Internal Parasites (Helminths) of Sheep
Internal parasites (helminths) cause clinically important disease (helminthiasis) in pastured and free-ranging sheep in most regions of the world. Nematodes, flukes, and lungworms can all adversely affect sheep and cause devastating production losses. Although tapeworms do not often cause disease, because they are visible to the naked eye they can be a useful opening to discuss parasitic infections with producers and develop a parasite control program for the flock. Hobby flocks that are allowed access to graze can have as many parasite-related disease complications as do large production flocks. General clinical signs of helminthiasis include weight loss, poor growth, anemia, poor hair coat, submandibular edema (bottle jaw), and diarrhea. Certain parasites (eg, lungworms) may cause specific clinical signs, such as coughing or bacterial pneumonia.
Environmental conditions and stocking density influence the number and severity of parasite-related problems in a flock. Overcrowded flocks in warm, moist conditions often struggle with severe helminthiasis, whereas flocks grazing in arid or semiarid environments may suffer few production losses due to helminths.
Overuse of anthelmintic drugs has led to increased parasite resistance and increased loss of drug efficacy. A 2008 study determined that, on 48% of small ruminant holdings in the southeastern US, Haemonchus contortus infections were resistant to all anthelmintic drugs available on the market. 1 References Internal parasites (helminths) cause clinically important disease (helminthiasis) in pastured and free-ranging sheep in most regions of the world. Nematodes, flukes, and lungworms can all adversely... read more In light of the loss of these drugs for treatment, the current paradigm for helminth treatment in sheep flocks is to 1) treat only when sheep show clinical signs of disease, 2) use fecal egg courts to determine drug efficacy, 3) use pasture and nutrition management to decrease the risk of exposure, and 4) select for sheep that have genetic resistance to parasites. A few head of sheep are often responsible for shedding a majority of the parasite eggs. Identification and removal of those high-shedding sheep should also be encouraged. The FAMACHA scoring system should be used to assess anemia, and thereby the severity of a H contortus infection. It is a low-cost, validated means of monitoring for some internal parasites.
Howell SB, Burke JM, Miller JE, et al. Prevalence of anthelmintic resistance on sheep and goat farms in the southeastern United States. J Am Vet Med Assoc. 2008 Dec 15;233(12):1913-1919. doi: 10.2460/javma.233.12.1913. PMID: 19072608
External Parasites of Sheep
Lice, mites, and keds (Melophagus ovinus) are all common external parasites of sheep. These organisms seldom cause severe disease; an infestation that is extreme enough, however, can cause anemia and loss of condition. Clinical signs of infestation are pruritus, alopecia, and visible presence of the organisms on the skin and in the hair.
Ovine Progressive Pneumonia
Ovine progressive pneumonia, or maedi-visna, is a lentiviral disease related to caprine arthritis and encephalitis Caprine Arthritis and Encephalitis Caprine arthritis and encephalitis (CAE) is a persistent lentiviral infection of goats. There are multiple clinical presentations: 1) leukoencephalomyelitis, affecting 2- to 6-month-old kids... read more that is thought to affect > 25% of sheep in North America. Clinical signs include difficulty breathing; exercise intolerance (trailing behind the flock); a hard, nonproductive udder; and chronic, progressive weight loss. Sheep are infected for life and can transmit this disease both vertically and horizontally. Serum tests can be used to determine the incidence of ovine progressive pneumonia in a flock.
Paratuberculosis (Johne's Disease) of Sheep
Paratuberculosis is a bacterial infection of the intestinal wall due to Mycobacterium avium paratuberculosis. Sheep can be infected with both the S and C strains of the microorganism. M avium paratuberculosis is shed in the feces, and sheep are often infected via a fecal-oral route, generally at a very young age. The pathogen can also be transmitted in utero or via colostrum and milk. Clinical signs, including weight loss, submandibular edema, and weakness, may not manifest until the animals are much older. Diarrhea is not as common in sheep as it is in cattle. Johne's disease cannot be cured, and decontamination of the environment is difficult because the bacterium can live for years in the soil. Fecal PCR testing is recommended to identify and remove infected animals. Necropsies on animals that show chronic weight loss are also recommended, to look for enlarged mesenteric lymph nodes and thickened intestine with prominent lymph vessels. On microscopic examination, acid-fast-staining bacteria are observed in infected tissue.
Caseous Lymphadenitis (Cheesy Gland) of Sheep
Caseous lymphadenitis is an infection of the lymph nodes due to Corynebacterium pseudotuberculosis. Infection is transmitted commonly through breaks in the skin that come in direct contact with pus or fomites (such as shears and feeders) contaminated with pus, and it can occur at any age. Two forms of the disease exist: an internal form and an external form. Internal abscesses are difficult to identify before death; however, they can lead to chronic weight loss and poor production. External abscesses form in the superficial lymph nodes and contaminate the environment with large amounts of infectious pus when they rupture. Treatment is not considered effective, and organisms can remain viable for months in the soil. External abscesses should be cultured to confirm disease, and animals with obvious swollen lymph nodes or abscesses should not be introduced into the flock.
Additional Clostridial Diseases of Sheep
Tetanus, due to infection by Clostridium tetani, is most likely to occur after procedures such as castration and tail docking in sheep, particularly if elastic bands are used. Lambs should be vaccinated against tetanus before or immediately after these procedures. Clostridium novyi is the causative agent for “big head” in rams. Tissue injured during head butting becomes infected, resulting in swelling and pain. Tetanus can be prevented with a vaccine; however, it is not included in the commonly administered CDT vaccines.