Histophilus somni Complex  in Cattle

Histophilus somni (formerly Haemophilus somnus) is a Gram-negative bacterium that is a member of the Pasteurellaceae family.  It appears microscopically as a cocco-bacillus, is a facultative anaerobe, non-motile, and a non-spore-forming bacteria.  H. somni is a commensal organism of cattle that may inhabit certain mucosal surfaces, including the upper airway and reproductive tract.

  H. somni was first recognized as a pathogen in cattle in 1956.  H. somni is a pathogen of cattle worldwide, but the greatest prevalence of disease is focused in the large beef producing countries of the world, such as the United States and Canada.  Infection with H. somni is most commonly a feedlot disease, but may be seen in dairy and grazing operations.  Young growing cattle age 6-12 months are most commonly infected and show clinical signs.  The actual prevalence of the bacteria is very high, and almost all cattle will be exposed at some point in their life.  This can be noted in certain herds where 100% of animals have circulating antibodies to H. somni.  Actual clinical disease, however, is uncommon in susceptible groups, with an incidence rate of 1-2% lower.  Clinical disease can be devastating when it occurs.  H. somni is capable of causing a variety of disease syndromes, including thrombotic meningoencephalitis (TME), respiratory disease (H. somni is a component of the Bovine Respiratory Disease Complex, BRDC), myocarditis, polysynovitis, otitis media, mastitis, and reproductive tract diseases.  Historically, the most common disease manifestation was TME but, in recent years, respiratory disease and myocarditis are becoming more prevalent.

  H. somni can be described as an opportunistic pathogen. The bacteria require a breakdown in mucosal immunity in order to cause disease.  Many different types of events can compromise the immunity of a beef calf.  These may include stress from transport, concurrent viral infection, inclement weather, weaning, etc.  Certain virulence factors play a role in the disease process as well.  H. somni exhibits a variety of different virulence factors, such as lipooligosaccharide (LOS, an endotoxin), induction of apoptosis (programmed cell death of bovine endothelial cells, the ability to inhibit destruction by phagocytic cells, histamine production, and many others.

  The pathogenesis of TME involves a bacterial septicemia.  Once the bacteria enter the bloodstream, they are able to evade host defenses and cause apoptosis of bovine endothelial cells.  This, in turn, causes a vasculitis and thrombosis  which, in the brain, leads to neutrophil infiltration and tissue necrosis (TME).  In the heart this can cause myocarditis with multiple infarcts, necrosis, and abscessation.

Clinical signs of cattle with H. somni infection can vary greatly depending on which form of the disease the calf has.  Calves with neurologic disease (TME) will often times be acutely affected and sudden death may be the only clinical sign.  Other clinical signs that may be observed are fever, depression, lateral recumbency, and closed eyes ("sleeper syndrome").  If animals are still able to stand they will be ataxic, weak, and may appear blind.  Regardless of the clinical signs, the course of the disease is rapid and most affected animals will die within 24 hours.  The progression of disease in calves with myocarditis is also very rapid and sudden death may be the only clinical sign.  A clinical diagnosis of myocarditis is rarely made, with affected cattle potentially showing signs of left heart failure (exercise intolerance, open-mouth breathing, cough, etc.).  Cattle exhibiting signs of the respiratory form of the disease show nonspecific signs consistent with any pneumonic calf.  Affected animals will be febrile, off-feed, show labored breathing, etc.  H. somni is capable of causing upper airway disease as well, meaning that calves may also cough and have a foul odor emitting from their mouths.

  Post mortem examination of cattle that have suffered from H. somni infection can reveal a variety of lesions.  The most striking gross lesions will be seen in cattle with either myocarditis or respiratory disease.  The left ventricular free wall is most commonly affected and will show full thickness myocardial pallor.  Evidence of pulmonary congestion and edema may be noted as well.  Lungs of infected cattle will exhibit a suppurative broncho-pneumonia with fibrinous pleuritis.  Gross lesions of cattle with TME may be difficult to see and can be highly variable.  Areas of hemorrhage and necrosis may be seen on the surface of the brain or on cut section.  The brain itself may also be swollen secondary to edema.

  Microscopic lesions in all affected organs include vasculitis, neutrophilic inflammation, and tissue necrosis.  Colonies of Gram-negative bacteria may be seen in thrombi.

  Definitive diagnosis of H. somni may be made in a variety of ways.  These include bacterial culture, serology, and immunohistochemistry.  H. somni can be cultured from a variety of tissues including blood, CSF, joint and pleural fluids, brain, liver, and kidney.  Selective culture media is needed to ensure growth of the bacteria and samples should ideally be taken from untreated animals.  Bacterial culture remains the gold standard for diagnosing H. somni infection.   Serology may also be used to make a definitive diagnosis of H. somni.  However, a high prevalence of seroconversion  exists  in many herds of cattle and may not reflect an acute infection.  Therefore, acute and convalescent titers are needed to make a definitive diagnosis which is often times impossible due to the rapid course of the disease.  Finally, immunohistochemistry may be used to identify H. somni in formalin-fixed tissues.

   H. somni is susceptible to a wide variety of antibiotics, and treatment decisions are often made depending on a veterinarian's experience with a certain drug.  Treatment for all forms of the disease complex have often been with oxytetracycline.  Oxytetracycline has been effective in the treatment of TME when the drug is given at the onset of clinical signs.  Treatment is often unsuccessful when antibiotic therapy is delayed or if the animal is already recumbent.  Treatment of bronchopneumonia associated with H. somni may be accomplished with a variety of antibiotics, including oxytetracycline.  The bacteria are also susceptible to many of the antibiotics commonly used to treat bovine pneumonia, including some of the newer, longer-acting medications.

  Prevention of the H. somni disease complex can be difficult due to the ubiquity of the organism.  Commercial bacterins are available, but their efficacy is questionable.  Field trials to test the efficacy of the vaccine are difficult to perform due to an inability to consistently recreate the disease process.  Most vaccines are labeled for protection again TME only, not the other forms of disease.  If calves are to be vaccinated it should be performed prior to entry into the feedlot.  Metaphylaxis has also been used to prevent H. somni infection.  Metaphylaxis involves pre-treatment with antibiotics prior to clinical illness.  This is often done when calves enter the feedlot.  They are given an injection of a long-acting antibiotic that is designed to protect them against infection in the early, most stressful time at the feedlot.  Metaphylaxis is widely used in the prevention of BRDC, which includes H. somni.

-by Dr. Nathan Ahlemeyer, Class of 2009
-edited by Dr. Ryan Jennings, ADDL Graduate Student

References:

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