Combating Resistance

Vaccination seeds houses with oocysts that are sensitive to commonly used in-feed coccidiostats

Poultry producers confronting resistance to in-feed coccidiostats can minimize the problem by incorporating coccidiosis vaccination into their management plan, says poultry veterinarian Dr. Linnea J. Newman.

Continuous and long-term use of infeed ionophore coccidiostats has resulted in resistance and less effective control of coccidiosis, which is caused by protozoan parasites of the Eimeria genus, says Newman, a consulting veterinarian for Schering-Plough Animal Health. Resistance results in impaired performance, particularly poor weight gain.

However, rotating coccidiosis vaccination with the in-feed products "seeds" the houses with oocysts that are more sensitive to the in-feed treatment, she says.

Changes Oocyst Population

Coccivac-B, Newman explains, is a live vaccine produced with oocysts that were isolated before currently used coccidiostats were even developed. Consequently, birds that receive this vaccine shed oocysts that are sensitive to the in-feed coccidiostats widely used today by poultry producers.

"In other words, the vaccine can be used to change coccidiostat-resistant populations into coccidiostat-sensitive populations," she says. "The oocysts that result from vaccination, in fact, are extremely sensitive to both chemicals and ionophores, yet they are not as virulent as some of today's coccidiostatresistant field isolates."

Clear Evidence

As evidence, Newman points to

Eimeria maxima
research by Dr. H. D. Chapman of the University of Arkansas, which demonstrates that vaccination of a single flock with Coccivac-B is sufficient to restore the sensitivity of field coccidia to monensin.

She also cites trials conducted by Dr. Harry D. Danforth of the USDA (see feature on page 6), which further demonstrate that vaccinating with Coccivac-B renews the sensitivity of an on-farm coccidial population to the ionophore salinomycin, the most widely used coccidiostat in the United States.

Eimeria acervulina
In the trials, oocysts were isolated from litter samples taken in houses where broilers had been vaccinated for three or more flocks with Coccivac- B. Oocysts also were collected from litter samples taken in houses where birds were not vaccinated, but were treated with ionophore coccidiostats.

Next, sensitivity to salinomycin was tested. One group of specific- pathogen-free (SPF) birds was challenged with oocysts from the vaccinated houses, and another group of SPF birds was challenged with oocysts from the ionophore-treated houses. During the challenge, all birds received 60 ppm of salinomycin.

Weight Gain Improved

Comparison of the two groups revealed much better weight gain in the birds

Eimeria tenella
challenged with oocysts from the vaccinated houses; only an occasional field sample demonstrated comparable salinomycin sensitivity, Newman says.

More recent studies by Danforth indicate that vaccination can not only restore the sensitivity of oocysts to coccidiostats, but that it changes the composition of mixed species oocysts in the field and their ability to cause intestinal damage, Newman says.

For instance, an aggressive strain of Eimeria tenella and a moderately pathogenic E. maxima were isolated from litter samples collected from a farm at a large broiler integrator. After the samples were collected, a new flock was
Mixed-species oocysts
vaccinated with Coccivac-B and placed in the same house. New litter samples were collected at the end of the growout period.

The mixed-species oocyst population from each sample was isolated and used to challenge 10-day-old SPF test birds fed nonmedicated or salinomycin- medicated feed. Six days after the challenge, the birds were weighed and intestinal lesions were recorded. "Following vaccination with Coccivac-B, the aggressive E. tenella population had virtually disappeared," Newman says. Before vaccination, the E. tenella was considered very aggressive and created high lesion scores, even in salinomycin-medicated birds. It should also be noted that 3-Nitro (arsenelic acid) had to be used routinely to augment the ability of ionophore coccidiostats to control E. tenella, she says.

Lesion Scores Improve

After immunization, lesion scores for the middle and upper intestine due to E. maxima and E. acervulina had improved in salinomycin- medicated birds, which indicates that vaccination had an impact on these species of Eimeria and that each species had improved sensitivity to salinomycin after vaccination.

Coccidiosis vaccination can not only be rotated with coccidiostats, it can be an alternative to these in-feed products, Newman says.

Vaccine Fosters Natural Immunity

In-feed coccidiostats prevent coccidiosis by disrupting the parasite's life cycle, but vaccination enables chicks to develop natural immunity to coccidiosis infection, Dr. Linnea J. Newman explains.

Immunity develops when birds are exposed to infected oocysts passed in droppings. "It takes about two or three cycles of mild infection to provide immunity adequate enough to protect chicks from later field exposure to coccidia," she says.

Vaccination is more likely to be successful today than ever before because better methods of administration have been developed. For example, spray cabinet administration on hatching day helps ensure the vaccine is administered evenly to chicks, which in turn helps development of immunity in a flock and protect against coccidiosis outbreaks, she says.

Source: CocciForum Issue No.2, Schering-Plough Animal Health.
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