Brucellosis: An Overview
Brucella abortus primarily grows in the uterus of female animals
Brucella abortus primarily grows in the uterus of female animals over an extended period, typically causing mid to late-term abortions (4-7 months of pregnancy) in calves that would otherwise be completely healthy. Infected cows and heifers remain infected for life and are a permanent source of infection for other cattle, as well as humans. To date, no treatment for animals is yet available. In the cattle industry, the occurrence of Brucellosis is a major cause for concern due to the financial implications of this dreaded disease. The elimination of infected animals, calf losses, and milk losses can have a significant impact on the economic sustainability of a farm, and as such, farmers must ensure that they do everything in their power to maintain a Brucellosis-negative herd. The first and only symptoms displayed by Brucellosis-infected animals are abortions, dead or premature calves, and retained placentas.
Although it is primarily female animals that show symptoms, there are rare cases where Brucellosis causes inflammation of the reproductive organs in bulls. Sometimes, adult animals will also have swollen, non-painful joints, but this is in the minority. One of the biggest concerns of Brucellosis is that it is a zoonotic disease affecting humans. Brucellosis is highly contagious, and any contact with an infected animal – or contact with the aborted foetus or placenta of an infected animal – carries a risk of infection. Infection can occur through direct contact, inhalation of infectious organisms, or particles that end up in the eyes.
The main danger of human infection with Brucellosis is that symptoms (joint pain, headache, depression, poor appetite, and weight loss) often appear months after infection, when treatment is almost too late. Human treatment for Brucellosis suppresses the bacteria, but for people with chronic brucellosis syndrome, infections will repeatedly resurface.
Treatment for Brucellosis is also very expensive and takes place over an extended period of time. A “closed” herd – one in which no animals brought in from outside and you only breed with your own animals – is seen as a good way to combat the possibility of Brucellosis infection in your herd. However, this is very impractical and not always feasible; bulls break boundary wires to reach cows, new breeding animals are needed for genetic progress, and flies can carry Brucellosis from one farm to another. As a result, many people believe that there is, in truth, no such thing as a “closed” herd; nevertheless, there is great value in controlling which animals are allowed on your farm and which are not.
There are three critical measures to control Brucellosis outbreaks: vaccination, testing, and record-keeping.
Vaccines reduce the risk of brucellosis-induced abortions and therefore also the contamination that can occur in the environment when a cow aborts. Currently, there are two vaccines available against Brucellosis; the OBP S19 vaccine and the RB-51 vaccine. By law, all heifers between 4 and 8 months must be vaccinated with S19 or RB-51, after which a proposed vaccination protocol with RB-51 between 12 and 16 months and then every 2 to 3 years is suggested.
Annual vaccinations of the entire cow herd are suggested in cases where new animals are regularly purchased and brought in. Follow-up vaccinations with S19 are strongly discouraged due to the possibility of false-positive tests. RB-51, on the other hand, does not cause false-positive results because the LPS side chain has been altered through biotechnology. The vaccine can thus be repeated a second time, but not within 3 months of the first mating and definitely not during pregnancy. This live vaccine can potentially interfere with conception.
The role of latent carriers must also be considered; these are animals that test serologically negative but are disease carriers. They are the offspring of carrier animals that developed in the uterus where Brucella abortus was present to such an extent that their immune systems do not identify it as “foreign” and do not produce diagnostic antibodies against it. Carrier cattle are cattle that are unvaccinated, tested positive, and have produced offspring. Record-keeping and traceability of your herd is therefore critical to identifying carrier cattle and eliminating them and their offspring. To date, no effective treatment for animals with a positive Brucellosis status is available. Unvaccinated animals that test positive for Brucellosis must be slaughtered by law, as infected cows and heifers remain infected for life and are a permanent source of infection for other cattle and humans. Because Brucellosis is an intracellular pathogen, treatment with antibiotics is not effective. This is due to the fact that the minimum amount of the antibiotic required to kill this bacteria cannot reach the inside of the cells in a short treatment duration, and therefore prolonged treatment would be required. Prolonged treatment with antibiotics for cattle is not possible because it would cause significant damage to their rumen microbes.
These microbes are essential for degrading the cellulose in the cattles diet into fatty acids, which are then converted in the liver into a critical energy source. Antibiotics would thus compromise the functionality of the rumen, without which the animal cannot live. In addition to this, antibiotic residues in the meat or milk of an animal treated long-term would pose a significant risk to food safety and would not be suitable for human consumption. The implementation of good management practices – such as following the suggested vaccination programs, adhering to state guidelines for transportation, and applying good record-keeping – is the only way to protect a herd against Brucellosis.
It is the responsibility of every farmer and breeder to ensure that these practices become part of their management program and are applied to give their herd the best possible chance to remain Brucellosis-free. An important part of Brucellosis prevention and control in your herd is testing. By the time you suspect Brucellosis in your herd, it is already too late. Therefore, testing of your entire female herd should take place on a regular basis (annually) and testing of newly purchased animals is non-negotiable.
Brucellosis tests can be handled by the state or by private veterinarians, but results are usually obtained faster when private services are used. To perform Brucellosis tests, blood is drawn and sent to the nearest accredited laboratory, where they carry out three tests. The first test performed is a Rose Bengal Test (RBT), a qualitative test that only gives a positive or negative result. In the case of a negative test, no further tests are required, and you can reasonably assume that your herd is clean.
A positive test will require a subsequent test – called the Complement Fixation Test – to be performed. This is a quantitative test that shows infection counts. Low counts (between 20 and 100) are a good indication that a vaccination reaction has occurred and the result was a false positive. This happens when animals are vaccinated too late. Counts of 200 and up indicate a positive Brucellosis case, after which the last test, a Serum Agglutination Test, provides a final answer for what is going on at your farm.
It is possible to get false-negative results as well, but this usually only happens in heifers. By the time heifers are tested for the first time, the bacterial count in the blood is not yet at a level that can be detected by the test, due to the slow-growing nature of the bacteria. Therefore, it is critical not to view blood tests with individual animals in mind, but to have the entire herd tested. This very reason makes it extremely important to insist on the Brucellosis status of the entire herd when purchasing heifers at auctions, and not just the individual animal’s Brucellosis status.
