Email from Dr Ruth Watkins February 21 2008
Unfortunately the wild circulating BTV-8 strain in Northern Europe can infect the foetus in utero. I understand this is not the usual property of wild BTV viruses, but when they are attenuated to use as a live vaccine this seems to be a property that can be found in the modified live vaccine (MLV) when not present in the wild virus.In fact any infection in a pregnant animal in which a phase of viraemia occurs (virus or other infectious organism in the blood stream, bacteraemia or parasitaemia) has the potential to cross the placenta and infect the foetus. Generally this occurs in the primary infection of the mother- so that the mother has no immunity during the early blood borne phase of infection so the levels of the infectious agent in the blood are very high. Examples are Listeria spp, toxoplasma gondii, chlamydiale psittaci, bovine viral diarrhoea virus etc.etc.First of all infection of the foetus does not always happen, that is the natural biological variation!Secondly the infectious agent may only infect the placenta. The placenta may or may not be damaged, damage resulting in a small malnourished newborn, or foetal loss.Thirdly the infectious agent crosses or infects and crosses the placenta. The foetus is infected. Shortly before birth the foetus can make some kind of immune response but early in gestation it cannot. It may be killed in-utero as a result of infection. (miscarriage or still birth). The foetus may be born alive, and exhibit disease at birth or later. It may be severely affected and die, such as if the bone marrow and liver is damaged it can have hydrops, if the brain is damaged it could be blind or paralysed, such as the brain cysts of toxoplasmosis. It may have both these affects and other evidence of damage such as to the heart or lungs. The sick newborn may struggle to live or die shortly after birth. However in many infections of the foetus there is no disease at all in the newborn, it comes to term and has a normal birth. Disease may be very mild and unrecognised, or unusually it may develop later as with BVD and mucosal disease.The congenitally infected newborn if it survives and is healthy will eventually get the virus or other infection under control with its immune system as it matures. However this may take much longer than if it was infected post natally when its immune system is mature when it is exposed to the infection. The congenitally infected newborn (infected in-utero) may be infectious for several years. The EU has been remiss in doing so little follow up and investigation into the natural results of BTV-8 infection. I understand that increase in foetal loss as well as stillbirths and disease with congenital infection in cattle and sheep have been seen with BTV-8 but I am not sure how many have been properly investigated in the virus laboratory. It may well be that calves congenitally infected are likely to have a persistent and high viraemia and be infectious to female midges for a year, or some other unusually long time.Private individuals are left to decide to investigate and pay for the investigations themselves- quite expensive in the case of a calf post mortem (PM) for example, with the tests to look for different possible infections to explain the disease and death. I don't think Sabine Zentis will object to my revealing that in one of her recent emails that she told me she has lost 2 calves in 2008 to cows infected in 2007 (she has paid for screening her herd. The majority of her herd were infected in 2007 and the remaining adult animals were infected in 2006. So far the cows infected in 2006 have had entirely healthy calves). Of those born in 2008 to mothers infected in 2007 one was stillborn, small and malnourished, the another had hydrops and died within 2 minutes of birth. There are more cows infected in 2007 to calve this spring. She is paying for a PM and investigations on one of the calves. Sabine has never seen this before in over 400 calvings. My sister's cow Duchess, infected with BTV-8 in Norfolk, is shortly to calve.If the calves born in N Ireland to the imported cows are documented to have bluetongue infection even whilst being apparently well, it signifies the cows have had a primary infection with BTV-8 in 2007 during their pregnancy and it is likely the calves will be viraemic and infectious to any biting female midge for an unknown period of time in the future, likely into the summer or throughout the summer of 2008.I would suggest that calves (or lambs) are tested of any imported ruminant into the UK if the cow (or ewe) was pregnant at the time of import, and has antibody to BTV-8 even if she has no virus present- one cannot know if infection in the antibody positive but virus RNA PCR negative mother was in 2006 or 2007- when there is antibody only present (none have yet been vaccinated).
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