Good Production Practice # 5
External and Internal Parasite Control
External parasitism, like internal parasitism, results in poor quality sheep products and lost income to producers. Common external sheep parasites include lice, keds, mites, and ticks. These parasites bite the animal and cause skin irritation, resulting in rubbing, scratching, and chewing of the skin. This can lead to damage to the wool and pelt. Some parasites also feed on the sheep's blood, causing blood-loss anemia, especially in lambs. The result is unthrifty, poor-performing sheep.
The best time to treat for external parasites is immediately after shearing. All sheep should be treated with an approved product according to label directions. If infestations are heavy, the treatment should be repeated 2 weeks later. Pyrethrin-type insecticides are safe and effective and can be used on pregnant ewes. Unapproved products should never be used; their use can result in illness in the sheep, as well as chemical residues in products that contain lanolin.
A regular program for treating and preventing external parasites is an important part of a flock health program. Treatment of sheep at shearing time is recommended. Benefits include increased comfort for the animals, improved performance, and higher quality wool and pelts.
Internal parasitism is a common problem. Scars in the liver, and cysts in the muscle, both due to internal parasites, cause tissue condemnation at the harvest plant. In addition, the presence of parasites can reduce the performance and productivity of affected animals. Animals with low parasite loads grow more efficiently than those with heavier parasite burdens. There are also fewer carcass condemnations from sheep with few parasites.
The control of parasites depends upon careful management of the flock's environment, as well as strategic use of dewormers. Avoiding exposure of sheep to parasites is the best method of control, but that is not always possible. Range flocks have fewer problems with internal parasites because they seldom regraze the same area during a season. More intensively pastured sheep tend to build up parasite loads as the season progresses. Sheep kept in dry lots and fed from feeders have a lower risk of being exposed to internal parasites than do grazing sheep.
There is little doubt that parasitism, either directly or indirectly, is the leading cause of death among goats, especially animals younger than 6 months of age. Removing goats from their normal dry, tropical habitat to humid, temperate regions has greatly affected the normal relationship between goats and internal parasites.
Because of a lack of many approved products for use in goats, most anthelmintics are used off-label. The following products should be considered: fenbendazole two to three times the label dose for cattle; ivermectin at 1 ml/88 pounds of body weight (1 percent cattle injectable may be administered orally, which will prevent abscess formation at injection sites); and levamisole as a drench or injection according to label specifications for cattle. A veterinarian should be consulted for advice on methods to avoid drug residue.
A regular program of deworming is essential for parasite control. Rotating anthelmintics at regular intervals in order to prevent parasite resistance is recommended. Thiabendazole is the only approved anthelmintic for goats, but unfortunately it is not very effective because of parasite resistance to the drug. Thiabendazole has also been associated with polioencephalomalacia in some breeds of goats.
Coccidiosis is a contagious disease of goats, especially young kids. In young animals, signs similar to those caused by stomach worms may be seen. Coccidiosis is greatly overlooked on many farms where deworming is directed only at traditional parasites and where programs of checking the feces for Coccidia are not in place. The classical signs of coccidiosis are most obvious in young kids and include diarrhea, severe weakness, and sometimes bloody stools. Also, Coccidia tend to predispose young animals to pneumonia. Coccidia in young kids are generally acquired from adult animals that shed the parasites from feces into the pens and yards. Although coccidiosis is typically a disease of young, growing kids, most adults are mildly infected and will continuously shed oocysts that infect young kids. Diagnosis can be based on clinical signs or microscopic fecal examinations. Coccidiosis should be suspected when kids older than 2 weeks of age have scours. Older kids and adults with diarrhea may have worms, coccidiosis, or both. Preventing coccidiosis in the herd is very important if young kids are to thrive. As soon as diarrhea has developed, most damage to the intestinal wall has occurred.
A variety of drugs, including sulfa drugs and amprolium, may be given orally to treat sick kids. Monensin in the feed has also been used as prophylactic medication. The FAMACHA© System was developed in South Africa to identify severely parasitized sheep and goats. A laminated color chart that shows five consecutive grades of conjunctival pallor, ranging from 1 (red color; not anemic) to 5 (very pale) is used to score the animals. Only the animals in the palest catagories are drenched. This approach decreases the use of dewormers, and allows the producer to identify animals that need frequent deworming to survive.