Histophilus somni: A bacterium that demands attention

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Estimated reading time: 4 minutes

  • The organism Histophilus somni is part of the normal bacterial population (normal flora) found on the mucous membranes in the upper respiratory, intestinal and genital tracts of ruminant animals.
  • If another infection – often viral – damages the upper and lower respiratory tract, such an infection can keep the immune system so busy that it creates the perfect gap for an H. somni infection to take hold.
  • When inhaled, H. somni attaches to the mucous membranes in the animal’s upper respiratory tract where it begins to proliferate. If other flora and the immune system fail to keep these bacteria in check, H. somni will multiply unabated.
  • Furthermore, H. somni is known to lead to infection of the joints. It can even catch a ride via the bloodstream to the placenta of a pregnant animal, where it proceeds to infect the fetus.
  • Producers can prevent the spread of H. somni in his/her animals by avoiding or preventing known stressors. This will ensure a strong immune system in animals.

The organism Histophilus somni is part of the normal bacterial population (normal flora) found on the mucous membranes in the upper respiratory, intestinal and genital tracts of ruminant animals. Although cattle are the main victims of disease, sheep can also be susceptible to infection.

Stockfarm spoke to Dr Annelize Jonker, a veterinarian and bacteriologist at the University of Pretoria’s Department of Veterinary Tropical Diseases, about this culprit.

Causes of infection

“Any event or factor that suppresses the animal’s immune system can lead to H. somni multiplying unabated and causing infection,” says Dr Jonker. If another infection – often viral – damages the upper and lower respiratory tract, such an infection can keep the immune system so busy that it creates the perfect gap for an H. somni infection to take hold.

Environmental factors such as cold and rain may have a detrimental impact on an animal’s immunity, she adds. The management practices of producers are a key factor in this regard, as they directly influence animals’ susceptibility to H. somni infections.

Because of the negative impact it has on the immune system, stress is a crucial factor requiring due consideration, warns Dr Jonker. “Any form of handling can cause stress. This includes the application of ear tags, dehorning, vaccination, castration and weaning of calves. Transport is also high on the stress factor list.”

Producers also need to pay attention to the way animals are kraaled, as indiscriminate kraaling can lead to stress. Examples include:

  • Overstocking.
  • Animals of different sizes kraaled together.
  • Animals from different herds kraaled together.

“Overstocking provides these bacteria with many more hosts since the animals are bunched close together,” says Dr Jonker. “H. somni spreads through droplets when sick animals cough or urinate.”

Animals in close proximity then inhale these droplets. This leads to more H. somni organisms settling on the mucous membranes in animals’ respiratory tract than their immune systems can handle.

Consequences of infection

Dr Jonker says that although a large number of animals within a herd may carry H. somni as part of their normal flora, infections are often only sporadic.

“When inhaled, H. somni attaches to the mucous membranes in the animal’s upper respiratory tract where it begins to proliferate. If other flora and the immune system fail to keep these bacteria in check, H. somni will multiply unabated. “Because of their sheer numbers, the bacteria start to spread down the airway until they reach the lungs. Once in the lungs, they multiply even further, eventually leading to pneumonia.

“Bacteria may end up in the animal’s bloodstream and cause septicaemia (blood poisoning),” she explains. “If the blood carries it to the brain, it can cause encephalitis (inflammation of the brain) and meningitis (meningo-encephalitis).”

Furthermore, H. somni is known to lead to infection of the joints. It can even catch a ride via the bloodstream to the placenta of a pregnant animal, where it proceeds to infect the fetus. “This infection can cause the animal to abort. However, abortions are usually sporadic. Infertility due to uterine infection does occur from time to time.”

In the case of sheep, producers must note that these animals can contract meningo-encephalitis, pneumonia, septicaemia or myocarditis (infection of the heart muscle).

From an economic point of view, these infections can erode some of a producer’s profit as animals can succumb due to pneumonia or an infection in the brain. Another consequence is poor growth due to pneumonia or joint inflammation, which means optimal yield is not possible. Furthermore, foetal losses due to abortion have a negative influence on a producer’s balance sheet.

Treatment and prevention

“Antibiotics are utilised for the treatment of animals with pneumonia as well as the early stages of meningo-encephalitis. However, animals that present with advanced meningo-encephalitis are usually beyond help.”

Producers can prevent the spread of H. somni in his/her animals by avoiding or preventing known stressors. This will ensure a strong immune system in animals.

Dr Jonker says that if the animals are bound for the feedlot, it is advisable to complete as many procedures as possible on the farm of origin. This includes dehorning, the application of ear tags, castration, deworming, vaccinations and weaning. “Vaccination and deworming usually prevent other infections from weakening the immune system.” – Susan Marais, Stockfarm

Send an email to Dr Annelize Jonker at annelize.jonker@up.ac.za for more information.

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