The only clinical sign that a heifer or cow has bovine brucellosis may be an abortion when the animal sheds millions of bacteria into the environment. Infected heifers/cows that carry their calves to full term will also shed bacteria at calving.
Alternatively, calves can carry the disease without showing any symptoms or reacting positively to blood tests (latent carriers) until they are pregnant, when they may abort or seroconvert (become positive on blood tests). A biosecurity plan should be drawn up in consultation with your vet. All plans will have to encompass the following aspects:
First establish if your herd is infected with bovine brucellosis. Blood samples should be taken from all heifers and cows over the age of 18 months and from bulls. Testing younger animals can lead to false-positive results if the animals were vaccinated with Strain 19 (see panel). The initial test should be followed by a second one three to five months later. Thereafter, the herd should be tested every one to two years. Tests for brucellosis are done at state laboratories for free. If the herd is infected, the state will take over the control and eradication of the disease.
Only buy in cattle from another negative herd with the same level of biosecurity (or higher). Ask the seller for a herd history of brucellosis and for copies of recent herd tests. Keep the new cattle isolated from your herd and have them tested for diseases (including brucellosis), using the opportunity to vaccinate them for other diseases. Treat for internal and external parasites as well. Once you have done this and are sure the animals are free of infectious diseases, introduce them into your herd.
Keep your fences in good condition to prevent unwanted cattle from straying onto your farm, as you may not know their health status. If your neighbour’s herd is infected, keep your cattle away from the boundary fence. If this is not possible, do not allow your cattle to graze in adjacent camps, as infected uterine material and afterbirths from infected cows could contaminate the camp. Ensure that any run off from your neighbour’s farm does not contaminate your camps or your water (streams, dams) as the bacteria can survive damp conditions for a few months and infect your cattle.
If at all possible, any abortion occurring on the farm should be presented to a laboratory so the cause can be identified and brucellosis excluded.
Cows must calve in isolation. Disinfect the calving stalls afterwards. As infected cows can calve normally and still excrete millions of bacteria, remove the afterbirth immediately and destroy it. Cows should be kept in smaller groups so if a cow aborts it will infect fewer cows.
Positive cows should be isolated from negative cows immediately to reduce the risk of the disease spreading. Heifer calves from infected cows should be marked as such and sent to an abattoir at slaughter weight. The risk of them being latent carriers of the disease is too high to leave them in the herd.
Colostrum from positive cows should not be collected and used for calves from negative cows. It can infect a calf, turning it into a latent carrier and become a source of infection when it aborts or calves. All milk should be pasterurised/boiled before consumption to prevent infection of humans.
The foregoing are merely guidelines for a biosecurity programme to exclude or eradicate bovine brucellosis from a herd and do not consider other diseases. A comprehensive biosecurity programme should be discussed with your vet.
Source: The Ruminant Veterinary Association of South Africa, a group of the South African Veterinary Association. For more information, contact Dr Sewellyn Davey at [email protected].