A comparison of the virulence of three strains of infectious bovine rhinotracheitis virus.

The virulence of three strains of infectious bovine rhinotracheitis (IBR) virus was compared in six-month-old Ayrshire-cross calves. The strains were an isolate from a recent severe outbreak of IBR in Scotland (Strichen strain), the prototype British strain (Oxford strain) and a North American isolate (Colorado strain). The Colorado and Strichen strains produced the characteristic clinical signs and pathological lesions of severe IBR three to four days post infection (p.i.). The Strichen strain was slightly more virulent, possibly as a result of its having been passaged fewer times in tissue culture. In contrast, the Oxford strain produced a mild clinical response with minimal pathological lesions. Virus was recovered from nasal swabs for a longer-period from the calves infected with the Strichen strain (up to 13 days p.i.) and Colorado strain (up to 12 days p.i.) than from the animals infected with the Oxford strain (up to 10 days p.i.). These findings support the suggestion that the recent epidemic of severe IBR in Britain had resulted from the importation of a "new" strain of virus.

Experimental infection of cattle of different ages with infectious bovine rhinotracheitis virus (Strichen strain).

The clinical signs and pathological lesions which developed in various ages of cattle experimentally infected intranasally with the "Strichen" strain of IBR virus were similar to, but generally milder than, those of the field disease. The clinical signs were most severe 4 days after infection and had almost wholly regressed after 12 days. Serum neutralizing antibodies were detected in every animal. Virus was isolated from nasal and ocular swabs for up to 13 days and 10 days, respectively, after infection. The clinical signs and the pathological lesions were more severe in the younger animals.

Reactivation and shedding of bovine herpesvirus 1 following Dictyocaulus viviparus infection.

Three groups of 4 bullocks which had recovered from infectious bovine rhinotracheitis (IBR) were infected 5 months later with Dictyocaulus viviparus larvae. Bovine herpesvirus 1 was recovered from days 7 to 21 post-infection from the nasal secretions of the group given 50 larvae per kilogram and on one occasion from those given 1000 larvae per animal (less than 5 L3 per kg). Virus was not isolated from the animals given 1000 irradiated larvae. Typical clinical signs and lesions of IBR developed in the group from which the virus was isolated regularly.

The prevalence of serum neutralizing antibodies to infectious bovine rhinotracheitis virus in Scotland.

The prevalence of serum neutralizing antibodies to infectious bovine rhinotracheitis virus in 1152 serum samples from adult cattle in 114 dairy and beef herds in different regions of Scotland was 12%. In the Grampian region, the number of seropositive cattle in the self-contained herds was significantly less (P less than 0.01) than in the "other" herds. Holstein cattle had been introduced into five of these "other" herds, and significantly more (P less than 0.01) of the samples from these five herds were seropositive compared with the samples from the rest of the "other" herds in the same region into which recently purchased cattle had been introduced. The introduction of Holstein cattle was also a major factor in the association between the prevalence of antibodies and herd size in the Grampian region. The prevalence of serum neutralizing antibodies to infectious bovine rhinotracheitis virus was significantly greater (P less than 0.001) in this survey than in those previously undertaken in the United Kingdom.

Scanning electron microscopy of the tracheal epithelium of calves inoculated with bovine herpesvirus 1.

Tracheal epithelium was examined by light, scanning electron and transmission electron microscopy at four, five and seven days after calves were infected intranasally with bovine herpesvirus. Similar areas were examined from uninoculated calves. Tracheal epithelial lesions were found in all inoculated calves and their degree of severity did not vary with the interval between inoculation and examination. Extensive loss of cilia leaving areas of tracheal epithelium covered by microvilli was the main feature.

Clinical and epidemiological features of 15 incidents of severe infectious bovine rhinotracheitis.

Fifteen incidents of infectious bovine rhinotracheitis (IBR) were studied in herds distributed widely throughout northern Britain. Fattening beef animals (10 outbreaks), dairy cattle (four outbreaks) and suckler beef cows (one outbreak) were affected and all bar one incident occurred in housed cattle during the winter. The first signs of illness noticed were a reduced appetite, dullness, coughing and oculonasal discharge. In 13 of the incidents they were observed in cattle purchased from a market within the previous four weeks. In every outbreak, affected animals developed a serous nasal discharge which became purulent in severe cases. In the early stages the nasal mucosa was congested but later yellow-brown diphtheritic plaques developed. In such animals halitosis was always detected. Soft coughing was frequently heard but pneumonia was rarely confirmed ante mortem. Conjunctivitis and ocular discharge were a major finding in 13 incidents and, in severely affected cases, conjunctival oedema was seen. The drooling of saliva was noticed in 14 incidents but congestion of the oral mucous membranes was the only abnormality found on examination of the oral cavity. Diarrhoea was a consistent feature in one outbreak. As a result of contracting this disease beef cattle failed to put on weight for a period of one to eight weeks and the milk yield of lactating dairy cattle decreased markedly. The morbidity rate was high, being more than 90 per cent in 10 incidents. The mortality rate varied considerably but 7 to 8 per cent of the animals died, or were culled, in three outbreaks. The clinical signs were most severe on intensive units with a high turnover of cattle.

The pathological features of severe cases of infectious bovine rhinotracheitis.

In recent outbreaks of infectious bovine rhinotracheitis (IBR) in Britain a proportion of the animals affected developed a severe clinical disease characterised, at necropsy, by widespread damage to the respiratory tract. They had necrotising rhinitis, pharyngitis, laryngotracheobronchitis with extensive pseudomembrane formation and severe pneumonia with or without interstitial emphysema. Renal infarction was seen in approximatley half of the cases. The central nervous system was not affected in any of the 25 animals with severe IBR examined in this study. Tissues from the respiratory tract of 14 animals were examined for the presence of bovine herpesvirus 1 and the virus was isolated from the nasal passages of 11 and the lungs of four. Mycoplasma bovis was frequently isolated in large numbers from both the upper and lower respiratory tract.