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Goals Clarify the concerns regarding parasite management Explain the history behind current deworming practices Discuss emerging issues in equine parasite control Review ways in which parasite control can be optimized Why do we care about parasites anyway? Parasites can affect multiple organ systems The intestine is the obvious primary site affected Can impair feed digestion and absorption Can alter normal intestinal function Can physically damage intestine Can impair blood supply to the intestine Can cause substantial blood loss and anemia The liver can be affected by migrating larvae The lungs are also a site for migration of some larval stages Why worry? For almost 30 years we have been somewhat complacent regarding parasite management As long as we did our two-month rotations everything was ‘okay’ Unfortunately this is not always true and this has led to a number of problems New types of parasites are becoming a problem Parasites that we have considered as ‘controlled’ are emerging as problems again Some that we never regarded as a serious problem are turning out to be associated with disease How did we get here? Three primary reasons Research in the 1960’s showed that deworming resulted Historical Perspective Forty years ago the primary parasite problem was with large strongyles in reduced shedding of strongyle eggs for ~8 weeks These were known as ‘blood worms’ This led to the current recipe for treating with Strongylus vulgaris was the worst anthelmintics every eight weeks The development of easily administered paste wormers simplified anthelmintic treatment Emphasis moved to routine treatment over monitoring Because we have Ivermectin! It was touted to be impossible for parasites to become resistant to this drug Large Strongyles Invaded the blood vessels supplying the large intestine resulting in clots which interrupted blood flow This resulted in severe and often fatal colic Even with surgery many horses died What happened to large strongyles? The good news was that they are not very complex parasites These parasites have become almost a historical Simple fecal-oral life cycle Eggs are shed in manure Hatch in manure and larvae attach to blades of grass where they are eaten by the horse Larvae migrate from intestine into bloodstream and then back into the intestine where adults suck blood from intestine wall and shed eggs Susceptible to a variety of anthelmintic drugs There is not any stage in the life cycle where the organisms are ‘protected’ from anthelmintics footnote for most North American horses thanks to routine deworming Why did treatment work so well? The simple life cycle of these parasites and susceptibility to most drugs meant that a single treatment would stop pasture egg contamination for 6 months The larvae could only survive for ~1 year in the environment After only 18 months of routine treatment of the herd the organism is effectively eliminated Other Parasites Did not seem to be a major problem back then… Tapeworms Seen occasionally at post-mortem, but did not seem to be causing much damage Ascarids Only present in younger animals Small strongyles Rarely present in large numbers, did not seem to Anthelmintic Therapy Historically this consisted of potentially toxic drugs that had to be administered by stomach tube Dichlorvos, phenothiazines, piperazine Realistically this meant that most horses were not dewormed very frequently Some highly managed farms would monitor for parasite eggs in the manure to guide them on when to deworm Many horses were never treated with an effective anthelmintic Due to the poor coverage with anthelmintics colic due to large strongyles was unfortunately common cause problems Paste Dewormers At first only a few drugs were available, but the ease of administration meant that owners could administer the treatment themselves The frequency with which horses received dewormers began to increase As more drugs became available the concept of ‘rotational deworming’ became widely accepted Rotational Deworming By using different drugs alternately we could try to prevent the parasites from becoming resistant to any single drug Not a bad concept, but because of the frequency of administration resistance still developed What we were seeing is evolution on an incredibly fast timeline Kill off the most sensitive organisms and only the resistant ones survive to reproduce Continue selecting for the MOST resistant organisms until the population does not include any that are sensitive to the drug anymore Along Came Ivermectin Suddenly we had a new drug that supposedly had a mechanism of action that parasites could not become resistant to… The rotational concept was then revised for two reasons So what happened next? First, if nothing can become resistant to ivermectin why not use it all the time? Second, by using ivermectin as the primary drug we could stop using the less effective drugs and improve our ‘kill rate’ Equine Tapeworms While the early thought was that they did not cause many problems for the horse this opinion began to change Veterinarians began to see more horses with obstructions of the small intestine at the ileum, where the small intestine empties into the large intestine Large numbers of tapeworms were often present at this site In some cases the wall of the intestine was very thickened, perhaps secondary to the tapeworms Research in the UK began to show that there might be a link between tapeworm infestation and colic Tapeworms So why were they an increasing problem? Most of the commonly used anthelmintics did not eliminate these parasites Exposure in the environment is widespread We did a better job controlling the other intestinal parasites Tapeworms may alter the normal motility of the intestine Tapeworm Antibody Positives Tapeworm Life Cycle The life cycle is more complex than for the large strongyles The parasite releases body segments known as proglottids These contain numerous eggs Eggs are released into the environment and then ingested by mites The infected mites are then ingested when the horse eats grass The tapeworm then matures in the intestine New Deworming Rotations Ivermectin was used every two months except for late fall/early winter when Strongid double dose given This is thought to be the best time to eliminate tapeworms Mites are unable to survive hard freezes in winter Some programs include tapeworm therapy once every six months Use of these rotations has decreased the incidence and severity of tapeworm infestations Tapeworm Treatment Even as the appreciation of the potential importance of tapeworms increased there were few treatments available The primary therapy was pyrantel pamoate (Strongid™) This treatment is only effective when administered at twice the normal dosage “Double-dose Strongid” - 13.2 mg/kg orally once A newer therapy is the drug praziquantel This treatment is highly effective Single dose of 1 mg/kg Marketed in combination with ivermectin Zimectrin Gold™, Equimax™ Quest Plus™ (moxidectin and praziquantel) So, now what? With the use of these new ivermectin and double dose Strongid (or praziquantel) regimens parasite problems should be a thing of the past, right? But, along come the small strongyles! No one paid them much attention, and they probably were not a major problem when they were competing with the large strongyles for resources in the intestine Even more importantly, these parasites have a complex life cycle Includes a stage where they are ‘protected’ from most drugs which allowed them to survive despite deworming Small Strongyles Cyathostomes Adult worms produce eggs Eggs shed in manure Eggs hatch and develop through three larval stages in environment Third stage larvae (L3) are infective and horse ingests them while grazing L3 larvae penetrate gut wall and ‘encyst’ The cyst wall protects them from most drugs L4 larvae ‘excyst’ and enter intestine where they develop to L5 stage and then to adulthood Small Strongyles The encysted L3 larvae do little harm But they tend to all excyst (‘hatch’) simultaneously, usually in the spring The cumulative effect is to damage the intestinal wall and cause substantial inflammation The inflamed gut wall can lose www.power-dose.com/strongyles.asp it’s normal motility pattern This may predispose to colic Do they cause a real problem? Small Strongyle Treatment The small strongyles are not typically a major The adult parasites are sensitive to all commonly used pathogen in healthy horses that are well nourished and maintained at low stocking densities Unfortunately low stocking densities are pretty rare in urban and suburban areas Most horses probably suffer from sub-clinical disease Poor feed efficiency, decreased performance Horses with severe infestations show clinical disease Cyathostomiasis Weight loss, hypoproteinemia, anemia, poor growth, colic drugs But the adults are not the problem Treatment of L3 and L4 stages requires drugs that can penetrate cyst wall Only two currently available drugs accomplish this Moxidectin Quest® – 0.4 mg/kg moxidectin Fenbendazole is only effective at higher doses and when given daily for five days Panacur® ‘Powerpack’ – 10 mg/kg once daily for 5 days What now? The tapeworms and the small strongyles should be effectively addressed with our new plans Yet another parasite was waiting in the wings Ascarids An emerging problem in young horses Typically weanling age Horses normally develop natural immunity by 2-3 years of age Clinical signs Infected animals are usually in good condition and are growing well Primary sign is typically acute colic Ascarids Parascaris equorum Interesting life cycle Eggs are shed in feces Very hardy (up to 10 years) Infective larva (L2) develop within the eggs Eggs are ingested and hatch Larvae migrate through the liver to the lungs Larvae are coughed up and then swallowed Mature, reproduce and lay eggs in intestine Ascarids Can be seen as an incidental finding Gastroscopy or ultrasound Can cause sub-clinical disease GI or respiratory May cause poor growth, poor hair coat, pot belly Can cause severe or even fatal colic Especially following deworming Why are ascarids a problem now? Ivermectin resistance Long term use of frequently dosed ivermectin has resulted in resistance This has been reported in the US, Canada and Europe More intensively managed farms are more likely to have resistant organisms develop Often treat foals monthly with ivermectin for first year The problem is hard to detect without monitoring Ascarid Ivermectin Resistance Typically resistant to moxidectin as well Only a few drugs appear to be effective Oxibendazole Panacur PowerPak for the full five days Pyrantel (Strongid) - variable Monitor response to treatment with fecal egg counts fecal egg counts on a farm-wide basis This will ensure that treatment is effective Typically sub-clinical, as these foals look and act pretty Will also allow for modification of treatment if needed healthy unless they develop acute signs What about Bots? Do not cause serious health issues for most horses But control is indicated to minimize damage to the stomach Best addressed during the winter Eggs are laid by Gasterophilus flies Flies don’t survive in winter Larvae migrate from mouth to the stomach and develop on the wall Ivermectin remains the primary treatment and is given in late fall or early winter What the heck am I supposed to do now? Manage to Minimize Parasitism Do not overstock pastures Designing Treatment Regimens Use anthelmintics most in the horses that Rotate pastures actually need to be treated Pick up manure from paddocks The horse population on any farm typically includes low, medium and high ‘shedders’ Do not allow overgrazing Avoid feeding from the ground In one study 3/16 horses shed 85% of eggs in the herd Pasture ‘picking’ of manure Compost manure for at least one year Graze pastures with other species of livestock The medium and high shedders are the ones most likely to be affected adversely by parasites By treating low shedders less frequently they are not harmed but serve to maintain a population of parasites that remain sensitive to the drugs we have available Cattle and sheep How do we determine risk of shedding? As few as 20-30% of the horses in the herd are responsible for harboring the majority of parasites Therefore responsible for most of the pasture contamination Use fecal egg counts to determine who they are Perform fecal egg counts on all members of herd after a pause in deworming (several months) High counts > 500 eggs per gram Medium = 200-500 eggs per gram Low < 200 eggs per gram When to Treat? When is the contamination of the pasture and infection of the horses the greatest? In our region this occurs in the Spring and Fall Cold winter temperatures limit hatching of larvae in pasture but do not kill small strongyle larvae Hot, dry summers have an even greater negative effect Temperatures over 85 degrees dramatically reduce transmission rates and will kill small strongyle larvae For this reason we can consider that the Fall, Winter and Spring are actually the ‘Deworming Season’ for us When to Treat? Adult Horses Due to seasonality of excysting of cyathostome larval All horses (Low, Medium and High) stages in areas with a ‘true’ winter you can time the treatments to best effect Larvacidal therapy in spring and fall Ideally one larvacidal treatment should be applied in the early spring, prior to excysting of the 4th stage larvae A second larvacidal treatment in the fall reduces the burden of encysted larvae and aids in controlling the other intestinal parasites Animals with higher risk of shedding or farms with severe pasture contamination may benefit from one or two treatments in the summer as well Quest (moxidectin) Panacur PowerPak (fenbendazole) Ivermectin (least effective) Combine a tapeworm therapy with these in the fall Praziquantel Double-dose Strongid Medium Shedders Heavy Shedders Same as above for all horses Same as for moderate shedders Add an additional adulticide treatment in early Add one more treatment in the late summer summer to minimize the spread of cyathostomes period Pyrantel pamoate (Strongid) This will minimize the degree to which they Oxibendazole (Anthelcide EQ) Ideally fecal egg counts should be monitored to contaminate the environment Best to use a different drug than earlier summer treatment ensure effective treatment Ivermectin (effective against adults) Combining these two drugs may increase Moxidectin effectiveness First Year of Life Targeted towards ascarids and strongyles Do not deworm foals before 60 days Treat at 8 week, rather than 4 week, intervals Rotate among all effective drugs If ivermectin resistance is not present then continue to use it and/or moxidectin as part of the rotation Also include: Oxibendazole (Anthelcide EQ) Fenbendazole (Panacur PowerPak) Strongid (pyrantel pamoate) Monitor Response to Treatment Fecal Egg Count Reduction Test FECR = (FEC prior to treatment – FEC after treatment) ___________________________________________ X 100 FEC prior to treatment Use this test to determine if the drug being used is effective FECR of at least 80% is minimum required FECR of 90-100% is ideal (>98% for ivermectin, moxidectin) Timing of testing is variable in the literature Two weeks post treatment represents the best time to assess for resistance Monitoring Response Remember, a negative fecal egg count does NOT mean the horse is free of intestinal parasites. Larval stages do not produce eggs Conclusions We cannot continue to do things the way we have for the last 30 years That is why it is important to test as many horses in Evolution is constantly shifting the lines on the Test before AND after treatment at each deworming Effective parasite control will require assessing the herd as possible for the first 1-2 years battlefield Can assess response to treatment and identify ‘high and monitoring the actual parasite problem on each farm Cost of testing should be recouped in savings on And monitoring the effectiveness of treatment shedders’ over time deworming products (average savings of 75%) Additional Resources The Marion duPont Scott Equine Medical Center www.vetmed.vt.edu/emc/ The Horse Magazine/AAEP www.thehorse.com The AAEP www.aaep.org Proceedings of the International Equine Parasite Drug Resistance Workshop, July, 2008. www.vet.uga.edu/ID/conferences/equine/ Questions?