By Michelle Arnold
In this era of advanced vaccine technology and long-acting, expensive, powerful antibiotics, why do cases of Bovine Respiratory Disease (BRD) continue to increase? One reason is the re-emergence of Histophilus somni (formerly known as Haemophilus somnus) as a major bacterial pathogen responsible for the development of pneumonia in feeder operations. While Mannheimia haemolytica is the bacteria known to cause the dramatic pneumonia signs of fever, depression, appetite loss and rapid death, Histophilus somni (HS) can cause similar symptoms and is proving very difficult to treat and control with traditional methods. The organism is often found in combination with Pasteurella multocida or other BRD bacteria in “biofilms” which are clusters of bacteria in a matrix that serves as protection from antibiotics and host immune system responses. Stress can trigger dispersal of large numbers bacteria from the biofilm that can then invade the lower respiratory system. Once it establishes infection in the lungs, it can travel in the bloodstream to joints, organs (especially the heart), and to the brain. These calves may develop pneumonia, pleuritis (infection of the membrane surrounding the lungs), myocarditis (infection in the heart muscle), thrombotic meningoencephalitis (infection in the brain), tenosynovitis (infection within joints), and otitis media (middle ear infection). The disease can happen anytime in the year but most clinical cases occur between October and January. Previously, disease due to HS or “histophilosis” was associated with primarily Western and Midwestern states in the USA and Canada but cases are now being diagnosed throughout the US, including Kentucky. Most comprehensive studies have been conducted in Canada where HS accounts for an estimated 40% of the death loss in feedlots. Unlike typical BRD outbreaks that peak at 14 days after arrival to the feedlot, HS acute pneumonia cases peak at 25 days on feed. In the absence of consistently effective treatment or vaccine options, management practices are crucial to controlling Histophilus pneumonia. Metaphylaxis, where treatment is applied to the whole group (either on arrival or once 10-20 % of the calves are showing clinical signs of BRD), along with prompt individual treatment of sick cattle is one recommended control approach. Vaccines are available against HS but their ability to prevent disease has not been proven.
The common BRD-associated bacteria (Mannheimia haemolytica, Pasteurella multocida, Histophilus somni and Mycoplasma bovis) are considered normal bacteria in the nasal passages of healthy calves but with stress (such as transportation and commingling) and viral infection, they can descend into the lungs and sometimes spread throughout the body, causing disease. This simple disease model is now under scrutiny with the realization that multiple factors contribute to combinations and complex interactions between the environment, the bacteria and viruses, and the calf’s immune system. Stressed cattle are more susceptible to the viral components of BRD, including Infectious Bovine Rhinotracheitis (IBR), Bovine Viral Diarrhea virus (BVD), Parainfluenza 3 virus (PI3), Bovine Respiratory Syncytial virus (BRSV) and possibly another agent commonly found, Bovine Coronavirus (BCV). Viruses are known to damage the lining of the respiratory tract and some will actually suppress the immune system, allowing secondary bacterial infection. Once established in the lung, the bacterial components are responsible for the inflammation and bronchopneumonia signs seen with BRD. Combinations of different bacteria can work synergistically to cause more severe disease than if operating alone. There are no early clinical signs that indicate Histophilus is part or all of the disease problem, other than it occurs later after arrival than most cases of BRD. The Histophilus somni Disease Complex (HSDC) is a term used to describe the respiratory disease when HS reaches the lungs and the complications from “septicemia”, when HS leaves the lungs and lands in the brain, heart and joints. Components of the HSDC include:
- Rapidly Fatal Pneumonia: Some animals with HS pneumonia are simply found dead due to a rapidly fatal type of damage to the lung called “severe fibrinous pleuritis”. Currently, this is the most common manifestation of HS in western Canadian feedlots and is seen 30-90 days after arrival. Rarely are abnormalities found in any other organ besides the lungs which are covered in a thick sheet of fibrin and the lungs are often remarkably collapsed underneath.
- Bronchopneumonia: Some calves with HS develop typical signs of BRD including depression, separation from the group, 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 although in the laboratory, the bacteria is susceptible to many antibiotics.
- Heart Muscle damage: HS 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 (a heart attack) or chronic forms lead to “poor doers”.
- TME: Thrombotic meningoencephalitis-myelitis or “TME” is a disease of older calves and yearlings affecting the brain. During an outbreak, individual cases occur sporadically in separate pens in a feed yard. Signs include depression, fever, blindness, coma and death; this can look similar to signs seen with polioencephalomalacia or “brainers”. Treatment is most often unrewarding.
- Other Manifestations: Arthritis with joint swelling may follow episodes of pneumonia. Middle ear infections with drainage from the ear canals may also be seen.
Diagnosis of Histophilus somni pneumonia in a live calf is challenging because it is present in the upper airways of both healthy and diseased calves so swabs taken from deep in the nose will not tell the story of what is actually happening in the lungs. Only lung fluid removed from the lung via bronchoalveolar lavage can be used for culture or molecular assays such as PCR to identify the organism. The organism is very difficult to grow in the laboratory, especially if the calf was treated with antibiotics, so PCR is strongly recommended for detection. Mixed lung infections with other disease- causing bacteria such as Mannheimia haemolytica and Pasteurella multocida can easily overgrow Histophilus somni on a culture plate so the identification is easily missed when relying on traditional bacterial culture methods alone.
Prevention depends on sound biosecurity and biocontainment practices. Because HS pneumonia or septicemia may develop secondary to viral infections (especially BVD) or in combination with other bacterial pathogens, control should begin with minimizing the effect of well-recognized factors predisposing to BRD. Stress plays a major role in disease through immunosuppression so excellent nutrition, sound vaccination protocols and management are critical. There are numerous challenges to production of an effective vaccine to prevent Histophilus somni infections. In spite of all the considerable amount of work on bacterin-based HS vaccines, there is no direct proof that these vaccines are effective under field conditions. Consult with your local veterinarian for the best management plan for your operation.
The following steps should reduce the risk of pneumonia in feeder calves:
- Vaccination with a 5-way respiratory virus vaccine and a Mannheimia haemolytica toxoid. Use of an intranasal vaccine for the viral fraction is recommended in high risk cattle to keep appetite up and reduce vaccine-induced fever.
- Cattle with extremely low blood concentrations of the trace elements selenium and copper have difficulty fighting any disease challenge. An injectable trace mineral supplement (such as Multimin 90) is often needed to boost the copper and selenium levels during the initial arrival period.
- Metaphylactic treatment (treatment of all calves on arrival) of high risk calves with long-acting antibiotics is a well-established, beneficial procedure that can reduce morbidity (sickness) and case fatality (death) by up to 50% in high risk calves. Be aware that overconfidence in and reliance on metaphylaxis and long-acting antibiotics can lead to delayed follow-up treatment if calves are not adequately monitored.
- Feed bunks and watering troughs are known areas for disease transmission. Keep sick cattle, especially chronic pneumonia calves, away from healthy calves and manage the feed and water separately.
- Eliminating exposure by reducing unnecessary traffic through the farm, isolating new arrivals, good sanitation and treating sick cattle early will help reduce the risk of disease spread.
- Buying preconditioned calves that have been weaned and vaccinated for respiratory diseases prior to weaning (especially BVD) and dewormed will help minimize sickness and death loss.
In summary, the clinical features of disease caused by Histophilus somni are often confused with other diseases commonly seen in the feedlot. Response to antibiotic therapy is frequently very poor due to biofilm protection. Diagnosis in a live calf can be difficult since H. somni is often recovered from nasal swabs of healthy calves so the presence of the bacteria in the nose of a sick calf does not prove it is causing disease. It is also a difficult bacterium to grow in the laboratory, especially if the calf has been recently treated with antibiotics. Future research will focus on the relationship between HS and other respiratory pathogens, developing improved diagnostic tests, monitoring antibiotic effectiveness and development of new vaccines for prevention of disease.