Management practices, particularly feeding practices, can be the primary determinant of cases or outbreaks of infectious or metabolic disease in all flocks of sheep.
Pregnancy toxemia (see Pregnancy Toxemia in Ewes and Does) may be seen in late-pregnant ewes bearing multiple fetuses subjected to a falling plane of nutrition, specifically energy. It is associated with simple starvation, ewes too fat in early pregnancy, ewes too fat in late pregnancy and that voluntarily reduce feed intake, poor quality feed, and ewes subjected to stress in late-pregnancy (eg, trailing or transport, or severe environmental changes). Ewes rarely survive after showing signs of pregnancy toxemia, even with excellent veterinary care, and it is difficult to stop losses even after interceding with adequate feed.
Hypocalcemia (see Parturient Paresis in Sheep and Goats) is seen in pregnant ewes or ewes in early lactation subjected to a period of temporary starvation or to feeds particularly low in calcium, especially ewes with multiple fetuses, as a result of decreased food intake in late pregnancy. It is also seen in feeder lambs on a grain-based ration without adequate mineral supplementation, and during drought conditions. Calcium deficiency can be confused with pregnancy toxemia and is often a part of the pregnancy toxemia syndrome. Fortunately, pure calcium deficiency can be easily treated and rectified, saving the ewe and her pregnancy.
Hypomagnesemia (see Disorders of Magnesium Metabolism) may be seen during a period of temporary starvation in late pregnancy or early lactation, and also after movement of lactating ewes to lush spring growth pasture (especially green cereal crops). As with hypocalcemia, this condition is easily treated and rectified if properly diagnosed.
Dermatophilosis (see Dermatophilosis) is associated with poor shearing practices leading to shearing cuts, particularly in areas that still use dipping vats for ectoparasite control (this practice has largely disappeared since the advent of the avermectin parasiticides). Sheep in long wool at times of high rainfall tend to be at higher risk of infection, and some sheep are also genetically predisposed.
Caseous lymphadenitis (see Lymphadenitis and Lymphangitis) may be associated with shearing by not separating infected or discharging sheep before shearing, not shearing affected sheep last, not changing blades and disinfecting shears after cutting into an abscess, dipping in contaminated dip, and by close confinement of infected sheep with noninfected sheep after shearing and at other times.
Other means of spreading this very contagious disease is the use of feed bunks or working chutes where the organism (Corynebacterium pseudotuberculosis) has been smeared on slats or railings by sheep with draining abscesses. The organism can survive for up to a year in the environment. It can be controlled by vaccination.
This disease is responsible for huge economic losses in North America due to the rejection or trimming of large parts of carcasses of affected animals when slaughtered.
Enterotoxemia (Clostridium perfringens type D infection, see Type D Enterotoxemia) is seen in weaned, unvaccinated lambs on a rising plane of nutrition high in carbohydrates, as when moved to better pasture or following a “flush” in pasture growth, particularly cereal grains and in feeder lambs fed grain. It generally causes a “sudden death” syndrome and can be confused with acidosis. These animals are found dead and even if found alive can rarely be saved. There is an enterotoxemia antiserum, which if administered early, can be used for treatment. The vaccine is very effective.
Acidosis is seen in adults and lambs exposed suddenly to a higher carbohydrate diet than normal and is often confused with enterotoxemia. It is seen commonly when late-pregnancy ewes are started on grain, when feedlot self-feeders run out of feed for more than a few hours, and when children not properly instructed in correct feeding of animals perform feeding chores.
C perfringens type C infection is generally a suckling lamb disease seen in very young single lambs suckling ewes that produce an overabundance of milk or in a lamb after its twin dies. It is also seen in artificially reared orphan “bummer” lambs that have not had sufficient colostrum. The disease can be prevented by vaccinating ewes close to parturition with enterotoxemia C and D vaccine to provide antibodies in their colostrum; saving frozen colostrum from vaccinated ewes, does, or cows for administration to lambs that received no colostrum; or giving those lambs injections of enterotoxemia antiserum.
Malignant edema (see Malignant Edema in Animals) and blackleg (see Blackleg in Animals) may follow wounds (eg, improper shearing, vaccination). However, these are clostridial diseases not commonly seen in sheep.
Tetanus (see Tetanus in Animals) may also be seen in unvaccinated sheep and lambs after a wound associated with procedures such as castration, docking (tailing), shearing, or vaccination performed in contaminated yards/corrals. Tetanus has been specifically associated with the use of elastrator (castrating) bands to remove tails and testicles of young lambs. Vaccinating ewes with a C tetani–containing vaccine shortly before lambing will prevent the disease for up to 60 days of age if the lambs receive their dams' colostrum.
Black disease (see Infectious Necrotic Hepatitis in Animals) is caused by C novyi type D. Sudden death is a hallmark of this disease seen among unvaccinated grazing sheep on pastures with wet areas that support the snail that is the intermediate host of Fasciola hepatica. The organism, which produces a potent toxin, colonizes the necrotic areas of the liver caused by the migrating flukes on their way to the gallbladder.
C novyi is also the cause of “big head” in rams, in which necrotic and anaerobic tissue on the head caused by the head butting of unvaccinated fighting rams becomes infected with the anaerobic clostridial organisms. Black disease and big head can be prevented by vaccinating.
Navel ill (E coli and Erysipelas arthritis) (see Erysipelothrix rhusiopathiae Infection) are associated with navel infections of lambs born in muddy or dirty areas and that have not had their navels dipped in 7% iodine at birth. These two infections are often confused with one another and are usually seen around 3 wk of age, although simmering infections can exacerbate later. Other causes may be contaminated dip or poor hygiene at docking/tailing or castration.
Poor lambing management and sanitation of the lambing area can result in significant economic loss due to varied problems ranging from baby lambs dying of hypothermia-related hypoglycemia, starvation due to mismothering or ewes with mastitis, and sudden death due to overwhelming coccidial infection at 3 wk of age. Lambs that get little or no colostrum for whatever reason are at significant risk of succumbing to disease because of the lack of passive immunity even after weaning. Diseases particularly common to lambs lacking passive immunity are E coli scours, septicemias, navel ill, coccidia (see above), pneumonia, tetanus, enterotoxemia, sore mouth, and arthritis.
Coccidiosis can cause a sudden death syndrome at 21 days of age if the infecting dose was large enough. Otherwise, the disease is characterized by the area around a young lamb’s tail smeared with diarrhea. If not treated, the diarrhea can turn bloody after a time and the lamb can dehydrate and die. The lambs' environment or the dam’s contaminated teats is the source of the infections; most adults carry small to moderate amounts with no signs. Lambs, too, can carry moderate infections and show only lack of vigor and weight gain. Coccidia is the most common cause of poor-doing lambs and significant loss to the producer.
Coccidiosis can be controlled by keeping the lambs' area clean and dry and by treating ewes before lambing with a coccidostat. Feeding coccidiostats in feed accessible only to the lambs such as decoquinolate, amprolium, lasalocid, or monesin further controls the disease. It is treated with sulfonamides.
Ovine posthitis (pizzle rot) is seen in merino wethers on high-protein pasture and in breeding rams kept in corrals or paddocks, particularly with moist areas where the animals prefer to lie during hot periods of the day. Ammonium salts in the urine are broken down by soil organisms contaminating the wool around the prepuce, and the resultant NH4 causes erosions and scabbing of the preputial orifice. It can interfere with breeding because of pain and discomfort. Clipping wool around the preputial opening, keeping animals in dry paddocks, and reducing protein in the diet will help eliminate the problem.
Actinobacillosis (see Actinobacillosis) is seen in sheep grazing on abrasive, thorny pasture.
For disease risks associated with pasture or with specific plants (eg, bloat, polioencephalomalacia, hemolytic anemia, esophageal obstruction, enterotoxemia in sheep, and goiter in the lambs born to ewes grazing Brassica spp), see Poisonous Plants. For the risk of nutritional deficiency or toxic disease associated with formulated feeds, see Nutrition: Sheep.