Histophilus Somni

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Histophilus Somni

Also known as

Hemophilus somnus and Histophilus somni complex


Histophilus somni is a bacterium that can cause a wide variety of disease conditions in cattle including respiratory disease, laryngitis, arthritis, foot rot, infections of the vagina and uterus, eye and ear problems, mastitis, and occasional abortion.

These bacteria are common residents in the respiratory tract of adult cattle (in some herds as many as 50% of the cows carry antibodies against these pathogens), and most often cause disease in young animals.

H. somni can survive for long periods in infected nasal discharges or on equipment contaminated with blood. These bacteria may be present in water troughs, feed bunks, cattle handling chutes, etc.
When immunity drops because of stress, any bacteria present in the upper respiratory tract of cattle quickly multiply and gain entrance to the bloodstream--to cause septicemia. They damage the inner lining of blood vessels, creating blood clots that break loose and lodge in smaller arteries.

The tissues where blood supply is blocked by infected clots are quickly invaded by these bacteria, creating abscesses and disease. In some instances this occurs in the brain and is highly fatal and this condition is called thrombotic meningoencephalitis (TME); death may occur within 36 hours after first signs appear.  

Clinical signs of cattle with H. somni infection can vary greatly depending on which form of the disease the animal has.  Calves with neurologic disease (TME) will often be acutely affected and sudden death may be the only clinical sign.

Other clinical signs that may be observed are fever, depression, lateral recumbency (animal flat on its side), and closed eyes (sleeper syndrome). If animals are still able to stand they will be uncoordinated, weak, and may seem blind. Regardless of the clinical signs, the course of the disease is rapid and most affected animals die within 24 hours.

The progression of disease in calves with myocarditis (inflammation of the heart muscle) is also very rapid; sudden death may be the only clinical sign. A clinical diagnosis of myocarditis is rarely made, however, with affected cattle possibly 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 that are fairly typical of any calf with pneumonia.  Affected animals will have a fever and be off feed, with labored breathing, etc.  H. somni is capable of causing upper airway disease as well; those calves may also cough and have a foul odor from their mouths.

 H. somni can be cultured from a variety of tissues including blood, cerebral spinal fluid, joint and lung fluids, brain, liver, and kidney.  

Septicemia (resulting from rapid proliferation of bacteria in the lungs) causes high fever, rapid breathing, lethargy and weakness. Clots lodging in small arteries of larynx, kidneys, feet and brain create problems in those parts of the body.

Young cattle in crowded stressful conditions may develop a cough about 3 weeks after being penned together, then suddenly develop pneumonia that rapidly progresses to difficult breathing, high fever, and sometimes disorientation. Unless treated quickly, the animals with pneumonia may die within 12 hours.
Laryngitis/diphtheria may occur as an epidemic within a group of infected calves and is different than typical diphtheria. Blood clots in the larynx cause severe swelling that shuts off airways. Affected calves have loud, difficult breathing as they force air in and out through the constriction.

Foot rot may also occur as an epidemic in a pen of calves; a high percentage of them may become lame within 48 hours. Blood clots in the feet make conditions ideal for bacteria that cause foot rot.
H. somnus may also invade the joints after an animal develops septicemia. Shoulders, stifles, hocks and elbows are most commonly affected, and tendon sheaths around the joints may be distended. The joints are very swollen, but the animal is usually not as lame as when affected by other forms of joint infection.
Infection in the uterine/vaginal lining can cause infertility in females. The disease may cause a prolonged interval between calving and conception, since females may not become pregnant until after several heat cycles and repeated breeding. This infection can occasionally cause abortion in late gestation. Bulls may carry the bacteria in their sheaths.

The brain infection generally affects cattle 1 to 3 years of age and is most common in feedlot cattle--especially weanlings and yearlings. Stress associated with weaning, shipping, handling or breeding may lead to outbreaks. Often the first sign is a dead animal, or a down animal unable to get up.

If a clot in the brain is large the animal may die suddenly of a massive stroke. More commonly, many small clots lodge throughout the brain, causing multiple signs--depending on which part of the brain is most damaged. Common signs are one-sided circling or animals lying down with head turned around toward one side.

If you find the animal before it dies, early signs include loss of appetite, fever of 105 to 107 degrees (though temperature often drops toward normal as the condition progresses), depression, lameness and incoordination, reluctance to move, stiffness and knuckling of the fetlock joints.
The head is often held up with nose extended, or tilted, with rolling eyes. The animal may walk in circles or seem blind in one or both eyes. Once it goes down it may have muscle tremors and make paddling movements with the legs.

Death usually occurs in a few hours after the animal goes down. Examination of the brain (to find the lesions created by clots) may be necessary for diagnosis, since symptoms may be confused with listeriosis.

The differential diagnosis includes other neurological diseases of cattle including polioencephalomalacia, lead poisoning, salt poisoning, listeriosis, bovine herpesvirus encephalomyelitis, and rabies.


  • Fever
  • Rapid breathing
  • Lethargy
  • Weakness
  • Lameness
  • Neurologic signs


This complex of diseases is caused by a gram negative coccobacillus (oval shaped bacterial cell) formerly called Hemophilus somnus.  

These bacteria thrive best in an environment containing carbon dioxide, are commonly found in the respiratory tract and probably enter with air breathed in by the animal. They inhabit certain mucosal surfaces, including the upper airway and reproductive tract.
Histophilus somni is a member of the Pasteurellaceae family and was first recognized as a pathogen in cattle in 1956. H. somni affects cattle worldwide, but the greatest prevalence of disease is in major beef-producing countries of the world, such as the U.S. and Canada. Infection with H.somni is most commonly found in feedlot cattle, but may be seen in dairy and grazing operations.  

Young growing cattle age 6 to 12 months are most commonly infected and show clinical signs. Almost all cattle are exposed at some point in their life; in some herds 100% of the animals have circulating antibodies, but actual clinical disease is uncommon, with an incidence rate of 2% or lower.  

Clinical disease can be devastating when it occurs, however, since 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 (inflammation of the heart muscle), polysynovitis (inflammation of the membranes lining multiple joint capsules), inflammation of the middle ear, mastitis, and reproductive tract diseases.

In past years, the most common disease manifestation was TME but in recent years, respiratory disease and myocarditis have become more prevalent.

Any form of H. somni disease can cause another form or contribute to the success of other invaders unless treated early.

The many forms of H. somni can include myocarditis (with signs similar to BRD or congestive heart failure, which may cause sudden death), pericarditis and pleuritis (which also may have as signs similar to BRD and may be associated with pulmonary edema), polyarthritis in one or both stifle joints (and may involve other joints, as well), reproductive failure (death of the fetus at any stage of gestation, with subsequent abortion and uterine infection) and septicemia, with clinical signs determined by where the organism attaches, colonizes and blocks blood flow.

H. somni is an opportunistic pathogen. These bacteria require a breakdown in mucosal immunity in order to cause disease. Many types of events can compromise the immunity of a calf, including stress from transport, concurrent viral infection, inclement weather, weaning, etc.

The development of TME involves a bacterial septicemia. Once the bacteria enter the bloodstream, they can evade host defenses and cause death of the cells lining the blood vessels. This, in turn, causes inflammation of the blood vessels and formation of blood clots. In the brain, this leads to tissue death and TME. In the heart it can cause myocarditis with tissue death and abscess formation.

Bovine thrombotic meningoencephalitis (TEM), formerly known as thromboembolic meningoencephalitis (TEME), is a neurological disease, primarily of feedlot cattle between 6 to 12 months of age. Calves are infected by carrier cows early in life and later spread the infection at the feedlot.

The organism probably gains entry to the body through the respiratory tract before moving into the nervous system. Onset of disease can range from acute death without much sign of illness, to chronic disease.

The neurological form of infection is characterized by fever, depression, ataxia (incoordination), and recumbency. Opisthotonus (muscle spasms causing backward arching of the neck), rapid involuntary eye movements, eyes not aligning with one another, blindness, increased sensitivity to sights and sounds, and convulsions may also be seen.

Signs usually develop in the first 3 to 4 weeks after arrival at the feed yard.

Thrombotic meningoencephalitis (TME) causes neurological signs such as knuckling of hind limbs, walking in circles, incoordination, hyperextension of the neck, and coma, as well as sudden death with no clinical signs.

Histophilus somni is a versatile bacterium associated with sudden death and involvement in various clinical conditions, and can also open the way for other diseases such as respiratory pathogens or clostridial diseases.

When H. somni colonizes organs or tissues, it can form blood clots that block oxygenation to those areas, creating anaerobic conditions that are ideal for proliferation of clostridial organisms. Cause of death in these cases is often attributed to a clostridial infection, yet H. somni infection may have set up the conditions for the clostridia to proliferate.


Prevention of the H. somni disease complex can be difficult due to presence of the organism in the environment. Most vaccines are labeled for protection against TME only, and not the other forms of this disease. If calves are vaccinated it should be done prior to entry into the feedlot.

Calves should be vaccinated at about 4 months of age, with an annual booster for feedlot animals. Two vaccinations are required, with at least a 3 week interval between doses. Vaccine is ineffective during an outbreak; it must be given before animals are exposed.  

Other ways to help prevent infection include minimizing stress, dust and overcrowding, careful cleaning of water troughs, feed bunks, handling facilities, trucks and trailers used for hauling cattle, and not mixing groups of cattle.

Since a high percentage of cattle are carriers of this pathogen, the less mingling of cattle or adding new ones to a group, the better. A good vaccination program to protect cattle against other respiratory diseases can help prevent secondary invasion of the lungs by opportunistic H. somni infection.

Metaphylaxis has also been used to prevent these infections. 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 designed to protect them against infection in the early, most stressful time at the feedlot. Prevention is always better than having to treat the disease, since fatal outcomes are common, particularly due to BRD in feedyards.


Treatment of affected cattle involves supportive care and use of antibiotics that will cross the blood-brain barrier. Treatment is more successful if the animal is still in early stages of the disease process, especially for brain lesions. The animal usually responds to high levels of antibiotics such as oxytetracycline, which should be administered intravenously to get high blood levels quickly. For brain infections, use an antibiotic that will cross the blood-brain barrier in high concentrations.   
H. somni is susceptible to a wide variety of antibiotics. Oxytetracycline has been effective in treatment of TME when given at 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 utilize a variety of antibiotics. These bacteria are susceptible to many of the antibiotics commonly used to treat bovine pneumonia, including some of the newer, longer-acting medications.

Early, aggressive treatment usually results in rapid recovery of acutely ill animals.  Dramatic recovery of animals that were near death is typical of this infection, but repeat treatments are usually needed.

Since it is difficult to find these animals early, however, 50% recovery rate is considered good. Animals with prolonged pneumonia may be slow to recover, but unlike cattle with pasteurella pneumonia, recovery is usually complete.

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EquiMed Staff

EquiMed staff writers team up to provide articles that require periodic updates based on evolving methods of equine healthcare. Compendia articles, core healthcare topics and more are written and updated as a group effort. Our review process includes an important veterinarian review, helping to assure the content is consistent with the latest understanding from a medical professional.