Bovine Herpesvirus 1 - Transmission

Transmission

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. Interferons are a component of innate immunity involved in inhibiting viral replication in a host cell, as well as activating immune cells. BoHV-1 is also able to evade the adaptive immune cells by inducing apoptosis in CD4+ cells, which assist in activating T cells when antigens are present. This down regulates the number of immune cells that recognize the virus, allowing the virus to evade detection and elimination. The virus has many other evasion strategies against the host’s immune system contributing to the virus being able to maintain lifelong infection in the animal.

After primary infection of BoHV-1, the latent infection is quite often found in the trigeminal ganglion of the cow, although on occasion infection can enter the central nervous system. These latent infections can possibly reactivate, with or without clinical symptoms, under conditions of stress or by experimental methods. Infected animals will be continuous shedders throughout their lifetime when the virus reactivates; therefore, successfully propagating the disease. The virus sheds in such high titers that it will spread rapidly throughout a herd. Even though cattle might not be showing clinical signs they can still spread the disease. Aside from cattle, studies experimentally infecting animals have shown that goats and buffalo can act as reservoirs for BoHV-1, as well as red deer, sheep, swine, and reindeer. Shedding begins from the nasal mucosa as soon as infection occurs, and the virus has replicated in the upper respiratory tract. During replication in the respiratory tract cells of the epithelial will undergo apoptosis. The necrosis in the epithelial will result in an entry site for secondary infections that may result in Shipping Fever.