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====Persistent Infections====

====Persistent Infections====

Foetal infection with a non-cytopathic BVDV virus before 120 days gestation may result in the birth of calves persistently 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 no 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 tests⁴².

Foetal infection with a non-cytopathic BVDV virus before 120 days gestation may result in the birth of calves persistently 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 no 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 or infection with a heterologous strain of BVDV can make these animals seropostitive, so care must be taken when timing and interpreting tests⁴².

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.

Persistently infected animals continuously shed large amounts of virus throughout their lives, providing a major source of infection for naive 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 and up to 13% of foetal calves are persistently infected³⁴.

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 diseases⁴³. Persistent infection with BVDV is the prerequisite for developing mucosal disease.

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 diseases⁴³. Persistent infection with BVDV is the prerequisite for developing mucosal disease.