Of course we can always learn from others, and certainly many biological principles are not species specific. Over the last forty years, cattle and sheep producers have been blessed with numerous anthelmintic products (i.e. dewormers) that have been safe, effective, and relatively convenient to use. These products have given helped us maximize the genetic potential for productivity in our animals by reducing or eliminating the effects of internal and external parasites. The cost of these products has been very reasonable in light of the potential production gains they have provided. These factors have encouraged their frequent and widespread use. However, it is becoming very clear that internal parasite control that relies almost exclusively on using anthelmintics is not sustainable.

The first reports of dewormer-resistant worms in sheep appeared in the 1960s only a few years after the new anthelmintic, thiabendazole, was introduced. By the 1970s, reports of worms in sheep (and horses) that were resistant to this class of drugs had appeared from all over the world. Two new and different classes of drugs were introduced in the 70s and 80s, levamisole and ivermectin, that were initially very effective on these resistant worms, but within only a few years, reports of worms resistant to these chemical classes were also becoming rather common. By the end of the 1990s, multidrug-resistant worms, of multiple species, were considered a major threat to the continued survival of sheep operations in the foremost sheep producing countries of the world. Reports from the United States over the last 15-20 years indicate that our country has not escaped this problem. Over the last 15 years we have documented drug resistant worms on several Ohio sheep farms and, recently, a few farms where none of the available classes of dewormers work very well.

Unlike sheep and goats, cattle tend to develop a much stronger immune response to gastrointestinal parasites after a season of grazing exposure. Mature, well-fed cows tend to be fairly resistant to worm challenge. In addition, until fairly recently it was believed that most farms were deworming cattle relatively infrequently, and not all production classes were receiving dewormer. Because of these things, it was once thought that the appearance of dewormer-resistant worms in cattle would not likely become a problem except perhaps to a very limited extent. This outlook is now changing.

From a global perspective, dewormer resistance has been detected in all the economically important gastrointestinal worms (the roundworm species) of cattle. These reports come principally from the more developed countries with sizeable cattle populations such as Australia, New Zealand, South Africa, Brazil, Argentina, some countries of Europe, and more recently, the USA. This is not to say that all worms of cattle in these countries are resistant to all classes of dewormer or that the level of resistance is complete, but it does indicate that we should be concerned about the sustainability of our current deworming practices. The title of a 2003 paper from Argentina by Mejia and colleagues "Multispecies and multiple anthelmintic resistance on cattle nematodes in a farm in Argentina: the beginning of high resistance?" is an example of the concerns being raised by parasitologists. In a review of the situation in New Zealand by Pomroy, which was published in the New Zealand Veterinary Journal in 2006, he states, "The results of recent national surveys, detailed elsewhere in this issue, have shown that farming with anthelmintic resistance is already a reality for a majority of sheep and beef farmers in New Zealand. Not only is the prevalence of resistance high for at least some species but the actual reduction in efficacy is also substantial in many cases."

There have been a few reports of resistant worms in cattle from the US over the past 5-7 years. Very recently Dr. Gasbarre and colleagues from the USDA's Agricultural Research Service unit at Beltsville, MD, published their discovery of certain resistant species of cattle worms being grazed on a stocker operation in Wisconsin. The source of the cattle was the southeastern USA. In December of 2009, the USDA's National Animal Health Monitoring System (NAHMS) published two information sheets describing results of their studies of parasite control practices on US cow-calf operations as part of the Beef 2007-08 study. (http://www.aphis.usda.gov/vs/ceah/ncahs/nahms/beefcowcalf/index.htm ) This overall study represented 79.6% of US cow-calf operations and 87.8% of US beef cows. Producers should read these info sheets themselves, but the highlights include the findings that for operations that dewormed cattle or calves at least occasionally, a regular schedule was the primary factor used to determine when to deworm on the majority of operations (85%), and only 0.6% used fecal tests as the primary factor. Only 5.7% of operations reported using fecal examinations to evaluate parasite burdens in their cattle. This is interesting because 91% of producers cited efficacy (effectiveness) as an important or very important reason for choosing a product. Considering that fecal egg counts (FEC) are the only practical way to really evaluate effectiveness at the farm level, one wonders how these producers determine effectiveness on their farms.

Of 291 operations who initially expressed an interest in submitting fecal samples for evaluation in the NAHMS study, 99 actually submitted samples in Phase 1 of the survey. These samples were collected from weaned beef calves, 6-18 months-of-age, which had not been dewormed in the previous 45 days. Examination of these samples revealed that 85% of operations had samples with strongyle-type eggs in them, and the average was 32.5 eggs per gram of feces. This family of worms contains perhaps the most important species of cattle worms, and this result would not be too surprising in this age group. What was more interesting is that in Phase 2, producers were able to send in a second set of fecal samples from these animals collected about 14 days after deworming them using the standard practices on their farms. Sixty-one operations provided samples for this phase. It is generally accepted that if the average FEC of the samples collected after treatment is not reduced by at least 90% over the pretreatment samples, some level of resistance in the worms is present or that the dewormer was administered improperly. In Phase 2, 44% of all operations had less than a 90% reduction in FEC (strongyle-type eggs); 31% of all operations had a reduction of less than 80% (strongyle-type eggs).

The study was completely voluntary and farmers are assured their information will be kept confidential. Therefore, we have only overall, descriptive data at this time. There are several important considerations in interpreting these results. 1) We do not know if the farms with some evidence for resistant worms participated because they already suspected that they had a problem. If this were true, it could make the overall results show a higher level of resistance than is the case. 2) We do not know whether the farms with evidence suggesting resistance were predominantly from a region of the country where parasite life cycles are more easily maintained such as the southeast. In these areas deworming tends to be more frequent and the selection pressure for drug resistant worms is greater. 3) Although pre and post treatment FEC are commonly used to test for resistance, it is usually recommended to have an untreated control group for comparison purposes because sometimes the egg counts in a group of animals will change over a two-week period, either up or down, for reasons unrelated to a dewormer use. 4) We do not know the predominant species of worms represented in these egg counts. Some worm species are potentially more important than others for cattle with respect to the losses they cause. We do not know if any of the farms with less than 90% reduction in FEC actually experienced production loss. And 5) 25 of 27 (92%) farms that had a less than 90% reduction in FEC used a pour-on dewormer. Under some management conditions, licking among animals can result in substantial differences in the amount of actual drug delivered to an animal and its worms. Under dosing of some animals can occur in those situations and could lead to the wrong conclusions. It has also been suggested that some generic pour-on dewormers may not have the same potency as the original name-brand products.

Taken together with the available reports from the US and from other countries, this new information suggests that we need more specific information to accurately assess the situation with regard to resistant worms in cattle in the US, and we should begin to take stock of what we do with respect to our use of dewormers and the possibility that resistant worms will become a greater problem. Similarly, individual farmers and ranchers should get better informed about modern parasite control strategies and should begin gathering information about the status of their own operations. Here is where we can learn from sheep producers, many of which have been struggling with this issue for several years. If you are interested, visit the OSU Sheep Team website, www.sheep.osu.edu under the link on parasites for more information. We plan several more articles on this topic but will close this one with a quote from the publication by Dr. Pomroy referred to above, "High-risk practices in relation to selection of [worm] resistance need to be identified and avoided or at least their use limited. These include: treating adult animals where there is no identified need, moving newly treated animals onto 'clean' pasture, and failing to effectively quarantine-drench bought-in animals. None of these are new concepts but many have not been adopted or practiced."

Source: William P. Shulaw DVM, Extension Veterinarian, Ohio State University