Haemophilus somnus has long been recognized by veterinary researchers, diagnosticians, and practitioners as a major cause of death in feedlot calves due to encephalitis (brain infection). In the past five years, H. somnus has also been found to be a major cause of pneumonia, arthritis, myocarditis, and reproductive problems. H. somnus infections occur most commonly in stress situations such as when cattle are grouped together in sale barns, feedlots, dairy herds, for shipping, etc., or when assembled for grazing fall and winter pasture.
The disease usually occurs one week to one month after cattle are grouped together. During an outbreak, some cattle will show signs of encephalitis, including blindness, staggering, and convulsions. Without prompt treatment with antibiotics, most of these animals will die. The brain damage caused by H. somnus is irreversible, so even if the animal is treated and survives it may have to be culled.
Concurrent with the encephalitis outbreak, other cattle will develop pneumonia. This pneumonia is indistinguishable from viral and Pasteurella shipping fever complex pneumonia, thus H. somnus pneumonia can be easily misdiagnosed.
H. somnus infection is usually a systemic disease. The organism causes disease by blocking capillaries, thereby restricting the blood flow to vital organs. While the most common manifestations of H. somnus are encephalitis and pneumonia, all organs of the animal’s body can be affected. Even if the animal is treated with antibiotics and recovers, damage due to loss of blood supply to various body organs can cause chronic, debilitating disease problems. Common manifestations of this damage include arthritis, myocarditis, abortion, and sterility.