OSU Sheep Team

by William P. Shulaw, Extension Veterinarian

(Originally Published in Sheep Team Newsletter April 2003)

Drug resistance has become a serious problem in the major sheep and goat rearing areas of the world. Resistance to all the chemical classes of dewormers has also been described in flocks of sheep and goats in the United States. Drug resistance in the worm develops as a genetic trait just as some production traits in sheep. Unfortunately, once resistance is detected in the flock, it will often be permanent. If resistance develops to all 3 chemical classes of dewormers, it may be very difficult to graze sheep on that property.

Resistance develops when worms are exposed frequently to the same drug, especially when there is significant pasture contamination. Although parasitologists disagree about whether drug classes should be rotated annually, continuous use of the same product usually results in resistance with enough time. Along with frequent use, an important cause of drug resistance is UNDER DOSING because this allows worms with the potential for drug resistance (for example, one copy of a resistance gene) to survive and reproduce while the fully susceptible worms are killed. Under conditions favorable to the worms, such as a switch to a clean pasture following deworming, the eggs from these resistant worms may constitute the majority and become larvae on the grass waiting for the sheep to ingest them. Once the frequency of resistance genes reaches a threshold in the flock, the producer may see failure of the dewormer and clinical disease. Underestimating the sheep’s weight, incorrect calculations of the dose to be given, incorrect dilution of products that must be mixed, and improperly calibrated equipment are common causes of under dosing. Using an average weight for calculating the dose to be given to all the sheep, leads to under dosing essentially half the flock.

Once resistance is present to a specific dewormer, other drugs in that chemical class may also be less effective; this is known as side resistance. As of the present time, only 3 drugs are licensed by the FDA for use in sheep. They are Ivomec Sheep Drench® (ivermectin – the avermectin/milbemycin class), Levamisol® and Tramisol® drench and oblets (levamisole hydrochloride), and Valbazen® (albendazole – the benzimidazole class). In the past, many shepherds have used fenbendazole (Panacur®, Safeguard®) or thiabendazole (Omnizole® or Thibenzole®; these products are no longer marketed). If resistance to one of them is present, some level of resistance to albendazole (Valbazen®) may already be present in the flock. All available dewormers in the US today are members of one of these three classes of drugs. It is important for shepherds to know whether each class is effective in their flock before finding out otherwise during an episode of clinical parasitism. If the flock is large enough, it is possible to test for resistance to all three chemical classes in a summer by using groups of sheep or lambs.

In addition to the injectable formulation of ivermectin labeled for cattle, several other long-acting formulations of drugs in the avermectin class have been developed for use in cattle in recent years, and they have been used by some producers in sheep and goats. These include doramectin, eprinomectin, and moxidectin. They are available as pour-on products, as injectables, or both. They may provide extended protection from worm infection to grazing animals because of their persistency in the body. However, some of these formulations are likely to select for drug-resistant worms because the systemic levels decline slowly over as much as 60 to 80 days or more. These drugs may provide protection from re-infection for 21-28 days, but toward the end of the protection period, levels fall below what is needed to kill most of the worms. Consequently, those worm larvae that sheep ingest that have some capacity for drug resistance will survive to produce eggs. In contrast to the relatively short duration of action of Ivomec Sheep Drench®, these persistent drugs have a greater potential to select for a population of resistant worms, and modeling by researchers in Australia suggests that resistance development and complete failure of ivermectin can occur in just 5 or 6 years. In 2002 we documented ivermectin resistance in Hemonchus contortus (the barber pole worm, a voracious blood feeder) in a flock where we suspect it developed as a result of the use of an injectable ivermectin product. Resistance to ivermectin appears to be inherited in Hemonchus contortus as a dominant trait, thus it can develop very quickly.

Some of these drugs might be useful for sheep in very special situations (by veterinary prescription only), but when used indiscriminately by the producer, will likely speed the development of drug-resistant parasites and create a much larger problem during subsequent seasons. In addition, reliable slaughter withdrawal times have not been developed for use of these products in animals other than cattle, and producers run the risk of creating violative drug residues in animals marketed for food. We have heard reports of the use of Cydectin® Pour-On (moxidectin) orally in sheep and goats. Information gleaned from these reports, and research data concerning tissue residues of moxidectin in lambs, suggest that it is possible that tissue residues might persist as long as six months in some cases.

Pour-on formulations were devised for cattle skin and are not appreciably absorbed across the skin of other ruminants such as sheep, goats and llamas. These topical formulations are easy to apply, but that does not mean they are effective in sheep as they would be in cattle. As an example, a study of drug absorption in llamas and alpacas showed that systemic levels of some of these drugs did not reach reliable levels for parasite control. This would indicate that use of such formulations is both a waste of the producer’s money as well as contributing to poor parasite control and development of drug resistance. Sheep producers and veterinarians should carefully weigh the risks posed by using any dewormers not specifically labeled for sheep.

It is interesting to note that the development of drug resistant parasites in sheep has many similarities to concerns now arising in Ohio about the development of glyphosate (Roundup® and others) resistance in certain weeds. Weed scientists and crop management specialists are recommending that producers know how their herbicide works and develop management strategies that delay the onset of resistance. This is good advice for sheep producers seeking to control internal parasites. Strategies that don’t rely on chemicals alone to control worms include the use of clean pastures; grazing pastures with alternate species, such as cattle or horses; the use of protein and energy supplements at strategic times; and the use of resistant breeds of sheep. Some of these strategies will be discussed in future newsletters. Portions of this article were taken from   Bulletin #883, “Parasite control in sheep: Biologic control for the new millennium” by Drs. Shulaw and Monahan.