Testing sensitivity: Study shows ‘widespread’ monensin resistance

Turkey producers should scrutinize their coccidiosis-control program to make sure it’s working now that a study has demonstrated monensin resistance in a majority of Eimeria isolates obtained from US turkey flocks.

“Our isolates were obtained from seven states, suggesting the problem is quite widespread,” says renowned coccidiosis expert H. David Chapman of the University of Arkansas, the lead investigator of the study.

In chickens, Chapman explains, it is well known that resistance to in-feed anticoccidials is prevalent because of long-term and continuous use. In turkeys, producers may have observed a decline in monensin efficacy over the years, but there have been few studies proving these observations, which prompted Chapman to initiate the study with co-researcher Thilak Rathinam.

The investigators tested the sensitivity of 23 field isolates of Eimeria — the family of parasites that causes coccidiosis — which they obtained from litter and intestinal samples taken from turkeys farms in Arkansas, California, Kansas, Missouri, North Carolina, Pennsylvania and Virginia, all major US turkey-producing regions.

To evaluate the sensitivity of the isolates to monensin, the investigators used 9-dayold female poults, which were weighed, allocated to cages and given feed that either was or was not medicated with monensin. Two days later, the researchers challenged all the poults with a maximum of 100,000 oocysts from each isolate, which they bred for the study. Next they evaluated the poults 6 days after infection for weight gain and mortality, which was expressed as a growth-and-survival ratio (GSR). Then they determined anticoccidial activity for each treatment to determine monensin efficacy.

The dose rate of monensin in the medicated feed, Chapman notes, was 99.2 ppm, which is the highest concentration used for coccidiosis control in turkeys and also most likely to achieve maximum efficacy.

RESISTANCE FOUND IN 17 OF 23 ISOLATES

Of the 23 Eimeria isolates tested, investigators determined that six were sensitive, seven were partially resistant and 10 were resistant to monensin.

“Significant differences were observed in the GSR of unmedicated poults, indicating that the isolates varied in pathogenicity,” Chapman and Rathinam write in an article about their study published in Avian Diseases (51:954-957, 2007).

The investigators also tested vaccinal oocysts of Coccivac-T, a live coccidiosis vaccine, for sensitivity to monensin because the Eimeria strains in the vaccine have been maintained in a laboratory and never exposed to anticoccidials. Those isolates were found to be sensitive to monensin, further demonstrating that Eimeria resistance is due to exposure to monensin, which is one of the most widely used anticoccidials in turkeys for coccidiosis control. In fact, about 70% of meat-type turkeys in the United States are reared with monensin in at least one of their feeds, Chapman and Rathinam point out in their article.

“This is the first report of the acquisition of resistance to monensin in isolates of Eimeria from turkey flocks in the United States,” they say.

In 1980, other investigators described resistance to monensin in strains of coccidia from Canadian turkey flocks, and the report was the first to document resistance to this drug in either chickens or turkeys.¹ At the same time, the investigators tested US isolates and found them to be sensitive to monensin, but the drug had not been used in areas where they obtained the isolates, say Chapman and Rathinam, who used the same testing methods as the earlier investigators.

“Our recent [published] results show that the situation has changed with the majority of isolates showing either resistance or partial resistance to monensin,” Chapman says. It is also “likely” that resistance to other anticoccidials has developed but, unfortunately, few studies have been conducted.

LOOK FOR SUBCLINICAL DISEASE

The consequences of resistance to any drug is usually subclinical disease, characterized by impaired performance such as reduced growth and poor feed conversion, even though poults may appear normal and show no obvious signs of disease. In poorly managed flocks, acute coccidiosis can occur, but is not often reported, Chapman says.

It is possible that turkey producers may not even realize their flocks have subclinical coccidiosis, he continues. “Coccidiosis in turkeys is particularly insidious because the disease does not cause obvious gut lesions and is therefore likely to be missed during a regular necropsy session — or it may be confused with some other condition.

“Often the losses due to subclinical coccidiosis are not apparent because there is no yardstick for comparison. However, change to a more effective drug can result in improved performance. This has sometimes been referred to as ‘the new drug effect,’ and indicates a loss of sensitivity to the drug used in previous flocks,” Chapman says.

As a result of the study, turkey producers should look carefully at their coccidiosis control program and make changes if needed, Chapman advises.

“Regular necropsies can indicate whether intestinal disease is contributing to poor performance and whether coccidia are responsible,” he says. Poor litter conditions, particularly wet litter, are often a contributory factor to coccidiosis and therefore attention to management and hygiene is always important.

If coccidiosis is found to be a problem, “It will be necessary to change to a more effective drug or utilize a coccidiosis vaccine,” Chapman says.

1. Jeffers, T.K. and Bentley, E. J. Monensin sensitivity of recent field isolates of turkey coccidia. Poult. Sci. 59: 1722-1730. 1980.

Chapman says the isolates tested were from seven states.

Poults received the highest dose of monensin used for coccidiosis control.