Johne’s disease in cattle herds

Introduction

Johne’s disease is caused by the bacteria Mycobacterium paratuberculosis.

In Australia, there are considered to be two strains — one infecting sheep and one infecting cattle. To date there has been no spread between sheep and cattle, and the diseases are considered separately for each species.

Survival of the germ

The Johne’s disease germ is very persistent, with the bacteria able to live for a period of up to 12 months on the ground. Contamination of the ground must therefore be considered in any control program.

Animals infected

In cattle, calves up to 30 days are very susceptible and readily infected if exposed to Johne’s germs. They are still susceptible up to 12 months — they do not often become infected after the first year. In most cases, cattle are infected on their property of birth.

Spread of the germ

The most common method of animal-to-animal spread is to the calf from its dam via germs on the udder. Calves can also be infected by cross-suckling. Animals up to 12 months of age may pick up infection from contaminated ground. Some calves may also be born infected.

Because of the risk of infection — before as well as after birth — all progeny of infected cows are highly likely to be infected.

Milk from infected cows is also considered a risk. Artificial insemination and embryo transfer are considered very low-risk sources of contamination.

The most common form of spread from farm to farm is via an infected animal (one generally showing no clinical signs). Environmental spread has to be considered but is relatively uncommon.

Features of the disease

• Long incubation before signs develop — from 12 months to 15 years (in most cases, 3–5 years). It also takes a long time for an animal infected as a calf to show up ‘positive’ using the current tests.
• Fall in milk yield.
• Diarrhoea which can be acute, chronic or intermittent.
• There may be remissions.
• Loss of weight in cattle.
• Emaciation.
• Eventual death.
• There may be several cases per year — up to 4% in very heavily infected herds.
• Production loss may be occurring in up to 5–10% of animals.

Herd status

‘Non-assessed’ herd

This is a herd that is not known to be infected, but in which there has not been enough testing to be confident that it is probably clean.

‘Monitored negative’ herd

This is a herd that has had one or more negative tests of an adequate sample of the herd and has implemented all of the other requirements of the Cattle Market Assurance Program.

CHECKTEST herd

This is a herd where a biased sample of 50 of the older breeders have been tested with negative results by an approved veterinarian. Test remains valid for 12 months.

CHECKTEST replaced the ‘movement test’ on 1 September 2000.

‘Suspect’ herd

This is a herd where information has been received that indicates that the herd might be infected and may therefore be a risk to other herds. It is in everyone’s interest that this suspicion be resolved as quickly as possible.

Because of the long incubation period of the disease and the unreliability of the available tests in small groups of animals, it may take some time to resolve this suspicion.

Possible reasons for suspicion:

• There is suspicion over the property of birth of an infected animal found in another herd.
• A herd has received cattle from an infected herd (sometimes the animals will no longer be in the herd).
• Reactors have been detected in a small test group (for instance, for movement to shows or interstate).

‘Infected’ herd

This is a herd where one or more infected animals have been found and which has not completed an eradication campaign. An infected animal will have been confirmed by faecal culture, or slaughter and histopathology, or culture of bowel tissue. Movement of animals from an ‘Infected’ herd will put other herds at risk and, for this reason, cattle from these herds are restricted to sale for slaughter only.

Reactors

A ‘reactor’ is an animal that has given a positive result to a blood test. This animal may be infected or not infected. It is expected that, even in clean herds, some animals will give a reaction to the blood test. A reactor will therefore not be diagnosed as infected until it has also given a positive result to culture of faeces, histopathology, or culture of bowel tissue after slaughter.

Further information

For more information, contact your local Rural Lands Protection Board, NSW Agriculture office, private veterinarian, or NSW Cattle Johne’s Steering Committee Executive Officer, Tim Jessep, Goulburn, on (02) 4828 6614.

Acknowledgment

The assistance of Rural Lands Protection Board Veterinarians Dan Salmon and Keith Hart is appreciated.