Baku, July 19 (AZERTAC). The usual procedure to create a vaccine against viruses is to use create a harmless version of the virus from the disease. In Australia, chickens were vaccinated with two vaccines. The two viruses from the vaccines have merged and formed a new virus, which is killing the animals.

Chickens worldwide are susceptible to a group of herpesviruses which target their upper respiratory tract. The resulting disease, known as infectious laryngotracheitis (ILTV), reduces egg production and can kill up to one-fifth of those infected. The chickens start to choke and suffocate on blood and mucus. Only chickens and closely related animals are susceptible to these viruses.

To combat ILTV, farmers vaccinate their chickens with attenuated herpesviruses that can still infect and replicate but do not lead to disease. Farmers have used two different vaccines, one from Pfizer and another from an European supplier. Shortly afterwards, two new strains of ILTV appeared which were also killing the animals. Originally it was thought that the vaccine reverted back to its disease-causing form, but now scientists have analyzed the genomes and found that the new viruses were actually stitched together from the two vaccines.

Basically the two vaccines which were developed independently from each other, have had two different ways to "break" the virus, i.e. to reduce it`s infectiousness, so that it does not cause the disease. As these two viruses recombined their genomes, a new virus emerged that was no longer "broken" and could therefore again cause the disease. According to a scientist, the viruses dominate over the previous strains.

Only well-characterized live vaccines, rendered harmless by mutations in the same or overlapping regions, should be used in order to minimize the risk of recombination to a new virulent strain. Humans normally get much less vaccines than chickens, for whom the risk of similar problems should be much lower.