D429 - Bovine herpes viruses

Bovine herpes viruses (BHV) cause large number of clinical diseases in cattle. Infectious bovine rhinotracheitis (IBR) caused by bovine herpes virus type 1 (BHV-1) is a contagious viral disease of cattle and is the most common form of BHV seen in North America. Symptoms include upper respiratory tract disease that predisposes animals to shipping fever and abortion. Coughing, anorexia, depression, decreased milk production (in milking cows), weight loss, and increased salivation may also accompany these respiratory tract problems. A nasal discharge along with nasal congestion may develop and is referred to as red nose. Animals in a stressful environment such as a feedlot will often develop more complicated conditions of IBR and death may result. A second bovine herpes virus type 1 syndrome, infectious pustular vulvovaginitis (IPV) in the cow or infectious balanoposthitis (IBP), causes pustular lesions of the genital tract in females or males but rarely causes abortions. IPV is a disease of the reproductive system, observed 1 to 3 days after mating, and often leads to painful inflammation. The first signs of IPV are frequent micturition (urination) and a tail out of normal position followed later by small pustules on the vulva. Outbreaks of the respiratory form (IBR) and genital disease (IPV) together are rare.
The virus replicates in the mucosal epithelia of the upper respiratory tract, eyes, and genital tract. Newly synthesized viral particles can spread via the air or by contact with infected secretions, such as nasal and ocular secretions, or semen and cryopreserved embryos, or transmitted to newborns during or after parturition. If a cow survives bovine herpes virus type 1 infection, the cow is considered a silent carrier of the disease and acts as a virus reservoir that can potentially cause infection in healthy animals. After recovery, BHV-1 establishes a state of latency primarily in trigeminal ganglionic neurons and in some cases, in the germinal centres of the pharyngeal tonsils. Although bovine herpes virus type 1 can persist as a latent infection throughout a host’s lifetime, it is also able to reactivate periodically, giving rise to new virus shedding and transmission to uninfected cattle. Reactivation from latency occurs through a complex and largely unknown mechanism, initiated by natural or artificial stress factors.

The availability of a reliable diagnostic method is a key requisite to establish an effective programme for the eradication of Bovine herpes virus. In the past, cell culture was often used to detect the presence of this virus in a suspected host. However, virus isolation by cell culture presents several disadvantages such as relatively low sensitivity and time-consuming process. Moreover, some specimens (e.g. semen) are cytotoxic or can be easily contaminated by bacteria, therefore affecting the this procedure and yielding inaccurate results. Current serological methods include the use of ELISA on bulk-tank milk from the herd and on individual blood specimens. However, it should be noted that antibody-based methods fail to detect bovine herpes virus type 1 latent carriers that are seronegative, but can still transmit infection. Fortunately, new generation DNA-based diagnostic tests avoid the problems associated conventional diagnostic procedures. By implementing an assay known as the PCR (polymerase chain reaction), specific genetic sequences unique to BHV-1 can be targeted and identified to definitively confirm the presence of this virus in a suspected host or carrier. Since the virus itself is targeted (not antibodies), this test provides veterinarians with a highly sensitive and specific means of detecting the presence of bovine herpes virus type 1 in a blood or semen sample.