Story: Diseases of sheep, cattle and deer
Page 5 – Reproductive and nervous system diseases
Bacterial infection of the testis and epididymis duct can cause infertility in rams. A significant cause is Brucella ovis, and there is a voluntary, national industry-based programme to record flocks which are free of this bacterium.
There are a number of causes of spontaneous abortion, which can result in high lamb losses. Toxoplasmosis and Campylobacter infections can reduce the lambing rate of some flocks by 25% or more. They can be prevented by vaccines.
Since 1996, in intensively stocked areas of the South Island, Salmonella Brandenburg has caused abortions, and in previously unaffected flocks 50% of the affected ewes have died. A vaccine is available.
Other less common causes of abortion are hairy shaker disease (Pestivirus), Listeria and Fusobacterium organisms. Toxoplasmosis, Campylobacter and hairy shaker disease can also cause lambs to die shortly after birth.
This disease is commonly seen in late summer and autumn, and is caused by sheep ingesting Lolitrem B, a toxin produced by an endophyte fungus in ryegrass. Affected sheep become unsteady on their legs, eat less and lose weight. Ryegrass pastures low in endophytes overcome this problem. Similar to ryegrass staggers, Phalaris staggers can affect animals grazing on new Phalaris tuberosa grass in summer and autumn. This is caused by alkaloids in the plant.
Polio-encephalomalacia can occur in sheep of all ages, but is most common from weaning to 12 months of age and is characterised by impaired vision, blindness, circling, tremors, convulsions and ‘star gazing’. It is thought to be caused by thiamine deficiency and has been associated with a feed change to less roughage. It has also been linked to sulfur intoxication after eating feed rich in sulfur.
Listerosis or circling disease is caused by Listeria monocytogenes, a bacterium which moves up cranial nerves to the brain, where it forms micro-abscesses. Poorly made silage and baleage (preserved feed) and hay bales are often a source of infection, as is the loss of teeth. The disease becomes evident about a month after exposure to the bacteria. Damage to the brain means that treatment is seldom successful.
Sheep clostridial diseases
Clostridial bacteria are found in the soil and intestinal tracts of animals and humans. Outside the body they are transformed into spores that are highly resistant to drying and temperature changes, and can live in the soil for many years. When environmental conditions become favourable, the spore germinates into a vegetative bacterial cell and begins to multiply. This may occur in damaged tissue, such as wounds, or in the intestinal tract, and especially after over-eating grain or lush pasture. When multiplying, the bacteria release powerful toxins into the system of the host animal. Their effects are usually fatal.
Sheep are prone to a number of clostridial diseases, but vaccination can usually prevent them.
Tetanus or lockjaw is a fatal disease of sheep caused by the bacterium Clostridium tetani. Symptoms are muscular stiffness in the neck, difficulty in swallowing, rigidity of abdominal muscles and fever in sheep as well as humans. Deep wounds with devitalised tissue are ideal sites for the proliferation of tetanus bacteria. Most cases occur after lambs have had their tails removed with rubber rings, when Clostridium tetani infests the oxygen-deprived tissue. Prevention is by injecting lambs with tetanus antitoxin at tailing, or by vaccinating ewes so the antibody is available to lambs in their colostrum (first milk).
Enterotoxaemia (pulpy kidney)
Enterotoxaemia is caused by type-D Clostridium perfringes. The disease causes convulsions, muscle tremors, then sudden death in well-fed, rapidly growing lambs.
Black leg and malignant oedema
Black leg and malignant oedema are caused by the bacteria Clostridium chauvei and Clostridium septicum, which enter wounds such as those from shearing, castration, docking, fighting and parturition. Both diseases can be prevented by vaccination.