Chinese study finds very few mild H7N9 cases

May 9, 2013 (CIDRAP News) – Testing of more than 20,000 Chinese who had influenza-like illnesses (ILIs) in March and April turned up little evidence of mild cases of H7N9 influenza, researchers reported, as Chinese authorities revealed that another H7N9 patient has died.

Writing in Emerging Infectious Diseases, a team of Chinese and US researchers said that testing of samples from 20,739 patients who had ILIs revealed only six cases of H7N9 infection, a rate of .03%.

"We did not find evidence of widespread mild disease, suggesting that the reported cases reflect the true distribution of infection and not a surveillance artifact," the researchers wrote.

Meanwhile, a 56-year-old man in Henan province whose H7N9 case was confirmed 2 weeks ago died today, according to China's official Xinhua news service. His death raised the apparent death toll to 33, out of 131 cases. The World Health Organization had not acknowledged the death at this writing.

Xinhua said the man had several preexisting conditions, including hypertension, coronary heart disease, and a cerebral infarction. He had had no direct contact with birds before he got sick, but bird cages hung in the corridor of the building where he lived, the story said.

Henan has had four H7N9 cases so far; the other three patients have recovered, according to Xinhua.

Nationwide surveillance
The H7N9 surveillance study involved testing of patients at hospitals throughout China that participate in the Chinese National Influenza-Like Illness Surveillance Network. Patients who sought care for an ILI between Mar 4 and Apr 28 were tested, the report says.

The 20,739 samples that were tested came from 141 sentinel hospitals in 10 provinces and cities.

The six patients who tested positive lived in five provinces already known to have H7N9 cases. Two of the six were children (ages 2 and 4) who were not hospitalized, and the other 4 were adults who required hospitalization for pneumonia. Four of the six had had contact with live chickens or been to a live-poultry market.

"These data demonstrate that avian influenza A(H7N9) virus is an uncommon cause of ILI in any age group and in the areas reporting confirmed cases of influenza A(H7N9) infection," the report says.

The authors found that the share of medical visits for ILI increased during the study period, but the proportion of samples that tested positive for flu decreased, and there was no increase in flu viruses that couldn't be subtyped. This suggests that the increase in ILI visits was prompted by worries about H7N9 or by other respiratory viruses.

The researchers note several limitations of the study, including that the sentinel hospitals are in cities and therefore may not detect H7N9 cases in rural areas.

Poultry market findings
In other developments today, the Chinese Ministry of Agriculture said an environmental sample from a poultry market in southeastern China's Fujian province tested positive for H7N9, according to Xinhua. That raised the total number of positives from such settings to 52.

The isolate was genetically similar to one found in a pigeon sample that was collected at a Shanghai market on Apr 4, the story said.

Meanwhile, Taiwan reported via Twitter today that it has deposited the genetic sequence of the H7N9 isolate from its one human case in the GISAID database.

In another development, an updated H7N9 risk assessment from the European Centre for Disease Prevention and Control (ECDC) said more than 3,000 contacts of H7N9 patients have been traced, without uncovering any more cases. The detailed risk assessment was announced yesterday and posted online today.

Among other advice, the ECDC said it does not recommend tracing and treating contacts of airline passengers who are sick during a flight home from China, since there is no evidence of efficient human-to-human transmission of the virus. However, such tracing "could be considered if the case is later confirmed."

The ECDC strongly advised Europeans living or working in China to avoid live-bird markets.

See also:

May 8 Emerg Infect Dis report

May 9 Xinhua story on H7N9 death

May 9 Xinhua story about poultry market virus finding

May 8 ECDC risk assessment

GISAID Twitter feed

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