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One of the great mysteries surrounding avian influenza (primarily, but not exclusively H5N1) is how often these viruses jump to and infect humans.
In the relatively short history of our observation of the influenza virus, only the H1, H2, and H3 strains have adapted well to people.
On rare occasions humans have been infected by other influenza strains, such as H5N1, H7N7, and H9N2. These strains are primarily seen in birds, but since they are constantly evolving and mutating, the worry is one might eventually adapt to human hosts.
The H5N1 bird flu virus remains at the top of our list of concerns, because it is endemic (in birds) in a number of countries, has shown the rare ability to infect humans, and when it does, it has a incredibly high fatality rate.
The two countries reporting the greatest number of human H5N1 cases are Indonesia and Egypt. The Case fatality rate in Indonesia remains well over 80%, but in Egypt, it runs a much lower (but still disturbing) 32%.
Which has led some researchers to wonder if their might be ongoing transmission of a `milder’ or even asymptomatic strain of H5N1 in Egypt.
In order to find out, a 3-year seroprevalence study has been designed that would track 1,000 people with various levels of exposure to poultry, and test them regularly for antibodies to the bird flu virus.
The full details are available at the link below. Here is the abstract.
Prospective study of avian influenza transmission to humans in Egypt
Ghazi Kayali , Richard J Webby , Xiaoping Xiong , Lobna S Sherif , Esmat A El-Ghafar and Mohamed A Ali
BMC Public Health 2010, 10:685doi:10.1186/1471-2458-10-685
Published:
9 November 2010Abstract (provisional)
Background
The highly pathogenic avian influenza (HPAI) H5N1 virus remains a public health threat and continues to cause outbreaks among poultry as well as human infections. Since its appearance, the virus has spread to numerous geographic areas and is now considered endemic in Egypt and other countries. Most studies on human H5N1 cases were conducted to investigate outbreak situations and were not designed to address fundamental questions about the epidemiology of human infection with H5N1 viruses. Our objective for this study is to answer these questions by estimating the prevalence and incidence rates of human cases and determine associated risk and protective factors in areas where H5N1 viruses are endemic.
Methods
We designed a 3-year prospective cohort study of 1000 individuals of various exposure levels to poultry in Egypt. At onset, we will collect sera to estimate baseline antibody titers against AI viruses H4-H16. Two follow-up visits are scheduled at 1-year intervals following initial enrollment. At follow-up, we will also collect sera to measure changes in antibody titers over time. Thus, annual prevalence rates as well as incidence rates of infection will be calculated. At each visit, exposure and other data will be collected using a specifically tailored questionnaire. This data will be used to measure risk and protective factors associated with infection. Subjects will be asked to contact the study team any time they have influenza-like illness (ILI). In this case, the study team will verify infection by rapid influenza A test and obtain swabs from the subject's contacts to isolate and characterize viruses causing acute infection.
Discussion
Epidemiologic studies at the influenza human-animal interface are rare, hence many questions concerning transmission, severity, and extent of infection at the population level remain unanswered. We believe that our study will help tackle and clarify some of these issues.
The few previous seroprevalence studies on H5N1 that we have to date show little or no asymptomatic or mild human infections.
Most of these are several years old, however, and are based on older clades of the virus.
In May of 2009 (see Cambodian Study Finds Rare Asymptomatic H5N1 Infections) we saw a seroprevalence study published in the Journal of Infectious Diseases conducted on more than 600 members of a Cambodian village where 2 human H5N1 cases were detected in 2006.
Antibody titers showed that only 1% (7 of 674) of the villagers tested had contracted, and fought off, the H5N1 virus. A figure much lower than many had expected.
Risk Factors Associated with Subclinical Human Infection with Avian Influenza A (H5N1) Virus—Cambodia, 2006
Sirenda Vong, Sowath Ly, Maria D. Van Kerkhove, Jenna Achenbach, Davun Holl, Philippe Buchy, San Sorn, Heng Seng, Timothy M. Uyeki, Touch Sok, and Jacqueline M. Katz
Abstract-Full Text-PDF Version (175 kB)
Another study – presented at the Options For Influenza Control VI Conference in Toronto in 2007, came up with similar results (see Seroprevalence Study).
This is how it was reported in MedPage Today .
OPTIONS VI: No Evidence For Mild Avian Flu
By Michael Smith, Senior Staff Writer, MedPage Today
Reviewed by Zalman S. Agus, MD; Emeritus Professor at the University of Pennsylvania School of Medicine.
June 19, 2007TORONTO, June 18 -- Highly pathogenic avian flu is rarely transmitted to people and apparently never in a mild or asymptomatic form, a Thai researcher said here.
A study of 901 people who lived near confirmed victims of the H5N1 avian flu strain showed no serological evidence that they had been infected and fought off the disease, Rapeepan Dejpichai, M.D., of the Thai Ministry of Health told attendees at the Options for Influenza Control meeting.
Given the relative dearth of good seroprevalence studies in the past, this prospective study could go a long way to help answer some very important questions regarding the threat posed by avian influenza viruses to the human population.
