Changes

In most countries, BVDV is endemic and studies detecting antibody have estimated that between 70 and 100% of herds are either currently infected or have recently been infected with bovine viral diarrhoea virus²³.

In most countries, BVDV is endemic and studies detecting antibody have estimated that between 70 and 100% of herds are either currently infected or have recently been infected with bovine viral diarrhoea virus²³.

BVDV can be transmitted from infected to susceptible cattle in several ways. Firstly, direct contact with a virus-shedding animal can cause disease. Both acutely and persistently infected animals shed virus, but levels of shedding are much higher in persistently infected cattle and transmission is more efficient. Transmission to heifers and cows may also occur venereally or via artificial insemination as acutely and persistently unfected bulls sheed bovine viral diarrhoea virus in their semen²⁴. The testes is an immunoprivileged site, and the virus can persist in this location despite otherwise systemic clearance²⁵. Indirect spread is possible: BVDV has been shown to spread through the re-use of needles, nose tongs²⁶ and rectal gloves²⁷, and blood feeding flies also give transmission.

BVDV can be transmitted from infected to susceptible cattle in several ways. Firstly, direct contact with an animal shedding BVDV in its secretions and excretions can cause disease. Virus is shed by both acutely and persistently infected (PI) animals but levels of shedding are much higher in persistently infected cattle, which are a natural reservoir for virus. It is estimated that the incidence of persistently infected animals may be as high as 1-2% of cattle less than one year of age. On a farm, PI cattle are often found in cohorts of similarly aged animals. This is because persistent infections arise when pregnant animals are acutely infected, and so an outbreak of acute, possibly subclinical, BVD in pregnant cattle can later result in a "batch" of PI calves.

Cattle that are persistently infected with noncytopathic BVDV serve as a natural reservoir for virus. Persistent infection develops when noncytopathic BVDV is transmitted transplacentally during the first 4 mo of fetal development. The calf is born infected with virus, remains infected for life, and usually is immunotolerant to the resident noncytopathic virus. Transplacental infection that occurs later in gestation results in abortion, congenital malformations, or birth of normal calves that have antibody against BVDV. The prevalence of persistent infection varies among countries and between regions within a country. In some areas, the prevalence of persistent infection in calves may be as high as 1-2% of cattle <1 yr of age. On a given farm, persistently infected cattle are often found in cohorts of animals that are approximately the same age. Persistently infected cattle can shed large amounts of BVDV in their secretions and excretions and readily transmit virus to susceptible herdmates. Clinical disease and reproductive failure often are seen after healthy cattle come in contact with a persistently infected animal. Biting insects, fomites, semen, biologic products, and possibly wild ruminants also can spread BVDV.

Transmission to heifers and cows may also occur venereally or via artificial insemination as acutely and persistently unfected bulls sheed bovine viral diarrhoea virus in their semen²⁴. The testes is an immunoprivileged site, and the virus can persist in this location despite otherwise systemic clearance²⁵. Indirect spread is possible: BVDV has been shown to spread through the re-use of needles, nose tongs²⁶ and rectal gloves²⁷, and blood feeding flies also give transmission.

==Pathogenesis==

==Pathogenesis==