– Dr. Michelle Arnold, UK Veterinary Diagnostic Laboratory
What is “cryptosporidiosis”?
Cryptosporidiosis, also known as “crypto”, is a disease primarily seen in calves due to a protozoan parasite, Cryptosporidium parvum or C. parvum for short. In its “clinical” or visible form, calves have profuse, watery diarrhea that can lead to dehydration and death. It generally affects calves from newborns up to 6 weeks of age but older animals may be asymptomatic shedders. There are no effective treatments or vaccines available in the US. Cryptosporidiosis is “zoonotic”, meaning humans may acquire C. parvum from infected calves and have watery diarrhea lasting up to 3 weeks in healthy people with strong immune systems but can be life- threatening in immunocompromised individuals.
How is the organism transmitted?
Cryptosporidium “oocysts” are thick-walled structures, similar to parasite “eggs”, that are passed in the feces of infected calves. These oocysts are spread between calves by the “fecal-oral route”, either directly through contact with feces from infected calves (for example, on manure-covered teats), or indirectly by ingestion of feces-contaminated feed or water. Very few oocysts are required to cause infection; in one study in calves less than 24 hours old, only 17 oocysts were needed to cause diarrhea. Following ingestion (swallowing) of oocysts by the calf, the organism begins a very complex reproductive cycle within the cells that line the calf’s intestinal tract (see Figure 1 for a complete review of the life cycle). The conditions inside the gastrointestinal tract of low pH and body temperature trigger the oocyst to “excyst” (hatch) and four sporozoites are released (Figure 1A). C. parvum sporozoites invade the cells that line the small intestine (Figure 1B), and undergo asexual (Figure 1D) then sexual (Figure 1F-I) reproduction phases to develop new oocysts. The newly formed oocysts are of two different types: thin-walled oocysts (Figure 1K) that stay inside the gut and keep infecting new cells, or thick-walled oocysts (Figure 1J) which are passed in feces and are immediately infective for other calves. The ability to produce thin-walled oocysts which stay in the calf and continue to infect cells in the small intestine is one of the reasons why the Cryptosporidium parasite is so successful. This self-infection means that the parasite can rapidly produce oocysts in a relatively short time. The thick-walled oocysts are released in the feces and can survive for months in cool, moist climates and are resistant to most disinfectants. A neonatal calf can shed 30 billion oocysts over 1-2 weeks but it only takes 10-30 oocysts to cause a new infection in a calf. Livestock (cattle and sheep) and wildlife (deer) can share the same genotype of C. parvum so transmission can occur between livestock and wildlife.
What does a calf with cryptosporidiosis look like?
A calf with cryptosporidiosis will have diarrhea that varies from small amounts up to profuse, watery diarrhea which is typically yellow or pale and sometimes contains mucus. Affected calves have little to no appetite, are lethargic, develop dehydration and sometimes death, especially when other bacteria or viruses are present. Severity of the illness depends on the number of oocysts ingested (infective dose), the immune status of the calf (colostrum absorption), nutritional status of calf, virulence of parasite and occurrence of co-infections with other bacteria and/or viruses. The diarrhea is due to destruction of the small intestinal lining, causing impaired milk digestion and nutrient absorption. The diarrhea begins 3-4 days after ingestion of oocysts and lasts 1-2 weeks. Infected calves have reduced intestinal absorption until recovery which is, on average, at day 21 post-infection. Oocyst shedding (passing oocysts in the feces) starts as early as 4 days after infection depending on initial dose. Peak shedding is at 2 weeks of age if infected near time of birth. Naturally infected calves can shed large numbers (over a billion) oocysts each day which are immediately transmissible to susceptible calves and may remain viable for 18 months in the environment under the right conditions. There is a direct impact on growth rate during infection and those calves with severe cases as newborns will have reduced weight gain for at least 6 months. A recent study found on average, a calf with severe disease weighed 75 pounds less at 6 months of age compared to a calf with no clinical signs of cryptosporidiosis.
Figure 1: Life cycle of Cryptosporidium species. From https://www.cdc.gov/dpdx/cryptosporidiosis/index.html Following ingestion by a suitable host, excystation occurs. The sporozoites are released and parasitize the epithelial cells ( , ) of the gastrointestinal tract. In these cells, usually within the brush border, the parasites undergo asexual multiplication (schizogony or merogony) ( , , ) and then sexual multiplication (gametogony) producing microgamonts (male) and macrogamonts (female) . Upon fertilization of the macrogamonts by the microgametes ( ) that rupture from the microgamont, oocysts develop and sporulate in the infected host. Zygotes give rise to two different types of oocysts (thick-walled and thin-walled). Thick-walled oocysts are excreted from the host into the environment , whereas thin-walled oocysts are involved in the internal autoinfective cycle and are not recovered from stools . Oocysts are infectious upon excretion, thus enabling direct and immediate fecal-oral transmission.
How is the disease diagnosed?
No type of diarrhea is considered “characteristic” of cryptosporidiosis in calves. Diagnosis is based on finding oocysts in the feces from infected calves that begins 4 days post-infection with peak intensity of oocyst shedding at 2 weeks and steadily declines as the calf ages. The oocysts are extremely small, making it a challenge to find them by traditional flotation methods and a microscope. A modified acid-fast staining method is widely used to help detect C. parvum in feces but it requires at least 500,000 oocysts per gram of feces to confirm by microscope. There are rapid antigen (ELISA) test kits available commercially for accurate detection in fecal samples. Polymerase Chain Reaction (PCR) assays are frequently used in diagnostic labs and provide the only method to distinguish between the four common species found in cattle (C. parvum, C. bovis, C. ryanae and C. andersoni). Of those four species of Cryptosporidium in cattle, only C. parvum has been found to cause disease in young calves. The UK Veterinary Diagnostic Laboratory has a PCR test to diagnose the cause of calf diarrhea. A small sample of scours (in a leakproof container) from a calf that has not been treated for diarrhea with antibiotics is needed to run the test. This PCR assay tests for bovine coronavirus, rotavirus, E. coli K99, Salmonella and Cryptosporidium parvum.
Is there a treatment available?
The disease is normally self-limiting in neonatal calves as long as 1) sufficient colostrum was absorbed in the first few hours of life, 2) the calves are kept warm and dry and given supportive treatment if required, and 3) there are NO co-infections with other causes of calf diarrhea (Rotavirus, coronavirus, E. coli K99 or Salmonella spp.). Calves with diarrhea need fluids and electrolytes (oral and sometimes SQ or IV) to correct dehydration and metabolic acidosis. It is critical to continue feeding milk to scouring calves to minimize loss of body weight and provide needed energy. No products are licensed in the US for treatment or prevention of cryptosporidiosis in livestock. Outside the US, there are two licensed drugs for prevention and treatment in calves, halofuginone lactate (Halocur®) and paromomycin. Halocur® is administered for 7 consecutive days and must be started within 48 hours of birth for prevention or must be started within 24 hours of onset of diarrhea for treatment purposes. This medication does not totally prevent or cure the disease but reduces oocyst shedding and duration of diarrhea. Paromomycin, an aminoglycoside antibiotic, has anti-protozoal properties and shows promise in controlling cryptosporidiosis in neonatal calves, lambs and goats. Several other treatments have been tested in livestock but none have worked consistently to reduce clinical signs. Of the natural/alternative treatments studied, garlic offered a promising effect on prevention and treatment through immune system stimulation and strengthening of body defense mechanisms. Currently no vaccines are available although studies are being conducted in pregnant cows vaccinated with recombinant C. parvum to produce antibodies against C. parvum that can be passed in colostrum. Results are promising in the laboratory but have not been tested in field conditions. Specific protection may be transmitted by vaccinated dams in their colostrum against viruses (Rotavirus, Coronavirus) and certain strains of E coli. Although there is no vaccine against cryptosporidiosis, after an infection, the recovered animals are immune.
Can cryptosporidiosis be prevented?
Cryptosporidiosis is one of the most challenging diseases to control due to the environmentally stable oocysts, the low dose needed for infection, the high level of oocyst shedding by infected calves and the resistance of oocysts to many disinfectants. This disease often occurs year after year on an infected farm. No vaccines are available and treatment is basically limited to rehydration therapy in the US. Sanitary measures and good management practices are the weapons used to minimize environmental buildup and to optimize the neonatal environment. Reduce environmental contamination through frequent removal of feces and contaminated bedding from calving areas; steam cleaning and disinfection of surfaces with hydrogen peroxide- based disinfectants followed by thorough drying can significantly reduce buildup. Oocysts are susceptible to extremes of temperature from -4⁰ F to 140⁰ F (-20⁰ to 60⁰ C) and drying. In beef cattle, infections are most often seen when cattle are housed or calved out in a barn and cases increase quickly during calving season. Calves born at the beginning of calving season may be infected from oocysts in the environment and show mild or no symptoms. However, calving pens can build up oocysts and other pathogens quickly over the duration of the calving season. Calves born later in the calving season are exposed to higher numbers of pathogens (bacteria, viruses and C. parvum oocysts) increasing the likelihood of diarrhea. A study to determine the most important factors on a farm that increase the risk of cryptosporidiosis include housing calves in a barn, having a larger herd size (>200 head), use of hay bedding, and precipitation (100-150 mm) increased risk of shedding C. parvum. Preventing calf scours must include a good scours vaccine program in the cow herd. If a rotavirus, coronavirus, or bacteria (E. coli K99, Clostridium perfringens Type C, Salmonella spp.) infects a calf in addition to the parasite Cryptosporidium parvum, mortality (death loss) can be very high especially in newborns. Controlling rotavirus, coronavirus and E. coli with vaccines can significantly reduce sickness and death losses due to calf scours.
Is cryptosporidiosis common in humans?
Most human cases of cryptosporidiosis are caused by either C. parvum or the human adapted species C. hominis. Virtually all infections in calves 8 weeks of age and younger are caused by C. parvum and it is recognized as the major zoonotic species (animal to human transmission). It is most often transmitted through contaminated water supplies because the hardy oocyst is resistant to most disinfectants and their small size makes it difficult to eliminate with filtration from drinking water. Infection in immunocompetent people causes self-limiting diarrhea for up to 3 weeks but can be life-threatening in immunocompromised individuals. The disease is commonly contracted by veterinary students, or may be acquired at petting zoos and farm visits by the public.
As always, your veterinarian is the best resource for diagnosis of all medical conditions, treatment, and prevention recommendations.