Intercontinental Movement of Bluetongue Virus and Potential Consequences to Trade

LETTER

Maan et al. (2) recently published the complete genome sequence of an isolate of bluetongue virus serotype 2 (BTV-2) and suggested movement of western-type BTV into India. Our recent analyses of the complete genome of BTV-9 (4) and some segments of BTV-10 (1) from India also imply introduction of western-like BTV or genome segments thereof into India. We comment on two important issues in relation to these observations.

First, live attenuated vaccine viruses may play a role in the movement of BTV gene segments. There is evidence now for movement from the West into India of all 10 segments of BTV-2 (2), at least five segments (including segments 2 and 5) of BTV-10 (1), segment 5 of BTV-3 (4), and at least segment 2 of BTV-16 (unpublished). Data from others have also indicated possible introduction of BTV-2 into the United States (3) and of BTV-16 into China (5). It is of note that the sequences of the segments in question from BTV-2, -3, -10, and -16 are all highly similar to sequences of live attenuated vaccine strains used in different parts of the world or BTV reference strains, which are probably the antecedents of most of the vaccine strains.

Second, these findings highlight the issue of livestock trade playing a role in the movement of BTV or its genetic elements. We have recently discussed the introduction of U.S. BTV-10-like virus into India (1). India imported cattle from 2002 to 2005 from the Belgium, France, Germany, Nepal, Russia, South Africa, Thailand, the United Kingdom, and the United States (Food and Agricultural Organization [http://faostat.fao.org]). Some of these countries used live attenuated vaccines of various serotypes at one time or another during this period. Introduction of various BTV genes into India from the West could be attributed to the movement of animals or vectors or to livestock trade. Thus, the use of live attenuated vaccines might be inadvertently contributing to the flow of BTV gene segments from one geographical region to another. What is of concern is the undetected movement of vaccine viruses or vaccine-like gene segments between continents in spite of elaborate trade regulations.

It is therefore very important to elucidate full genome sequences of live attenuated vaccine strains as well as more field isolates of BTV, especially those from various parts of Asia. This should aid in (i) informing predictions on virus movement and incursion, (ii) further developing or refining reagents as well as molecular typing techniques, and (ii) redefining BTV topotypes.