Bovine Respiratory Disease (“BRD”) or “shipping fever”, also known as bronchopneumonia, continues to be the most common cause of illness and death in postweaned (stocker) calves despite significant improvements in the vaccines and antibiotics available today. Traditionally, disease prevention through vaccination was thought to be the answer to improving stocker health outcomes but the current vaccination recommendations are not meeting the challenge as morbidity and mortality rates continue to rise. There is an increasing amount of research focusing on the importance of the normal, healthy “microbiota” (bacterial population) in the upper respiratory tract to maintain calf health and improve immunity. This normal microbial population modulates, or controls, host immune defenses through several mechanisms including 1) competition with the pathogenic organisms (the bad bugs) for nutrients, 2) through production of compounds toxic to the pathogens, 3) through recruitment of white blood cells to defend the lung tissue, and 4) by stimulating antibody production, specifically IgA, to protect the mucosal surface of the respiratory tract. However, cattle diagnosed with BRD have a significantly disrupted microbiota that instead allows pathogenic bacteria to flourish. Examining ways to preserve the normal microbiota while stimulating the immune system is the new frontier currently under exploration to reduce sickness, death loss and antimicrobial use, especially in the stocker calf sector. Is it time to limit the management procedures and treatments that have a profound influence on the respiratory microbiota to better the health of high-risk stocker calves? The stocker industry is vital to the economic success of cow/calf operations in KY. Calves marketed off the farm through the sale barn are generally in no way, shape, or form ready to enter feed lots to be fed to slaughter weight. These calves often arrive to the yards in small groups, sometimes 10 or fewer calves, that were weaned on the trailer on the way to the sale. Many calves are lightweight (<400#) and in poor nutritional and trace mineral status, unvaccinated, males are intact bulls, and a portion of the heifer calves are pregnant. On arrival at the yards, the calves are commingled with calves from multiple sources, most with unknown vaccination and deworming history, then are weighed, sold and eventually transported to a stocker or backgrounder operation. Either before leaving the yards or on arrival at the stocker facility, calves are administered a modified-live (MLV) respiratory vaccine, a blackleg vaccine, dewormed, and implanted. Additionally, calves commonly receive a long-acting antibiotic to prevent bronchopneumonia, a practice known as “metaphylaxis”. The time it takes to move through this critical transition period, from when the calf leaves the farm of origin to arrival at the stocker/backgrounder operation, can vary greatly depending on how long they stand at the yards before and after the sale and the distances they are transported. The longer calves stand without rest and with limited access to feed and clean water, the more likely they will arrive at their new facility dehydrated and in a negative energy balance. These stocker procurement decisions to buy unweaned, unvaccinated, intact calves at the sale and then add stress through weaning, transport, commingling and diet change, administration of MLV vaccines and metaphylaxis, profoundly change the normal respiratory microbiota (Figure 1) and set the stage perfectly for shipping fever pneumonia.
We have known for a long time that “stress” weakens the calf immune system, allowing viruses to invade and compromise lung defenses, enabling bacterial pathogens to reach areas deep within the lungs and initiate bronchopneumonia. Stress also negatively affects vaccine effectiveness in the field. But is all stress the same? “Acute” or short-term stress that lasts 24 hours or less affects the immune system differently than chronic stress that may last for days up to weeks. A good example of acute stress is vaccinating calves prior to weaning as part of a preconditioning program. Calves experience a short term stress from being worked through the chute and given the MLV vaccine, but the immune system responds as it should to the challenge and there is adequate time for protection to develop prior to weaning. Chronically stressed calves, on the other hand, administered MLV vaccine have too much immune system suppression from cortisol (the stress-induced hormone), that can result in replication and nasal shedding of vaccine virus, more symptoms of BRD and more antibiotic treatment. Killed respiratory virus vaccines do not have enough antigens to stimulate a timely protective response in stressed calves. So, what is the answer to prevent disease if vaccines cannot do it alone?
To maintain health, the focus should shift towards preservation of beneficial microbiota during the critical transition from the home farm to the stocker operation through stress reduction, nutritional management, strategic vaccine use, and limiting antibiotic therapy to only the individuals that need treatment. Stress reduction and nutritional management at the cow-calf level may be in the form of preconditioning programs that require pre-weaning vaccinations, castrations, weaning, then feeding on the farm for a 45-60 day period to prepare them for sale. At the yards, stress reduction may include allowing calves access to good grass hay and clean water while at the facility, periodically wetting the ground if dust is an issue, not overcrowding pens, regularly cleaning pens and alleyways, and facilitating quick and easy loadout after calves are sold. Strategic vaccine use may be delaying the 5-way modified live respiratory vaccination up to 21-30 days post-arrival at the stocker facility to give high-risk cattle the opportunity to overcome stress-induced immune dysfunction. The impact of chronic stress, dehydration, and lack of energy experienced on arrival at stocker operations has a known, profoundly negative effect on immune function, vaccine effectiveness, and overall health. Similarly, metaphylaxis (administering long-acting antibiotics to calves upon arrival at the stocker facility) disrupts the beneficial bacteria in the upper respiratory tract, resulting in greater colonization of the lungs by bacterial pathogens, namely Mannheimia haemolytica, Pasteurella multocida, Histophilus somni, and Mycoplasma bovis, as well as increasing resistance in those bad bugs to antibiotic therapy. Limiting treatment to calves showing signs of BRD (depression, off-feed, fever > 104 degrees, increased respiratory rate) will preserve the efficacy of antibiotics when they are truly needed for survival.
The importance of the stocker/backgrounder sector to the vitality of the KY cow/calf industry cannot be overstated. These operations provide western feedlots a year-round, steady supply of calves ready to be fed to slaughter weight. Acknowledging the importance of preserving the normal microbiota through stress reduction, better nutritional management, strategic vaccine use, and limited antibiotic use throughout the transition from farm to stocker facility could significantly reduce sickness, death loss and antimicrobial use in KY calves. Talk with your veterinarian about where your operation fits in the beef cattle production system and how you can contribute to calf health and productivity.
