Parasites in Sheep Flocks 1 WHAT IS NEW IN PARASITE CONTROL FOR SHEEP FLOCKS? Drew E. Hunnisett, DVM Honeywood and Warder Veterinary Services 132 Commerce Park Drive, Unit N Barrie, Ontario L4N 8W8 705 722 3232 info@hwvs.ca 1. New Goals: A. Maintain a level of parasite burden in the sheep that does not cause disease (NOT, Keep sheep parasite-free ). B. Maintain a parasite burden on pasture that remains susceptible to anthelmintic drugs (NOT, Keep all pastures parasite-free ) C. Preserve effectiveness of anthelmintic drugs. 2. New challenges: A. Anthelmintic Resistance among gastrointestinal parasites. B. Changing climate 3. New methods: A. Reduced reliance on anthelmintic drugs B. Increased reliance on pasture management strategies C. Combining use of pasture management strategies with use of anthelmintic drugs at optimal times. GASTROINTESTINAL PARASITES OF CONCERN TO ONTARIO SHEEP FLOCKS 1. Haemonchus contortus (Barberpole worm) found in abomasum, causes anaemia, ill thrift, and death, especially in lambs 2. Teladorsagia (Ostertagia) found in abomasum, causes loss of blood protein, ill thrift, and death, especially in lambs. 3. Trichostrongylus species one species found in the abomasum, others in the small intestine, they cause ill thrift, diarrhoea, and death, especially in lambs. 4. Coccidia not worms, but single-celled parasites of lambs causing ill thrift, poor weight gain, dysentery, and occasionally death.
2 ESSENTIAL BIOLOGY OF GASTROINTESTINAL NEMATODES (WORMS) 1. Adults lay eggs in the abomasum (4th stomach) or intestine (depending upon location of worm). 2. Eggs pass out in manure pellets and are not infectious to other sheep. 3. Eggs hatch and mature into 1st and then 2nd stage larvae, neither of which are infectious. They are susceptible to the effects of hot, dry conditions on pasture, but survive longer in cool, moist conditions. 4. 3rd stage larvae are infectious to sheep and more resistant to hot, dry conditions than previous stages. Species other than Haemonchus can also survive freezing in a dormant stage. Eggs shed in the fall of the year frequently mature only to the 3rd stage and then go dormant, not becoming infectious until the following spring. This provides an important source of infection surviving from one grazing season to the next in Ontario. 5. Dormant 3rd stage larvae reactivate after 2 weeks of 10C weather, and remain infectious until approximately 1 July. Most of those that have not been consumed by a sheep by then will have died. 6. Infectious 3rd stage larvae consumed by a sheep mature in the abomasum to 4th and 5th stage larvae before becoming adults which begin producing eggs and starting the cycle over again. 7. Larval development can be arrested at the 4th stage, which frequently happens in the fall and winter in Ontario. 4th stage larvae of all three of the major gastrointestinal nematode species can enter a hypobiotic state in the sheep, causing no signs of clinical disease. This is the major method that these parasites survive from one season to the next. 8. Periparturient rise in egg production occurs as immunity declines in late pregnancy, allowing many hypobiotic larvae to mature to the adult stage and cause serious clinical disease in lambing ewes. Ewes with young lambs can be shedding large numbers of parasite eggs per gramme of faeces, and are the major source of pasture contamination for their lambs.
9. Anthelmintic drugs (dewormers) are considered effective if they kill 99% of the parasites in an animal. This means that 1% that are resistant survive to produce the next generation of resistant parasites. Over time, with only the susceptible parasites being killed, the resistant parasites become a larger and larger part of the population. This is why resistance inevitably develops to any killing drug, whether it be an antibiotic or an anthelmintic. 10. Low to moderate numbers of gastrointestinal parasites are not harmful to adult sheep, and may serve to stimulate immunity to parasitism. Sheep having faecal egg counts of less than 500 eggs per gramme are not usually at risk for disease. In any flock, some sheep will be genetically more resistant to parasitism than others. Resistant sheep will harbour low numbers of parasites and shed few eggs, while resilient sheep may shed large numbers of eggs but be unaffected by disease. 11. Infectious 3rd stage larvae migrate up the leaves of pasture plants, and are consumed during grazing. Allowing pasture to be grazed very short increases the number of larvae ingested. Dry lot feeding reduces risk of parasitism unless sheep are fed on the ground. 3 INTEGRATED PARASITE CONTROL STRATEGY: PART OF YOUR VETERINARY FLOCK HEALTH PLAN 1. Monitor parasite burden with quantitative faecal egg counts, giving results as number of eggs per gramme. July and August are the best months in Ontario to assess parasite burdens by faecal egg counts. Discuss the results with your veterinarian. There is no one-size-fits-all parasite control strategy. 2. Treat with anthelmintic drugs when parasite burdens become high (over 750 eggs per gram), or when clinical signs appear, or in the fall to prevent periparturient rise in egg shedding during the following spring lambing season. Remember that since larvae cause disease, but only adults shed eggs, a parasitised sheep can have a low or negative faecal egg count. Your veterinarian will recommend the most appropriate drug for your flock following consultation with you about the conditions in your flock. 3. Ensure that treatments have the possibility of being effective: Dose for the largest sheep in the group, ensure that a drench is delivered over the back of the tongue and not spit out, use only drugs that are licensed for use in Canada and prescribed by your veterinarian. Administer drugs by the route described on the label.
4. Deworm additions to the flock sequentially after arrival first with an avermectin and then with a benzimidizole. This will reduce the chance of introducing parasites that are resistant to one or both of those drug groups. 5. To ensure that sheep are not left with only resistant parasites, allow sheep to graze the pasture for 2 days after deworming before moving them to cleaner pasture. This is in contrast to the practice of deworming and immediately moving the sheep to clean pasture that was recommended in the past. 6. Investigate any apparent failure of deworming treatments with a faecal egg count reduction test. Take faecal samples prior to and 14 days after deworming; a 95% reduction in faecal egg count shows that deworming was effective. If it was not, check bodyweight of a sample of the flock against the dose of dewormer that was given, and review administration technique. Check that the drug was not past its expiry date, and that it had been stored in accordance with the label directions. If none of these can account for failure of deworming, anthelmintic resistance is likely present. 7. Pasture that has not been grazed by sheep in May and June can be considered clean. Consider taking a first cut of hay off a field and using the second growth for clean pasture. Again, remember to leave sheep in their old pasture for 2 days after deworming to allow them to ingest a low number of parasites that have not been exposed to the dewormer. 8. Pasture that has been grazed by cattle or horses after sheep will be clean again for sheep, since few parasites are infectious to those species also affect sheep, and larvae of sheep parasites consumed by them will not mature. Grazing cattle or horses with sheep will have a similar but less dramatic effect on pasture larval counts.
4 ANTHELMINTIC DRUGS FOR SHEEP IN CANADA Ivermectin: The most commonly used deworming drug licensed for sheep in Canada, available as a drench or as an injectable. Drench is preferred, as it delivers high levels of drug to the gastrointestinal tract and should lead to more complete killing of parasites. The injectable form persists longer at lower levels as the drug is metabolized, potentially allowing exposure of parasites to concentrations of drug that are not adequate for killing many parasites. This can select for resistance. Ivermectin is highly effective at removing 4th and 5th stage larvae, including hypobiotic larvae, as well as adults. Resistance to ivermectin occurs in Ontario. Fenbendazole: Licensed for use in cattle and horses, it is effective at removing adults and some 4th and 5th stage larvae. Resistance occurs in Ontario, and can develop quickly, as with all drugs of this class (benzimidizoles). Use in sheep is extra-label and requires a veterinary prescription. No matter where you buy it, a producer does not have the authority to use a drug in a manner not specified on the label without veterinary advice. Albendazole: Similar to fenbendazole but can harm the foetus in the first 1/3 of gestation. It should not be used in sheep during the breeding season or early pregnancy. Use of albendazole in sheep is also extra-label and requires veterinary advice. Levamisole: No longer available in commercially in Canada, some veterinarians have had it compounded by pharmacies. It is potentially toxic, and dosing must be accurate to avoid this. It is not effective against larval stages. Closantel: This drug is now available in Canada and has activity against liver flukes and Haemonchus. It is not effective against other gastrointestinal nematodes. Startect: Abamectin and derquantel, for use in flocks where anthelmintic resistance has been confirmed, has now been approved in Canada.