“Oocysts passed in manure are tiny and much smaller than worm eggs,” Faries says. “They aren’t infectious at that stage. They have to sporulate and develop into four cells, each of which has two sporozoites. These eight sporozoites are the infective stage.”

How long it takes for the oocyst to sporulate depends on weather and temperature. “If it’s cooler than 35°F, or hotter than 85°, development is prolonged. But at optimal temperatures, the oocysts form sporozoites in 2-4 days,” Faries says.

Once a calf ingests sporozoites, they invade the lining of small and large intestines, down into the mucosa where they multiply by asexual development.

“Then they form a second stage. These merazoites rupture out of the mucosal cells and invade new cells, multiply again and produce a second generation. This process continues for at least four asexual multiplication cycles. After a few days with all this damage, they re-enter the mucosal lining and form male and female gametes. These unite and secrete a wall around themselves and form an oocyst, which is passed out with feces to start the life cycle again.”

By the time oocysts are detected in a manure sample, damage is underway in the calf’s intestines, and the immune system is kicking in to fight it.

“If the calf is healthy, immunity becomes fairly high fairly quickly and there’s no further damage,” Faries says.

If a calf only ingests a few coccidia, it’s beneficial because it stimulates immunity against that species. This allows a calf to counter a higher level of exposure down the road.

“If a larger number are ingested and there is some damage (signs of coccidiosis), immunity kicks in and shuts down the asexual multiplying and the disease is self-limiting and runs its course. If immunity stays strong, due to good nutrition and good health, the calf won’t get the cattle disease again if exposed again later,” Faries explains.

Treatment tips

There is no vaccine against coccidiosis; prevention or treatment consists of traditional products that kill or hinder coccidia. Coccidiocidal compounds include amprolium and sulfas. Preventive drugs are called coccidiostats because they hinder multiplication of coccidia. These include decoquinate and the ionophores.

“If the calves are doing okay, we should let it run its course, instead of treating it and shutting down the asexual multiplication – which would interfere with development of immunity,” Faries says. “But if some calves are passing blood, with watery diarrhea, shedding some mucosal lining, or are anemic, weak and dehydrated because they can’t absorb fluid and nutrients, they need supportive treatment,” he adds.

These calves need treatment to shut down multiplication of coccidia, along with supplemental fluids and nutrients. Serious cases need intravenous therapy if the intestine is too damaged to absorb oral fluid and electrolytes.

Several products are effective for treating coccidiosis; these include amprolium (Amprol® or Corid®), as well as decoquinate (Deccox®). “These work well if infection is still in the asexual stage. Some of the sulfonamides, such as sulfaquinoxalene, are also still used,” Faries says.

If calves are brought into a contaminated facility, such as weaning pens or a feedlot, they may break with coccidiosis within four weeks unless administered a coccidostat.

“A preventive drug would be fed at a low level during that period. Some producers give every new group of calves 21 days or more of a preventive level of Corid,” Hawkins says. “The label for Corid as a preventive is 21 days at half the treatment dose (which is given for five days),” he says.

Ionophores, such as monensin (Rumensin®) and lasalocid (Bovatec®), are often given to feedlot-age calves because they help increase feed efficiency and also act as a mild coccidostat, hindering multiplication of coccidia. 

Joe Dedrickson, DVM and Merial’s field director of veterinary services, considers Corid, which is labeled for prevention and treatment of coccidiosis, as being “probably the best treatment for calves exhibiting symptoms, and Deccox for control is probably the next choice.” Deccox, which is labeled for prevention but not treatment, prevents immature coccidia from growing while the calf heals, and is unique because it’s safe at any level and not toxic to mammals and birds. Meanwhile, monensin and lasalocid are also labeled for prevention of coccidiosis, while sulfaquinoxaline and sulfamethazine are labeled for both treatment and control.