Infectious Bovine Rhinotracheitis
Infectious Bovine Rhinotracheitis (IBR) is a clinical syndrome seen in cattle associated with Bovine herpesvirus-1 (BHV-1) infection. BHV-1 has worldwide distribution and is associated with a range of respiratory, reproductive and enteric diseases.
Worldwide, BHV-1 shows considerable variation in virulence and it is possible to classify isolates into sub-types. Sub-type BHV-1.1 is the most virulent and is associated with severe upper respiratory tract infection and abortion in North America and Europe. Sub-type BHV-1.2 is also capable of causing severe respiratory disease but is not associated with abortion. In Australia, only sub-type BHV 1.2 has been identified.
IBR is the most common clinical syndrome associated with BHV-1 infection in Australian cattle herds. IBR varies in severity from a mild disease with little more than a nasal discharge to a severe fatal disease with extensive ulceration of the trachea and a severe pneumonia. Clinically, it is difficult to differentiate from other common causes of the bovine respiratory disease complex (BRD).
More severe cases of IBR can sometimes be differentiated from other forms of BRD on the basis of characteristic clinical signs, although mild infection may predispose cattle to subsequent bacterial infection with agents like Mannheimia haemolytica which subsequently cause a severe and often fatal fibrinous pneumonia.
As well as causing IBR, BHV-1 can also cause venereal disease. In bulls the virus causes infectious balanoposthitis with reddening and ulceration of the prepuce and in cows it causes infectious vulvo-vaginitis.
Several surveys have confirmed that BHV-1 infection is common in Australian cattle herds. Approximately 80-90% of herds have evidence of past exposure to the virus with one or more animals having antibodies to BHV-1 infection. The seroprevalence in Queensland surveys conducted in the mid-90’s averaged 30% of cattle sampled. As with most infectious diseases, the proportion of animals seropositive to the virus increases with increasing age, with young cattle having the lowest level of infection.
A percentage of cattle that have recovered from BHV-1 infection develop latent infections which can reactivate when animals are stressed. On arrival at a feedlot, these latently infected cattle will begin to shed virus under stress and rapidly infect naïve cattle that have been commingled with them. This same scenario may also occur in cattle at pasture, although outbreaks of disease are sporadic in pastured cattle and generally only associated with stressful situations like weaning, extremely dusty conditions or particularly inclement weather.
As IBR is a viral disease, the only feasible means of controlling the disease in feedlot cattle is via vaccination. Modified live intranasal vaccines have been shown to give the most rapid and best protection against IBR. Because they only need to be given once to confer strong immunity with reasonable duration, they are generally given as part of the feedlot induction process, although their use as a part of backgrounding programs is also indicated where practical.
Control of IBR in grazing cattle is generally not necessary as infection is largely asymptomatic.