In early April, Bovine Veterinarian ran an article titled “Detecting and preventing drug-resistant parasites,” based on an FDA publication outlining possible causes for the development of resistant parasites and potential management solutions.

Knowing drug resistance in parasites is a complex issue with no clear consensus among the experts, we asked for responses from veterinarians and parasitologists to the FDA’s conclusions and recommendations.

Soon after, we received a response from veterinary parasitologist Lou Gasbarre, PhD, in which he questioned several of the recommendations in the FDA publication, specifically as they apply to beef and dairy cattle. In response, FDA’s Center for Veterinary Medicine sent the following letter, further explaining the publication and its recommendations:

“FDA’s Center for Veterinary Medicine thanks Bovine Veterinarian for featuring our brochure, “Antiparasitic Resistance in Cattle and Small Ruminants in the United States: How to Detect It and What to Do About It,” in your article published online on April 10, 2013. We also thank Dr. Lou Gasbarre for his comments, published online on April 29, 2013, on both your article and our brochure. 

We want to take this opportunity to expand on and clarify some issues raised by Dr. Gasbarre. 

Dr. Gasbarre mentions that the brochure is an over-simplification of the complex issues surrounding antiparasitic resistance in the cattle industry. He further mentions that although FDA lumped small ruminants and cattle together in the brochure, the differences between the two industries require fundamentally different strategies to address antiparasitic resistance. 

We agree with Dr. Gasbarre that because the U.S. cattle industry is complex and stratified, veterinarians and producers have to manage parasites differently at each step. However, we think the general concepts of managing antiparasitic resistance outlined in our brochure, such as preserving refugia, not treating the entire herd, avoiding under-dosing, and reducing stocking density, can be practically applied to the cattle industry.

We intended for the concepts and recommendations in the brochure to be high-level. By presenting information about antiparasitic resistance in an easy-to-read format, such as a brochure, we hope to reach more veterinarians. We also hope that veterinarians will use our brochure to educate their clients about the emerging problem of antiparasitic resistance.

On the last page of the brochure, we included the links for information from FDA’s public meeting on “Antiparasitic Drug Use and Resistance in Ruminants and Equines,” held March 5 and 6, 2012. At this meeting, seven expert parasitologists and pharmacologists gave presentations on antiparasitic drug use and resistance in grazing species in the United States and worldwide. Dr. Gasbarre was one of those invited speakers. A transcript of the meeting, the presentations, and a meeting overview are available on our website for those seeking more in-depth information on antiparasitic resistance.

We acknowledge that our brochure does not discuss the use of combination antiparasitic drugs as a tool for slowing the development of antiparasitic resistance. Dr. Gasbarre notes that mathematical models as well as international experiences, such as those in Australia and New Zealand, indicate that appropriate combination antiparasitic drugs can help slow the development of antiparasitic resistance. Several speakers at FDA’s public meeting presented similar findings, and the use of combination antiparasitic drugs was an item of considerable discussion at the meeting.

We welcome drug companies to pursue approval of appropriate antiparasitic drug combinations with highly or completely overlapping indications. Contrary to Dr. Gasbarre’s assertion, we do not harbor any aversion to antiparasitic drug combinations. However, a drug company has to show us that the proposed combination is safe and effective.

We did not mention antiparasitic drug combinations in our brochure because, currently, none with highly or completely overlapping indications are FDA-approved for use in cattle or small ruminants in the United States. Without such an approved combination antiparasitic drug available, it would have been irresponsible for us to recommend combination therapy because of the lack of safety and effectiveness data and an established withdrawal period. It would have also been confusing for veterinarians and producers who are left wondering what combinations to use.

Dr. Gasbarre is correct that FDA does not adhere to the concept of antiparasitic drug treatment to enhance production goals. Antiparasitic drugs in cattle and small ruminants are only FDA-approved for therapeutic uses. Using antiparasitic drugs for production purposes, such as increased weight gain, is inconsistent with their labeling and illegal, as federal regulations do not permit extra-label use of any drug for production purposes. Furthermore, studies suggest such usage hastens the development of antiparasitic resistance due to the practices of treating the entire herd and not selecting animals based on therapeutic need.

We recognize that using antiparasitic drugs for production purposes is prevalent in the U.S. cattle industry, as Dr. Gasbarre suggests. We think there needs to be a paradigm shift within the cattle industry away from these illegal production uses. This type of proactive change will help approved antiparasitic drugs remain effective for longer.   

We also want to point out that Dr. Gasbarre’s comments address statements made in both Bovine Veterinarian’s article and our brochure. For clarity, the opening statement that Dr. Gasbarre takes exception to (“Today’s antiparasitic drugs generally remain effective against cattle parasites in the United States…”) is the opening statement of Bovine Veterinarian’s article, not our brochure. In fact, like Dr. Gasbarre, we also take exception to this statement, and our brochure clearly states that resistance to antiparasitic drugs is emerging in the United States.   

Finally, we agree with Dr. Gasbarre’s closing statements, “We need to make sure that producers are using sustainable parasite control programs. This requires the use of the right classes of drugs at the right times, and proper pasture management again done at the right time.” He gets at the crux of sustainable antiparasitic drug use that we are promoting in our Antiparasitic Resistance Management Strategy (ARMS).

Our goals in publishing the brochure were twofold: (1) to make veterinarians more aware of the problem of antiparasitic resistance in cattle and small ruminants in the United States; and (2) to provide veterinarians with a starting point for managing antiparasitic resistance by using appropriate antiparasitic drug therapy along with good management practices.

We realize antiparasitic resistance is a complex topic and encourage further dialogue between bovine practitioners, producers, members of academia, and FDA.”