Johne's Disease is caused by the bacterium Mycobacterium avium subspecies paratuberculosis (MAP) and estimated to cost New Zealand $40-88 million in lost production each year.
What are the signs?
Infection affects the small intestine in ruminant animals (cattle, deer, sheep, goats and wildlife) and leads to a gradual thickening and inflammation of the intestinal wall eventually preventing the uptake of nutrients.
Most new infections occur in the first 6 months of life and calves are particularly at risk in their first 30 days. The onset of clinical signs usually occurs between 2 and 6 years of age in cattle.
Infected animals excrete the Johne’s bacteria in faeces in increasing amounts over years as the infection develops. Even then, the disease usually remains sub-clinical in dairy cows, with only occasional animals showing the characteristic clinical signs of ill-thrift, progressive weight loss and profuse diarrhoea. In these older and clinically affected animals the bacteria are also likely to be found in milk or colostrum. No cure is available and the condition is eventually fatal.
How is it spread?
Johne's bacteria are commonly spread from the dam or other adult cows to the calf, usually through faeces, colostrum or milk.
Because the bacteria may persist in the environment, spread through infected pastures and waterways may also be significant. The bacteria are robust and can survive for up to 18 months in the environment.
How common is it?
Johne’s Disease is present in all countries with ruminant populations, and many of these, including New Zealand, have research programmes underway on aspects of Johne’s disease management.
Research indicates that more than 50 percent of New Zealand dairy farms have had cases of Johne's Disease, however it is likely to be present, but not identified, on more farms.
Johne's Disease prevention
Infection can only be limited by preventing exposure to the bacteria. This requires good hygiene practices - separating calves from dams after initial feeding of colostrum, and minimising contact between young and old animals.
Keeping uninfected animals separate from potentially infected herd mates is important.
Diseased animals should be removed from the herd as soon as possible and new animals coming to the farm should be tested to show they are free of infection. (see guide to laboratory testing)
Due to the protracted nature of the subclinical stages of Johne’s disease infection, the full effect of changed management practices that reduce transmission will only be seen after several years.
This disease can be transmitted between generations.
Management starts with the planning of breeding, including only purchasing bulls free of Johne's Disease and only breeding replacements from cows in good health.
Hygiene with young calves is paramount in Johne's Disease control programmes, and in particular avoiding exposure of calves to adult faecal matter:
- Immediate post-calving management needs care. Each calf has to receive sufficient colostrum from a healthy dam. Where dam health has not been properly considered then cross feeding of colostrum from another dam, or mixture of dams, potentially carries a significant risk.
- Early separation after birth of the calf from the cow is important provided it can be guaranteed that adequate colostrum has been fed.
- Calves should not graze in hospital paddocks.
The dynamics of Johne’s Disease are complex, however progress is possible with the right approach.
DairyNZ and the Johne’s Disease Research Consortium have developed a tool box to assist farmers and veterinarians to manage the disease in New Zealand dairy herds, see Guide to Johne’s Diease Management for NZ Dairy Herds. The toolbox includes a series of management strategies to minimise transmission of the disease, as well as specific, targeted interventions for farmers with a high level of Johne’s Disease in their herd.
Your veterinarian will be able to help you work out the right approach for your herd.
How do I find out more?
The Johne's Disease Research Consortium has its own website.