Ruminant Diseases and Immunology Research Unit, National Animal Disease Center, USDA, ARS, 1920 Dayton Ave, Ames, Iowa 50010, USA.
Virol J. 2012 Aug 6;9:150. doi: 10.1186/1743-422X-9-150.
Bovine viral diarrhea virus (BVDV) strains circulating in livestock herds show significant sequence variation. Conventional wisdom states that most sequence variation arises during acute infections in response to immune or other environmental pressures. A recent study showed that more nucleotide changes were introduced into the BVDV genomic RNA during the establishment of a single fetal persistent infection than following a series of acute infections of naïve cattle. However, it was not known if nucleotide changes were introduce when the virus crossed the placenta and infected the fetus or during the acute infection of the dam.
The sequence of the open reading frame (ORF) from viruses isolated from four acutely infected pregnant heifers following exposure to persistently infected (PI) calves was compared to the sequences of the virus from the progenitor PI calf and the virus from the resulting progeny PI calf to determine when genetic change was introduced. This was compared to genetic change found in viruses isolated from a pregnant PI cow and its PI calf, and in three viruses isolated from acutely infected, non-pregnant cattle exposed to PI calves.
Most genetic changes previously identified between the progenitor and progeny PI viruses were in place in the acute phase viruses isolated from the dams six days post-exposure to the progenitor PI calf. Additionally, each progeny PI virus had two to three unique nucleotide substitutions that were introduced in crossing the placenta and infection of the fetus. The nucleotide sequence of two acute phase viruses isolated from steers exposed to PI calves revealed that six and seven nucleotide changes were introduced during the acute infection. The sequence of the BVDV-2 virus isolated from an acute infection of a PI calf (BVDV-1a) co-housed with a BVDV-2 PI calf had ten nucleotides that were different from the progenitor PI virus. Finally, twenty nucleotide changes were identified in the PI virus of a calf born to a PI dam.
These results demonstrate that nucleotide changes are introduced into the BVDV infecting pregnant cattle at rates of 2.3 to 8 fold higher then during the acute infection of non-pregnant animals.
牛病毒性腹泻病毒（BVDV）在牲畜群体中流行的毒株显示出显著的序列变异。传统观点认为，大多数序列变异是在急性感染过程中，由于免疫或其他环境压力而产生的。最近的一项研究表明，在单个胎儿持续性感染的建立过程中，BVDV 基因组 RNA 中引入的核苷酸变化比在一系列新生牛的急性感染后更多。然而，尚不清楚这些核苷酸变化是在病毒穿过胎盘感染胎儿时引入的，还是在母体的急性感染时引入的。
将从四头因接触持续性感染（PI）小牛而急性感染的怀孕母牛中分离的病毒的开放阅读框（ORF）序列与源自 PI 小牛的病毒序列以及源自感染胎儿的病毒序列进行比较，以确定何时引入遗传变化。这与从怀孕的 PI 母牛及其 PI 小牛中分离的病毒以及从接触 PI 小牛的三头急性感染非怀孕牛中分离的病毒中发现的遗传变化进行了比较。
在母体感染后六天从母体中分离的急性感染病毒中，先前在亲本和后代 PI 病毒之间鉴定出的大多数遗传变化已经存在。此外，每头后代 PI 病毒在穿过胎盘和感染胎儿时都引入了两到三个独特的核苷酸取代。从接触 PI 小牛的公牛中分离的两种急性感染病毒的核苷酸序列显示，在急性感染过程中引入了六个和七个核苷酸变化。与 BVDV-2PI 小牛共同饲养的 PI 小牛急性感染中分离的 BVDV-2 病毒的序列与亲本 PI 病毒有十个核苷酸不同。最后，在与 PI 母牛所生小牛的 PI 病毒中发现了二十个核苷酸变化。
这些结果表明，在感染怀孕牛的 BVDV 中引入核苷酸变化的速度比感染非怀孕动物的急性感染速度快 2.3 到 8 倍。
