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Control, Transmission, and Prevalence of Natural Infections of Internal Parasites of Equids and Ruminants
Department of Veterinary Sciences
Effects of internal parasites of domestic animals range from causing no perceivable clinical problems to death of these hosts. Historically, many species of internal parasites have shown resistance to chemical compounds after a period of usage. Research is necessary to continuously monitor the level of activity of the drugs. The purpose is to provide up-to-date information to animal owners and veterinarians about the the most active or inactive compounds for parasite control.
2009 Project Description
Certain aspects of small strongyles and ascarids in horses with notes on H. contortus in sheep. The main emphasis is on drug resistance and especially research at the University of Kentucky. It highlights the pioneer, Hal Drudge, DVM , ScD, and his almost 50 years of research on this subject, alone or with colleagues, at UK. He was the first to do research and publish on drug resistance of a nematode species. This was on phenothiazine resistance of the "barber pole" stomach worm (H. contortus) in sheep.
Later, Drudge and others in England found small strongyles resistant to this drug in horses. Thiabendazole was highly active initially against small strongyles and H. contortus, but resistance was found later in Kentucky and other geographical areas. A fairly short time after their extensive usage, benzimidazoles (e.g., thiabendazole), pyrimidines (eg., pyrantel pamoate), and piperazines became ineffective against small strongyles. This phenomenon has been studied continuously for several decades at the University of Kentucky and continues now (2009). Numerous other researchers in many parts of the world have documented the resistance problem with these compounds for control of these parasites.
Recently, research in various locations, including Kentucky, has recorded that small strongyle EPGs in horses treated with macrocyclic lactones (IVM or MO X) are returning sooner than when the drugs were first marketed. At the University of Kentucky (Lyons et al, 2009), critical tests in horses treated with IVM revealed that removal of small strongyles in the lumen of the large intestine was 100% for adults and 36 to 80% for immatures.
Results of this study indicate the following probable reason EPGs of these parasites now are returning sooner than initially in horses treated with IVM . Apparently, lessened activity on immatures in the lumen of the large intestine has led to a shorter "completion" of the life cycle. Ascarids are now resistant to activity of IVM and MO X according to research in various locations in the world, including Kentucky.
Most chemical classes of parasiticides have become inactive, especially against nematodes, after a period of use in domestic animals. This has resulted in a major dilemma because no new broad-spectrum classes of chemicals have been marketed for nematode control since the avermectins in the early 1980s. Adding to the situation is the apparent lack of new compounds becoming available commercially in the near future. Besides lowered efficacy of drugs, the number available for horses has declined substantially in the past 20 or so years.
It is obvious that there has been too much reliance on, and in many cases, unnecessary overuse of drugs. For instance, for strongyle control, profiles of EPG counts for individual horses could be established and then only the horses with high EPG values could be selectively treated. In other words, don't routinely treat all horses on a farm for strongyles. Currently, there is more leeway with strongyle control because the many years of usage of the benzimidazoles/ macrocyclic lactones/pyrimidines have dramatically reduced the prevalence of the most pathogenic nematodes (Strongylus spp) in horses on farms with routine deworming programs.
This situation could change if drug resistance of these species occurs or if treatment programs change. Under current circumstances, parasite control is in general a "throw-back" to over 50 years ago before the advent of effective drugs. This essentially means that, with exceptions where drugs are still effective, horse parasites cannot be as controlled now as in the past several decades. It is highly important that practical measures other than, or in addition to, using drugs need to be developed and implemented to aid in parasite control.
Lyons, E.T., and Tolliver, S.C. (2009) Some historic aspects of small strongyles and ascarids in equids featuring drug resistance with notes on ovids-emphasis on research at the University of Kentucky. SR-102 (Bulletin) Agricultural Experiment Station, University of Kentucky College of Agriculture, Lexington, KY.
Yeargan, M.R., Lyons, E.T., Kania, S.A., Patton, S., Breathnach, C.C., Horohov, D.W., and Howe, D.K. (2009) Incidental isolation of Setaria equina microfilariae in preparations of equine peripheral blood mononuclear cells. Vet Parasitol. 161(1-2):142-145.