Bovine Johne’s ‘suspect’ herds

Introduction

This Agnote explains the factors that determine when a herd of cattle is considered ‘Suspect’ of being infected with Johne’s disease.

A ‘Suspect’ herd is generally one where there may be a reasonable chance of Johne’s infection in the herd. It can also apply to a herd:

• that has been infected but the disease is on the way to being eradicated;
• that has no testing history;
• with some testing history that has been negative.

Suspicion may be aroused about cattle that may or may not still be in the herd.

‘Suspect’ herd situations

Herds may be considered ‘Suspect’ in the following situations:

• Trace back. An animal at some other location is found to be infected with Johne’s disease and the possible source is investigated. If the infected animal was introduced as an adult, the property where the animal was born is the most likely source of the infection and would be considered ‘Suspect’.
• Trace forward. A new herd is found to be infected, and movements of stock out of that herd are traced. If any of the stock were sent to other properties, for example for restocking or agistment, those properties may have introduced an infected animal and may have to be considered ‘Suspect’.
• There is a legal requirement to report suspected cases of Johne’s disease, as it is a notifiable disease under the Stock Diseases Act 1923. Therefore, owners are obliged to notify their Rural Lands Protection Board (RLPB) of any animals that may be showing signs consistent with Johne’s disease. Unless the veterinarian is fairly sure the problem is some other condition, the herd may have to be considered ‘Suspect’ until the problem has been properly diagnosed.
• Where one or more of a small group of sale or movement animals has/have reacted to a test, this situation is considered ‘Suspect’ until faecal cultures or post-mortems have been completed. As false positives to blood tests (reactions in animals that are not infected) can occur, a reactor animal is not classified as ‘Infected’ unless supplementary testing confirms infection.
• Occasionally, a herd may have to be considered ‘Suspect’ because of the possibility of spread via the environment (for instance, effluent from an infected farm may have been sprayed onto another area where cattle may have been).

Important facts

• Cattle more than 12 months of age are normally, though not absolutely, resistant to infection.
• Most infected cattle become infected on their property of birth (unless they were sold as very young animals).
• The most common form of animal-to-animal spread is from a dam to its own calf — also there may have been some cross-suckling to other calves in the group.
• Some spread may occur before birth but most occurs when a calf suckles an udder contaminated by faeces.
• It is unlikely (but not impossible) for a Johne’s-infected animal younger than 2 years to shed and spread Johne’s germs.
• Johne’s disease germs can remain alive on the ground for about 12 months.
• Semen and embryos are unlikely to spread Johne’s disease.
• Most property-to-property spread is via introduction of an infected animal (generally one showing no clinical signs).

Investigation of ‘Suspect’ herds

Cattle from ‘Suspect’ herds should only be sold for slaughter. This makes it essential for the true status of the herd to be determined as soon as possible. In most cases, this will take some time. This is because Johne’s disease has a long and variable incubation period and the available tests are not precise. This means that a long and complicated investigation is often required to determine whether or not the herd is infected.

Testing can involve blood testing, faecal sampling, or slaughter of suspected animals. The cattle owner and veterinarian will have to work out a plan to confirm whether or not infection is present. This plan will depend on:

• the reason for the suspicion;
• the timing of the event causing the suspicion;
• the presence or absence of suspected animals.

These factors will be different for each property, and therefore no two properties will have exactly the same plan.

Note: The investigating veterinarian has a responsibility to resolve the suspicion as quickly as possible for the benefit of the owner. However, the vet also has a responsibility to potential buyers not to allow continuing sale of restockers while there is a possibility that some animals in the herd may be infected.

Further information

For more information, contact:

• your local Rural Lands Protection Board
• NSW Department of Primary Industries (NSW DPI) office
• your private veterinarian
• NSW DPI Technical Specialist (BJD), Sally Spence, phone (02) 6391 3630.

Acknowledgment

The assistance of RLPB veterinarians Dan Salmon and Keith Hart is appreciated.