H7N9 virus is more transmissible and harder to detect than H5N1, say experts

BMJ 2013; 346 doi: (Published 22 April 2013) Cite this as: BMJ 2013;346:f2568

Re: H7N9 virus is more transmissible and harder to detect than H5N1, say experts

The outbreak of H7N9 avian influenza in China had killed 22 people and infected 108 by 24 April. Of the remaining patients, 14 have been discharged from hospitals after receiving treatment, and the other 72 people are being treated in designated hospitals. Thirty three cases occurred in Shanghai city, forty two cases occurred in Zhejiang province, twenty four cases occurred in Jiangsu province, four cases occurred in Anhui province, three cases occurred in Henan province, one cases occurred in Shandong province, one cases occurred in Beijing city, the official Xinhua news agency reported.

Though the virus is less deadly in humans than H5N1 – a deadly bird flu that has killed more than 60 percent of the people it has infected but has not become transmissible between humans – it is still an extremely dangerous strain. If this virus mutates to be transmissible from human-to-human it will be a major issue. More than 40% of the H7N9 cases involve victims that have not handled poultry, state media said. As the Chinese government openly begins to speculate about the possibility of human-to-human transmission of the H7N9 bird flu strain, an international team of experts, including some from the World Health Organization (WHO), have been deployed to investigate the disease in the country. If true, the development would quickly raise concerns of the disease outbreak leading to a pandemic. Flu experts have long warned once a particularly deadly strain of the flu, as H7N9 appears to be – it has killed 22 of the 108 people it has infected – becomes transmissible between humans, it can quickly spread. Although there was no evidence of human to human transmission, it does not mean that human to human transmission does not occur. And experts feared the prospect of H7N9 mutating into a form easily transmissible between humans, which would have the potential to trigger a pandemic. WHO said last week there was as yet no evidence of human-to-human transmission of H7N9. But it remained unclear how in a few cases, caregivers or neighbors of patients have also become ill. Even within the rare and isolated examples of potential clusters it is hard to determine if one person got it from another or if they were all exposed to the same source of infection.

And not all the H7N9 cases have clinical symptoms. For example, one 4 year old boy in Beijing, who displayed no symptoms and tested positive for the H7N9 virus, was considered a carrier of the strain and has been placed under observation to see if he develops symptoms. And medical teams have found that he had contact with a 7 year old girl, who was confirmed as Beijing’s first case of H7N9. Beijing Health Bureau deputy director Zhong Dongpo said, “This is very meaningful because it shows that the disease caused by this virus has a wide scope. It's not only limited to critical symptoms. There can also be slight cases, and even those who don't feel any abnormality at all. So we need to understand this disease in a rational and scientific way”. The appearance of case with no symptoms in human could make tracing more difficult, and may also mean that many people infected don’t get seriously ill and recover quickly, making the virus is less deadly than it appears.

H7N9 strain was not previously known to infect humans before cases turned up in China1. The studies from our scientists have found that a novel reassortant avian-origin influenza A (H7N9) virus was isolated from respiratory specimens obtained from all three patients and was identified as H7N92. Sequencing analyses revealed that all the genes from these three viruses were of avian origin, with six internal genes from avian influenza A (H9N2) viruses. Substitution Q226L (H3 numbering) at the 210-loop in the hemagglutinin (HA) gene was found in the A/Anhui/1/2013 and A/Shanghai/2/2013 virus but not in the A/Shanghai/1/2013 virus. A T160A mutation was identified at the 150-loop in the HA gene of all three viruses. A deletion of five amino acids in the neuraminidase (NA) stalk region was found in all three viruses. And close contact with infected birds is a likely source of transmission. But epidemiologists haven't yet been able to establish a clear and strong link. The source of the virus remains unclear because only a handful of birds - out of tens of thousands that have been tested - have been found to carry the H7N9 virus. And making the H7N9 strain hard to detect is that infected poultry display slight or no symptoms, unlike the H5N1 strain which kills birds and raged across the region in last decade.

Also, many of the patients have no reported history of contact with birds. And now many cities have closed markets that sell live poultry and ban poultry trading in a move to try to halt the spread of the H7N9. At the same time, Chinese health authorities have closely monitored hundreds of family members, caregivers, health workers and friends who have been in contact with patients and that only a handful have signs of H7N9 infections. In 2003 the SARS (Severe Acute Respiratory Syndrome) killed several hundred people worldwide. And the H5N1 avian flu virus has killed 371 people in 15 countries since 2003. So we should pay more attention to the spread of H7N9.

1. Parry J. H7N9 avian flu infects humans for the first time. BMJ. 2013; 346: f2151.
2. Uyeki TM, Cox NJ. Global Concerns Regarding Novel Influenza A (H7N9) Virus Infections. N Engl J Med. 2013.

Competing interests: No competing interests

09 May 2013
dai cong
min jiang
Department of Cadre Ward V, First Affiliated Hospital, China Medical University
No. 92 of Beier Road, Heping District, the city of Shenyang, Liaoning Province, China.