Diagnosis of Bovine Brucellosis

Brucellosis the disease

Brucellosis is a bacterial disease of cattle caused by Brucella abortus, which may cause abortions. The disease is most commonly spread between herds by the movement of infected animals and between animals by contact of susceptible animals with infective discharges at the time of calving or abortion of infected animals, and for up to 1 month thereafter. Other less common means of spread occur.

Clinical signs

Acute outbreaks of brucellosis, during which 30 to 40% of pregnant animals may abort, occur in a susceptible herd after exposure to the infection. Abortions usually occur from five months pregnancy to full term. Once brucellosis is established in a herd and has become chronic, the incidence of abortion usually decreases until it is only seen in replacement stock.

Hygromas (fluid accumulation in the knee joint) may occur in some cases.

How to diagnose Bovine Brucellosis

Foetal specimens

Specimens of choice for culture are aborted foetuses (foetal abomasal fluid, lung, spleen, liver) and foetal membranes. Smears should also be made of the foetal abomasal fluid, lungs, of the aborting cow’s lochia as well as of the cotyledons of the placenta. These samples must be submitted to the laboratory on ice as soon as possible after abortion has occurred.

Bacteriological specimens

Specimens listed above, the afterbirth or parts thereof which must include the cotyledons, must be submitted to the laboratory on ice (cool pack). They should reach their destination within 48 hours and each organ must be packed separately in sterile specimen jars.

Serological Tests

Blood samples are collected by a veterinarian or registered, authorised technician in vacuum tubes that are properly marked with the animal’s identification number. The samples are sent to a SANAS (South African National Accreditation System) accredited laboratory for testing. A negative result immediately after abortion may be a false negative as the immune system may not yet have developed antibodies, so a repeat serological test after at least a 2 week period will give a more informed result.

Infected animals are generally detected by serological tests, in particular the Rose Bengal Test (RBT), and complement fixation test (CFT), which measure the presence of antibodies to B. abortus.

Other tests: Other tests such as the milk ring test (MRT), ELISA, and culture and typing may be used in certain situations.

Most infected animals become serologically ‘positive’ soon after infection but some may not sero-convert until after the first abortion or calving post-infection. Latent infection refers to heifer calves that are infected with brucellosis but do not react to serological tests. They are extremely dangerous in transmitting infection as although they are carriers of the disease, they do not show that they are infected on serological tests.

Factors other than exposure to infection (e.g. strain 19 vaccination or infection with antigenically similar organisms) may result in the production of antibodies, which are detected by the serological test and give a false positive reaction on serology.

Herd test

Brucellosis diagnostics should be interpreted based on the status of the herd of origin, as brucellosis is a herd disease. If an individual animal tests negative, it means nothing without the herd status. If an individual animal tests positive, it means that the whole herd is seen as potentially positive and has to undergo further testing under quarantine. Testing and retesting at the herd level will also assist to clarify suspicious reactions or reactions caused by late S19 vaccination only.

Zoonotic risk of sampling

There is a zoonotic danger involved, especially when collecting samples for culture. It is recommended to at least wear gloves when sampling. Goggles/ glasses are also recommended to prevent splashes into the eyes (conjunctiva). If aerosolization is anticipated a respiratory mask is recommended as well. Correctly package and clearly label samples that are sent for Brucella culture to ensure that laboratory staff handles it appropriately.

Laboratories

The laboratory tests used in South Africa may be broadly grouped as follows:

Tests to demonstrate the pathogen itself (Brucella abortus) – direct methods
Tests to demonstrate the presence of specific antibodies in the blood, milk or semen – indirect methods (Impression smears are also an indirect method of testing)

All samples may only be tested at a Brucella SANAS accredited laboratory where technologists are qualified and tests validated against international norms and standards. Records are kept of these tests on the relevant Laboratory Information System and signed off by Laboratory State Veterinarians as part of the Quality Assurance System.

Sample submission forms have to be filled in correctly and completely (including the details of the local State Veterinarian) when submitting samples. Laboratories may not share results if all the relevant information is not provided.

In the case where blood was collected by a private veterinarian, the test results are sent by the laboratory to him or her and the relevant State Veterinarian.

If the results are positive, the State Veterinarian of the area will take action as stipulated in the Brucellosis manual.

For further details contact your local State Veterinarian or private veterinarian. Also visit: