B103 - BRUCELLOSIS (BOVINE)

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B103 - BRUCELLOSIS (BOVINE)

Nature of the disease
Bovine brucellosis is a highly contagious bacterial disease, almost exclusively caused by Brucella abortus causing late term-abortion and infertility in cattle. The disease is also a serious zoonosis, causing undulant fever in humans.
Classification
OIE List B disease
Susceptible species
Main natural hosts are cattle, horses and humans. Infection has been reported in a range of other species but these are not considered important in the maintenance of the disease. Infection with abortion occurs rarely in pigs, sheep and goats.
Distribution
Until recently, bovine brucellosis was widely distributed throughout the world. A number of countries, including several in Europe and Scandinavia, Australia, New Zealand, Canada, Israel and Japan have succeeded in eradicating the disease.
Clinical signs 
The dominant feature of the disease in cattle is abortion.

Cattle

  • Usually occurs at about 5-7 months. Full-term calves may die soon after birth
  • Abortion rates in herds vary. In fully susceptible herds rates may vary from 30% to 80%, although in some cases, abortions may be more insidious
  • Retained placenta and secondary metritis is common and may lead to permanent sterility
  • In bulls acute or chronic infections of the reproductive tract may occur (orchitis, epididymitis, seminal vesiculitis · hygromas, particularly of the carpal joints, occur in some animals in chronically affected herds 

Horses

  • Chronic bursal enlargements of the neck and withers with chronic draining sinuses
  • ‘Poll evil’ localisation of infection between the nuchal ligament, atlas and axis
  • ‘Fistulous withers’ inflammation of bursa between the nuchal ligament and dorsal spines of thoracic vertebrae

Humans

  • ‘Undulant fever’ (fluctuating, irregular fever)
  • Chills, depression, weakness
  • Headache, joint pains, generalised aches
  • May be complications affecting cardiovascular or central nervous systems
Post-mortem findings 
In cattle there is considerable variability in uterine lesions after abortion
  • Mild to severe endometritis
  • Placenta is thickened, oedematous, yellow-grey and may have exudate on surface
  • Mammary gland lesions
  • Inflamed regional lymph nodes 

While some aborted foetuses will appear normal, others may show: 

  • Varying degrees of sub-cutaneous oedema
  • Blood stained fluid in body cavities
  • Enlarged orange-brown discoloured liver
  • Fibrous pleuritis and focal pneumonia
Differential diagnosis 
Other cause of abortion and reproductive failure in cattle include: 
  • Trichomoniasis
  • Vibriosis
  • Leptospirosis
  • Listeriosis
  • Infectious bovine rhinotracheitis
  • Mycoses 

Brucellosis should always be suspected when there are multiple late-term abortions in a herd.

Specimens required for diagnosis 
Live animals 
  • Milk samples from each quarter and vaginal swabs from affected cows
  • Blood samples (for serum) from a number of cows in the herd, together with a pooled milk sample

At post-mortem

  • Samples of lymph nodes, spleen, mammary gland and uterine tissues from cows
  • Spleen, lung and stomach contents from aborted foetuses, and cotyledons from foetal membranes
Transmission   
Bovine brucellosis is almost always transmitted from herd to herd through the movement of infected cattle. Cows shed large numbers of organisms when they abort. Bacteria are also excreted intermittently in milk throughout the lactation. 

Animals become infected through ingestion of contaminated feed or water, or after licking an infected placenta, foetus or genitalia of another cow, after it has aborted. 

Most cows remain chronically infected. Urine, faeces and hygroma fluids are also sources of bacteria. Infected bulls may excrete the organism in their semen. Congenital transmission may occur through in utero infection.

Humans are infected through handling infected cows or their tissues, or through drinking infected milk. Pasteurisation will prevent the latter.

Risk of introduction   
Importation of infected animals is the highest risk for introducing bovine brucellosis. The disease could also be introduced through semen from infected bulls. 

Imports should be restricted to brucellosis-free countries, or if genetic stock from infected countries is desired, from certified free herds. In the later case, strict quarantine procedures including testing should be applied (see OIE International Animal Health Code).

Control / vaccines  
Infected cattle can be treated with tetracyclines. Treatment is not used as part of control schemes.

Live attenuated vaccines are available which provide good protection. However, vaccination can produce persistent antibodies, which interfere with subsequent diagnostic tests. 

A number of countries have successfully undertaken control programs that have led to eradication. The programs use combination of vaccination, test-and-slaughter, surveillance and abattoir traceback. Experience has shown that vaccination should be maintained well into the advanced stages of the program to protect clean herds from serious breakdowns.

References
  • Brucellosis in Cattle, In Merck Veterinary Manual, National Publishing Inc. Eight ed, 1998, Philadelphia, p 999-1000
  • GEERING WA, FORMAN AJ, NUNN MJ, Exotic Diseases of Animals, Aust Gov Publishing Service, Canberra, 1995, p301-306
  • Office International des Epizooties, 2002