All cattle are infected to some degree by coccidiosis protozoa, a one-celled parasite that lives in the intestine. Young cattle that have not yet developed immunity can become ill if environmental conditions are right for picking up large numbers of oocysts. The parasite may cause illness when calves are confined in small areas, as for weaning or going into feedlots, allowing the oocysts passed in manure to build up to infective levels. Coccidia are found in all healthy cattle and usually cause trouble only when animals are stressed or have overwhelming numbers of protozoa.

Stress inhibits the immune system; the parasites are then able to overcome the animal's resistance. The stress of drought, cold or adverse weather, weaning, etc. allows the parasite to divide more rapidly and go through more life cycles. If a calf's immune system is hindered by stress, the number of cycles is greater before the calf can begin to resist the parasite, creating more damage to the gut lining.

Research in Alabama
Veterinary researchers and parasitologists at Auburn University in Alabama have looked at various effects on the immune system that can hinder coccidiosis in young calves. Dr. Byron Blagburn and Dr. Jim Sartin completed a study in 1997 on the effect of hormones on calves' immune systems. They took day-old calves and raised them in a parasite-free environment. At 8 weeks of age they gave the calves Synovex-C (a commercial growth implant containing estrogen and progesterone, two female hormones). At 11 weeks of age they infected the calves with coccidia oocysts--enough to create a sub-lethal infection with fever and diarrhea. The calves eventually got over it, in two or three weeks.

Dr. Sartin says that after being infected with coccidia, "at day 15 they developed fever. The ones that had been given the hormone had fever for only two days, while the others had fever for five days. The calves that did not receive the implants (no hormones) had 9 or 10 days of diarrhea, while those with implants had diarrhea for only fivedays; they had the diarrhea only half as long.

Twenty days after we gave them the coccidia infection, those without hormones were off feed, with a 60 percent reduction in food intake. Those with the hormone decreased their food intake by only 25 to 30 percent, and recovered more quickly; four days later they were back to 95 percent of their feed consumption and were almost normal. The others had lowered food intake for more than eight days and had not fully recovered when we stopped the study."

There were three groups of calves. "The controls (no hormones, no coccidia infection) gained 40 pounds in the two weeks; the ones with the coccidia infection alone lost seven pounds in the two week study, and those with the coccidia and the implant gained 27 pounds."

The researchers are not sure why hormones help. "We've looked at a lot of immune parameters," says Sartin. "We know estrogen will increase antibody production, but coccidia antibodies are not of much benefit in fighting the infection. We are doing another study to see if hormones block the cytokines--a family of molecules that send the body signals during infectious disease, telling it to turn on the fever, stop eating, and trigger immune functions."

The researchers don't know if the protective benefit from the hormone implant is from the estrogen, the progesterone, or both. "We do know from our literature search that female cattle seem to have more resistance to parasitic diseases than males, and it may be due to these female hormones," he says.

This fact was also noted by Dr. Ellis Greiner (Department of Pathobiology, University of Florida), citing a trial some years ago on calves in a feedlot situation that showed there was a cost benefit in bull calves for a preventative coccidiosistat given in the feed, but not in heifers. The heifers may not have been as adversely affected by coccidiosis due to their female hormones.

Dr. Sartin is not sure if calves could be given the hormone after they become sick, to gain any protective benefit, but he is convinced the implants help if given before exposure. "People are already using growth implants, so this may be a secondary benefit they are not even aware of," he says. "The hormones are protecting against severity of coccidiosis and also bacterial infections, and also helps with vaccination success, so you are getting a triple level of benefit." He says these anabolic hormones do not really prevent disease, but they do help the animals recover faster and keep them from losing so much weight. He also says coccidiostatic drugs really do work well to protect calves from getting sick with coccidiosis. "One year we tried to do a study on dairy calves, infecting them with coccidia, and couldn't get them sick. Later we found out they had been given a coccidiostat when they were younger." He says that if producers use the coccidiostat, a growth implant and a good vaccination program against common infectious diseases, it can protect calves quite well from coccidiosis.

There is no vaccine for coccidiosis. The coccidiosis antigen (which stimulates the calf to build immunity) is not a strong one, so it has been impossible to create an effective vaccine, explains Dr. Sartin. The calf has to have continuous low exposure to stay immune; the immunity created lasts only a few months.

It is impossible to eliminate the parasite from the soil; disinfectant will not kill it. Under favorable conditions the oocysts can survive on the ground for two years. Control depends primarily on cleanliness, avoidance of stress and overcrowding, and the use of a good coccidiostat in situations where calves are apt to be exposed to more than a minimal level of infection. Hormonal growth implants can be added to the list of things a producer can use to reduce the problems caused by coccidiosis. ©