Infectious bovine rhinotracheitis (IBR) is caused by bovine herpesvirus-1 (BHV-1). Other types of herpes viruses affect humans and cause diseases we know as chicken pox, cold sores, and shingles. One of the characteristics of herpes viruses is the ability to infect cells and then lie dormant for long periods before some stressful event lowers the immune response and triggers reactivation. Carrier animals generally do not develop clinical disease when infections resurface, but shed large amounts of infectious virus in nasal and other secretions. BHV-1 is highly contagious and spreads rapidly when cattle are crowded and commingled. Reactivation of latent virus and subsequent transmission of virus to susceptible animals is responsible for persistence of IBR in cattle populations, and is a reason why producers shouldn’t get smug about IBR no longer being a threat in breeding herds.
Because BHV-1 virus can invade a variety of organ systems following exposure several disease syndromes are recognized as part of IBR outbreaks. In feedlots, IBR is commonly observed with eye infections (conjunctivitis) and respiratory disease. While all ages and breeds are susceptible, the disease is most common in calves over six months of age when the stress of weaning, transport and placement in feedlots often coincides with receding levels of natural immunity. IBR is often one of the agents in the cocktail of respiratory pathogens causing the complex respiratory disease syndrome labelled as shipping fever. Abortion storms in susceptible herds, a serious production disease made inconsequential by the evolution of highly effective vaccines, are starting to reappear in cow herds where vaccination success over three decades triggered complacency. Pregnant animals exposed to BHV-1 between 5.5 and 7.5 months of gestation are especially at risk. BHV-1 can also cause a variety of pustular-like genital diseases in both males and females, and serious systemic infections in calves, including involvement of the central nervous system. In unvaccinated, susceptible populations, IBR can affect up to 30 per cent of exposed animals. Herpes viruses in general have a relatively short duration of clinical immunity. Regular revaccination of the cow herd is important to establish an ongoing shield of protection against IBR infection.
Problems typically start with some breakdown in revaccination programs. Herds re-experiencing IBR abortions after the virus seemed to have disappeared have added a group of previously infected animals shedding virus, or a group of susceptible, unvaccinated replacements. In completely naive herds exposed to the virus, abortions can appear as storms. Abortion rates of five to 60 per cent of pregnant animals have been reported. The picture varies with the herd’s vaccination history and the residual immune status of all age groups.
BHV-1 spreads rapidly within herds. Abortions from susceptible cows are generally not evident until 23 to 53 days following exposure to the virus. Usually, by the time the first abortion occurs all cows have been exposed. Abortions may occur sporadically. Vaccination in the face of an outbreak can alter the course of the disease, but usually does not stop or prevent additional abortions.
Abortion has been reported in herds on two successive years indicating that immunity following recovery or vaccination doesn’t approach 100 per cent. Abortion has also been reported in herds that institute regular vaccination programs. Sources of virus in these cases include: new additions (shedders), use of modified live virus vaccines, and in some cases involvement of wild ruminants (deer, elk). Abortion in herds can occur without observed signs of illness although mild respiratory signs and eye infections are not uncommon. Abortion can occur at any stage of gestation, but typically occurs through the second half. Death and absorption of the fetus may occur in early pregnancy and look like simple infertility. Following abortions, most animals recover and normal pregnancies follow.
As with all causes of abortion, starting with an accurate diagnosis is very important. IBR abortion occurs as a result of fetal death, and the fetus is usually partially decomposed.
Producers should consult with their veterinary practitioner regarding submission of samples to a laboratory. BHV-1 can be easily isolated from lesions and detected in fetal tissue.
Preventing IBR in breeding herds
Vaccinating pregnant females with certain types of modified live virus vaccine may produce abortions. Choice of vaccines, timing of vaccination, and managing vaccination programs in different age groups are important considerations. Producers really need to work with a veterinarian in designing herd vaccination strategies — both short- and long-term. At last count, around 30 vaccine combinations contain an IBR fraction. There are both modified live and killed vaccines; some administered subcutaneously, others intramuscularly, still others via an intranasal route. Some vaccines are designed for use in feedlot situations, while others are safe and effective in cow-calf herds. There are some cautions concerning using a modified live IBR vaccine on pregnant animals, unless the vaccine is a temperature-sensitive, intranasal vaccine. Heifers vaccinated only once with a killed product without receiving a booster may not develop protective immunity. Many contain bovine virus diarrhea (BVD) antigens, while a number are combinations of elements covering a variety of bovine respiratory diseases.
Factors to consider: design of vaccine use in the face of an outbreak, routine vaccination of herds pre-breeding, vaccination of cows with calves at side and vaccination of replacement breeding stock being introduced into the herd.
It is generally agreed that pre-breeding vaccination with a modified live vaccine, or a two-shot series with a killed product, is the most effective way of preventing IBR abortions.
Producers buying breeding replacements should only buy from credible sources and not be shy about asking for letters of certification regarding IBR vaccination along with those signifying clinical freedom of Johne’s disease and BVD.
— Dr. Ron Clarke prepares this column on behalf of the Western Canadian Association of Bovine Practitioners. Suggestions for future articles can be sent to Canadian Cattlemen ([email protected]) or WCABP ([email protected]).