Livestock production: Testing your herd for brucellosis (contagious abortion)


By Digital team | 8 January 2018
cattle; brucellosis; antagonists; livestock

Question: How to I test my herd for contagious abortion (brucellosis)?

Ask your veterinarian to help you collect the samples you will need for testing.

  • The milk ring test (MRT) is an extremely sensitive screening test that can be a very good indication of brucellosis in a herd.
  • The MRT could, however, also give false positive reactions if there are high cell counts in the milk.
  • It is also possible to isolate the causative organism (a bacterium) from the milk of a cow that tests positive, but the test gets less dependable, the longer after calving the sample is taken.
  • A blood sample could also be sent to Onderstepoort Veterinary Institute for testing.

WHICH FACTORS COULD AFFECT THE TEST RESULTS?

  • If a heifer is inoculated a little too late, she will test “suspicious” for a certain period – usually until she has given birth to her first calf.
  • This is why it’s so important to inoculate heifers with Brucella S19 (Onderstepoort vaccine) when they are between 4 and 8 months old – preferably closer to 4 months in breeds that show early sexual maturity.
  • It is also important that the syringe used to inject the S19 vaccine should not be used for anything else, such as antibiotics.
  • This is because remnants of the vaccine could remain in the needle, and cause a vaccine reaction that is difficult to distinguish from a real reaction in a routine blood test.
  • It is important to indicate which animals are heifers so that, if reactions do occur among the younger animals, they are not incorrectly considered to have tested positive.
  • Heavily pregnant cows could also test “suspicious” or positive.
  • Always establish whether there really is a wild strain of Brucella abortis (the bacterium that causes contagious abortion) in the herd, by having a culture made from either colostrum or – preferably – from the calf or afterbirth.
  • If it is confirmed that the herd is, in fact, infected with the wild strain, do not take a chance to try to differentiate between the wild strain and the vaccine strain, as 20% or more of heifers born from infected mothers are latent carriers and may only test positive after their first, or subsequent, calves.
  • If there are incorrect results from Onderstepoort, the mistake usually crept in when the blood sample was taken.
  • The farmer should be there when the blood samples are taken, to write the animals’ numbers on the form opposite the number of the specimen bottle used by the person drawing the blood.
  • This article was written by Dr. Matt Ekron and appears in Ask the vet: What cattle farmers should know (1), compiled by Dr. Faffa Malan.