Internal Parasite Control in Sheep

James E. Strickland, Extension Veterinarian

Contents

Types of Internal Parasites

The Life Cycle of Internal Parasites

Symptoms

Diagnosis

Pasture and Weather Factors in Control

Animal Factors in Control

Control Programs

Pasture Control

Treatment

Preventive Treatment and Control

Coccidiosis in Sheep

Internal parasites and predators have limited sheep production in Georgia. However, with the improve-ments made in electric fencing, predator control has improved and better internal parasite control products and methods are available. With these improvements, Georgia's sheep population is increasing. Popular interest in the 4-H Lamb Project has increased the awareness of better internal parasite control needs in sheep.

Internal parasites account for the greatest economic loss to sheep producers through deaths and loss of efficiency. With sheep concentrated in small areas, intensive grazing habits make the animals very vulnerable to internal parasite infection.

Types of Internal Parasites

The stomach worms of sheep are responsible for causing the greatest losses for Georgia sheep producers. The barberpole worm (Haemonchus contortus) causes the most harm. It has a very short life cycle, is a ravenous blood sucker and may be inhibited or buried in the stomach glands. Ostertagia and Trichostrongylus cause problems in the stomachs of sheep.

Several species of Trichostrongylus are found in the small intestines. Sheep hookworms (Bunostomum) are found in the small intestine and are blood suckers. Roundworms (Nematodiriasis) are also found in the small intestine. Another internal parasite of sheep found in the small intestine is the nodular worm (Oesophagostomum). Whipworms (Trichuriasis) and large mouth bowel worms (Chabertia) are found in the large intestines.

The threadworm (Strongyloides) penetrate the skin or are consumed in the milk from an infected ewe. Skin damage between the claws of the feet may predispose sheep to foot rot. The tapeworm (Moniezia expansa) affects young sheep by causing digestive upsets. Liver flukes (Fasciola hepatica) are uncommon in Georgia's sheep, but may be a problem when sheep are brought in from other areas.

The Life Cycle of Internal Parasites

A good understanding of the life cycle of internal parasites is a must for effective control. Because the life cycle of Haemonchus is the shortest, it provides a good pattern for control measures for all internal parasites.

Adult worms stay in the abomasum (true stomach) and lay tremendous numbers of eggs, which are passed in the manure.

Weather conditions begin to play an important role in the continuation of the life cycle. Warm, wet conditions are necessary for the eggs to hatch into larvae and become active and infective. If the weather is too hot, the eggs will hatch and die. If the weather is too cold the eggs will not hatch. When conditions are dry, the larvae will die. Moisture and warmth are also necessary for the infective larvae to move from the manure onto the grass, where they are consumed by the sheep.

The parasite's life cycle begins when sheep graze on a blade of grass containing the infective larvae in dew or raindrops. Favorable weather conditions are necessary for the manure to break up and for larvae to float or splash on the grass to be consumed.

Short pasture conditions are favorable for more severe internal parasite infestations because more infective larvae are normally found on the lower parts of the plants near the soil surface. Sheep are short grazers.

This free-living stage of the life cycle of these internal parasites can vary from several weeks during cooler months to only a few days in the warmer months.

Extreme temperature will affect the development or survival of the free-living larvae. In warmer seasons the development is rapid, but larvae survival is brief.

Free-living larvae remain viable during winter months and are capable of infecting the sheep in the spring. When warm weather brings on grazing, parasite infection for the sheep increases.

This host-parasite-environment relationship -- especially the development and survival of free-living larvae on the pasture -- affects the severity and time of internal parasite infection in sheep. This relationship is responsible for light, chronic infection or severe, acute infection.

On severely contaminated pastures, this will account for severe losses due to parasites with early spring grazing.

The parasitic phase of the life cycle of internal parasites in sheep begins when the infective larvae are eaten and is completed when eggs are passed in the manure.

The stomach worms, Haemonchus, require 14 days to complete this part of the life cycle. This is important to treatment and control because the sheep will be passing larvae in the manure and recontaminating the pasture. Ostertagia and Trichostrongylus normally take 21 days after the infective larvae are eaten before the adult matures and starts to produce eggs.

As with the free-living phase of the life cycle, the weather can play a role in changing the parasitic phase in the sheep. Stomach worms have the ability to go through a stage in their development in which the larvae are inhibited or arrested. Nature has a way to predict, according to weather conditions, whether the life cycle can be continued after eggs are passed.

In northern areas of the country this inhibited stage occurs during the cold winter. Georgia has mild winters and hot, dry summers, so the larvae in the stomach are usually inhibited during the summer.

These larvae burrow into the stomach glands and are dormant and don't cause any harm to the sheep. When the weather is favorable for completion of the free-living stage of the life cycle in the environment, the larvae will come out of the stomach glands. The larvae mature to adults, egg shedding starts and the life cycle is completed in a favorable environment. Damage occurs when the inhibited larvae come out of the stomach glands.

When the inhibited larvae are released, a digestive disturbance usually occurs. When the inhibited Haemonchus larvae are released and become adults, they begin to satisfy their blood-sucking appetite.

If the number of larvae is large, they remove more blood than the sheep can replace and severe anemia develops without notice. This anemia may develop quickly and cause acute symptoms and death before the animals can be treated. After both types of stomach worms reach the adult stages, there is an enormous increase in egg shedding and pasture contamination.

Symptoms

Symptoms will vary according to the types of worms present and ages of the affected sheep. Lambs with heavy infection of Haemonchus may be found dead, with pale mucous membranes from acute, massive blood loss. More chronic cases of stomach and intestinal parasites of sheep will show muscular weakness, pale mucous membranes, diarrhea and swelling. Seldom do Haemonchus cases have diarrhea; however, constipation may be a symptom. Swelling or edema occurs in the lower body parts. The swelling is most evident under the jaw, resulting in "bottle jaw."

Weaker animals lie around, have a depressed appetite and lose weight quickly. Unthriftiness and poor bloom are symptoms just before the young sheep start to die. More chronically affected sheep may show a break in the wool, followed by fleece loss.

Diagnosis

If an adequate parasite control program is not being carried out, and any of the above symptoms are observed, a quick diagnosis must be made. If dead sheep are found, a post-mortem should be done by a veterinarian as soon as possible. This is the most reliable method of diagnosis.

Types and numbers of internal parasites can be determined and used in a control program immediately before other losses occur. In chronic cases, fecal counts should be made, but must be interpreted according to history and symptoms. If the parasites are inhibited, egg counts will be low.

Blood tests can be an aid in diagnosing parasite problems in sheep. Age, nutritional plane, season of the year and response to treatment should also be used in diagnosing parasite problems. If coccidiosis is the cause of the problem, a different treatment is required.

Waiting until sheep look wormy is not a very efficient management or parasite control process.

Pasture and Weather Factors in Control

With the recent development of many new dewormers, much research has been done on parasite control, mainly in Georgia cattle. Average dates, in terms of seasonal occurrence of different parasite stages, vary according to weather conditions.

Normally, in early March, infective larvae that have overwintered on contaminated pastures start to increase. The warmer, wetter climate increases available grazing and high numbers of infective larvae are taken in.

As the temperature increases and moisture decreases in early or mid-summer, the number of infective larvae on the pasture begins to drop. At this stage, the larvae in the stomach start to be inhibited in the glands. If untreated, sheep harboring adult worms continue to pass hundreds of eggs in the manure. This continued development is dependent on weather factors.

Later in the fall, the inhibited larvae may exit the stomach glands, causing acute symptoms in some sheep.

Having survived the time when the environmental and climatic conditions were not favorable for completion of the life cycle, the larvae now mature into adults. These adults, along with adults not inhibited, lay eggs and cause heavy pasture contamination under favorable conditions to produce infective larvae.

Animal Factors in Control

Most adult animals, including sheep, are not as susceptible to parasite infection as younger lambs are. They are capable of developing immunity to parasites and, after some exposure, will be able to slow parasite development and egg-laying to a degree.

However, during times of environmental or nutritional stress, and especially where high numbers of infective larvae are ingested, adults may have severe problems.

Mature ewes infected with adult worms also serve as environmental contaminators, providing infective larvae for susceptible lambs. When sheep are shipped into areas where seasons are different and where exposure is higher, resistance may be reduced and parasite infection may be a problem.

Special factors in the reproductive cycle of sheep must be considered: Two weeks before lambing and up to eight weeks after lambing, pregnant ewes have a factor called periparturient (pre-lambing) rise in fecal egg count.

Any immunity or resistance they might have decreases and they are not able to cope with the parasite problem. The potential to prevent the development of infective larvae, expel worms and stop egg production by adult worms is impaired.

An increased number of eggs passed -- normally occurring when environ-mental conditions are favorable -- places the new lamb in a very dangerous parasite environment.

Control Programs

Because sheep internal parasites cannot be completely eliminated, they must be kept under control. This means keeping the parasite burden in the sheep below a level where economic loss may occur. This includes levels where symptoms may not be evident but production efficiency is lessened.

To do this, the host-parasite-environment relationship must be considered. The development and survival of the infective larvae on the pasture and the inhibited larvae in the sheep must be controlled.

To effectively control internal parasites of sheep, treatments must be used along with good pasture management. Treatment alone is costly and time-consuming, and the more it is used, the greater the chance for parasites to develop resistance to the dewormer.

Pasture Control

Pasture rotation is not very effective if the pastures are not clean or at least safe. A safe pasture is not without infective larvae, but is low enough in numbers for infection to be minimal.

Pastures that are tilled, such as small grains for winter grazing and millet for summer grazing, are excellent to use in pasture control.

Permanent pastures may be used where hay cutting is rotated annually or during spring and fall. These pastures will not be as clean as tilled pastures, but should be considered safe.

With Georgia's mild winters, a heavily contaminated pasture may not be considered safe even after a year without grazing. To keep pastures safer and cleaner, deworm at least 24 hours, preferably longer, before placing sheep on clean or safe pastures. Remember: As soon as the dewormer is ineffective, new infections start on contaminated pastures. Controlled or limited grazing, with a majority of the time spent in dry lots, keeps exposure down and causes less pasture contamination.

Treatment

Until recently, only three products -- Thiabendazole, Phenothiazine and Levamisol -- were approved to treat sheep for internal parasites. With the recent approval of Ivermectin, a better choice of approved sheep dewormers is available. Tables 1 and 2 list approved and experimental dewormers.

On contaminated pastures during favorable parasite development weather conditions, deworming may be necessary every two weeks to control Hae-monchus infections in non-resistant sheep.

Ivermectin has been shown to stay in the blood system at an effective treatment level two weeks or longer. Significantly higher weight gain, better fleece quality and no death losses were reported in a trial in which weaned lambs were dewormed monthly and at six-week intervals using Ivermectin. Ivermectin is also effective against the inhibited larvae.

Other, non-approved, dewormers are being used as dewormers in sheep. To be used legally, these non-approved products must be prescribed by a licensed veterinarian. Some of these products are used if tapeworms are a problem.

Parasite resistance can be a problem with internal parasites in sheep. However, lack of response is not necessarily caused by internal parasite resistance. Resistance has developed against Thiabendazole and Levamisol with frequent use of dewormers over extended time.

Factors such as improper dosing, underdosing, rapid reinfection on contaminated pastures, or inhibited larvae not responding to certain dewormers may be confused with resistance. Following label directions will eliminate most of these factors. With the effectiveness of most dewormers being only about 24 hours, rapid reinfection will occur on contaminated pastures and where treated sheep aren't moved to clean or safe pastures in time.

Egg counts used before deworming and one week after deworming are an effective way to determine the resistance and effectiveness of dewormers.

Preventive Treatment and Control

Limited treatment may be satisfactory if clean or safe pastures are available. If not, continuous deworming will be necessary when weather conditions are favorable for infective larvae development on contaminated pastures.

To control Haemonchus, treatment must be given every two weeks starting in early spring and continuing until the hot, dry summer, when infective larvae will not develop. In the late summer and fall, this schedule will need to be repeated.

Take fecal egg counts in the summer to be sure the life cycle has been interrupted by the weather change.

When using Ivermectin, the interval between deworming treatments can be extended to four weeks or longer.

If clean or safe pastures are available, a single treatment is sufficient, giving one to three days for adults to pass, preferably in a dry lot, before moving to a clean area. If necessary, fecal egg counts can be used to monitor infection.

When using either program, give pregnant ewes a deworming treatment about two weeks before lambing. This will prevent high contamination resulting from the periparturient egg rise. Rams and replacements should be dewormed when they are placed with the breeding flock and included in the deworming program being followed.

Lambs purchased for 4-H projects should be dewormed before they are moved. Repeating the treatment two weeks later should provide a parasite-free environment for the lamb.

In many cases, a continuous schedule is followed to ensure the best performance. Because the lamb projects are educational, fecal egg counts, safe pasture concepts, weather factors and other environmental concerns should be stressed in the project.

Three-day or continuous deworming with some products in block or mineral form are being used in cattle and may be effective in sheep.

At present, these products are not approved for use in sheep. This could cause a resistance problem later, if some of these products are approved.

Phenothiazine in free-choice mineral has been used to prevent completion of the life cycle in the manure. This practice is questionable, especially with better dewormers being approved.

Normally, sheep in dry lots or pens aren't reinfected after deworming, but they still need to have periodic fecal checks.

Coccidiosis in Sheep

Coccidiosis is a very common parasitic disease in sheep and is mainly caused by two out of approximately 15 species of coccidia that can affect sheep.

Young lambs (one to six months of age) are the most severely affected. Older animals are more resistant. Young lambs stressed by weaning, han-dling, shipping, adverse weather conditions and overcrowded pasture conditions are predisposed to coccidiosis. Many lambs, especially 4-H lambs, are kept in close confinement where exposure is greater.

The life cycle of coccidia takes about 21 days to complete and is very complex, developing both inside and outside the sheep.

The infective or egg stage is called oocyst. The eggs are passed in the manure, where they are infective to other sheep. Coccidia will spread fast because of the short life cycle and very high numbers of eggs produced.

The small intestine and upper large intestine are mainly involved and are damaged as the life cycle is completed. Signs of coccidiosis include diarrhea (sometimes containing blood or mucus), dehydration, off feed, weight loss, anemia, wool breaking and

death. The symptoms may be confused with other internal parasite conditions. The eggs or oocyst may be found on fecal examination. If untreated, death losses can be high. Permanent gut damage will cause stunting and poor performance.

Lasalocid, at two mg/lb daily, and Monensin, at 15 parts per million (PPM) in starter grain, are coccidiostats approved for prevention of coccidiosis. Decoquinate and amprolium are not approved, but have been used successfully for treatment and prevention and can be used by prescription from your veterinarian.

Bulletin 1064 / Reviewed April, 2009

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