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Rapid Death in Feeder Calves? It May be Histophilus Somni

By  Dr. Michelle Arnold

October and November are known as two of the most difficult months to buy feeder calves in KY due to major health challenges. Weather is often blamed but is just one of many risk factors that play a role in Bovine Respiratory Disease (BRD) development. This fall, the bacterium Histophilus somni (formerly known as Haemophilus somnus) has emerged as a major bacterial pathogen responsible for the rapid development of disease and death in feeder operations. While Mannheimia haemolytica, commonly called “Pasteurella”, is the bacterial species known to cause bronchopneumonia and rapid death, Histophilus somni (HS) can cause similar symptoms and is proving very difficult to treat and control with traditional methods. In addition, a more severe form of disease, known as the “septicemic form” of Histophilus somni has been seen in several cases submitted to the UKVDL over the last month. This septicemic form typically hits calves 30-60 days after arrival and the bacteria may affect the brain, heart, larynx, muscles, joints, liver and kidney in addition to the lungs. In some cases, the calves are simply found dead with no clinical signs. The septicemic form usually results in rapid death, generally less than 48 hours from development of symptoms up to the time of death. The joints are frequently involved, and calves may become noticeably lame or wobbly and the joints may become extremely swollen, like Mycoplasma bovis joint involvement. On necropsy, several of these cases have had laryngitis and edema (swelling) in the back of the throat because the bacteria have attacked the laryngeal muscles. The HS 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), laryngitis, 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 Northwestern 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 mortality. 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 protection is by no means complete from these products.

The common BRD-associated bacteria (Mannheimia haemolytica, Pasteurella multocida, and Histophilus somni) are found 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 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 tends to occur 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:

  1. 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.
  2. 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 usually susceptible to many antibiotics.
  3. 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 (similar to a heart attack) or chronic forms lead to “poor doers”.
  4. 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 with antibiotics is most often unrewarding.
  5. Other Manifestations: Arthritis with joint swelling, laryngitis and 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 necessarily tell the story of what is causing disease in the lungs and elsewhere. The HS organism can be difficult to grow in the laboratory, especially if the calf was treated with antibiotics, so the PCR assay is strongly recommended in addition to culture 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 may be missed when relying on traditional bacterial culture methods alone. Culture is still important to identify what antibiotics should be effective. Unfortunately, the transfer of genetic elements that caused antimicrobial resistance to develop in Mannheimia haemolytica is also thought to occur with HS.

Prevention is difficult. This is not a disease complex managed through a needle. What we do in the first 14 days after arrival is crucial because we can’t reverse the damage done by this disease. Biosecurity and biocontainment practices must be followed. Because HS pneumonia and septicemia often develops along with viral infections (especially BVD) or in combination with other bacterial pathogens, control should begin with minimizing 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, but they may help lessen the severity. 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:

  1. Vaccination with a modified live (MLV) 5-way respiratory virus vaccine and a Mannheimia haemolytica toxoid (“Pasteurella”) after calves have rested 24 hours. Because vaccines usually cause calves to have a slight fever, research has shown the MLV vaccine may be delayed 2-3 weeks in high risk calves without affecting the morbidity and mortality rate.
  2. 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 one option to boost the copper and selenium levels during the initial arrival period.
  3. Metaphylactic treatment (treatment of all calves on arrival or when 10-20% show signs of BRD) of high risk calves (high risk = lightweight, weaned on the trailer, green 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 longacting antibiotics can lead to delayed follow-up treatment if calves are not adequately monitored. In other words, calves still need to be watched very closely in spite of having an antibiotic on board.
  4. Feed bunks and watering troughs are known areas for disease transmission. Keep sick cattle, especially chronic pneumonia calves, away from healthy calves and manage their feed and water separately. Do not allow nose-to-nose contact between sick and healthy groups.
  5. Do not pen new arrivals next to calves that were purchased last week! Try to load your farm with calves as quickly as possible rather than buy a load every week for 4 weeks and combine them in same area. This is a sure-fire recipe for disaster.
  6. Management is key. In a recent article by Chad Engle from the US Meat Animal Research Center, he wrote, “I like to think of our feedyards as five-star hotels. Once these calves step into our “hotel”, they should be greeted by knowledgeable handlers, fresh feed, clean waterers and clean pens. We never put new calves into pens that do not have fresh hay and ration in the feed bunks. It is our job in the feedyard to show those calves that the feedyard is the best place on earth for them to be.” Enough said.
  7. Good sanitation, especially regularly cleaning and sanitizing waterers, feed bunks and working chutes is imperative.
  8. Buying preconditioned calves that have been weaned and vaccinated for respiratory diseases prior to weaning (especially BVD) and dewormed will help decrease, but does not eliminate, sickness and death loss.

In summary, the disease syndrome caused by Histophilus somni may have many different clinical signs besides the typical pneumonia case. Disease onset is rapid and 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 efficacy and development of effective vaccines for prevention of disease. In the meantime, make your feedyard a five-star hotel for calves to prevent, rather than treat disease.

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