Changes

====Persistent Infections====

====Persistent Infections====

Immunotolerant Animals.

Foetal infection with a non-cytopathic BVDV virus before 120 days gestation may result in the birth of calves pesistently infected with and tolerant to bovine viral diarrhoea virus. At this stage in gestation, the immune system is partially competent and recognises the BVDV antigen as self, meaning that response is mounted. The calf therefore becomes tolerant to the virus, which persists into neonatal life. Persistently infected animals can be identified at birth as being antigen-positive but seronegative. However, colostral transfer of maternal immunity through colostrum or infection with a heterologous strain of BVDV can will make these animals seropostitive, so care must be taken when timing and interpreting tests⁴¹.

Infection of the foetus with non-cytopathic virus before 120 days gestation may result in the birth of immunotolerant and persistently infected (PI) calves. The immune system, although competent, recognises the antigen as “self” rather than “foreign” and no response is mounted. The calf therefore develops a tolerant state to the virus which persists into neonatal life. Although no antibodies are produced against the original, transplacental-infecting strain, heterologous BVDV strains can elicit a response in PI cattle. Therefore, these may prove seropositive if tested (Bolin, 1985).

Persistently infected animals continuously shed large amounts of virus throughout their lives, providing a major source of infectious virus for naïve cattle²³. Persistently infected dams produce persistently infected calves, resulting in family lines capable of maintaining the virus in a herd³⁵. It is estimated that 1-2% of the cattle population up to 13% of foetal calves are persistently infected³⁴.

While they may appear clinically healthy, PI animals continuously shed large amounts of virus throughout their lives, providing a major source of infectious virus for naïve cattle. (Houe, 1999). PI dams produce PI calves, resulting in PI family lines which maintain the virus in a herd (Baker, 1995). 1-2% of the cattle population are PI (Houe, 1999), rising to 13% in foetal calves.

50% of persistently infected cattle die within the first year of life<Sup>33</sup>. Animals may be undersized and slow-growing, and are predisposed to other diseases⁴².Persistent infection with BVDV is the prerequisite for developing mucosal disease.

 

PI cattle are predisposed to other diseases, and have a reduced survival rate (Houe, 1993) with 50% dying within their first year (Duffell and Harkness, 1985). This increased susceptibility may be due to BVDV-associated immunosupression, considered in section 1.2.5. Animals may be undersized and slow-growing, and persistent infection is the prerequisite for mucosal disease.

====Mucosal Disease====

====Mucosal Disease====