How is the Virus Transmitted?

MVV virus can infect by a variety of routes but the most important are through respiratory secretions and through ingestion of infected colostrum and milk. 

Colostrum and Milk

When infected colostrum and milk is consumed by a lamb – the intestinal epithelial cells become infected and transfer the virus to monocytes which migrate to Peyer’s patches and then to the mesenteric lymph nodes. Lambs that are born and raised by infected ewes are 2.3 times more likely to be infected by weaning than lambs born and raised by uninfected ewes.

Respiratory Secretions

Although colostrum and milk are very important, overall lambs are more likely to pick up the infection from respiratory fluids if raised within an infected flock.  Respiratory aerosols generated by coughing contaminate the air which is then inhaled but may also infect through the conjunctival (eye) route.  The intimate contact that nursing lambs have with their infected dam, greatly increase the risk of infection above and beyond the risk from drinking infected milk. Additionally, if the dam is not infected - the lamb may easily pick up the infection from other ewes in the flock. The further down the respiratory tract the virus is inhaled, the more likely the animal is to become infected.

Other Routes

In utero (i.e. infection before the lamb is born) reports of transmission rates vary between 0 and 10%.  Therefore it is possible that a lamb can be born infected. This is why even if lambs are snatched at birth and reared with safe milk and colostrum, they should be tested before you assume they are not infected. 

There is no evidence that sperm or embryos in themselves transmit the virus.  Although it has been shown that secondary sex organs and semen contain proviral DNA, experimental venereal transmission of virus has not yet been successfully demonstrated. To make sure that flushed embryos are safe, they need to be washed to make sure there is no remaining cells or free virus from the uterine flush fluids.

Contaminated needles either from vaccinating or taking blood, don’t appear to successfully transmit the virus.  Infective cell-free virus is not present in the blood, and infected monocytes are only present in very low numbers (1 infected cell per 10,000 cells).  However, those infected cells do contain viral antigen as provirus and RNA. 

Virions are excreted into the environment through feces and urine but do not survive long – making environmental contamination of drinking water and feed troughs much less important as a way of transmission.  Contaminated pasture is thought to be virus-free within 1 to 2 weeks, although in reality likely a much shorter time.

As the virus is easily killed by disinfectants and does not survive long periods in the environment, the major source of transmission then, is other infected sheep.  Thus, MV control programs focus on removing the infected sheep and preventing new infected sheep from entering the flock.