Problem brewing: Could pigs be part of new strain?

Avian strains are in the mixing vessel 

The recently released influenza pandemic plan for the United States couldn’t be much more timely because there are some pigs on the other side of the world that already may be brewing up the next global outbreak.

A researcher from China’s Harbin Veterinary Research Institute has presented initial evidence that pigs from farms in parts of China have been infected with the H5N1 strain of avian influenza.

Pigs are known to be susceptible to infection with avian influenza viruses, but natural infection of swine with the H5N1 strain previously has not been reported, the World Health Organization (WHO) in Geneva stated.

"The cocirculation of avian, human, and pig viruses in pigs is of significant concern because of the potential for a genetic exchange, or reassortment of material between these viruses," the WHO reported. "Such an occurrence has the potential to produce a new, pandemic influenza strain."

Indeed, pigs have been implicated in the emergence of new influenza viruses in at least two previous pandemics. If a pig is coinfected with both human and avian strains, it can serve as a mixing vessel. The end result might be a new part-avian, part-human strain of flu that can be transmitted easily and cause disease in a completely susceptible population.

"It is a concern because we know that pigs have receptors for both avian and human influenza, so the chance of a reassortment occurring and the humanization of an avian strain increases if pigs are involved," says Arnold Monto, PhD, professor of epidemiology in the school of public health at the University of Michigan in Ann Arbor.

"One of the theories behind why the avian virus — even though it has infected humans directly — has not gone into human to human transmission is the fact that we haven’t had these kinds of events taking place," he adds. "That’s one of the reasons we recommend that anyone who has contact with patients who have avian influenza or is involved in culling chickens, etc., get vaccinated with regular influenza vaccine to cut down on the possibility that some kind of event like that will take place."

Indeed, the feared reassortment to produce a pandemic strain could occur within a human co-infected with avian and human influenza strains, says Scott Harper, MD, a medical epidemiologist in the influenza branch at the Centers for Disease Control and Prevention.

"You can have the same thing in people now, which we didn’t [always] realize," he explains.

"So that, for instance, a human could be infected with a human virus and then subsequently also infected with an avian virus and the mutation could occur in people without having the pig [involved] at all. It does add, I guess, one more in the way of species that could be coinfected and that mutation could occur, but it is something that is less ominous than if we had heard about pigs being infected [with avian flu] 10 years ago," Harper points out.

Monto says the question of why it is happening in China and not Vietnam is a mystery. "Some investigators have tried to infect pigs with the avian virus and have, in fact, been able to," he notes.

"But it doesn’t spread from pig to pig. The question is why is this occurring in China when they really haven’t been able to identify it to that great of an extent in Vietnam?" Monto adds.