Bovine Respiratory Disease continues to plague producers, remains insidious in herds
Bovine Respiratory Disease, more commonly referred to as BRD, is the most costly disease of beef cattle in North America, states University of Nebraska-Lincoln (UNL) Assistant Professor J. Dustin Loy.
“Some studies attribute 70 percent of cattle mortality to BRD, while others show 57 percent. That’s a significant contributor of mortality in feedyards,” comments Loy. “Other studies show 12.5 deaths per 1,000 head of cattle. BRD can also be insidious, which is where a lot of these BRD cases are getting missed.”
Loy further mentions studies show 68 percent of untreated steers in a multi-year survey had evidence of BRD at slaughter. However, there aren’t any polls of treatment on those cases.
While costs have increased, Loy cites a 2002 study showing the cost for one treatment of BRD was $40.64 per calf. If three or more treatments were needed for the calf, the treatment cost was a minimum of $291.93.
“BRD is a multifactorial disease, meaning it needs a host, pathogen and a suitable environment to contribute to a disease outbreak,” explains Loy.
Loy notes the host needs to be susceptible to the pathogen, which is usually achieved by an immune suppression from shipping and then subsequent co-mingling at feedyards. Another thing to consider with incoming calves is making sure they are vaccinated against BRD prior to arriving at the feedyard.
“People need to think about herd immunity,” recommends Loy. “Feedyards have to determine if a group of preconditioned calves has a high level of antibodies for the respiratory viruses or if they are naïve to them.”
Loy further warns, “When calves are persistently infected, they continuously shed their virus, expose their pen mates and put them at a high risk for BRD.”
The first factor of BRD is generally associated with a virus, which is then followed by bacteria, notes Loy.
“Viruses are metabolic parasites that cause cell death or dysfunction in the lungs of calves, compromising their tissue, which may lead to death or malfunction of their immune cells,” he states.
The cilia in an animal’s trachea reduce the ability of bacteria to colonize inside their lungs. Once bacteria are able to colonize, they multiply rapidly and secrete toxins that compromise the lung tissue further.
Within 24 hours, the infection can go from one organism to a million, or sometimes even a billion, organisms.
Viral infections usually take two to three days before an animal to start showing sickness and fever, while bacterial infections take three to six days.
“The main culprit for BRD seen in our diagnostic center is Mannheimia haemolytica,” described Loy. “Patients who contract this start to get a fibrous bronchopneumonia and acquire exudate on top of their lungs to try and deal with the infection.”
If producers are using a Pasteurella vaccine, they need to make sure they have a leukotoxin and cell antigen component to help reduce the severity of the disease, notes Loy.
The second culprit Loy named as a big contributor for BRD is Pasteurella multocida, a bacteria that is opportunistic anywhere there is compromised tissue and strives to cause a chronic infection. It has also been isolated and seen in several chronic pneumonias.
To treat these bacteria, antibiotics or antimicrobials are needed. Antibiotics are generally made of natural or synthetic chemicals. Antimicrobials can be bacteriostatic, which means they are going to inhibit further growth but are unable to kill the bacteria already present.
“These drugs mainly weaken the bacterial cell wall and pre-expose the bacteria to environmental stresses, such as salts and different ions, which allows them to be more susceptible to the host’s immune system and environment, so they may be killed,” explains Loy.
The drugs Penicillin, Ceftiofur, Naxcel, Excede and Excenel work in this fashion against BRD.
The class of drugs called tetracyclines can also help to treat BRD by inhibiting the protein synthesis of the virus, as well as binding to the bacteria’s cell mechanisms that synthesizes the cell wall.
These drugs are broad spectrum and can be placed in an animal’s feed or are injected into the animal.
Drugs in this group used to treat BRD are Chlortetracycline, Oxytetracycline, Aureomycin and LA200.
The newest group of drugs for the treatment of BRD is macrolides, and they inactivate key mechanisms inside the bacterial cell. Drugs of this class are Micotil, Draxxin, Zactran and Zuprevo.
“Bacteria have been multiplying and reproducing for millions of years, so it’s not necessarily the invention of these new drugs or therapies that we have that are causing this antibiotic resistance,” states Loy.
Bacteria are constantly looking for competitive advantages by sampling the environment to try and find genes or mechanisms they can exploit to give them a competitive advantage, mentions Loy.
“The antimicrobial is merely exerting a selective pressure to evolve,” he adds. “A lot of times this comes from exposure to insufficient concentrations of antimicrobials at the site of infection.”