Bovine alphaherpesvirus 1

Species of virus

Bovine alphaherpesvirus 1 (BoHV 1) is a virus of the family Herpesviridae and the subfamily Alphaherpesvirinae, known to cause several diseases worldwide in cattle, including rhinotracheitis, vaginitis, balanoposthitis, abortion, conjunctivitis, and enteritis. BoHV 1 is also a contributing factor in shipping fever, also known as bovine respiratory disease (BRD). It is spread horizontally through sexual contact, artificial insemination, and aerosol transmission and it may also be transmitted vertically across the placenta. BoHV 1 can cause both clinical and subclinical infections, depending on the virulence of the strain. Although these symptoms are mainly non life threatening it is an economically important disease as infection may cause a drop in production and affect trade restrictions. Like other herpesviruses, BoHV 1 causes a lifelong latent infection and sporadic shedding of the virus. The sciatic nerve and trigeminal nerve are the sites of latency. A reactivated latent carrier is normally the source of infection in a herd. The clinical signs displayed are dependent on the virulence of the strain. There is a vaccine available which reduces the severity and incidence of disease. Some countries in Europe have successfully eradicated the disease by applying a strict culling policy. Infection can occur in cattle of any age, but it is most commonly seen between the ages of 6 and 18 months.

BoHV 1 enters the animal through the mucous membrane in the respiratory tract or genital tracts. The main mode of disease transmission is direct nose to nose contact between an infected and a susceptible animal. This is made possible because of the virus sloughing off into the mucus. Aerosols have to be exhaled, sneezed, or coughed from an infected animal during viral shedding in order for transmission to occur. Transmission also originates from contaminated semen through use of live breeding or AI; bulls that have been affected genitally may shed the virus in their semen. Once infected it is hard for the animal to get rid of BoHV 1 because it has many mechanisms to evade the hosts’ immune systems involved in both innate immunity and adaptive immunity. The virus degrades interferon regulatory factor 3 (IRF3), effectively halting transcription of interferon type 1. These latent infections can possibly reactivate, with or without clinical symptoms, under conditions of stress or by experimental methods. The necrosis in the epithelial will result in an entry site for secondary infections that may result in shipping fever.

The respiratory disease caused by BoHV 1 is commonly known as infectious bovine rhinotracheitis. This disease affects the upper respiratory tract as well as the reproductive tract of cattle, and is commonly found in feedlots across North America. IBR can also cause abortion. This generally occurs in mid gestation when a susceptible cow is infected with BoHV 1. A viraemia occurs and subsequently the virus crossed the placenta and causes organ necrosis in the fetus. BoHV 1 also causes a generalized disease in newborn calves, characterized by enteritis and death.

The genital disease causes infectious pustular vulvovaginitis in cows and infectious balanoposthitis in bulls. Symptoms include fever, depression, loss of appetite, painful urination, a swollen vulva with pustules, ulcers, vesicles and erosions in cows, and pain on sexual contact in bulls. In both cases lesions usually resolve within two weeks.

Clinical signs and history are normally enough to make a preliminary diagnosis. To definitively diagnose the infection the virus should be identified in the tissues by virus isolation or PCR, or in bulk milk samples by ELISA. Serology may be performed to identify latent carriers, but it is not completely reliable as not all of the infected animals have detectable antibodies.

Treatment is symptomatic. Anti inflammatories reduce fever and provide pain relief. Antibiotics may be necessary if a secondary infection occurs. Vaccination is widely used both to protect cattle clinically in the case of infection and significantly reduce the shedding of the virus. Vaccination provides herd immunity, which lowers the likelihood of an animal coming into contact with an infected animal. Both inactivated and live attenuated vaccines are available. Immunity usually lasts approximately six months to one year. Marker vaccines are also available and recommended. Marker vaccines, also known as DIVA (differentiation of infected from vaccinated animals), have become popular in order to distinguish vaccinated animals from infected animals. A marker vaccine uses either deletion mutants or a virion subunit, such as glycoprotein E. Studies show that vaccinating after an animal has been infected decreases shedding of the disease and reduces reactivation of the latent virus, although not completely. Using a killed gE deleted marker vaccine after infection will reduce viral excretion following reactivation, using a dexamethasone treatment. In a small, vaccinated herd BoHV 1 can be eradicated within one or two decades, whereas in a large herds eradication is unlikely. Eradication was accomplished in Switzerland in 5 years using a four step protocol focusing on preventions on transmission with trade barriers, slaughtering animals with antibodies to BHV 1, detection and eradication of further reservoirs (feedlot cattle), and then putting in place a monitoring program to maintain the control. This cost Switzerland a lot of money as they compensated producers who culled their seropositive animals. Eradication is a difficult goal for this disease, but being disease free opens barriers of trade with countries that have eradicated it as well, and saves producers money from the virus’ effects.