Transplacental transmission of bluetongue virus 8 in Cattle, UK

15 December 2009

To determine whether transplacental transmission could explain overwintering of bluetongue virus in the United Kingdom, we studied calves born to dams naturally infected during pregnancy in 2007–08. Approximately 33% were infected transplacentally; some had compromised health. In all infected calves, viral load decreased after birth; no evidence of persistent infection was found. Bluetongue virus (BTV) is generally transmitted between ruminant hosts by Culicoides biting midges, and infection may result in the disease called bluetongue. In 2006, a strain of BTV-8 caused the first outbreak of bluetongue in northern Europe. Although adult Culicoides midges are absent from this region during winter for long enough to interrupt normal transmission, BTV-8 survived the winters of 2006–07 and 2007–08.

Several mechanisms have been suggested to explain the overwintering of BTV, one of which is transplacental transmission. Tissue-attenuated strains of BTV are sometimes capable of crossing the placenta and infecting fetuses in utero, and transplacental infection has been reported from the field after use of live attenuated vaccines. However, many wild-type strains of BTV failed to cross the placental barrier when cows were infected during pregnancy. Additionally, although a few studies have reported experimental transplacental infection with wild-type strains, these studies did not recover infectious virus from live offspring (although many field strains do not grow in tissue culture) and suggested that fetal infection often resulted in deformation, stillbirth, or abortion. Collectively, this information led to the assumption that only viruses passaged in tissue culture had the potential to overwinter by transplacental transmission. However, in 2008, nonlethal transplacental transmission of BTV-8 was detected in Northern Ireland. To examine the occurrence, rate, and consequences of transplacental BTV-8 transmission in the United Kingdom, we studied calves born to dams naturally infected with BTV-8 during pregnancy.

Emerging Infectious Diseases
December 15, 2009

Original web page at Emerging Infectious Diseases