Bovine Respiratory Disease

Bovine Respiratory Disease (BRD) is the number one health issue in cattle. It is a function of the relationship between stressors, susceptibility, and pathogens in the herd. Each of these factors is important, but it is the relationship between them that determines whether an animal will be healthy or sick.

Stressors are those things that cause stress to the animal’s system. It includes things like shipping, weaning, moving, intermingling, drought, weather change, heat, cold, poor nutrition, processing, and on and on. It is all of those things that require energy to overcome.

Susceptibility refers to the animal’s immunity to a particular disease. There are many forms of immunity. Passive immunity is created by the transfer of antibodies from one individual to another such as through the placenta from the mother to the offspring or through the colostrum. Innate immunity is resistance to disease resulting from good general health. If the immune system is functioning properly it will often be able to fight off many diseases. Acquired immunity results from vaccination or from previous infection.

Pathogens are those infectious agents that cause disease or illness for its host. These infectious agents may be airborne, in the soil, or passed from animal to animal. They include bacteria, viruses, fungi, protozoa, parasites, and some proteins (prions).

Sickness occurs when the pathogen load exceeds the animal’s ability to resist. Calves may be perfectly healthy at the ranch, but when weaned and taken to the auction, the stress may cause them to succumb to pathogens they may be exposed to at the auction barn. This can occur whether they have been vaccinated or not. If the animal’s threshold of resistance falls below the level at which it can overcome the pathogen load to which it is exposed, it will become sick.

Calves that have been weaned and pre-conditioned prior to shipment typically have a higher threshold of resistance than those which are shipped immediately after weaning. They are usually de-wormed and vaccinated against common pathogens in order to boost the antibody levels in their blood. They also have had time to overcome the stress of weaning. These calves will typically experience fewer problems in the stocker phase or at the feedlot.

Generally, unless exposed to extreme pathogen loads, pre-conditioned cattle perform well with little additional treatment. However, there continues to be risk of sickness even for those animals that have been handled properly. Recall that sickness occurs any time the threshold of resistance is lower than the pathogen load. This can occur because of a large number of stressors causing the resistance threshold to fall, or because the pathogen load is high. The more frequently cattle are handled, shipped, or co-mingled – exposed to stressors – the lower their threshold of resistance.

One of the primary sources of exposure to high pathogen load is cattle that are persistently infected with the bovine viral diarrhea virus (BVD-PI). These calves are carriers of the BVD virus which is one of the primary causes of BRD in cattle. BVD acts as a pathogen, but also acts as an immuno-suppressant, causing the immune system to fail to react properly to pathogens for a period of time. BVD-PI calves shed the BVD virus at an extremely high rate – approximately 1,000,000 to 10,000,000 viral particles daily throughout their life. Such high viral shed rates can overcome the immune system of healthy cattle that may be exposed to them.

Cattle health is important to the profitability of the operation. It is a function of many things, creating a complexity that often affords no simple solution. It is always recommended that the producer seek the advice of a qualified professional nutritionist and veterinarian to implement a program that will improve their chance of success.