Published Date: 2006-09-07 00:00:00
Subject: PRO/AH> Avian influenza, human (142): Cambodia
Archive Number: 20060907.2543
AVIAN INFLUENZA, HUMAN (142): CAMBODIA
A ProMED-mail post
ProMED-mail is a program of the
International Society for Infectious Diseases
Date: Thu 7 Sep 2006
From: ProMED-mail <firstname.lastname@example.org>
Source: CIDRAP News, Thu 7 Sep 2006 [edited]
Researchers who tested 351 Cambodian villagers after they had
extensive contact with avian influenza-infected poultry in 2005 found
that none had antibodies to the H5N1 virus, suggesting that it
doesn't easily spread to humans and that mild cases are rare.
"Our findings suggest that asymptomatic and mild H5N1 virus
infections had not occurred in the population we investigated,"
states a report published by Emerging Infectious Diseases [See: Vong
S, Coghlan B, Mardy S, et al. Low frequency of poultry-to-human H5N1
virus transmission, southern Cambodia, 2005. Emerg Infect Dis
The study was done by an international team of scientists, with
Sirenda Vong of the Institute Pasteur in Phnom Penh, Cambodia, as 1st
author. Preliminary results were reported at a conference last March.
Some experts have speculated that the current case-fatality rate for
H5N1 avian flu, about 59 percent, could be inaccurate because mild or
asymptomatic cases have gone undetected. Few such cases have been
found in the limited number of studies on the topic so far. The
village involved in the new study was the home of a 28-year-old man
who had Cambodia's 2nd confirmed H5N1 case. The man, from Banteay
Meas district in southeastern Cambodia, died in March 2005. After his
death, Cambodian health officials launched an investigation to find
additional cases and identify how the patient was exposed. The
researchers said the rural village setting allowed them to study the
epidemiologic features of the H5N1 virus in poultry and humans.
Focusing on a 1-kilometer radius around the patient's house, the
researchers conducted a retrospective survey of poultry deaths
immediately after the patient's H5N1 infection was confirmed. Two
months later they conducted a serologic investigation of members of
Villagers were questioned about recent illnesses in their chickens,
and researchers collected samples from sick, dead, and randomly
selected healthy chickens. The investigators also interviewed
villagers about their exposure to animals and the environment during
the previous 12 months and took blood specimens from each of them.
The poultry survey revealed that 42 households were likely to have
had an outbreak of H5N1 in poultry, reflecting an overall attack rate
of 27 percent among households with chickens. Only 2 households
reported the simultaneous deaths of ducks, and raising ducks with
chickens was not associated with deaths in chickens.
Cloacal specimens from 2 sick chickens tested positive for the H5N1
virus. The owner of the chickens reported that the man who died of
avian flu had spent daylight hours in his compound.
In the serologic survey, investigators found that substantial numbers
of villagers had had regular, close contact with poultry or pigs over
the previous 12 months. Despite this, none of the villagers reported
having had a febrile or respiratory illness during the same period,
and none of the residents had antibodies that suggested they had been
infected with the H5N1 virus. The researchers said their finding of
little H5N1 transmission between infected poultry and humans is
consistent with recent studies among healthcare workers and household
contacts of patients. The findings also suggest that none of the
villages had an asymptomatic or mild H5N1 infection, they added.
The authors said they couldn't explain why only one person became
infected when so many others had similar exposure and did not have
evidence of infection. "H5N1 virus transmission in humans may be rare
because it only occurs in exposed persons with unique host
susceptibilities and a predisposition to an abnormal inflammatory
response that results in severe and fatal outcomes, rather than
causing a broad spectrum of illness with mild disease and subclinical
infections," the article states.
The scientists found that some poultry-keeping practices appeared to
affect a flock's chance of contracting the H5N1 virus. Handling
poultry, cleaning their stalls and cages, and collecting feathers
seemed to reduce infections, while purchasing live poultry seemed to
increase the risk.
Though the H5N1 virus does not spread easily to humans, the
researchers cautioned that this could change as the virus continues
to circulate and evolve, and they called for routine, extensive
investigations of all H5N1 outbreaks among humans and animals.
[This investigation addresses some of the questions raised by S.M
Apatow and the moderator in the ProMED-mail post of 4 Sep 2006
entitled, "Avian influenza, human (140): atypical infections
20060905.2522". The result of the Cambodian study carried out by
Sirenda Vong et al., although limited in scope, provides no support
for any appreciable frequency in a population at risk of
asymptomatic, mild or atypical infections. The observations are
consistent with a situation in which rare genetic susceptibility of
the human host is the major determinant of outcome rather than virus
Other relevant discussion can be found in the CIDRAP News story of 27
Mar 2006, entitled "Mild H5N1 cases weren't missed in Cambodian
outbreak", and the WHO comment on the second of the 2 cases recorded
in Cambodia in 2006
<http://www.who.int/csr/don/2005_03_29b/en/index.html>. - Mod.CP]