Water Quality Information Center: Water Quality -- Cryptosporidium parvum and Cattle

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Water QualityCryptosporidium Cryptosporidium parvum and Cattle

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	Cryptosporidium parvum and Cattle: Implications for Public Health and Land Use Restrictions

Medical Ecology & Environmental Animal Health
University of California, Davis
U.C. Cooperative Extension / Veterinary Medicine Extension
Department of Population Medicine & Reproduction
Veterinary Medicine Teaching & Research Center
Tulare, CA 93274

Jun 1995

Cryptosporidium parvum (C. parvum) is a tiny protozoal parasite that can cause gastrointestinal illness in a wide variety of mammals, including humans, cattle, sheep, goats, pigs, and horses. It also occurs in various wildlife species such as deer, raccoons, opossums, rabbits, rats, mice, and squirrels. In cattle, clinical disease and shedding of the parasite is usually limited to calves under a few months of age, but there are a few reports of subclinical shedding in adult cattle fro m researchers in England and Spain. In humans, clinical disease and shedding appears to occur at all ages, but is most common among children. The predominant clinical sign is profuse, watery diarrhea lasting up to several weeks in normal (immunocompeten t) calves and humans. While this disease is usually self-limiting in immunocompetent calves and humans, it can be prolonged and life-threatening among immunocompromised people such as AIDS patients since an effective treatment for eliminating this parasi te from the gastrointestinal track still does not exist. A few antibiotics may show some promise in reducing the amount of oocyst shedding in AIDS patients, but further clinical trials are needed to fully evaluate their efficacy. This lack of an effecti ve treatment and the fact that this parasite was implicated in recent large-scale waterborne outbreaks of gastroenteritis in humans (e.g., 400,000 suspect cases in Milwaukee, Wisconsin, 1993; 13,000 suspect cases in Carrollton, Georgia, 1987) has prompted public health officials and state and federal agencies to consider ways to reduce drinking water contamination of this parasite. Some of this attention has focused on identifying the primary sources of C. parvum in surface water. Furthermore, th e US Environmental Protection Agency intends to include C. parvum in the proposed Enhanced Surface Water Treatment Rule (Giardia lamblia, Legionella and coliform bacteria, and enteric viruses are already regulated) and have warned tha t such an inclusion will likely result in new restrictions being placed on the location and management of livestock operations situated within watersheds. Presented below is a brief summary of the medical ecology of C. parvum in calves and in hum ans and the existing scientific evidence that addresses the claim that calves put humans at significant risk for water-borne infection of C. parvum.

In calves and in humans, this parasite is primarily transmitted by the fecal-oral route. Infection typically begins when a susceptible individual ingests water or food contaminated with the oocysts (eggs) of this parasite. The parasite then invades the epithelium of the intestine, replicates, and through sequential reproductive cycles can result in up to 10 (9th) -10 (10th) oocysts being shed in the feces per day. Shedding of oocysts can last for 3-12 days in calves and for an average of 18 days ( range of 9-50 days) in humans. Given the large number of oocysts shed per day, the immediate environment can become quite contaminated with these oocysts. These oocysts are immediately infective to another individual, allowing for the rapid spread of th is parasite within a group of susceptible individuals.

Oocysts shed from one species of mammal appear to be infectious to other species of mammals. Oocysts from humans have been shown to be infectious to calves, lambs, goats, cats and dogs. Oocysts from cats, calves and pigs appear to be infectious to humans. People working with diarrheic calves infected with C. parvum have themselves become infected with C. parvum, presumably from the calf. However, working with diarrheic calves is not common for the general public. A second crypto sporidium species, C. muris, has been identified in the abomasum of adult cattle, but to date it has not been shown to be infectious to humans.

The critical issue is how would C. parvum from calves gain access to surface waters and end up in drinking water supplies. The essential steps must include calves becoming infected and shedding the oocysts in their feces. These oocysts must then enter a surface water supply and remain infective as they journey downstream to water treatment plants and distribution systems.

How common is it for calves with access to surface water to shed this parasite? Very little work has been done in beef calves, with most research conducted on dairy calves. The largest survey to date on shedding of C. parvum in beef calves w as conducted by the USDA's National Animal Health Monitoring System in cooperation with USDA's National Veterinary Services in which 20% of diarrheic calves (n=391) and 11% of non-diarrheic calves (n=1,053) from a total of 210 operations were found to be shedding C. parvum oocysts at the time of sampling. Around 40% of these 210 operations had one or more calves shedding C. parvum oocysts at the time of sampling. In this same study shedding of oocysts was documented in 9% of asymptomatic c alves between 3 and 6 months of age, indicating that shedding can occur in these older age groups and without clinical signs. In Manitoba, Canada, 22% of beef calves from 148 herds known to have problems with neonatal diarrhea were found to shed C. pa rvum. In England, 36-39% of diarrheic beef calves tested positive for C. parvum while only 8% of healthy beef and dairy calves tested positive. In San Bernardino county, California, 5.6% of predominately Holstein dairy calves were found to b e shedding C. parvum. More specifically, the prevalence of shedding was 21% in calves with diarrhea and 2% in healthy calves. From across the United States, 22% of 7,369 dairy calves tested positive for this parasite. This parasite appears to be relatively common in dairy calves, but we need much better information on the distribution of C. parvum shedding in beef cattle herds located on open range with access to important watersheds.

Little is known about the prevalence of shedding among wildlife species with access to surface waters or what contribution humans themselves make to surface water contamination.. In a survey of 100 raccoons, 13 juveniles had oocysts in their feces. Cryptosporidial infection has been confirmed in a large variety of neonatal captive deer, including mule and fallow deer. Experimental infection has been demonstrated in opossums. Thirty percent (35/115) of wild mice trapped at a dairy shed oocysts. O ocysts obtained from mice have been shown to be infective to calves, perhaps indicating a mouse-calf cycle. The prevalence of shedding among groups of people is highly dependent on which country, which population, and can range from 0-60%, with the highe r proportion of shedding among diarrheic individuals. The CDC estimates that the overall background rate of shedding in the United States is around 1%, but the relationship between shedding in humans and surface water contamination remains unknown. Also unknown is what proportion of cryptosporidiosis in humans is due to water-bome infection as opposed to human-to-human direct infection.

How long do C. parvum oocysts survive in the environment once they are shed in feces? Oocysts that dry out appear to become non-infective in just a few hours. If fecal material thoroughly dries before reaching water, the oocysts would presu mably become non-infectious for animals and humans. Ten or more days of freezing causes over 90% of oocysts to become noninfective. Oocysts in distilled water became non-infective if heated to 72.4C or higher for I minute or if heated to 64.2C or higher for 2 minutes. What if fecal material is deposited directly in a stream? One study found that after 33 days in river water, an estimated 34-40% of purified oocysts were incapable of excystation. After 176 days, 89-99% were estimated to be incapable of excystation. In conclusion, it may be that most oocysts do not remain infective as they journey from calves to surface water to water treatment plant to human consumption.

What evidence directly links the presence of C. parvum in surface water supplies to livestock production? Environmental studies to date have had a difficult time determining if the Cryptospordium found in surface water is C. parvum or some other Cryptospordium species not infectious to humans yet detected by one of the laboratory assays used for environmental testing. For example, it appears that C. serpentis which is shed by reptiles and C. meleag ridis whi ch is shed by turkeys, both of which are not infectious to humans, can cross-react with the immunofluorescent assay produced by Meridian Diagnostics Inc. With this in mind, 77% of general surface waters collected from the westem United States were found to have oocysts, with a geometric mean ranging between 0.91 and 28 oocysts/L. In comparison, one study found no difference in the concentration of Cryptospordium oocysts from protected surface waters (0.3-4.0 oocysts/L) as compared to surface waters subject to agricultural run-off (0.1-2.0 oocysts/L). Moreover, 68% of these oocysts had become non-viable. Another study measured 5,800 oocysts/L in irrigation canal water running through agricultural acreage with cattle pastures (beef or dairy not s pecified), compared to 127 oocysts/L in river water subject to human recreation and 0.8 oocysts/L for strewn water exposed to ranch land runoff. The ID, for human infection with C.parvum, estimated from a small group of volunteers was 132 oocysts. As fe w as 30 oocytsts were capable of inducing infection. Although there are severe environmental pressures for oocysts to remain infective when excreted on land, apparently only a few oocysts would need to remain viable in drinking water to pose a risk to hu mans. Finally, concentrations of Cryptospordium oocysts from pristine surface waters have been found to contain 0.005-18 oocysts/L, indicating that this organism occurs naturally in pristine watersheds. This would suggest that wildlife will need to be carefully examined for their role in contaminating surface water with this parasite.

The scientific evidence supporting the claim that cattle are a significant source of C. parvum for surface water is incomplete and contradictory in some cases. Therefore, it would be premature at this time to claim that cattle production is t he leading source of C. parvum in surface water. If, in the words of US EPA, we are to "minimize the potential for source water contamination" by C. parvum, then we must first identify the primary quantitative source(s) of this parasite in the environment, be it livestock, wildlife, humans, companion animals, or human associated sewage effluent.

	Rob Atwill, D.V.M., M.P.V.M., Ph.D. Tel: 209-688-1731 ext 218 Fax: 209-686-4231 E-mail: ratwill@vmtrc.ucdavis.edu
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Last Modified: Wednesday, 03-Dec-2008 17:22:33 EST