Coccidiosis presents health challenge
Coccidiosis is an intestinal disease is caused by protozoa, picked up by the calf from the environment. Most cattle have encountered these protozoa and have developed some immunity but may continue to shed a few oocysts – the egg form of the protozoa – in their feces, which can then contaminate feed or water.
Calves are most vulnerable to this disease because they don’t yet have much immunity, and if they ingest a high number of protozoa in a dirty environment, they may break with coccidiosis.
Dr. Gary Zimmerman, veterinary researcher in Livingston, Mont., says there are several species of coccidia that affect cattle.
“There are also different susceptibilities in the individual animals to the various coccidia. If new animals are brought into the herd, they may bring new coccidia that the rest of the herd has not yet been exposed to,” he says.
All cattle have subclinical infections of coccidian, held in check by the host immunity, Zimmerman says.
“There is no cross-species immunity that works against all species of coccidia. A calf that has been exposed to one species and developed immunity will not have immunity to the others,” he explains. “Some coccidia seem more pathogenic than others.”
While researchers can’t look under a microscope and determine which species is worst, Zimmerman says, “There is something inherent in that particular coccidia that is more virulent to the host.”
Zimmerman also adds that coccidia are host specific, with only a few strains that are pathogenic to cattle.
“Some producers think cattle can get coccidiosis from birds, but this is not true,” he says.
The coccidia in cattle are continually shedding oocysts into the environment.
“The oocysts accumulate on the ground and become infective to other cattle,” Zimmerman explains. “The oocysts are long-lived. They are resistant to varying degrees of heat and cold. Introduction of new animals into a location where there are coccidia they have not encountered before could result in disease.”
The immune system is continually battling parasites.
“For a parasite to survive in an otherwise immuno-competent host, it must have evolved some mechanism to avoid, evade or suppress immunity. Otherwise the host’s immune system would kill them,” Zimmerman explains.
He continues, parasites can suppress the host’s immune response directed against them, however or create a generalized immune suppression.
“This is one reason why animals that have parasites can be more susceptible to other pathogens,” he says. “The parasite may be knocking down the entire immune system, in addition to causing problems with the host’s ability to absorb nutrients.”
Parasites can also cause a decrease in the host’s response to vaccine.
“When we vaccinate populations of animals or humans, not all of them will have a positive response to the vaccine. This is not due to vaccine failure but to the inability of some individuals to develop a protective response,” he explains.
Zimmerman adds, “Some children are always sick, because they do not have a completely optimal functioning immune system.”
Some of the things that allow coccidiosis to develop include factors that increase the risk.
“Various stressors like weaning, poor nutrition, poor feeding practices such as confining the animals and feeding them where oocysts might be present, etc. can increase the risk,” Zimmerman comments.
As an example, he notes producers often feed hay in the same place every day.
“Feeding round bales in feeders, even if feeders are periodically moved, concentrates cattle in the same area. The big bale lasts longer, so cattle are in that same spot longer, and it builds higher concentration of oocysts,” he says.
The presence of other pathogens, change in diet, inadequate colostrum for a newborn calf and more are all factors that may play a role.
Environmental factors could include use of permanent buildings and small pastures used year after year, crowding, poor hygiene, adverse weather, transport, frequent regrouping of calves and bringing in new animals.
If a group of calves becomes exposed to a high level of contamination, they all will be infected, but only some show clinical signs. There may be only a few that break with diarrhea, but in some instances, morbidity – or the proportion of the group showing disease – may be as high as 75 percent.
“Morbidity and mortality rates of coccidiosis are quite variable, depending on herd factors such as overall health, as well as nutritional and immune status, and on external factors such as crowding, weather, exposure to other disease agents and other stressors,” Zimmerman says
While the mortality rate in a group of cattle is usually much lower than morbidity, it can be as high as 24 percent.
“Even if none of the animals die, the economic impact of coccidiosis can be devastating,” he adds.
There is no vaccine for coccidiosis. Prevention or treatment still consists of traditional products that kill or hinder the coccidia.
The drugs for treatment include coccidiocidal compounds like amoprolium or sulfas that kill the organism.
There are also drugs for prevention, and these are called coccidiostats because they hinder the multiplication of coccidia. These include decoquinate and ionophores like lasolocid and monensin.
European countries and other places in the world have some additional drugs that are not used in cattle in the United States.
“Most of the time coccidia are held in check by the host, but sometimes we must resort to treatments. Times of year when we tend to see coccidiosis are in young calves after three weeks of age, especially when environmental stressors play a role or when weaning age calves are stressed. We often do things at the worst time, such as calving or weaning during cold or wet weather,” says Zimmerman.
Heather Smith Thomas is a correspondent for the Wyoming Livestock Roundup. Send comments on this article to firstname.lastname@example.org.