Overview of Cryptosporidiosis
Cryptosporidiosis is recognized worldwide, primarily in neonatal calves but also in lambs, kids, foals, and piglets. Cryptosporidia cause varying degrees of naturally occurring diarrhea in neonatal farm animals. The parasites commonly act in concert with other enteropathogens to produce intestinal injury and diarrhea.
There are currently 19 species and 40 genotypes of Cryptosporidium. C hominis (formerly C parvum type I) is a specific human pathogen. C parvum (formerly C parvum type II) is zoonotic and infective to many animals, including people and calves. Four cryptosporidial species have been isolated from cattle (C parvum, C andersoni, C bovis, and C ryanae). C andersoni infects the abomasum of older cattle; C bovis and C ryanae are cattle adapted (cattle are the major host). C parvum is a common cause of calf diarrhea, and cryptosporidial oocysts have been detected in the feces of 70% of 1- to 3-wk-old dairy calves. Infection can be detected as early as 5 days of age, with the greatest proportion of calves excreting organisms between days 9 and 14. Many reports associate infection in calves with diarrhea occurring at 5–15 days of age.
C parvum is also a common enteric infection in young lambs and goats. Diarrhea can result from a monoinfection but more commonly is associated with mixed infections. Infection can be associated with severe outbreaks of diarrhea, with high case fatality rates in lambs 4–10 days old and in goat kids 5–21 days old. Cryptosporidial infection in pigs is seen over a wider age range than in ruminants and has been seen in pigs from 1 wk old through market age. Most infections are asymptomatic, and the organism does not appear to be an important enteric pathogen in pigs, although it may contribute to postweaning malabsorptive diarrhea. Cryptosporidial infection in foals appears less prevalent and is seen at a later age than in ruminants, with excretion rates peaking at 5–8 wk old. Infection is not usually detected in yearlings or adults. Most studies indicate that cryptosporidiosis is not a common disease in foals; infections in immunocompetent foals are usually subclinical. Persistent clinical infections are seen in Arabian foals with inherited combined immunodeficiency. Cryptosporidiosis is also recorded in young deer and can be a cause of diarrhea in artificially reared orphans.
The source of cryptosporidial infection is oocysts that are fully sporulated and infective when excreted in the feces. Large numbers are excreted during the patent period, resulting in heavy environmental contamination. Transmission may occur directly from calf to calf, indirectly via fomite or human transmission, from contamination in the environment, or by fecal contamination of the feed or water supply. A periparturient rise in the excretion of oocysts may occur in ewes. C parvum is not host-specific, and infection from other species (eg, rodents, farm cats) via contamination of feed is also possible.
Oocysts are resistant to most disinfectants and can survive for several months in cool and moist conditions. Oocyst infectivity can be destroyed by ammonia, formalin, freeze-drying, and exposure to temperatures <32°F (0°C) or >149°F (65°C). Ammonium hydroxide, hydrogen peroxide, chlorine dioxide, 10% formol saline, and 5% ammonia are effective in destroying oocyst infectivity. Infectivity in calf feces is reduced after 1–4 days of drying.
Concurrent infections with other enteric pathogens, especially rotavirus and coronavirus, are common, and epidemiologic studies suggest that diarrhea is more severe in mixed infections. Immunocompromised animals are more susceptible to clinical disease than immunocompetent animals, but the relationship between disease and failure of passive transfer of colostral immunoglobulins is not clear. Age-related resistance, unrelated to prior exposure, is seen in lambs but not calves. Infection results in production of parasite-specific antibody, but both cell-mediated and humoral antibody are important in protection, as well as local antibody in the gut of the neonate.
Case fatality rates in cryptosporidiosis are generally low unless complicated by other factors (eg, concurrent infections, energy deficits from inadequate intake of colostrum and milk, chilling from adverse weather conditions).
The life cycle of Cryptosporidium consists of six major developmental events. After ingestion of the oocyst, there is excystation (release of infective sporozoites), merogony (asexual multiplication), gametogony (gamete formation), fertilization, oocyst wall formation, and sporogony (sporozoite formation). Oocysts of Cryptosporidium spp can sporulate within host cells and are infective when passed in the feces. Infection persists until the host’s immune response eliminates the parasite. In natural and experimentally produced cases in calves, cryptosporidia are most numerous in the lower part of the small intestine and less common in the cecum and colon. Prepatent periods are 2–7 days in calves and 2–5 days in lambs. Oocysts are usually passed in the feces of calves for 3–12 days.
Calves with cryptosporidiosis usually have a mild to moderate diarrhea that persists for several days regardless of treatment. The age at onset is later, and the duration of diarrhea tends to be a few days longer than are seen in the diarrheas caused by rotavirus, coronavirus, or enterotoxigenic Escherichia coli. Feces are yellow or pale, watery, and contain mucus. The persistent diarrhea may result in marked weight loss and emaciation. In most cases, the diarrhea is self-limiting after several days. Varying degrees of apathy, anorexia, and dehydration are present. Only rarely do severe dehydration, weakness, and collapse occur, in contrast to findings in other causes of acute diarrhea in neonatal calves. Case fatality rates can be high in herds with cryptosporidiosis when the calf feeder withholds milk and feeds only electrolyte solutions during the episode of diarrhea. The persistent nature of the diarrhea leads to a marked energy deficit in these circumstances, and the calves die of inanition at 3–4 wk old.
Calves with persistent diarrhea have villous atrophy in the small intestine. Histologically, large numbers of the parasite are embedded in the microvilli of the absorptive enterocytes. In low-grade infections, only a few parasites are present, with no apparent histologic changes in the intestine. The villi are shorter than normal, with crypt hyperplasia and a mixed inflammatory cell infiltrate.
Diagnosis is based on detection of oocysts by examination of fecal smears with Ziehl-Neelsen stains, fecal flotation techniques, ELISA, fluorescent-labeled antibodies, a rapid immunochromatographic test, and PCR. Sheather’s flotation sedimentation staining is the most sensitive (83%) and specific (99%) of these techniques, with a relatively low cost per test. This technique requires centrifuging a fecal sample in Sheather’s solution, aspirating the top layer and diluting the fluid in phosphate buffer saline, centrifuging, and placing the sediment on the slide and performing a modified Ziehl-Neelsen technique to look for cryptosporidial oocysts. It has been suggested that if the diarrhea is caused by cryptosporidia, there should be 105–107 oocysts/mL of feces. The oocysts are small (5–6 mm in diameter) and relatively nonrefractile. They are difficult to detect by normal light microscopy but are readily detected by phase-contrast microscopy.
There are no currently licensed therapeutics available in the USA for C parvum infection in food animals. Anecdotal reports of success with extra-label use of various compounds have not been replicated in controlled trials. Experimental treatments have for the most part been toxic or ineffective. Halofuginone is reported to markedly reduce oocyst output in experimentally infected lambs and naturally and experimentally infected calves; therapy was also reported to prevent diarrhea. Paromomycin sulfate (100 mg/kg/day, PO, for 11 days from the second day of age) proved successful in preventing natural disease in a controlled clinical field trial in goat kids.
Affected calves should be supported with fluids and electrolytes, both orally and parenterally, as necessary until recovery occurs. Cows’ whole milk should be given in small quantities several times daily (to the full level of requirement) to optimize digestion and to minimize weight loss. Several days of intensive care and feeding may be required before recovery is apparent. Parenteral nutrition may be considered for valuable calves.
The disease is difficult to control. Reducing the number of oocysts ingested may reduce the severity of infection and allow immunity to develop. Calves should be born in a clean environment, and adequate amounts of colostrum should be fed at an early age. Calves should be kept separate without calf-to-calf contact for at least the first 2 wk of life, with strict hygiene at feeding. Diarrheic calves should be isolated from healthy calves during the course of the diarrhea and for several days after recovery. Great care must be taken to avoid mechanical transmission of infection. Calf-rearing houses should be vacated and cleaned out on a regular basis; an “all-in/all-out” management system, with thorough cleaning and several weeks of drying between batches of calves, should be used. Rats, mice, and flies should be controlled when possible, and rodents and pets should not have access to calf grain and milk feed storage areas.
Hyperimmune bovine colostrum can reduce the severity of diarrhea and the period of oocyst excretion in experimentally infected calves. Protection is not related to circulating levels of specific antibody but requires a high titer of C parvum antibody in the gut lumen for prolonged periods. Many research groups have attempted to develop effective vaccines against cryptosporidia. Unfortunately, to date, vaccinations have not been effective.
Infections in domestic animals may be a reservoir for infection of susceptible people. C hominis and C parvum are considered to be relatively common nonviral causes of self-limiting diarrhea in immunocompetent people, particularly children. In immunocompromised people, clinical disease may be severe. The infection is transmitted predominantly from person to person, but direct infection from animals and waterborne infection from contamination of surface water and drinking water by domestic or wild animal feces can also be important. Animal handlers on a calf farm can be at high risk of diarrhea due to cryptosporidiosis transmitted from infected calves. Immunocompromised people should be restricted from access to young animals and possibly from access to farms.