Bovine Respiratory Disease (BRD) is the most common and financially damaging illness affecting the North American cattle industry. This disease complex causes significant illness and death, particularly in young, recently weaned, or transported animals. Because of its association with the stress of transport and commingling, BRD is also known as “Shipping Fever.”
Defining Bovine Respiratory Disease
BRD is not a single infection but a multi-factorial syndrome. It results from the simultaneous presence of three main factors: environmental or management stress, a primary viral infection, and a subsequent secondary bacterial infection. This interplay compromises the animal’s natural defenses, allowing infectious agents to overwhelm the respiratory system.
The syndrome targets the upper and lower respiratory tracts, often resulting in a severe form of pneumonia known as bronchopneumonia. Viral agents initially damage the respiratory tract lining, impairing natural clearance mechanisms. This damage allows opportunistic bacteria, often already present in the upper respiratory tract, to rapidly colonize the lungs and cause severe inflammation.
Recognizing the Signs
Identifying BRD early is important for successful intervention, though cattle instinctively hide signs of illness. Early indicators include a slight fever, often exceeding \(104^\circ\text{F}\) (\(40^\circ\text{C}\)), and depression, where the animal appears lethargic. These signs are often accompanied by a diminished appetite, causing the animal to stand apart from the rest of the herd.
As the disease progresses, respiratory signs become more obvious. Initially, there may be a clear, watery nasal or ocular discharge that thickens and becomes cloudy or purulent later. Affected animals may exhibit a soft cough that progresses to a harsh, prominent “honking” cough. They will also display labored or rapid, shallow breathing, sometimes extending their neck to aid air intake.
Key Contributing Factors
The development of BRD relies on factors that compromise the animal’s immune system and introduce pathogens. These factors are divided into viral agents, bacterial agents, and environmental stressors.
Viral Agents
Several viruses initiate the BRD syndrome by damaging the respiratory lining and suppressing the immune system. The four most common viral pathogens include:
- Bovine Herpesvirus-1 (BHV-1), which causes Infectious Bovine Rhinotracheitis (IBR).
- Bovine Respiratory Syncytial Virus (BRSV).
- Parainfluenza-3 (PI3) virus.
- Bovine Viral Diarrhea (BVD) virus, which causes profound immunosuppression.
These viruses disrupt the protective mucociliary escalator, allowing bacteria to migrate into the lung tissue.
Bacterial Agents
The most severe damage and the majority of BRD mortality are attributed to secondary bacterial infections. These bacteria typically reside harmlessly in the upper respiratory tract until the immune system is weakened by stress and viral insult. Mannheimia haemolytica is the most aggressive and frequently isolated pathogen in severe BRD cases.
Other bacterial culprits include Pasteurella multocida and Histophilus somni, which are members of the Pasteurellaceae family. Mycoplasma bovis is also a factor, sometimes causing a more chronic form of pneumonia. These agents multiply rapidly in the compromised lungs, leading to the inflammation characteristic of pneumonia.
Stress Factors
Environmental and management factors act as stressors that suppress the animal’s immune response. Weaning is a stressor, involving separation from the mother, a change in diet, and social restructuring. Transportation, which can involve long periods without feed and water, is a major trigger, leading to the nickname “Shipping Fever.”
Other stressors include commingling cattle from different sources, which increases pathogen exposure, and adverse environmental conditions. Poor ventilation, extreme weather fluctuations, and routine procedures like dehorning or castration all contribute to a weakened immune state. Minimizing these burdens is an important step in BRD prevention.
Management and Prevention Strategies
Management and prevention strategies aim to reduce disease incidence by targeting both stress and infectious components. Programs focus on preparing the animal’s immune system before high-risk periods. This requires an approach developed in consultation with a veterinarian.
Vaccination Programs
Vaccination is a foundational component of BRD prevention to prepare the immune system against common pathogens. Calves benefit from preconditioning, which involves vaccinating and weaning them before they are shipped to a feedlot. Vaccines should target the primary viral pathogens (IBR, BRSV, PI3, and BVD) and key bacterial agents (Mannheimia haemolytica and Pasteurella multocida).
In young calves, vaccination timing is influenced by maternal antibodies from colostrum, which can interfere with injected vaccines. Veterinarians may recommend intranasal vaccines, which stimulate a localized immune response faster and are not blocked by maternal antibodies. Ensuring calves receive adequate, high-quality colostrum immediately after birth is the foundational step in long-term immunity.
Husbandry and Environment
Management practices should focus on minimizing stress and maintaining a clean, low-pathogen environment. Low-stress handling techniques during processing and movement help keep the immune system functioning optimally. Providing clean, dry bedding and ensuring proper air exchange reduces airborne pathogens and environmental irritants.
Reducing the commingling of cattle from diverse sources limits the spread of infectious agents. If mixing is unavoidable, minimizing transport duration and providing rest stops with feed and water are beneficial. Preconditioning calves also involves performing surgical procedures, like castration and dehorning, at least 30 days before transport, reducing simultaneous stressors.
Nutritional Support
A balanced diet ensures the animal’s immune system has the resources to mount a defense. Nutritional programs should ensure adequate intake of essential vitamins and trace minerals, as deficiencies can impair immune function. Providing high-quality feed and clean water, especially upon arrival at a new facility, helps cattle acclimate and minimizes nutritional stress.
Treatment Protocols
Once an animal shows clinical signs of BRD, immediate intervention is necessary to improve recovery and limit infection spread. Early detection is the most important factor in achieving a positive outcome. Treatment protocols primarily aim to eliminate the bacterial component of the syndrome.
Antimicrobial Therapy
The main treatment for BRD involves broad-spectrum antimicrobial agents to target the severe bacterial pneumonia. A veterinarian must diagnose the disease and prescribe the appropriate antibiotic, often choosing from classes like macrolides or florfenicols. Drug selection is based on disease severity and the history of antibiotic use on the farm to mitigate antimicrobial resistance.
Supportive Care
Antimicrobial therapy is paired with supportive care to manage the systemic effects of the disease. Non-steroidal anti-inflammatory drugs (NSAIDs) are administered alongside antibiotics to reduce fever and alleviate pain and inflammation. Reducing fever is important because it can quickly lead to dehydration and a depressed appetite. Supportive care also involves ensuring the sick animal has access to fresh feed and water, as they are often reluctant to move.
Isolation
Isolating sick animals is a fundamental biosecurity measure to prevent pathogen transmission to healthy herd mates. Animals undergoing treatment should be moved to a designated hospital pen that is clean, dry, and provides shelter from adverse weather. This separation allows for closer monitoring of the animal’s response to treatment and ensures they receive necessary individual care.