Parasitic Causes of Gastrointestinal Illness

Parasites are single celled microorganisms that are usually associated with contaminated water or food grown in contaminated water.

 Cryptosporidium spp.

Cryptosporidium parvum is an obligate intracellular parasite. It is currently thought that the form infecting humans is the same species that causes disease in young calves. The forms that infect avian hosts and those that infect mice are not thought capable of infecting humans. Cryptosporidium sp. infects many herd animals (cows, goats, and sheep among domesticated animals, and deer and elk among wild animals). The infective stage of the organism is the oocyst. The sporocysts are resistant to most chemical disinfectants, but are susceptible to drying and the ultraviolet portion of sunlight. (Center for Food Safety and Applied Nutrition, 2001)

Intestinal cryptosporidiosis is characterized by severe watery diarrhea that is particularly severe in immune compromised individuals. Healthy adults may be asymptomatic. The infectious dose is less than 10 organisms and, presumably, one organism can initiate an infection. Oocysts are shed in the infected individual's feces. Cryptosporidium sp. could occur, theoretically, on any food touched by a contaminated food handler. The incidence is higher in child day care centers that serve food. Fertilizing salad vegetables with manure is another possible source of human infection. Large outbreaks have been associated with contaminated water supplies suggesting that contaminated irrigation water could be another route of contamination. Produce- and juice-associated outbreaks of cryptosporidiosis have occurred (Table O-5, O-8). (Center for Food Safety and Applied Nutrition, 2001)

 Cyclospora spp.

Raw raspberries and possibly blackberries imported from Guatemala have been associated with several large Cyclospora cayetanensis outbreaks (Table O2). The natural host for this parasite has not been identified; however, contaminated water used for pesticide application and poor harvester hygiene has been suggested as the most likely routes of contamination. (Center for Food Safety and Applied Nutrition, 2001)

Cyclospora cayetanensis is a unicellular parasite previously known as cyanobacterium-like or coccidia-like body (CLB). The first known human cases of illness caused by Cyclospora infection (for example, cyclosporiasis) were reported in the medical literature in 1979. Cases have been reported with increased frequency from various countries since the mid 1980s, in part because of the availability of better techniques for detecting the parasite in stool specimens. (Center for Food Safety and Applied Nutrition, 2001)

Infected persons excrete the oocyst stage of Cyclospora in their feces. When excreted, oocysts are not infectious and may require days to weeks to become infectious (for example, to sporulate). Therefore, transmission of Cyclospora directly from an infected person to someone else is unlikely. However, indirect transmission can occur if an infected person contaminates the environment and oocysts have sufficient time, under appropriate conditions, to become infectious. For example, Cyclospora may be transmitted by ingestion of water or food contaminated with oocysts. Outbreaks linked to contaminated water, as well as outbreaks linked to various types of fresh produce, have been reported in recent years. Raspberries and possibly blackberries imported from Guatamala have been implicated in at least five outbreaks, two involving numerous states and Canadian provinces (Table O2). The route of contamination was not conclusively determined, but was suspected to be related to contaminated water used for irrigation or pesticide application. Berries imported in the spring but not in the fall were associated with illnesses suggesting a seasonality to the illness. In addition, fresh basil and products made from the basil were implicated in an outbreak in 1997 (Table O8). The source of contamination for this outbreak was not determined. How common the various modes of transmission and sources of infection are is not yet known, nor is it known whether animals can be infected and serve as sources of infection for humans. The incubation period between acquisition of infection and onset of symptoms averages 1 week. Cyclospora infects the small intestine and typically causes watery diarrhea, with frequent, sometimes explosive, stools. Other symptoms can include loss of appetite, substantial loss of weight, bloating, increased flatus, stomach cramps, nausea, vomiting, muscle aches, low-grade fever, and fatigue. If untreated, illness may last for a few days to a month or longer, and may follow a remitting-relapsing course. Some infected persons are asymptomatic. (Center for Food Safety and Applied Nutrition, 2001)

 Gardiasis

In 1985, G. lamblia was the most frequently identified pathogen for the seventh consecutive year, causing three (20%) of 15 waterborne outbreaks in addition to two outbreaks that resulted from unintentional ingestion of water in swimming pools. Giardia has been the cause of nearly all reported outbreaks of waterborne parasitic diseases in recent years, during which time this class of agents has increased as a proportional cause of all waterborne outbreaks (Table 5). In each of the outbreaks, as in well-characterized waterborne outbreaks of giardiasis in the past (3,4), water chlorination had been maintained at adequate levels to make outbreaks of bacterial diseases unlikely, but the lack of an intact filtering system capable of filtering Giardia cysts, distribution system problems, and mechanical deficiencies allowed drinking water to become a vehicle of giardiasis. Efforts are continuing to develop practical and efficient ways to detect Giardia cysts in water (5-7).  (CDC, 1988)

Gardia is associated with drinking contaminated water or eating foods that have been grown or washed in contaminiated water.  Cool, moist environments are condustive to the growth of this microorganism. (Balch, 2000)

Two outbreaks of giardiasis were associated with swimming in pools. .  (CDC, 1988)

Human giardiasis may involve diarrhea within 1 week of ingestion of the cyst, which is the environmental survival form and infective stage of the organism. Normally, illness lasts for 1 to 2 weeks, but there are cases of chronic infections lasting months to years. Chronic cases, both those with defined immune deficiencies and those without, are difficult to treat. Different individuals show various degrees of symptoms when infected with the same strain, and the symptoms of an individual may vary during the course of the disease. (Center for Food Safety and Applied Nutrition, 2001)

Ingestion of one or more cysts may cause disease. Giardiasis is most frequently associated with the consumption of contaminated water. Cool moist conditions favor the survival of the organism. Produce-related outbreaks have been linked to lettuce, tomatoes, and onions (Table O7, O8).  (Center for Food Safety and Applied Nutrition, 2001)

During 1999, 47 swimming pools, water parks, or aquatics centers were enrolled in the survey by telephone. Sample collection began Memorial Day weekend (May 29) and ended after Labor Day weekend (September 6). Samples of each fecal accident were collected into vials containing 10% formalin. Labels included no pool-specific identifiers. Samples were tested for Cryptosporidium- and Giardia-specific stool antigen without prior concentration. All positive specimens were verified using an immunofluorescent antibody mixture specific to Cryptosporidium and Giardia followed by microscopic identification. (CDC, 2001)

None of 293 formed stools from fecal accidents collected by pool operators contained Cryptosporidium. Giardia was found in 13 (4.4%) of the samples. Because this study addressed parasite prevalence in only formed stool, no information relating to disinfection procedures for diarrheal fecal accidents was obtained. (CDC, 2001)

Three outbreaks were attributed to G. lamblia; all were associated with drinking chlorinated but unfiltered water. In eight other reported outbreaks of acute gastrointestinal illness no agent was convincingly demonstrated. No waterborne outbreaks of documented viral diseases were reported in 1985. (CDC, 1988)

Organisms that appear identical to those that cause human illness have been isolated from domestic animals (dogs and cats) and wild animals (beavers and bears). A related but morphologically distinct organism infects rodents, although rodents may be infected with human isolates in the laboratory. Human giardiasis may involve diarrhea within 1 week of ingestion of the cyst, which is the environmental survival form and infective stage of the organism. Normally, illness lasts for 1 to 2 weeks, but there are cases of chronic infections lasting months to years. Chronic cases, both those with defined immune deficiencies and those without, are difficult to treat. Different individuals show various degrees of symptoms when infected with the same strain, and the symptoms of an individual may vary during the course of the disease. (Center for Food Safety and Applied Nutrition, 2001)

Ingestion of one or more cysts may cause disease. Giardiasis is most frequently associated with the consumption of contaminated water. Cool moist conditions favor the survival of the organism. Produce-related outbreaks have been linked to lettuce, tomatoes, and onions (Table O7, O8).  (Center for Food Safety and Applied Nutrition, 2001)