Anticoccidial resistance found in 13-state sampling

Evidence of resistance to several in-feed anticoccidials emerged in a recent sampling of coccidia from commercial turkey farms in 13 US states, according to a presentation at the 7th International Symposium on Turkey Diseases held recently in Berlin.

In addition, vaccination provided nearly 100% control during testing, said Dr. Steve Fitz-Coy, a parasitologist with Intervet/Schering-Plough Animal Health.

For the study, Fitz-Coy collected over 20 coccidial isolates from litter, feces and gut samples submitted from commercial turkey flocks. The samples were then fed as an inoculum to coccidia-free turkey poults that were 3 to 5 weeks of age, he said. From days 5 to 8 after inoculation, oocysts were harvested from droppings, and after they were cleaned and sporulated, anticoccidial sensitivity battery testing was conducted, he said. Each sample was maintained as an isolate.

Before they were inoculated, birds received one of several in-feed anticoccidial medications for 36 to 48 hours. Two additional groups served as controls: one group was not medicated or infected, and the other group was not medicated but was infected, Fitz-Coy said.

The effectiveness of each drug was evaluated according to a formula that considered the ratio of coccidial species present in the inoculum, as measured by weight gain, gross lesion scores, microscopic parasitic load and mortality.

Fitz-Coy confirmed that the isolates used to challenge birds did, in fact, cause disease because the control groups that were infected but not medicated had an average weight depression of 37% by 6 days after challenge.

EFFICACY AS LOW AS 9%

Of the anticoccidials tested, and using the infected but unmedicated group as a baseline for comparison, Fitz-Coy found average efficacy to be 67% for diclazuril, which was the most effective of the drugs tested, followed by monensin at 64% and then roxarsone at 63%. He said the least effective anticoccidial was amprolium, which was only 9% effective (Table 1).

The parasitologist also found geographic differences in resistance levels. Amprolium was found to be -25% (negative) effective in the Northwest but 54% effective in the West. Monensin had the best efficacy against isolates in the Midwest and was less effective against isolates from the West, Fitz-Coy said.

When the researcher immunized young, coccidia-naive birds with the live coccidiosis vaccine Coccivac-T and then challenged them after 28 days with the field isolates, the level of control was better. Efficacy was 99% compared to an overall average of 50% for anticoccidials (Figure 1). The vaccinated birds had better weight gain, a lower parasite burden and improved livability compared to birds that received anticoccidials, he said.

A large part of Fitz-Coy’s study involved identifying the types of coccidia infecting commercial flocks. The most prevalent species were Eimeria meleagrimitis, followed by E. adenoeides then E. dispersa, E. meleagridis then E. gallopavonis (Figure 2). The most pathogenic of these species — and the most costly to commercial turkey operations — are E. adenoeides, E. gallopavonis and E. meleagrimitis, he said.

Other evidence that turkey coccidia are becoming less sensitive to anticoccidials has been found by parasitologist H. David Chapman of the University of Arkansas, who discovered widespread monensin resistance in a study involving isolates from seven states (Turkey Health, Issue 3, page 1 and Avian Pathology, vol. 51:954-957, 2007).

Despite using the highest concentration of monensin administered for coccidiosis control in turkeys, 17 of 23 isolates he tested were either partially or totally resistant to monensin.

In chickens, resistance to in-feed anticoccidials due to long-term use of the drugs is well known, and studies on the topic have been well documented.

“It looks like the pattern of anticoccidial drug resistance in turkeys is similar to that in chickens,” Fitz-Coy said. “It would be wise for turkey producers to monitor their coccidiosis control program to be sure it’s working effectively.”

Subclinical coccidiosis in particular can be present but overlooked, eroding turkey performance and producer profits, he said.

TABLE 1. Average efficacy of in-feed anticoccidials as determined by sensitivity battery testing.

FIGURE 1. Coccidiosis vaccination yielded better control of recent field isolates from commercial turkey farms than in-feed anticoccidials.

FIGURE 2. The prevalence of various turkey coccidian species based on sampling from 13 US states.