Internal Parasites
Internal parasites can be a major problem for sheep in Ontario. Effective control of internal parasites will make a large difference in the productivity and profitability of your flock. These organisms cause a range of problems including decreased growth of lambs, poor reproductive performance of ewes, and can kill animals if left unchecked. Internal parasites of sheep include worms (e.g. roundworms, lungworms, etc.) and protozoa (e.g. coccidia) that live within the digestive and/or respiratory tract. Damage to the lining of the stomach and/or intestine can be severe enough to prevent nutrient absorption causing diarrhea and severe weight loss. Certain types of worms ingest blood, causing anaemia and weakening the animal.
Although there are a few notable exceptions, the life cycles of most internal parasites are somewhat similar. The adult organisms live and produce eggs in the stomach or intestines of the infected animal. The fertile eggs are passed with the manure and hatch outside the body. After a number of changes, a mobile infective larva is formed, attaches itself to vegetation, and may be eaten by a grazing sheep. The worm reaches sexual maturity after entering the stomach or intestine. The period from when the eggs are passed in the manure until the larvae have reached the infective stage is 3-4 weeks.
With repeated exposures, sheep do develop a natural resistance to parasite infections. This resistance is not as effective as the immunity developed against viruses and bacteria and does not kill the adult organisms.
However, the reproductive rate of the parasite is decreased, ultimately reducing the parasite load and the drain on the animal. Therefore, animals most susceptible to parasites are lambs (immature immune system and limited exposure) and animals with weak immune systems (disease, poor nutrition, other stresses). Even though adult animals will be relatively immune, if they are not managed properly they can contaminate the environment increasing the likelihood that vulnerable animals will be infected. Implementing a well-planned parasite management program is a very important aspect of your flock health program.
Chemical dewormers:
Although there are a number of management practices that are important in controlling worms, most effective programs also involve the use of chemical dewormers. Using dewormers at times of the year and/or production cycle when sheep are most vulnerable to parasitism helps increase the effectiveness of treatment and reduce the overall use of chemicals. This will decrease costs and help to maintain the effectiveness of the chemical. (Note: protozoal infections are not treated using dewormers. Discuss treatment with your veterinarian).
One of the most important times to deworm is just after lambing, when there is a sudden release of infective eggs within the ewe’s intestinal tract. Treating animals at this time minimizes the exposure of larva to newborn and young lambs.
Examples of deworming schedules are (Consult with your veterinarian and/or producers in your area for other suggestions):
Ewes: Early lambing (January through February)
• Deworming ewes shortly after lambing will help to protect young lambs.
• Deworm ewes again before they go to pasture to prevent pasture contamination.
• If possible, deworm flock again 3 weeks after the have been on pasture
• Deworm ewes at weaning time
Ewes: Late Lambing (April through May)
• Deworm ewes in mid-winter (January, February) before the spring thaw, to prevent heavy contamination of pens or corrals during spring thaw
• Deworm ewes shortly after lambing (before sheep go to pasture) and at weaning time
Weaned lambs:
• Deworm lambs at weaning time or before they enter feedlot or new pasture.
Replacements
• Deworm all newly purchased stock when they arrive on farm at beginning of quarantine period and again 3 weeks later.
After deworming it is advisable keep animals in the same area for 12-24 hours, and then move them to a clean pasture or pen. Moving animals to a new pasture immediately after deworming will contaminate the pasture, as eggs will be passed with the dead worms. Animals will become re-infected sooner if they are put back into the original pasture.
Resistance to dewormers:
Repeated use of the same dewormer can promote the development of strains of resistant parasites. This means that the dewormer will not be very effective and a large number of the worms will survive after treatment. These worms may pass their resistance to their offspring, creating a new line of resistant parasites.
Resistance to dewormers is a growing problem for the sheep industry and as dewormers lose their effectiveness, there are fears that economic losses from parasites will increase. In addition, there are concerns that certain types of these medications may be having a harmful effect on dung beetles (thought to naturally disperse and reduce worm eggs). Reducing the amount and the number of times medication is given will slow down the build up of resistant worms on the farm. The following strategies can help avoid this problem.
Fecal Egg Counts:
Fecal egg counts (done by your vet) are important to determine the level and type of parasite infection. Fecal egg counts before and 10 days after the deworming also help ensure that the dewormer is effective. There should be a decrease of least 85 percent.
Rotating De-wormers:
There are three main groups of wormers—benzimidazoles (e.g fenbenzadole); imidazothiazoles (e.g levamisole); and the avermectins (e.g ivermectin). Using a different family of wormer each year (for a full year) will help keep the parasite resistance down in your flock. Performing yearly fecal samples will help to determine if resistance to a particular group of dewormers is occurring.
Nutrition and Health
Parasites are opportunists; as long as the animal is healthy and well cared for, the parasite load will be minimal. However, a poor immune response in stressed or malnourished animals will give parasites a chance to thrive. Animals in this situation are doubly affected, as they must fight the parasite infection as well as the original problem. A high parasite load is often a sign of other health problems, usually poor nutrition. Infected sheep provided good nutrition are, in some cases, able to reduce their worm load significantly.
Pasture Management:
Many parasite larvae do not climb higher than a few centimetres from the ground. Since sheep are able to graze close to the ground, they are generally more susceptible to parasite infection than other livestock. Good pasture management can help reduce parasite problems in grazing sheep. Allowing pastures to ‘rest’ for 6-12 months will help break the parasite lifecycle and clear the pasture of worms, if you have the land base and/or other species on your farm. Other effective methods are to graze hay fields, cropland, or grazing pastures with livestock not affected by sheep parasites (cattle and horses are good, not goats).
Although time between rotations may not be long enough to have a direct effect on larvae populations, rotational grazing programs may help reduce the effect of parasites by providing better nutrition. In addition, as mentioned earlier larvae do not crawl far from the ground, therefore it is important to prevent sheep from grazing the grass down too far. Pasture rotation will also help reduce the amount of fresh manure in the pasture. When possible, livestock avoid grazing near their own feces. As larvae will migrate only ~30cm from the manure, livestock will eat fewer larvae if stocking rates and rotation times are appropriate. Larvae from most worms will die when exposed to dry conditions. Therefore, although overgrazing will increase parasite infections, allowing pastures to become overgrown isn’t the answer either, as the larvae population will increase if not exposed to direct sunlight. Maintaining grass at ~6 cm is ideal.
Harrowing fields aids in dispersing manure, as do dung beetles and earthworms in the soil, helping to dry and kill the larvae quickly. Since lambs are the most susceptible to infection, manage your pasture rotations to minimize their exposure to parasites. Use a “clean” pasture (not grazed the previous year) if possible when lambing on pasture. If pasturing weaned lambs, move them to a clean pasture and allow ewes to graze the infested areas.
Sanitation
For sheep in confinement housing, cleanliness is the best defence against parasites. To help minimize fecal contamination, use feeders that prevent animals from walking on feed or pulling feed onto the ground. Locate waterers to prevent sheep from defecating in water and in areas with sufficient drainage. Pens, especially lamb pens, should be clean and dry.
Genetic Resistance
Resistance to parasites has a strong genetic link, with certain sheep showing a greater resistance to infection than others. Being able to cull susceptible animals would be an ideal means of not only controlling parasitism, but also eliminating the problem completely. Unfortunately, an on-farm method of effectively and efficiently identifying susceptible sheep has not been developed.
Alternative Dewormers
There is little scientific evidence demonstrating that alternative dewormers (e.g. herbs, diatomaceous earth etc.) have positive effects on lamb growth and/or parasite load.
Summary
Parasites will thrive with:
• mild winter
• moist warm spring and summer
• overgrazed or permanent pastures
• marshy or wet pastures
Sheep are vulnerable when:
• very young or old
• poor immune status (stressful conditions, poor nutrition, weather, overcrowding)
• type and numbers of worms
Preventing parasite infestation in sheep
• strategic use of chemical dewormers
• manage pasture rotations to avoid pasturing young animals on fields recently housing older sheep
• avoid marshy or boggy pastures
• prevent overstocking of pasture, but keep grass reasonably well grazed
• break parasite life cycle (rest period, grazing with cattle etc.)
• maintain sanitary conditions in confinement housing
• discuss parasite control with your vet.
