Major Finding: At least one novel influenza strain was discovered among swine tested during the height of the flu pandemic; neither antibodies from pandemic flu vaccine nor natural infection conferred protection against the novel strain in vitro.
Data Source: Study of 4,101 nasal and tracheal swabs gathered from swine from a Hong Kong slaughterhouse between June 2009 and February 2010.
Disclosures: None was reported.
Researchers are warning that the pandemic 2009 influenza (A)H1N1 strain has been quietly combining with other influenza strains among Hong Kong swine, and that further viral reassortment among global swine populations could once again cause a pandemic among humans, with unpredictable results.
“The 2009 pandemic, although mild and apparently contained at present, could undergo further reassortment in swine and gain virulence,” wrote Dr. Dhanasekaran Vijaykrishna and associates at the State Key Laboratory of Emerging Infectious Diseases at the University of Hong Kong.
The investigators called for “surveillance in swine [that] is greatly heightened, and that all eight gene segments are genetically characterized so that such reassortment events are rapidly identified.”
In their study, Dr. Vijaykrishna and colleagues looked at tracheal and nasal swab samples taken from swine at a Hong Kong slaughterhouse between June 11, 2009, and Feb. 4, 2010.
Samples were taken every 2 weeks on up to 252 swine per sampling occurrence, for a total of 4,101 samples of unique swine. Overall, H1N1 and H1N2 viruses were isolated from 32 samples (Science 2010;328:1529).
Pandemic flu viruses “isolated on the same sampling occasion were genetically identical, suggesting transmission of viruses occurred within swine herds,” Dr. Vijaykrishna and associates said.
However, “viruses from different sampling dates were genetically distinct from each other and also from [2009 H1N1]–like swine viruses isolated in other countries, indicating multiple independent introductions of these viruses from humans to swine,” the researchers said.
But the greatest concern comes from a January 2010 sampling where a novel reassortant was discovered; the new strain was named A/swine/Hong Kong/201/2010(H1N1).
Dr. Vijaykrishna and colleagues determined that this novel strain—whose hemagglutinin gene most closely resembled European avian-based influenzas, and whose neuraminidase gene was likely derived from the 2009 swine-derived H1N1 strain—could be particularly contagious.
“Neither [the 2009 H1N1] vaccine nor natural infection reliably elicits cross-protective antibody to A/swine/Hong Kong/201/2010,” the investigators wrote.
Further laboratory testing of the new strain revealed that while the virus was susceptible to oseltamivir, it was resistant to adamantanes such as amantadine or rimantadine.
“Experimentally infected swine developed mild illness and seroconverted,” according to the researchers. Additionally, they determined that viral shedding occurred among the infected swine for up to 13 days.
“Our results show that the introduction of [pandemic H1N1] virus to swine has provided it with opportunities for reassortment,” wrote Dr. Vijaykrishna and associates. This “reservoir of reassortment” could, if left unchecked, “produce novel viruses of potential threat to public health.”