AVIAN INFLUENZA, HUMAN (15): CHINA, TRANSMISSION QUESTIONS

AVIAN INFLUENZA, HUMAN (15): CHINA, TRANSMISSION QUESTIONS
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ProMED-mail is a program of the
International Society for Infectious Diseases
<http://www.isid.org>

Date: Wed 21 Jan 2009
Source: CIDRAP News [edited]
<http://www.cidrap.umn.edu/cidrap/content/influenza/avianflu/news/jan2109birds-jw.html>


China's recent spike in human H5N1 avian influenza cases [see
ProMED-mail archived reports below - Mod.CP] appears to lack the
hallmark of nearby poultry outbreaks, a development that some public
health officials worry could signal asymptomatic infections in birds.
Veterinary experts, however, suggest the pattern could point to
surveillance gaps or the consequences of routine vaccination. China
has reported 4 human cases so far this year [2009], 3 of them fatal.
According to World Health Organization (WHO) reports:

- The 16-year-old boy [treated in] Hunan province who died yesterday
[20 Jan 2009] had been exposed to sick and dead poultry [in Guizhou province].

- Investigators found that a 19-year-old girl from Beijing who died
on 5 Jan 2009 had contact with poultry before she got sick, but they
did not say whether the birds were ill.

- Authorities are still investigating the virus source in the other 2
cases, a 2-year-old girl from Shanxi province who is in critical
condition and a 27-year-old woman from Shandong province who died on
17 Jan 2009. The country's agriculture ministry said on 18 Jan 2009,
after the 2-year-old's infection was confirmed, that no H5N1
outbreaks have been detected in Shanxi or Hunan provinces [see
ProMED-mail reports, archived below - Mod.CP].

York Chow, Hong Kong's secretary for food and health, has called on
China to release more epidemiologic information on the recent human
infections and said that an apparent lack of poultry outbreak reports
against the backdrop of human cases raises questions about a possible
change in the virus or that asymptomatic H5N1-infected chickens might
be contributing to the spread of the virus. Chinese officials have
said they have found no evidence that the virus has mutated to allow
easier human-to-human transmission, according to media reports.

This isn't the 1st time that health officials have voiced their
suspicions about asymptomatic poultry infections in China. In 2006,
Zhong Nanshan, director of the Guangzhou Institute of Respiratory
Diseases, suggested that 2 victims might have caught the virus from
chickens that were carrying it asymptomatically.

Avian influenza experts say the size and nature of China's poultry
population creates a difficult surveillance task. Jan Slingenbergh, a
senior animal health officer for the United Nations Food and
Agriculture Organization (FAO) told CIDRAP News that China has
roughly 4.6 billion chickens, 700 million ducks, and 300 million
geese that are distributed somewhat unevenly throughout the country.
He said the ducks gravitate toward the double-crop rice growing areas
in southern and southeastern China, which are thought to be the main
risk areas because the H5N1 virus keeps circulating in ducks. Geese
head toward single-crop rice growing areas in the less rainy
northeastern and extreme western part of China, Slingenbergh added.
Meanwhile, he said chickens are kept everywhere people live,
particularly in urban areas and coastal ports.

Slingenbergh links the low level of poultry outbreak reports to
China's poultry vaccination policy. "The entire national flock is
kept under a rigid vaccination blanket amounting to 11 billion
applications per annum," he said.

He said he doubts that H5N1 in China is evolving toward a low-
pathogenic virus. "Vaccination creates a rather sparse geospatial
mosaic of susceptibles, which may even enhance the pathogenicity
level," he said, adding that evidence from Viet Nam, where most
Chinese viruses spread to, suggests that the virulence increased
between 2002 and 2007 when measured by infecting and gauging shedding
in young mallards.

Vincent Martin, a senior technical adviser in the FAO's Beijing
office, told CIDRAP News that Chinese officials obtain a lot of
samples from farms and live bird markets each year to monitor
asymptomatic H5N1 infections among the birds. "Regularly, they find
the virus but do not detect any outbreak in the surrounding areas,"
he said. However, a combination of factors makes detecting the virus
difficult, Martin said. Several strains of the virus are circulating
in China, and ducks can excrete the virus without showing symptoms or
only exhibiting mild ones. In addition, suboptimal vaccination can
mask the symptoms without stopping viral shedding.

More intensive surveillance and monitoring efforts are needed in
China to detect new outbreaks and identify viral circulation that is
going unnoticed, Martin said, "to avoid a situation where humans
serve as sentinels and reveal infection in birds. The concern is,
therefore, that the current surveillance is unable to provide a
complete picture of the [high-pathogenic avian influenza]
epidemiological situation in domestic birds and should be
strengthened and improved in order to meet the challenge we are
currently facing," he said.

Three Chinese government ministries yesterday [20 Jan 2009] issued a
joint order for local health, agriculture, and commerce offices to
work together to improve surveillance and management of the country's
live poultry markets, Xinhua, China's state news agency, reported
today [21 Jan 2009]. The government urged local offices to close live
poultry markets in urban areas, if possible, or disinfect the markets
daily if they can't be shuttered. The offices were also ordered to
conduct daily surveillance and reporting and collaborate when they
detect the H5N1 virus.

Les Sims, from Australia's Asia-Pacific Veterinary Information
Services and a consultant to the FAO, said though humans are once
again acting as sentinels for infections in poultry, so far there is
no evidence to support asymptomatic disease as the reason for absence
of reported poultry outbreaks in China. "Vaccination will alter the
clinical appearance of disease if the flock is infected, but on a
flock basis, some disease will be detected. Infection is not silent,"
he said. He added that infected vaccinated flocks, for example, have
lower mortality rates with fewer birds showing classical symptoms of
the disease. "If vaccines are used, veterinary and medical
authorities have to accept that one of the signals they used to rely
on for detecting infection in poultry -- high mortality -- needs to
be modified," Sims said. Infected poultry can still shed small
amounts of the virus, even when the vaccine is a good match and the
birds are vaccinated properly, he said. "This has always been the
case and so can't explain the current situation in China."

The issue of less severe infections in vaccinated poultry is creating
negative sentiments about the measure, Sims said, but he added that
China has maintained a close match between the circulating strains
and the vaccine antigen, which greatly diminishes the viral load in
poultry. "The benefits of vaccination in reducing viral load need to
be considered and balanced against the changes in disease appearance
that will occur if a vaccinated flock is infected," he said. "The
situation in China would almost certainly be much worse if
vaccination was not used."

Sims said he's not surprised that some poultry infections go
undetected, given the size and make-up of China's poultry population,
along with the modified appearance of the disease in vaccinated
poultry. He suspects, though, that under-reporting of the disease
might be one factor that keeps the number of outbreak reports low.
Farmers who raise poultry for their livelihood have little incentive
to report the disease. A seasonal surge in poultry and human H5N1
cases in the winter isn't unexpected, he said. "Winter peaks have
been seen previously and are probably linked to the increased trade
in poultry for various festivals and enhanced viral survival due to
cooler conditions," Sims said.

[Byline: Lisa Schnirring]

--
Communicated by:
ProMED-mail <promed@promedmail.org>

[The 4 human cases (and 3 fatalities) of avian A/H5N1 influenza
reported in China since the beginning of the year (2009), after a
lapse of 10 months with no human cases, has raised fears that the
virus may have evolved into a form more easily transmissible to
humans, particularly as the cases were not coincident with large
scale outbreaks of disease in poultry. However, this interpretation
can be discounted on several grounds, namely: The cases appeared in 4
different provinces; there was no contact between the patients, no
onward transmission to other humans, and no evidence of genomic
changes. Furthermore, at least 2 of the 4 cases had contact with
poultry (although not confirmed to be virus-infected).

The CIDRAP article is in part an in depth discussion of the problems
of disease surveillance and veterinary control in the poultry
industry in China. This discussion is extended by an evaluation of
the consequences of vaccination of poultry and considers the
possibility that humans may play a role as sentinels in detection of
continued circulation of virus in vaccinated populations in the
absence of overt avian disease. - Mod.CP]