In early April, we 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.

Veterinary parasitologist Lou Gasbarre offered the following comments:

I have read with interest your article dated 4/10/2013 in Bovine Veterinarian concerning detecting and preventing drug-resistant parasites.  I feel this is a very important topic and applaud you’re efforts in bringing this topic to the attention of the cattle industry.  At the same time I feel it is very important to provide the industry with the best information we can.

I feel that there are a number of misconceptions in the FDA report that if these recommendations were followed by the cattle industry, they could pose very serious problems for the individual producers.  I would like to briefly outline several of these misconceptions/over-simplifications and would be more than happy to discuss them in detail with you if you so desire.

First, the information sheet developed by FDA is a very good set of recommendations for the sheep and goat industry, but has serious flaws when applied to the cattle industry.  As you are well aware, the cattle industry is not one management system but is stratified into several steps, i.e. cow/calf, backgrounder/stocker, feedlot and dairy.  The recommendations in this article refer to only the cow/calf sector, and even here have some very severe errors.  It is completely wrong to assume that the basic biology of the cattle-nematode system is the same as that of the sheep/goat- nematode system.  The parasites are in some cases closely related but they may not have the same behaviors.  

For instance the dominant parasites in cattle are cool weather parasites unlike the dominant H. contortus of small ruminants, as such treatment based on “high temperature, high humidity’ in cattle versus sheep can have very different results.  Similarly, dragging the pasture at the wrong time would only spread the parasites over the pastures and alleviate the advantage of keeping stocking rates low to avoid having the cattle graze near fecal pats.  There are similar errors in thinking, but I hope these two examples outline the danger in assuming the systems are identical.

Secondly, the procedures outlined will only work if the individual producer first ensures a relatively low overall infection rate in the herd.  This means that the producer would first need to develop an aggressive program to lower overall parasite numbers, then institute a good quarantine program for cattle coming into the operation, and finally have an active monitoring program for the operation.  Without these tools, the adoption of some of the ideas presented could have very severe effects on animal productivity.

Thirdly, the article completely ignores the use of drug combinations to manage resistance.  I am sure in great part this is due to FDA’s aversion in principle to such treatments.  The simple fact is that in areas where anthelmintic resistance has been a long standing affair (most notably Australia and New Zealand), the use of multiple drug classes at the same time has become the standard procedure for managing resistance.  In addition, all mathematical models indicate that this is the best way to prevent the development of resistance.  

Here in the US where resistance in cattle is mainly against the macrocyclic lactones it is important for us to protect the other drug classes by using such combinations.  In a similar vein, FDA does not adhere to the concept of drug treatment to enhance production goals, while we know that this has been the main focus of the cattle industry since ivermectin hit the markets nearly 40 years ago.  As such, as phrased the argument for refugia will fall on deaf ears.  The concept of refugia is an important concept and can be inserted into at least some aspects of the American cattle industry, but to do so takes much more sophisticated and precise implementation than the examples alluded to in the article.

Finally, I do take some exception to the opening statement that here in the US the existing drugs remain generally effective.   While this is true for the benzimidizoles and Levamosole there is widespread resistance to the macrocyclic lactones especially when used in a pour-on formulation - which as you know is the most commonly used treatment in the US.  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.   We must be exceptionally careful to give the right information.  


Lou Gasbarre, PhD