Study: H7N9 shows limited aerosol transmission in ferrets

May 23, 2013 (CIDRAP News) – The first study in animals to test-drive transmission of the new H7N9 virus found that it can spread by the airborne route in ferrets, but not very efficiently, an international team reported today in Science.

Transmission experiments in ferrets and other animals are used to help assess the risk and understand the possible spread in humans, which is especially helpful when new threats emerge, such as the H7N9 virus in China. Ferrets are thought to be the best animal model for studying influenza in humans, because there are parallels in clinical symptoms and transmissibility. However, some experts doubt how well they predict what will happen in humans.

Though the number of new H7N9 cases has tapered off, 131 people were been sickened by the new virus since the end of March, all but one of them in China. Clinical case studies have found that H7N9 can cause severe respiratory disease, and 36 deaths have been reported so far, with many patients still hospitalized.

In the new study, researchers from China, the United States, and Canada infected ferrets intranasally with the H7N9 virus, then placed unexposed animals in the same cages to see if the virus spread by direct contact. The H7N9 strain they used was from an index case-patient in Shanghai who died.

They also put uninfected ferrets in cages next to the infected ones to gauge aerosol spread, which plays a role in human-to-human transmission of flu viruses.

For comparison, the researchers repeated the same experiments with the 2009 H1N1 virus, and to further explore how the virus may spread to other animals, they infected pigs with the H7N9 virus and did tests to see if it can spread by direct contact and aerosol routes.

The study also assessed virus's infectivity and pathogenicity in ferrets.

Clinically, the H7N9 and 2009 H1N1 ferret groups had similar symptoms. Both groups were back to their normal activity levels 14 days after infection but had some residual sneezing and respiratory discharge. Virus shedding patterns were similar as well, with the viruses detected 1 day after infection and continuing at high titers for 7 days.

In the transmission part of the study, uninfected ferrets were put in cages with the infected ones 1 day after inoculation. Uninfected ferrets were placed in adjacent cages 10 cm away, with airflow directed at them from the infected ferrets' cages.

All three ferrets placed in the same cages with a pair of H7N9-inoculated ferrets got sick and shed the virus within 3 days of exposure. But only one of the three ferrets placed in adjacent cages became ill and shed the virus. In the H1N1 arm of the study, all uninfected ferrets got sick in the direct-contact and airborne experiments.

Pathogenicity testing on the H7N9-infected ferrets found viral RNA in the nasal turbinates, trachea, lungs, hilar lymph nodes, and brain. The team wrote that the findings suggest the H7N9 virus can infect the upper and lower respiratory tracts, lymph nodes, and possibly the brain. The tropism features might shed light on the disease's clinical spectrum. "And it may be advisable to examine human cases for signs of central nervous system effects," they added.

Since pigs are mammalian hosts of influenza A viruses and can serve as mixing vessels for novel viruses, including pandemic ones, the team was eager to test how transmissible and infective the H7N9 strain was in those animals.

Experts have wondered if there are other animal hosts of the H7N9 virus, and early in the outbreak suspicion turned to pigs, because thousands had been dumped in a river not far from the outbreak area and because pigs can mingle with poultry in farms and markets. However, no pigs in China have tested positive for the virus, including those that were dumped in the river.

The researchers found no virus shedding in pigs that were placed with or next to inoculated pigs. The H7N9 virus was able to easily infect the pigs only after intranasal inoculation. When they did the same experiment with the 2009 H1N1 virus, all direct-contact and airborne-exposed pigs got sick.

To see how the virus affected the pigs, the team looked for viral RNA in major organs, finding it at relatively low levels in the nasal turbinates, heart, liver, spleen, and intestine.

One of the main findings of the study was that H7N9 virus shedding occurred before most clinical signs developed, a pattern seen with pandemic and seasonal flu." 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 report says.

If H7N9 becomes endemic in China's poultry, the likely source of the of the virus, the risk of the virus evolving to become more transmissible among humans or spreading to pigs would increase, the team concluded. They suggested that health officials rethink how live poultry markets are managed, especially in urban settings.

Zhu H, Wang D, Kelvin DJ, et al. Infectivity, transmission, and pathology of human H7N9 influenza in ferrets and pigs. Science 2013 May 23; [Abstract]

See also:

Dec 23, 2011, CIDRAP News story "Fears about mutant H5N1 hinge on ferrets as flu model"

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