Infectious bovine rhinotracheitis (IBR) is a disease of cattle caused by infection with bovine herpesvirus 1 (BoHV-1). Clinical signs after infection with BoHV-1 can be observed in the respiratory, ocular, reproductive and nervous systems. Eight distinct herpesviruses are known to infect cattle, with BoHV-1 being the most common. Being an alphaherpesvirus, a persistent latent infection of BoHV-1 is established in sensory ganglion or pharyngeal tonsils after infection. Various stimuli can remove the virus from dormancy and the resulting excretion ensures infection remains within a herd. Reactivation of the virus in cattle typically occurs when animals are stressed (inclement weather or poor husbandry) or after management events (calving or mixing). Latently infected animals need not shed virus, although, in all but rare cases, they will produce antibodies against BoHV-1.
Clinical signs
IBR is the most common form of BoHV-1 infection resulting in upper respiratory disease in animals, generally aged over six months. The initial presenting signs are often relatively mild, including reduced appetite, pyrexia, milk drop, coughing and dullness, with or without serous oculonasal discharge. Where infections are more severe, coughing can be present for up to three weeks before subsiding. In these cases, mortality can be increased by a secondary bacterial infection producing a severe pneumonia. Complicated cases can be observed to develop congested nasal mucosa followed by halitosis due to the presence of necrotic tissue in the larynx and oesophagus. IBR may affect young calves with a high mortality. These animals may present with encephalitis, enteritis and pneumonia.
Although rare in the UK, venereal transmission of BoHV-1 results in the development of infectious pustular vulvovaginitis (IPV) and infectious pustular balanoposthitis (IPB). This pustular inflammation of the genital mucosa causes oedematous swelling with vesicles and pustules evolving into ulcers (Figure 1). Mid- to late-term abortion is not commonly observed in the UK but can occur up to 100 days after an outbreak of respiratory disease. Between 2012 and 2019, only 21 abortions were confirmed due to IBR foetopathy out of 28,801 sampled (VIDA, 2020). Also uncommon are the digestive presentations seen in calves. Respiratory signs are typically co-observed. Necrotic ulcerative lesions of the oropharynx and oesophagus can be seen on post-mortem. Nervous manifestations appear to be an extension of oropharyngeal infections in calves, and in older cattle may be blood-borne or from the nasal mucosa resulting in encephalitis (Penny et al., 2002).
Diagnosis
The basis of a successful diagnosis of BoHV-1 infection is a combination of serology (detection of antibodies in milk or serum) and virus detection by PCR. If virus detection is the chosen route, only swabs of serous, rather than mucopurulent, discharges should be taken. Swabs of the nasal cavity or conjunctiva should be submitted to the laboratory as soon as possible. Confirmation of the preferred transport media should be sought from the local investigation centre before testing. Bronchoalveolar lavage (BAL) fluid can also be tested but with adequate results being obtained by swabs there should be no indication to perform a BAL in a live animal. Paired “red top” serum samples can be used to show seroconversion via ELISA testing for antibodies. Acute samples at the time of illness followed by convalescent samples two to three weeks later will show a significant increase if infection was the causative agent. Alternatively, convalescent animals can be used to indicate the presence of virus within the individual/ herd. Bulk milk detection of antibodies is useful for the detection/monitoring of IBR at a herd level within dairy enterprises. However, due to the lower sensitivity of ELISAs for IBR gE glycoprotein, a negative result merely implies less than 10 to 20 percent of the herd is positive for IBR. Prior to entering the milking herd, pooled milk samples from heifers can be tested for more specific screening or investigation. Vaginal and preputial swabs can be obtained from early cases of IPV/IPB for virus detection.
Post-mortem diagnoses can also be made by a variety of means. Classic IBR resulting in death will produce lesions of necrotising laryngotracheitis (Figure 2), often accompanied by secondary bacterial pneumonia. Tracheal and lung tissue can also be collected in formalin for histopathology. In situ virus can also be shown using immunohistochemistry.
No specific treatment exists for IBR and only supportive treatment of antibiotics and NSAIDs is indicated. Appropriate antibiosis should be selected due to the frequent involvement of Mannheimia haemolytica as a secondary invader. Any animal showing clinical signs should be isolated from the remainder of the herd due to the levels of virus excreted.
Control and prevention
The level of control within a herd is specific to each farming enterprise and should feature in the herd health plan. Control of IBR depends upon the initial infection status of the herd. In an ideal scenario, a free herd would remain free by being truly closed with stringent biosecurity protocols in place to prevent introduction of disease. Free herds which cannot remain closed can maintain their status by isolation and antibody testing of purchased animals, along with suitable biosecurity. IBR may be introduced on fomites, so protocols should be in place to deal with this. Various cattle herd certification schemes are available under the Cattle Health Certification Standards (CHeCS) umbrella to provide a guided approach to control and eradication. Infected herds can remove infections without vaccination by culling of seropositive animals and establishing a seronegative breeding herd of uninfected youngstock (Corkish, 1988).
Vaccination can also be used to control infection within a herd or aid in eradication. BoHV-1 vaccines are efficient at preventing clinical disease and reducing virus spread, but latency is not prevented. Live attenuated vaccines are licensed for administration via the intranasal and intramuscular routes. These are used to protect animals at risk. Use of the intranasal route can reduce the clinical signs of disease in the face of an outbreak. Both live and inactivated vaccines are available. Marker vaccines allow exposure to “wild” virus to be distinguished from vaccinal antibody on serology through the detection of a specific gE glycoprotein. Evidence shows that live gE-deleted vaccines confer higher protection in naïve animals than the inactivated vaccine. However, the inactivated vaccine proved better at reducing virus excretion after recrudescence.
