The most widespread modern health problems in sheep are caused by nematode parasites, which live in the gastrointestinal tract. They cause diarrhoea (also called scours), poor growth and ultimately death, especially in young stock. Some species of worm can cause anaemia and death from blood sucking. Internal parasites can be controlled by an anthelmintic given orally or injected. However, resistance to anthelmintic products has become an increasing problem in sheep as well as goats.
Survival of the resistant
Drench resistance is when parasitic worms in the alimentary canal develop an inherited resistance to specific types of anthelmintic drenches or injections. This occurs when a few worms survive a dose of drench and go on to breed resistant worms, which eventually dominate the population. Drench resistance was common on New Zealand sheep and beef farms in the early 2000s.
Although internal parasites generally have little effect on healthy adult ewes, they contaminate pasture with their eggs, excreted in ewes’ dung. Some of the techniques farmers use to reduce pasture contamination can increase anthelmintic resistance in the parasites, so care must be taken to follow recommended practices and be aware of the resistance status of nematode worms on each farm.
Scours (diarrhoea) can be caused by a number of organisms other than nematode parasites. Gastrointestinal salmonellosis in ewes generally occurs in summer and autumn, and can be controlled by vaccination. A clinically similar disease seen in winter is caused by the bacterium Listeria monocytogenes, which sheep ingest in poor-quality silage or baleage (preserved feed) and hay bales. Enteric yersiniosis is a mild form of diarrhoea that occurs in winter in sheep 9–12 months old.
Johne’s disease in sheep is caused by the bacterium Mycobacterium avium spp. paratuberculosis, which affects the intestine and results in wasting, often without diarrhoea. The intestinal wall slowly thickens and the animal has trouble absorbing nutrition from its food. Infected sheep initially continue to eat and remain bright, but over the months and years lose condition. There is no cure. Lambs usually contract the infection, and shed the bacteria well before signs of sickness show up in two- and three-year-olds.
The disease is endemic, with an estimated 60–70% of New Zealand flocks being affected. Often, less than 1% of the sheep will show clinical signs in any one year. Vaccines can help control the disease.
Pneumonia and pleurisy
Pneumonia (an infection of the lungs causing inflammation) and pleurisy (an infection of the layers around the lungs) in lambs are common in late summer and autumn. These diseases have two forms: an acute fibrinous pneumonia, and a chronic non-progressive pneumonia. They are considered to be initiated by para influenza-3 virus (PI3) and adenoviruses with a secondary bacterial infection by Mannheimia (Pasteurella) haemolytica. Infections lasting more than one month are likely to be caused by another bacterium, perhaps Mycoplasma ovipneumoniae or Bordetella parapertussis.
Scrapie is a fatal, degenerative disease affecting the central nervous system of adult sheep and goats. The disease creates sponge-like spaces in the brain in genetically susceptible sheep, who often show signs of nervousness, and in some cases itching – hence the name scrapie.
Badly affected flocks can mean significant production losses, but more importantly the presence of scrapie in New Zealand would prevent the export of sheep products. Despite stringent regulations and quarantine requirements, scrapie appeared in 1952 in a South Island flock with imported stud animals, and there was a second outbreak on another property before the disease was eradicated. Sheep imported for research purposes were diagnosed in 1976 and 1977 while in quarantine on Mana Island. All animals involved in the project were destroyed.