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Buying Feeders? How “Histophilus somni” or “Somnus” is Changing the Game
Ohio Ag Connection - 02/09/2024

Most KY-born calves leave the farm and enter marketing channels, usually through auction markets, into stocker and backgrounding operations. Not surprisingly, late fall and winter are difficult seasons to keep feeder calves alive in KY due to major health challenges. Weather is just one of many risk factors that play a role in Bovine Respiratory Disease (BRD) or “Shipping Fever” development. Most auction market calves are sold as “high risk calves”, meaning they are lightweight (≤ 500#), young (estimated 6-8 months), unweaned (or abruptly weaned on the trailer on the way to the yards), unknown health history, never or poorly vaccinated and most are trace mineral (copper and selenium) deficient. At the auction barn, they are mixed or “commingled” with similar weight calves from multiple farms then sold, allowing respiratory “bugs” to spread prior to delivery to the stocker/backgrounder facility or feedlot. After arrival and a brief rest period, these calves are usually processed through the chute and receive multiple vaccines, deworming, are implanted and the bulls are castrated. These calves will typically break with respiratory disease within the first 2 weeks after arrival and require at least one antibiotic treatment. It is estimated that 60-70% of calves marketed through sale barns are considered at high risk for disease.

Over the last few years, the bacterium Histophilus somni (formerly known as Haemophilus somnus) has emerged as the major bacterial pathogen responsible for the rapid development of disease and death in feeder operations. While Mannheimia haemolytica, often referred to as “Pasteurella”, has traditionally been the most important bacterial species in “shipping fever” bronchopneumonia, Histophilus somni (HS) can cause similar disease symptoms but is proving very difficult to treat and control with traditional methods. Unlike typical BRD outbreaks that peak at 14 days after arrival to the stocker or backgrounding facility, “histophilosis” cases start at 3-4 weeks on feed. “Somnus” is normal flora in the upper respiratory tract and survives on mucosal surfaces in “biofilms”, a jelly-like matrix that serves as protection from antibiotics and the host immune system.

The Histophilus somni Disease Complex (HSDC) is a term used to describe the disease when “Somnus” reaches the lungs and extends systemically to the brain, heart and joints. Components of the HSDC include:

Rapidly Fatal Pneumonia: Some animals with “Somnus” pneumonia are simply found dead due to lung damage called “severe fibrinous pleuritis”. This is a common finding in western Canadian feedlots and is seen 30-90 days after arrival. The lungs are usually the only organ affected and are found covered in a thick sheet of fibrin, a tough protein substance composed of long fibrous threads, while the lungs themselves are collapsed underneath.

Bronchopneumonia: Some calves with “Somnus” develop typical signs of BRD including depression, off feed, cough, excessive nasal discharge and difficult or rapid breathing. Respiratory signs with fever of 104°F or above confirms the diagnosis of BRD but detection of the bacteria and/or viruses involved must be done at a diagnostic laboratory. Treatment is often very difficult and unrewarding in the field when “Somnus” is involved because it can hide within neutrophils and macrophages that normally destroy bacteria. In the laboratory, the “Somnus” bacterium is usually susceptible to many antibiotics. Heart Muscle damage: “Somnus” can cause a “necrotizing myocarditis” when it localizes in the muscles of the left ventricle of the heart. Death can be rapid with no previous signs (similar to a heart attack) or chronic heart failure leads to “poor doers”.

Thrombotic meningoencephalitis-myelitis or “TME”: This is a disease of older calves and yearlings that affects the brain. During an outbreak, individual cases occur sporadically in separate pens in a feed yard. Signs include depression, fever, blindness, down and death; this can look like signs seen with polioencephalomalacia or “brainers”. Treatment with antibiotics is most often ineffective.

Other Manifestations: Arthritis with joint swelling, laryngitis and middle ear infections with drainage from the ear canals may also be seen, similar to Mycoplasma bovis infections.

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