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Bovine Respiratory Syncytial Virus (BRSV) and Parainfluenza-3 (PI-3)1
E. J. Richey2Bovine Respiratory Syncytial Virus (BRSV)
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.
Treatment
As with other viruses, antibiotics have no affect on the BRSV infection. However, antibiotic treatment is indicated in attempts to control the secondary bacterial infections. In addition to antibiotics, treatment during an outbreak of the late syndrome in calves (also feedlot calves) would include withholding concentrate feeds or silage for 2-3 days and feeding only enough hay to identify animals that are off feed. Severely affected calves that are unable to drink will need supportive treatments consisting of fluids and electrolytes.The decision to vaccinate your herd against BRSV should be based upon the susceptibility of the herd. If on testing (serological) the herd is found to be negative to BRSV, you have a very susceptible herd. A vaccination program would be required to raise the resistance of the herd against BRSV. If, on the other hand, a significant number of adults were found to be test-positive, one could assume that the BRSV virus was present in the herd. Chances are that in such herds the heifers kept as replacements will become infected, possibly undergo the mild form of BRSV, and become nonsusceptible to BRSV disease as they age. However, calves shipped from those herds may leave the herd before they become infected; thus they are susceptible to BRSV disease. Herd replacements such as purchased heifers, cows and bulls could be very susceptible to BRSV. Adding them to an infected herd could be a problem for them. Vaccination of herds in which BRSV is known to exist will slow down the spread of the virus through the herd, allowing susceptible animals to get a low exposure rather than a high exposure to the BRSV virus. Unfortunately, maternal antibodies against BRSV passed to the calf via colostrum from vaccinated or infected cows will not protect the calf from BRSV infections. Whether those antibodies will reduce the severity of disease if the calf gets infected, or to what extent the antibodies interfere with vaccination of calves under four months of age has not been determined. Therefore, vaccination of cows against BRSV would not only reduce transmission to susceptible cows and slow down the spread of the virus through the herd, it would also reduce the spread of the disease to the calves. Vaccination of the cow herd provides a barrier. Calves should be vaccinated prior to weaning.
Presently all BRSV vaccines are nonreplicating and can be administered to both pregnant and nonpregnant cattle, regardless of whether they are modified live or killed virus vaccines. Vaccinating animals properly is different than just injecting an animal with a vaccine. Proper BRSV vaccination requires two doses initially, followed by an annual booster of one dose. The initial two doses should be administered at least 21 days apart and started when the animal is over 4 months of age. BRSV vaccine can be purchased alone or in several combinations with other vaccines such as IBR, BVD, PI3, and Lepto-5.
Several Florida ranchers have added BRSV to their vaccination programs, not only to provide a barrier for their herds but also to satisfy customers that have experienced BRSV outbreaks in the newly arrived calves.
The clinical and epidemiological picture in major BRSV outbreaks is quite distinctive; however, the symptoms and lesions noted are generally inadequate for diagnostic purposes. Diagnosis is usually after the fact and must be based on laboratory findings. Because of the difficulty in trying to make a positive field diagnosis, if BRSV is suspected, it is recommended that your veterinarian submit samples for laboratory examination.
Parainfluenza-3 virus (PI-3)
The PI-3 virus is relatively common in cattle and is found worldwide. Affected animals exhibit watery to yellow-colored discharges from the eyes and nose, coughs, increased respiration rates and fever. By itself, PI-3 is a relatively mild infection; death loss to the disease is rare, maybe even nonexistent. However, it generally works in concert with IBR, BVD, BRSV, pasteurella pneumonia and Haemophilus somnus infections, making the mixed infections more damaging and dangerous. Because PI-3 can enhance the damage of the other diseases, PI-3 vaccines are almost always found combined with IBR, BVD and/or BRSV vaccines.As with most diseases, calves are the most susceptible to PI-3; therefore, vaccination of cows to provide maternal antibodies to the newborn is recommended. In addition, calves should be properly vaccinated before weaning and/or shipment.
PI-3 vaccines are available in replicating, nonreplicating, and intranasal forms. All three forms are safe for use in any age cattle, regardless of the pregnancy status.
Replicating PI-3 vaccine.
Modified Live Virus - usually requires only one injection to provide protection. Vaccinate calves when they are over 4 months of age.Non-replicating PI-3 vaccine.
Killed Virus and Chemically Altered Virus vaccines require two doses initially and an annual booster to provide adequate protection. Vaccinate calves when they are over 4 months of age.Intranasal PI-3 vaccine.
The resistance stimulated by intranasal PI-3 vaccine is rapid but short lived and generally will stimulate resistance in calves of any age. This is the vaccine form of choice when a PI-3 exposure is anticipated. A booster vaccination with a replicating or nonreplicating form of PI-3 vaccine is required to provide longer protection.Adding PI-3 to a vaccination program is cheap. In fact, it is easier to find respiratory vaccines that contain PI-3 than it is to find one that does not contain PI-3.
Footnotes
1. This document is VM64, one of a series of the Veterinary Medicine-Large Animal Clinical Sciences Department, Florida Cooperative Extension Service, Institute of Food and Agricultural Sciences, University of Florida. Original publication date December 1, 1990. Revised May, 2002. Reviewed March, 2002. Visit the EDIS Web Site at http://edis.ifas.ufl.edu.2. E.J. Richey, Extension Veterinarian, College of Veterinary Medicine, Cooperative Extension Service, Institute of Food and Agricultural Sciences, University of Florida, Gainesville, 32611.
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U.S. Department of Agriculture, Cooperative Extension Service, University of Florida, IFAS, Florida A. & M. University Cooperative Extension Program, and Boards of County Commissioners Cooperating. Larry Arrington, Dean.
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