Coccidiosis is a protozoan disease that most cattle develop some immunity to, while continuing to shed a few oocysts in their feces. Calves are the most vulnerable because they have the least immunity, particularly if they ingest a high number of immature protozoa in a dirty environment that overwhelm their immune system.

James Hawkins, DVM, Jackson, MS, says young calves kept in a drylot or small pen are most at risk because the environment’s contamination tends to be high.

“People who run into problems with coccidiosis are usually backgrounders who buy small calves from different sources, keep them 90 days or less, and then resell them. They then buy another group of calves and move them into the same facility,” he says.

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Gary Zimmerman, veterinary researcher in Livingston, MT, says there are several different genera and many species of pathogenic coccidia, but only a few affect cattle. “There are also different susceptibilities in individual animals to various coccidia. If new animals come into the herd, they may bring coccidia the rest of the herd hasn’t been exposed to,” he says.

All cattle have subclinical infections, which are held in check by host immunity. “There is no cross-species immunity. A calf that’s been exposed to one species and developed immunity to it won’t have immunity to the other species. In addition, coccidia are host-specific. Some producers think cattle get coccidiosis from birds, but it isn’t true,” Zimmerman says.

Floron (Buddy) Faries, Texas A&M AgriLife Extension Service, says the three species of coccidia of most concern in cattle are Eimeria bovis, E.zuernii, and E. auburnensis. “Their ability to produce cattle disease is dependent upon the degree of exposure,” he says.


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A calf can handle a few oocysts, but a heavy load is another story. Meanwhile, adult cattle rarely develop the disease, though it can occur in thin, malnourished or severely stressed cows, or those compromised by another cattle disease. All cows, however, carry coccidia in the small and large intestines, Faries explains. 

“We distinguish between infection and disease,” Faries says. “When there are no clinical signs, I prefer to call it coccidiasis – simple infection in which coccidia are a normal inhabitant of the intestine. If there is deep damage to the intestinal mucosa, there will be symptoms and it becomes disease – which we call coccidiosis. The definition of disease implies infection and damage, plus clinical symptoms,” Faries says.

“For a parasite to survive in an otherwise immunocompetent host, it must have some mechanism to avoid, evade or suppress immunity,” Zimmerman adds. “Otherwise, the host’s immune system would kill the parasite. But parasites can suppress the immune response directed against them, or create a generalized immune suppression. This is why animals with a heavy parasite load are more susceptible to other pathogens.”

Stress, such as weaning, confinement and cold, wet or hot weather, can hinder the immune system. Nutritional stress also can be a factor, as many cattle experienced during drought in many parts of the country.

“We see more problems in young cattle coming from drought areas,” Hawkins says.  “Over the past dozen years, we’ve also started to see more problems in pastured cattle with coccidiosis – which we wouldn’t normally expect – and I’m not sure why.”

Conditions vary for different producers. “Someone buying calves in Florida may have the worst time during a hot, dry summer when calves are stressed by heat and dust,” Hawkins says. In some climates, however, it might be wet, sloppy spring weather that nips baby calves.

The presence of other pathogens, a change in diet, inadequate colostrum for a newborn calf, etc., are host factors that can play a role. Environmental factors include the use of permanent buildings and small pastures year after year, crowding, poor hygiene, adverse weather, transport, frequent regrouping of calves and bringing in new animals. 

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In a group exposed to a high level of contamination, all will be infected by the cattle disease, but only some show clinical signs with diarrhea. In some instances, morbidity (proportion of the group showing disease) may be as high as 75%.

“The morbidity and mortality rates of coccidiosis are variable, depending on overall herd health, nutritional and immune status, and external factors such as crowding, weather, exposure to other diseases, and other stressors,” Zimmerman says. “Mortality rate in a group is usually much lower than morbidity, but can be as high as 24%. Even if none of the animals die, the economic impact can be devastating.”