This disease is caused by bovine herpesvirus 1 and is characterised by reduced appetite, dullness, fever, nasal discharge, and ocular discharge with conjunctivitis. Clinical signs often develop soon after the arrival of newly purchased animals. The course of the illness is usually short, the morbidity is high and the mortality is low. 

AETIOLOGY

IBR results from infection of the respiratory tract with bovine herpesvirus 1 (BHV1).

The most common route of viral transmission is by air, but because close contact between infective and susceptible individuals is necessary, the spread of infection is comparatively slow. Consequently, in a large group of susceptible animals, clinical cases may continue to arise for several weeks.

Two to three days after infection fever, conjunctivitis, dullness, reduced appetite and coughing develop. In mild cases, recovery will have taken place after 7 to 10 days although affected individuals can continue coughing for a couple of weeks. 

The reasons for the sudden appearance of a severe form of IBR in Britain are not known, but there is circumstantial evidence to suggest that a "new" strain of virus may have been imported into Europe from North America, probably in latently-infected Holstein cattle.

EPIDEMIOLOGY

Following the initial diagnosis of the disease in Britain in 1962, mild incidents of IBR were recorded sporadically throughout the country. During the 1977-78 winter a severe form of IBR was recognised in Scotland and, since then, this form of the disease has spread nationwide

The incidence of disease is highest in fattening beef cattle and in dairy cows although beef cows and young calves can also be affected. In many outbreaks, the source of infection has been animals which have been on the farm for only a short time, often less than 4 weeks after purchase from a market. For this reason, IBR has been particularly prevalent on beef farms in northern and eastern Britain. In contrast, the incidence of disease has been greater in dairy animals in the south and west of the country because, in these areas, replacement heifers are mostly bought-in whereas, in other regions, they are usually home-bred. In addition, infection can also be introduced into a self-contained herd by individual animals returning from livestock shows having been infected by virus excreted from neighbouring, recovered animals.

Although IBR can occur in grazing cattle, severe incidents have only been confirmed during the winter housing period. It appears that "stress" factors such as weaning, mixing, overcrowding as well as the fear and fatigue associated with being sold, contribute significantly to the severity of the field disease. It is thought that those factors are largely responsible for the high morbidity and mortality in animals kept under intensive conditions.

CLINICAL SIGNS

Cattle which have been purchased from a market and have been on the farm less than 4 weeks are frequently those in which clinical signs are first seen. Initially, affected animals are dull with a reduction in appetite, which may be confined to roughage only, a serous nasal and ocular discharge and frequent coughing. In lactating dairy cows, there is a sudden reduction in milk yield. 

In mild cases, recovery can take place within 7-10 days without treatment. However, in a small proportion of cases, the clinical signs become progressively more severe.

In the acute stage of the disease, beef cattle can lose up to 30 kg bodyweight and, during the convalescent period, they may fail to put on weight for up to 8 weeks although they have a normal appetite. The milk yield of dairy cows is reduced for several days or for several weeks if they have been severely affected. Pregnant cows often abort as a result of foetal death either within a few days of being infected or even after a delay of several months.

The morbidity rate is usually greater than 50 per cent and, in many outbreaks, it is over 90 per cent. In the majority of incidents, fatalities do not occur. However, up to 7 per cent of severely affected cases may die or have to be culled in outbreaks involving large numbers of purchased, intensively-managed, beef animals. It is on these latter units that IBR is seen at its most severe and where there is a regular high turnover of purchased animals, IBR can become endemic. Within a susceptible group or herd, it takes from 3 to 5 weeks from the time the first case has been seen until clinical signs regress completely.

DIAGNOSIS

The clinical picture is characteristic and differential diagnosis is seldom a problem. Nevertheless, the following conditions should be considered: foot and mouth disease, pneumonic pasteurellosis, malignant catarrhal fever, photosensitisation and infectious bovine keratoconjunctivitis.

TREATMENT

During the early stages of the disease, it has been shown that the severity of the clinical signs can be minimised by the daily administration of a broad spectrum antibiotic until the animals temperature returns to normal. However the treatment of severely ill animals with antibiotics, even for a prolonged period, does not invariably result in a complete clinical recovery.

CONTROL

Susceptible cattle should be vaccinated whenever there is a high risk of exposure to BHV1. At present, there are several vaccines available:

These vaccines, which are given on one occasion, have the advantage that (i) they give almost immediate protection and significant immunity within 48-96 hours, (ii) they do not produce abortion and (iii) they can be used to protect non-infected, in-contact animals even after disease has been confirmed within a group.

All ages of cattle can be vaccinated and, when the challenge is likely to be great, annual revaccination may be desirable.