Evidence of Ivermectin Resistance by Parascaris equorum on a Texas Horse Farm

By collecting fecal samples every 2 weeks beginning at 2 months of age, 32 foals from a single Texas farm were monitored. The foals were administered ivermectin paste at the time of the first collection and again monthly. When foals had Parascaris egg counts higher 2 weeks after ivermectin treatment than at treatment, they were administered pyrantel pamoate at the manufacturer's recommended dose (6.6 mg/kg) or at twice the recommended dose (13.2 mg/ kg) when tapeworm eggs were also detected. An elevation or only minimal reduction (less than 75%) in Parascaris egg counts was seen 2 weeks after ivermectin treatment until the foals were 8 months of age, at which time there was an 85% reduction in fecal egg count after treatment. When pyrantel was administered at the manufacturer's recommended dose, a 42% to 84% reduction in egg counts occurred, but at 13.2 mg/kg there was a 98% to 100% reduction in fecal egg counts 2 weeks posttreatment. However, pyrantel failed to control strongylate egg counts even at the elevated dose, whereas ivermectin reduced strongylate fecal egg counts by greater than 99%, determined 2 weeks posttreatment. Pyrantel, but not ivermectin, lowered Parascaris egg counts. Ivermectin, but not pyrantel, lowered strongyle egg counts 2 weeks post administration. A single drug for all ages of horses approach to parasite control requires rethinking. Combinations of drugs or more careful evaluation of anthelmintics in foals may be necessary for continued parasite control.     

Evidence‐based considerations for control of Parascaris spp. infections in horses

Parascaris spp. infection is virtually ubiquitous in young foals and worm burdens can achieve high numbers. The most important disease manifestation is impaction of the small intestine, which occurs in a small proportion of infected foals but is associated with a guarded prognosis for survival. Control of Parascaris spp. is complicated by emerging resistance to currently available anthelmintic drug classes. Resistance to macrocyclic lactones has been reported worldwide and a few studies have also documented signs of resistance to pyrantel salts and benzimidazoles. Foals generally develop immunity to Parascaris spp. parasites around age 6 months, but a proportion of weanlings and yearlings can harbour smaller burdens at age 8–10 months. Older horses have occasionally been reported with substantial ascarid burdens as well. Qualitative detection of ascarid eggs has good diagnostic value whereas an actual egg count does not correlate well with the size of the worm burden. A recent investigation documented the applicability of a transabdominal ultrasound technique for semiquantitatively monitoring ascarid burdens in foals. Control of ascarids is complicated by the limited drug classes available for treating this parasite, and by the fact that foals are often concurrently infected with strongyles. In many cases, none of the 3 available anthelmintic classes are simultaneously effective against both parasite groups, so close monitoring is required to select the most appropriate anthelmintic in each case.     

Practical aspects of equine parasite control: A review based upon a workshop discussion consensus

Development of resistance of several important equine parasites to most of the available anthelmintic drug classes has led to a reconsideration of parasite control strategies in many equine establishments. Routine prophylactic treatments based on simple calendar‐based schemes are no longer reliable and veterinary equine clinicians are increasingly seeking advice and guidance on more sustainable approaches to equine parasite control. Most techniques for the detection of equine helminth parasites are based on faecal analysis and very few tests have been developed as diagnostic tests for resistance. Recently, some molecular and in vitro based diagnostic assays have been developed and have shown promise, but none of these are currently available for veterinary practice. Presently, the only reliable method for the detection of anthelmintic resistance is a simple faecal egg count reduction test, and clinicians are urged to perform such tests on a regular basis. The key to managing anthelmintic resistance is maintaining parasite refugia and this concept is discussed in relation to treatment strategies, drug rotations and pasture management. It is concluded that treatment strategies need to change and more reliance should now be placed on surveillance of parasite burdens and regular drug efficacy tests are also recommended to ensure continuing drug efficacy. The present review is based upon discussions held at an equine parasite workshop arranged by the French Equine Veterinary Association (Association Vétérinaire Equine Française, AVEF) in Reims, France, in October 2008.