H7N9: Ferrets Show Transmissibility by Direct Contact

Troy Brown

May 23, 2013

Ferrets infected with H7N9 influenza shed the virus in high amounts before showing signs of illness and transmitted the virus efficiently by direct contact, according to recent testing in ferrets and pigs. The virus spread inefficiently in ferrets by airborne exposure.

H. Zhu, from the Joint Influenza Research Centre at Shantou University Medical College, State Key Laboratory of Emerging Infectious Diseases at Shenzhen Third People's Hospital, and State Key Laboratory of Emerging Infectious Diseases/Centre of Influenza Research, School of Public Health at the University of Hong Kong in Hong Kong SAR, China, and colleagues report their findings in an article published online May 23 in Science Express.

Between March 31 and May 1, 2013, more than 125 cases of a novel H7N9 influenza virus have been reported in Eastern China. More than three fourths of infected individuals had contact with, or exposure to, poultry before becoming ill, suggesting that the infections have a zoonotic origin. Some family clusters have been identified, and this, along with sequence analyses showing that the H7N9 viruses may have mutated to become more capable of replicating in mammals, has caused concern that the virus may be capable of human-to-human transmission.

Therefore, the researchers evaluated the infectivity and transmissibility of A/Shanghai/2/2013 (SH2), a human H7N9 virus isolate, in ferrets and pigs. "Efficient transmission of influenza viruses in ferrets is considered as a predictor of human-to human transmissibility," the authors write.

They inoculated 6 influenza-free ferrets intranasally with the virus. Two ferrets were placed in each of 3 transmission cages. At 1 day postinoculation, an influenza-naive ferret was placed in each cage to evaluate direct contact transmission. Another naive ferret was placed in an adjacent cage to measure airborne exposure.

The virus was able to replicate in the upper and lower respiratory tracts of the ferrets. The ferrets shed the virus at high titers for 6 to 7 days, during which time the ferrets showed mild clinical signs. The virus was efficiently transmitted between the ferrets via direct contact, but it was transmitted less efficiently by airborne exposure.

The fact that the ferrets shed the virus before showing significant clinical signs may be clinically significant. "This trait has been observed previously for pandemic and seasonal influenza " the authors write. "If this virus acquires the ability to efficiently transmit from human-to-human, extensive spread of this virus may be inevitable, as quarantine measures will lag behind its spread," the authors write.

The researchers detected viral RNA in the ferret nasal turbinate, trachea, lungs, hilar lymph nodes, and brain. The researchers note that inoculation of a different site, such as intratracheal, could cause a different pathology. "Clinically, the cellular tropism of H7N9 viruses may determine its spectrum of clinical disease and it may be advisable to examine human cases for signs of central nervous system affects," the authors write.

The team conducted similar tests in pigs. The researchers were able to infect the pigs with SH2, and the pigs shed the virus for 6 days. The pigs were unable to transmit SH2 infections to naive pigs or ferrets, either by direct contact or airborne exposure, although 1 direct-contact pig and 2 airborne-contact ferrets seroconverted after exposure to inoculated pigs.

The researchers believe that if poultry is the source of the H7N9 virus, it could become endemic in poultry. "If so, the opportunities for the H7N9 virus to evolve to acquire human-to-human transmissibility, or to be introduced to pigs, would greatly increase. To prevent this from happening, it may be advisable to reconsider the management of live poultry markets, especially in the urban areas," the authors conclude.

This study was supported by the National Institutes of Health, Li Ka Shing Foundation, the Area of Excellence Scheme of the University Grants Committee of the Hong Kong SAR, Shenzhen Peacock Plan High-End Talents Program, and Emergency Research Project on human infection with avian influenza H7N9 virus from the National Ministry of Science and Technology. The authors have disclosed no relevant financial relationships.

Sci Express. Published online May 23, 2013. Abstract

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