Anxiety over the possible spillover to humans of the bird flu strain H7N9 grew larger after a study noted that pigs that caught the flu were likewise growing in numbers.

According to one of the authors of a study published in Proceedings of the Royal Society B, infected pigs that were raised for slaughter on farms in south and southeastern China, which also happened to be plagued by bird flu, are increasing.

In a process known as reassortment, pigs act as a "mixing vessel," developing new flu strains from swine, poultry and human viruses in areas where they especially live near to each other.

Although the bird flu strain H7N9 has not yet been traced to pigs and has not yet shown capable to jump from person to person, scientists continue to monitor it for genetic changes that can make the mutation possible.

In April, an article published in the science journal Nature highlighted the supposed correlation of the pig and bird populations, noting H7N9 seems to be present in China areas were large number of the two types of animals were present.

It is "providing opportunities for further adaptation to mammals and for reassortment with human- or pig-adapted viruses."

China's death toll from the bird flu strain H7N9 has jumped to 31, with the number of infected cases also rising to 129 now. First detected in March among patients in China, it has crossed the border to at least one case in Taiwan.

James Lloyd-Smith of the University of California in Los Angeles, co-author of the new study, told AFP they saw higher rates of influenza circulation on the farms where pigs are raised, which meant "that they have already been infected (and so they're immune) by the time they're going to slaughter."

"The prevalence of infection in swine has not decreased and so the risk of spillover to humans or birds is constant or growing," Mr Lloyd-Smith added.