were found to be infected with the parasite, and at 44% of the locations tested, oocysts were found in all cattle samples. DNA testing confirmed that the cattle downstream were infected with a "relative" of the parasite found in cattle upstream, indicating that the parasites were indirectly transmitted through the water

Another experiment dealing with the susceptibility of a water system to environmental contamination of C. parvum was carried out in Chesapeake Bay. This experiment used infected oysters for information. The objectives of this experiment were to find the most practical way of screening Chesapeake Bay oysters for C. parvum infection, and to assess the role that other factors, such as cattle farms and wastewater treatment facilities, played in contamination of the Chesapeake Bay. After collecting