Story: Diseases of sheep, cattle and deer
Page 7 – Bovine TB and other cattle diseases
One of the earliest serious threats faced by New Zealand farmers was bovine pleuro-pneumonia in cattle, which appeared in the South Island in 1864. The gravity of the disease was recognised, and it was eradicated by slaughtering the affected animals.
Bovine tuberculosis (TB) is a dangerous illness, and controlling it is important for the farming industry as well as the health of farmers. TB is a chronic, infectious bacterial disease caused by Mycobacterium bovis, which can take years to develop and results in weakness, coughing and weight loss.
In 1961 a compulsory control programme was set up for herds supplying town milk, and by 1970 all cattle were being tested. Voluntary testing of beef herds began in 1968, and by 1977 it was compulsory for all beef cattle to be tested. The Animal Health Board has responsibility for TB control.
A herd is classified infected if TB has been confirmed by testing or post-mortem. A herd is classified as ‘suspended’ if TB is suspected and further confirmation is being sought, if it has received stock from an infected herd or one with unknown status, or if testing obligations have not been met. For an infected herd to be reclassified as clear, the whole herd must have two consecutive clear tests, with a minimum of six months between tests and no further evidence of disease. A herd’s classification will determine what tests are carried out and whether controls are put on its movement.
Eradicating TB in cattle is complicated because the disease can be transmitted by ferrets and wild possums, which are abundant. In the late 1960s TB was diagnosed in brushtail possums (Trichosurus vulpecula), and since 1972 major effort has gone into controlling them. Infected possums tend to occur in clusters, depending on an area’s vegetation type and topography. Estimates of infection therefore vary, and usually range from 2–5%, although one five-year study in an area of the North Island indicated that 11% were infected. Cattle contract the disease when they come across a dying possum and sniff or lick it.
Although the disease has not been eradicated from cattle, regular testing means it is well under control. Cattle cannot be moved from one district to another without a permit. The control programme aims to have less than 0.2% of herds infected by 2013.
Bovine brucellosis causes abortions in cattle. In 1989, after 23 years of control measures, it was declared eradicated.
Leptospirosis or lepto (Leptospira pomona) causes abortions, red water (haemoglobinuria or red urine) and death in calves. The bacteria is excreted in the urine and can infect humans if it comes into contact with scratches or cuts, or through soft tissues. There is always a risk of people becoming infected at slaughtering plants. Vaccinating cattle can help control the diseases in both cattle and humans.
No mad cows in New Zealand
Mad cow disease (bovine spongiform encephalopathy) is a chronic degenerative disorder affecting the central nervous system of cattle. It was first diagnosed in the UK in 1986, and has not been found in New Zealand. It is probably caused by a prion (self-replicating protein) spread through contaminated feed. In 1996 researchers linked the disease with the human variant, Creutzfeldt-Jakob disease.
Enzootic bovine leucosis
Enzootic bovine leucosis (EBL) is a virus that causes enlarged lymph nodes and can infect almost any organ. A voluntary eradication scheme funded and operated by the dairy industry since 1996 has almost completely controlled the disease.
Mannosidosis is where an inherited, defective enzyme fails to break down complex sugar molecules, resulting in the harmful accumulation of these products. It particularly affects calves of Angus and Murray Grey breeds, and may be evident at birth or develop later. Calves show unsteadiness and lack of co-ordination (ataxia), have a high-stepping gait in the forelegs, or may not be able to stand to suckle. By using blood tests to detect and cull heterozygote animals (those with one normal and one mutant gene), the disease has been reduced to a negligible level.