For many years, the feedlot industry identified a respiratory problem in cattle that was diagnosed as an allergic reaction to changes in feed. Because of the microscopic lesions found in the lungs, a virus was suspected, but when tissue samples were submitted to the laboratory, no virus could be found. It was later determined that the virus would not survive the transport techniques. Only after taking the lab to the field was the virus isolated. When grown in the lab, the virus caused specific changes in the culture cells; the same changes found in the microscopic lesions of the lungs. The lesion, called a syncytium, became incorporated into the common name of the virus. A virus causing lung disease in the bovine and producing syncytial lesions became known as the Bovine respiratory syncytial virus (BRSV). After many years of research, several vaccine manufacturers developed vaccines, introduced diagnostic laboratory tests, and began extensive marketing campaigns. BRSV infections appear to be common in the United States. Nationwide studies have shown that BRSV is present in 38% to 76% of beef and dairy herds.
Like other viruses that attack the respiratory tract, BRSV reduces the resistance of the respiratory tract and makes the animal susceptible to secondary lung infections. If sickness caused by BRSV is not diagnosed early, secondary lung infections may mask the true cause of the illness.
BRSV disease occurs in cattle of all ages, but unless the herd has been completely isolated from the disease agent, most adult animals show little if any signs of the disease. BRSV, however, has been identified as an important disease agent in both nursing and weaned calves.
In cow/calf operations, we see at least two different disease syndromes in calves associated with BRSV infection. For convenience, we refer to them as an early and a late syndrome. February- to April-dropped calves are affected by the early syndrome in the summer while they are still nursing their mothers. The number of calves affected is usually low unless the calves are early-weaned. Coughing, some nasal discharge, and body temperatures of 103°F to 105°F are about the only signs noticed. A few days after weaning, calves in susceptible herds frequently show respiratory disease signs similar to those seen in the early BRSV syndrome. Deaths may occur in some calves affected with this milder form of the disease, but in most cases the deaths appear to be the result of secondary bacterial pneumonia.
The late syndrome, as seen in ranch operations that hold calves over, typically occurs from three weeks to three months after fall weaning. At the beginning, there is a high-pitched dry cough, a clear nasal discharge, and frequently a clear discharge from the eyes. The calves continue to eat, but back away from the feed before filling. They may show signs of depression when left alone, but when approached, they immediately brighten up and move around normally. Producers who have not dealt with the disease before can easily miss calves in this stage of the outbreak. In these situations, the first noticeable sign of disease may be a dead calf. As soon as some calves are recognized as being sick, a temperature check on a group of calves helps determine the extent of the outbreak. If many of the calves are going to break with the disease within a day or two, a large percentage will have body temperatures greater than 104°F. As the disease progresses and feed and water consumption decreases, the calves become increasingly gaunt. The area around the eye may begin to swell due to fluid accumulation under the skin, and body temperatures can rise to around 108°F. As this fluid accumulation worsens, the area under the jaw and throat begins to swell. There is frothy saliva around the mouth and breathing becomes very labored, especially when the calf is exerted. Eventually the tongue is extended and the neck is stretched as breathing becomes more difficult. At this point, preventing death is unlikely. The same late syndrome occurs in feedyard calves; however, because of the high incidence of secondary infections, many calves may die before the advanced stages of the disease are observed.
Clinical signs in adult cattle vary considerably; adult cattle raised in herds where the virus is present tend to become infected as they mature and exhibit few if any clinical signs. In contrast, adult cattle that have not been vaccinated nor exposed to the virus are very susceptible to clinical disease and can exhibit severe respiratory distress. These very susceptible adults can be gasping for breath within an hour or two of the first signs of respiratory problems. Frequently, they collapse and die with little or no struggle. The short time in which these animals die is astonishing; the day before they looked healthy and vigorous. For years we diagnosed these sudden deaths in adults as "fog fever" or "acute pulmonary emphysema," and they were thought to be caused by an allergic reaction to certain plants. It may have been an allergic reaction, but an allergic reaction associated with the virus can not be ruled out. One theory is that an initial exposure to the virus in some cattle caused them to be sensitized to the virus; a subsequent exposure would then result in a severe allergic reaction. As we say, it's only a theory.
BRSV-infected cattle are considered the principal reservoirs of the disease. Since the disease is manifested as a respiratory disease, transmission from one animal to another is thought to be via aerosol droplets from the nose and throat. Due to the mode of transmission, the length of time for the disease to progress through an exposed herd depends upon the confinement status of the herd. In feedyards and dairies, where cattle are in close confinement, the disease can spread rapidly through the cattle in 3 to 10 days. However, in pastured cattle it may take several weeks or months to get through the entire herd. Once exposed, it requires 2-4 days for a susceptible animal to begin showing clinical signs of the disease. In susceptible herds undergoing a BRSV outbreak, you can expect 100% of the animals to become infected with the virus, 20-50% to show clinical signs, and less than 5% to die.
Source: E.J. Richey, Extension Veterinarian, College of Veterinary Medicine, Cooperative Extension Service