Johne’s disease (also known as paratuberculosis) is a common cause of death and reduced growth rates among deer.
The disease is caused by a bacterial infection (Mycobacterium avium subspecies paratuberculosis) in the gut. The infection starts in the small intestine and spreads to the related lymph nodes. As the disease advances the intestinal walls thicken, reducing digestion and absorption of nutrients and causing the characteristic diarrhoea.
How does the disease spread?
Johne’s disease typically enters an uninfected farm via the purchase of apparently healthy but infected stock. Diseased animals can shed millions of bacteria in every gram of their faeces, contaminating their environment.
Young deer are by far the most susceptible to infection, especially in the first few months of life. Infection occurs mainly when they consume feed, milk or water contaminated with faecal material from diseased animals. But once infected, they remain apparently healthy for an indefinite period until something triggers the disease phase, often stress related.
Sporadic cases also occur in mixed age deer. Severely diseased hinds can also pass the bacteria on to their fawns through the placenta, colostrum or milk.
What are the symptoms?
The first sign detected on-farm is usually a slowing and then reversal of growth rates in infected individuals. Muscle wasting, ill thrift, diarrhoea and eventually death follow.
A variety of compounding stressors can trigger the disease phase to begin (for example, a tough winter, a parasite burden, transport, social stress, drought, lactation, or the rut). Most infected deer show only early symptoms and are culled or processed before they become severely diseased and die. This means it can be hard to see the full scale of a Johne’s disease issue.
Effect on deer production
Slow growth rates, a ‘tail-end’ and deaths in mobs of young deer being finished for processing are the main effects on production. In extreme cases up to 20% of weaner mobs have been affected.
Any unusual drop in productivity, or emergence of signs of poor health should be diagnosed quickly and thoroughly by a veterinarian. The vast majority of testing for Johne’s disease is with the ‘Paralisa’ blood test offered by Disease Research Ltd (DRL) at the Invermay research centre near Dunedin. It is low cost (approximately $15), quick and effective but not absolutely perfect. A negative test result indicates a low chance of infection but does not guarantee freedom.
For high value animals or cases of particular interest a test measuring the number of bacteria in the faeces (qPCR test) is also offered by Disease Research Ltd and some other laboratories. It is approximately $50 per test. The faeces of up to 10 animals can be ‘pooled’ together to check in a single qPCR test. Contact DRL by phone (03) 4894832 or email: email@example.com.
DeerPRO, an industry-wide programme run by Deer Industry New Zealand, checks every processed deer for signs typical of Johne’s disease (enlarged lymph node lesions in the gut). It reports back to farmers about these lesions, the subclinical effect of the disease and other venison productivity measures. It also has a range of resources and services to assist farmers and their veterinarians in controlling the disease. Contact DeerPRO by phone 0800 456 453 or email firstname.lastname@example.org
Control of Johne's disease
- Contact your veterinarian to establish an effective control plan, especially if you are euthanasing clinical cases of disease every year
- Immediately cull or euthanse and carefully dispose of deer with clinical signs of the disease – they are super-infectious!
- Ensure young stock do not graze areas contaminated in recent months by diseased deer
- As far as possible keep a ‘closed’ deer herd
- Ensure any purchased deer are blood test negative prior to arriving on your farm.
- Annually or bi-annually test a ‘tail-end’ or R2 hind mob to monitor infection rate
- Use the Deer Health Review booklet to document herd health including JD control.
- Vaccination is a possible option but interferes with the test for bovine tuberculosis. It may be an option for finishing animals destined for slaughter. A single dose at weaning is required and it reduces the incidence of clinical disease among the herd (it does not prevent infection altogether). All vaccinated animals must be ear-marked and the slaughter plant notified. It must not be given to breeding animals because it is very likely to interfere with TB testing and give false positives.
Contact your local veterinarian for expert advice. Alternatively, contact DeerPRO any time by phone (0800 456 453) or email (email@example.com) for free confidential advice.
|Information on Johne's disease is available in a convenient DINZ Deer Fact sheet (August 2015). Print off your own copy here >>|