Cryptosporidium parvum is the pathogen most often diagnosed in preweaned scouring calves, says Daryl Nydam, DVM, PhD, Cornell University, speaking at the 2010 Western Veterinary Conference. Under the right group of conditions, infection can cause severe diarrhea and death in young calves.

Clinically affected calves – as well as calves with no outward signs of infection -- can shed large numbers of oocysts. In fact, during an average infection a calf may excrete oocysts for six to nine days, scour for three days, and shed a total of approximately 40 billion oocysts.Mature cattle have also been shown to shed oocysts, albeit in lower concentrations, in their manure, especially around parturition. Thus, this pathogen can be very prevalent in the calf’s rearing environment and can be present in the maternity area as well.

Calves are primarily infected via the fecal-oral route and it likely takes less than 50 oocysts to infect a healthy calf. Oocysts survive very well in the environment with a portion of the oocysts retaining infectivity after freezing, and they are also resistant to many disinfectants at farm-friendly concentrations, e.g., sodium hypochlorite (bleach), peroxygen (Virkon), chlorine, and iodophores. Six percent hydrogen peroxide and 10% formalin have shown activity against oocysts, but hydrogen peroxide is readily deactivated in the presence of organic matter and have minimal activity against oocysts.

The potentially large number of oocysts that survive well in the environment leads to a high likelihood of a susceptible calf being exposed to an infectious dose of oocysts. Once the intestine is colonized, the life cycle of the parasite allows for auto-infection of nearby cells, further decreasing the number of ingested parasites required to initiate infection and possibly leading to chronic disease.

Control of cryptosporidiosis will rely on an integrated approach to reduce flies, curb transmission, and decrease environmental loading. Diligence in proper calf care and animal husbandry (i.e., clean, dry, isolated, well-ventilated calves that have received suitable colostrum) will serve calves well until age-related resistance associated with the normal development of the rumen and intestinal flora begins.