Bovine respiratory disease is associated with a large number of pathogens, including viruses (BVD, IBR, BRSV, Coronavirus, and many others) and bacteria (Mannheimia haemolytica [formerly Pasteurella haemolytica], Pasteurella multocida, Haemophilus somnus, Mycoplasma sp, and others). These pathogens nearly always gain entry to the lungs through the upper respiratory tract (nose, throat, and trachea).

Before birth, the respiratory tract is sterile, but pathogens begin to inhabit the upper respiratory tract soon after birth. If samples from several healthy calves are collected, some will yield major bacterial pathogens. Pathogens can be present and cause no illness because host defenses limit them to the upper airways. The ability to recover organisms from apparently healthy cattle is increased by "stressing" them - moving to a new location, commingling with others, withholding water, changing feed, etc.

When calf defenses cannot contain pathogens to the upper airways, respiratory disease will result. Calves with pneumonia act as "incubators", expelling pathogens in respiratory mucus. Presence of a sick calf forces healthy penmates to increase their host defense to compensate for increased exposure. If the healthy calf can do this, it will remain healthy; if it cannot increase host defenses, then it will become sick too.

Why are young calves so susceptible to bovine respiratory disease (BRD) compared to yearling age and older cattle? It has a lot to do with host defense. While older cattle aren't "bulletproof", they are far less likely to be affected with BRD, unless they are commingled from multiple sources and stressed.

Young calves have a less capable defense system. The antibodies in the colostrum they received shortly after birth decay by 50% every three weeks - so many calves may have little antibody left at weaning. Calves are less able to cope with adversity. They are not accustomed to other animals, feedbunks, and water tanks, so they may not eat well. They may not find a place to rest. They are more susceptible to the stress of change.

The control of BRD relies on control of stressors in the environment, and improvement of the calf defense response.

Stress control:

1. Weaning (withdrawal of milk and replacement with solid feed) is an unavoidable stress. Generally, it is not recommended to wean prior to four weeks of age. Weaning stress can be minimized by transitioning onto high quality feedstuffs over a period of time. Transition options include:

a. Decreasing milk feeding to 50% for 1 week then withdrawing milk.

b. Leave calf in the individual pen or hutch for one to two weeks or more after weaning to avoid stresses associated with grouping.

c. Leave calves on calf starter for an additional 2 weeks after re-grouping. This avoids a dietary stress on top of a group stress.

2. Dietary changes should be made slowly, bearing in mind that the digestive system may take two weeks to adapt to new feedstuffs. A dense, palatable diet is essential. The BRD will be a constant problem if major nutrients cannot be delivered to calves. Weaned calves should be consuming at least 2 lb of concentrate daily and should be gaining at least 1.25 lb/day.

3. For housing, weaned calves need a dry environment protected from the elements to lay down to rest and access to adequate clean feed and fresh water. They do not need a heated space. Do not allow humidity in calf facilities to go above outside humidity. Weaned calves should not have direct contact with older animals. Provide at least 30 ft2/animal and 18 inches of feeding space per calf, with dividers to define eating positions.

4. Group and social stress should be managed. Calves are best weaned as groups of four to six calves of similar age. Once this group is established, new calves are not added. This procedure allows calves to become exposed to pathogens in a controlled manner. Starting with large groups of calves or adding new calves to a group allows more risk of pathogen transmission. The result is a seemingly "chronic" pen of calves that take a long time to return to health.

5. Dehorning, ear tagging, vaccinating, and other stressful treatments should be timed so they do not add to weaning stress.

6. Weather is commonly blamed for respiratory disease outbreaks, yet controlled research to measure the effect of weather on BRD is practically non-existent. Day-to-night temperature extremes and moisture can be stressors, and these conditions are typical of fall weather. Often, weather is blamed in a BRD outbreak, when in reality, either housing is inadequate or calf nutrition is poor.

Host defense:

1. Colostrum - Calves that do not get adequate colostrum are at increased risk of disease pre- and post-weaning.

2. Nutrition - It takes energy and protein to support an immune response. Again, adequate nutrition is essential.

3. Vaccination can be a useful adjunct to other good management procedures. However, immunity resulting from vaccination of young calves is often less than that in adult animals. Selection and timing of specific vaccines can overcome part of this difficulty. Producers must work with their veterinarian to establish a good program.

4. Administration of antibiotics before illness (metaphylaxis) can significantly decrease BRD. Producers should consult with their veterinarian about this and should ask their veterinarian to recommend a legal, approved product. Some products used for beef cattle are not legal for use in dairy cattle.

Other points:

1. Even with great conditions, some calves will be affected with BRD. Watch calves carefully after weaning. Realize that in high stress environments, BRD may appear within 7 to 14 days after weaning. In moderate to low stress environments, BRD may not appear until 3 to 4 weeks after weaning.

2. Antibiotic treatment for BRD is used to control bacterial infection. While both bacteria and viruses cause respiratory disease, controlling the bacterial infection will greatly decrease clinical signs and limit lung damage. Antibiotics work by reducing bacterial load in the respiratory tract. Antibiotics decrease bacterial load to the point where the calf's defense system can clear the remaining bacteria. Calves that are extremely immune deficient may not mount a normal response and may either not respond to the antibiotic or require more treatments. Ask your veterinarian for a specific recommendation of a modern antibiotic to use for pneumonia treatment.

3. Calves that die of suspected respiratory disease should be necropsied by a veterinarian, especially when an outbreak is in progress. Why?

a. To confirm that respiratory disease was the cause of death. Sometimes, the actual cause of death was something else. Confirmation of the true cause of death may dictate a change in treatment programs. It is not uncommon to find liver failure, kidney failure, certain poisonings, and lungworms in calves that have died from what the producer assumed was simple respiratory disease. In these cases, other treatments can be applied to the group, and productivity can be returned to normal.

b. Find other disease. The Bovine Virus Diarrhea (BVD) is a very common infection that may present as respiratory disease. The fact that the calves were vaccinated does not preclude BVD infection. If BVD is diagnosed in calves, a major change in herd management may be needed.

Source: Dr. William B. Epperson, Extension Veterinarian, Department of Preventive Veterinary Medicine, Ohio State University