Bovine Brucellosis

Bovine Brucellosis

What causes brucellosis in bovines?

Bovine brucellosis is caused by Brucella abortus, less frequently by B. melitensis, and occasionally by B. suis. These organisms are subdivided into different biovars based on cultural and serological properties.

Where the disease is found?

Infection is widespread globally except in several countries of Northern and Central Europe, Canada, Japan, Australia and New Zealand.

What symptoms or signs can be seen in affected animals?

The main symptoms are abortion, retained placenta, orchitis, epididymitis and, rarely, arthritis, with excretion of the organisms in uterine discharges and in milk. The disease is usually asymptomatic in non-pregnant females.

Following infection with B. abortus or B. melitensis, pregnant adult females develop a placentitis usually resulting in abortion between the fifth and ninth month of pregnancy. Even in the absence of abortion, profuse excretion of the organism occurs in the placenta, fetal fluids and vaginal discharges. The mammary gland and associated lymph nodes may also be infected, and organisms may be excreted in the milk. Subsequent pregnancies are usually carried to term, but uterine and mammary infection recurs, with reduced numbers of organisms in cyetic products and milk. In acute infections, the organism is present in most major body lymph nodes. Adult male cattle may develop orchitis and brucellosis may be a cause of infertility in both sexes.

How the disease is diagnosed in animals?

Presumptive diagnosis can be made by assessing specific cell-mediated or serological responses to Brucella antigens. Tests like Rose Bengal test (RBT) and the buffered plate agglutination test (BPAT) and ELISA are suitable screening tests. Diagnosis depends on the isolation of Brucella from abortion material, udder secretions or from tissues removed at post-mortem.

What are the public health / occupational risks associated with this disease?

Brucella abortus, B. melitensis and B. suis are highly pathogenic for humans, hence all infected tissues, cultures and potentially contaminated materials must be handled under appropriate biosecurity conditions. The World Health Organization (WHO) laboratory biosafety manual classifies Brucella in Risk group III. Brucellosis is readily transmissible to humans, causing acute febrile illness – undulant fever – which may progress to a more chronic form and can also produce serious complications affecting the musculo-skeletal, cardiovascular, and central nervous systems. Precautions should be taken to prevent human infection.

Infection is often due to occupational exposure and is essentially acquired by the oral, respiratory, or conjunctival routes, but ingestion of dairy products constitutes the main risk to the general public where the disease is endemic. There is an occupational risk to veterinarians and farmers who handle infected animals and aborted fetuses or placentas.

How to prevent the disease? Does vaccines available?

Brucella abortus strain 19 vaccine:

The most widely used vaccine for the prevention of brucellosis in cattle is the Brucella abortus S19 vaccine, which remains the reference vaccine to which any other vaccines are compared. It is used as a live vaccine and is normally given to female calves aged between 3 and 6 months as a single subcutaneous dose of 5-8 × 1010 viable organisms. A reduced dose of from 3 × 108 to 3 × 109 organisms can be administered subcutaneously to adult cattle, but some animals will develop persistent antibody titres and may abort and excrete the vaccine strain in the milk. Alternatively, it can be administered to cattle of any age as either one or two doses of 5 × 109 viable organisms, given by the conjunctival route; this produces protection without a persistent antibody response and reduces the risks of abortion and excretion in milk when vaccinating adult cattle. Brucella abortus S19 vaccine induces good immunity to moderate challenge by virulent organisms. The vaccine must be prepared from USDA-derived seed and each batch must be checked for purity (absence of extraneous microorganisms), viability (live bacteria per dose) and smoothness (determination of dissociation phase). Seed lots for S19 vaccine production should be regularly tested for residual virulence and immunogenicity in mice.

Brucella abortus strain RB51 vaccine

Since 1996, B. abortus strain RB51 vaccine has been used for prevention of brucellosis in cattle in some countries. However there is disagreement in regards to the efficiency of strain RB51 compares to protection induced by S19 in cattle. Each country uses slightly different methods to administer the vaccine. In the USA, calves are vaccinated subcutaneously between the ages of 4 and 12 months with 1-3.4 × 1010 viable strain RB51 organisms. Vaccination of cattle over 12 months of age is carried out only under authorization from the State or Federal Animal Health Officials, and the recommended dose is 1-3 × 109 viable organisms. In other countries, it is recommended to vaccinate cattle as calves (4-12 months of age) with a 1-3.4 × 1010 dose, with revaccination from 12 months of age onwards with a similar dose to elicit a booster effect and increase immunity. It has been reported that full doses of RB51 when administered intravenously in cattle induce severe placentitis and placental infection in most vaccinated cattle, and that there is excretion in milk in a relevant number of vaccinated animals. Field experience also indicates that it can induce abortion in some cases if applied to pregnant cattle.

What happens if accidental exposure of vaccine occurs in humans?

Utmost care should be taken while vaccinating the animals. As the vaccine strains are live and they are not fully attenuated can cause disease in human beings. Immediate medical attention should be taken in such situations. Both RB51 and S19 can infect humans and cause undulant fever if not treated. The RB51 strain is highly resistant to rifampicin.

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