|Click on image to enlarge.|
H7N9-positive birds and humans (see MOA report) in
April 2013. 17x more humans were virus-positive
than humans were PCR/symptom positive. Based on
Li et al's April 24th New England Journal of Medicine
article from a similar time period which uses observation
for signs of disease among 1,251 followed contacts of 81 cases and
sentinel surveillance PCR data from 5,551 humans to
identify H7N9 cases).
In a more detailed report from MOA from 30th May 2013, 88 of 899,758 [0.009%] duck, pigeon, chicken (722,380 or 80% of all the samples tested), wild bird, pig, geese, "other" animal or environmental samples were virus [197,389 of the samples tested this way] &/or antibody [702,369 of the samples] positive (chicken, duck and pigeons were the positives; 3 were positive for both). The report presented by Zhang Zhongqiu does not make clear how many swabs and bloods were tested per animal so I'll just talk about sample numbers. The report notes that there were no clinical cases reported from 44 million farming households and no positives from 51,876 samples of 746,212 samples (?chickens) sent to Hong Kong; monitored by the General Administration of Quality Supervision, Inspection and Quarantine, China) nor among the 120/samples being tested per day in Hong Kong. In 1,874 samples collected from Henan and Jiangxi provinces, none were positive. Transmission among chickens was possible but was not efficient among ducks.
- Lam and colleagues (previously reviewed) identified 8 avian H7N9 strains from 1,308 (0.6%) chickens (95% of samples), ducks, pigeon and geese samples collected from live bird markets (LBMs) in Rizhao, Shandong province (about 9 times more than the 1st MOA study above, if they can be compared directly).
- Yang and colleagues (previously reviewed) found H7N9 antibodies in 25 (6%) of 396 humans poultry workers (none prior to 2013) but only 9 of 1,129 (0.8%) members of the general community showed some weak sign of past exposure (or cross-reaction with another influenza). No viral RNA was found in these poultry workers.
- Wang and colleagues, writing in the Journal of Infectious diseases, recently traced the source of some cases in the Hangzhou region of Zhejiang, to LBMs. 95 samples from chickens (n=47 samples), ducks (n=9), quails (n=2), pigeons (n=3) and poultry handlers and 4 from water were inoculated into eggs and were tested by real-time RT-PCR, within the first 2-weeks of April 2013. H7N9 RNA was found in 41/85 (48%) of samples. 40% of the chicken samples, 89% of the duck samples and a third of the pigeon samples. No human or environmental samples were positive. The authors concluded that migratory birds would continue the spread of H7N9 viruses and that their findings highlight LBMs as the major source of infection an as such control measures are needed.
- Shi and colleagues reached a similar conclusion in April in the Chinese Science Bulletin. "Strong measures" were needed to control the spread of H7N9 in order to prevent more infections. This followed the testing of 970 samples of drinking water, soil, cloacal and tracheal swabs from LBM poultry in Shanghai and Anhui province using egg inoculation. All 20 (10 from chickens) of the H7N9 isolates came from LBMs in Shanghai, confirming high genetic homology across the H7N9 genome from human H7N9 cases.
Today's Bloomberg article quotes researchers' concerns that the cooler weather will drive the re-appearance of H7N9, since influenza usually reaches epidemic levels during cooler months. In other words they believe this particular strain of H7N9 (the one infecting humans) was never removed from the ecosystem.
Re-opening of the LBMs has been ongoing since June in Shanghai municipality and Zhejiang and Jiangsu provinces, albeit in a more regulated fashion. The cleansing of the markets after culling more than 560,000 poultry from LBMs as of May 2013 combined to precede the precipitous decline in what had been an alarming rate of new cases in those regions. Is testing of these markets an ongoing process?
With the markets refilling from farms located in rural regions with exposure to mobile wild bird populations that may (albeit infrequently) carry H7N9 (and many other influenza viruses including its components), the risk of fresh outbreaks among humans is also growing.
It's a numbers game.
Even 1 human case, like the one we saw infected this week could signal an even wider level of circulation of H7N9. Let's hope testing will make sure our number's not up this time around.
Editor's Note - the figure was altered 01.02.14 to correct an error in the proportions and to adjust down the number of contacts since not all had been followed.