Bovine gammaherpesvirus 4 (BoHV-4)

Also known as

Passenger virus

Description

Bovine gammaherpesvirus 4 is a member of the Herpesviridae family and part of the subfamily Gammaherpesvirinae and genus Rhadinovirus.

In 2009, a BoHV-4 strain was isolated from cattle with respiratory disease from Quebec, Canada, and its genome was sequenced, though this wasn’t the first time the virus had been found in cattle.

Bovine gammaherpesvirus 4 (BoHV-4) is a herpesvirus that seems to be widespread in cattle populations, but with no clear disease association.

It has been found in animals with a variety of clinical signs, and seems to produce a lifelong and asymptomatic (no clinical signs) infection in most of the infected animals. BoHV-4 has been described mainly as a secondary pathogen implicated in reproductive disorders of cattle, rather than the cause of these disorders.

Infection is normally sub-clinical but can cause reproductive disease in cattle such as endometritis (inflammation/infection of the uterine lining), vulvovaginitis (inflammation of the vulva and vagina) and can also cause mastitis (infection in the udder). The virus has been found in cattle with neurological disorders, diarrhea, skin lesions, and rare bovine dermatitis. The viral DNA has also been detected in semen, in aborted fetuses, and milk. The potential for transmission via embryo transfer has also been reported.

Transmission is both vertical (from a cow to her fetus in the uterus) and horizontal (from one individual animal to another). It can also be indirectly spread by fomites (objects touched by the infected animal). Distribution is worldwide and the virus infects a wide range of ruminants, including bison, buffalo, sheep and goats.

This pathogen is sometimes referred to as a passenger virus, which means it is present in certain diseases but may not be the actual cause of the disease.

BHV-4 infection is often subclinical, with no observable clinical signs. However the virus may cause abortion and retained placenta, and if an infected fetus is born alive it may be weak. It may be found in cattle with conjunctivitis and respiratory disease but we still don’t know whether this virus is a primary or opportunistic pathogen in these cases.

Lab tests can find the virus in body tissues or secretions, but because BoHV-4 infection can be asymptomatic, the presence of the virus in a lab sample does not confirm that it was the cause of the observed clinical signs.

Although there is no established direct link between BoHV-4 and specific diseases, the seroprevalence data from lab tests suggest the possible involvement of BoHV-4 in a number of dairy cattle diseases.

Signs

• Usually no signs, but sometimes abortion
• Retained placenta
• Birth of weak calves
• Mastitis
• Respiratory disease

Cause

Unlike most other gammaherpesviruses, BoHV-4 can easily replicate in a variety of cells and in a broad range of host species. The natural host of the virus is cattle, but several ruminant and non-ruminant species are susceptible to BoHV-4 infection. Wild African buffalo are considered a reservoir species, in which the virus is highly prevalent. This virus was first isolated in Europe from cattle with respiratory diseases and later in the U.S. Since then, more than 40 BoHV-4 strains have been isolated worldwide.

As with other herpes viruses, BoHV-4 is able to hide in the body and establish latent infection in white blood cells and certain nerve tissues. Virus reactivation may occur following dexamethasone treatment or stress; this virus frequently becomes reactivated even without other clinical disease processes. The latent character of BoHV-4 is a major hindrance to clinical diagnosis.

Currently, it is still uncertain whether BoHV-4 plays a primary role in pathological processes in diseased animals or simply increases the likelihood of clinical manifestations associated with infection by another pathogen. At this point, scientific research suggests that the pathogenic potential of the virus is low.

BoHV-4 may exacerbate the impact of other infectious pathogens, however, which may induce reactivation of BoHV-4 from latency. BoHV-4 is currently considered a cofactor for and contributing to the development of diseases usually initiated by bacteria.

The antibody titers against BoHV-4 are often higher in diseased animals compared to healthy animals. BoHV-4 can potentially play a role in respiratory and reproductive diseases but we don’t know if this virus can actually cause them. Nevertheless, an increasing number of studies consider BoHV-4 as a risk factor for reproductive tract infections. Several other studies have proposed that BoHV-4 was a secondary agent, often associated with concurrent or secondary bacterial infections.

In the case of BoHV-4 persistently infected animals, prostaglandin secretion in the uterus may reactivate viral replication, which in turn could lead to inflammation of the uterine lining. Bacterially-induced metritis (uterine infection) in cattle persistently infected with BoHV-4 could possibly be exacerbated or become chronic.

Prevention

It is difficult to prevent infection. The virus can remain latent in recovered animals; they may only shed the virus during times of stress, allowing the spread of disease to other individuals. Appropriate hygiene and isolation measures may help control the disease. Vaccines exist but are not widely used.

Treatment

There is no treatment for the virus. Animals with disease signs are given supportive treatment to help alleviate those particular signs.