Transmission of H7N7 avian influenza A virus to human beings during a large outbreak in commercial poultry farms in the Netherlands

Summary

Background

An outbreak of highly pathogenic avian influenza A virus subtype H7N7 started at the end of February, 2003, in commercial poultry farms in the Netherlands. Although the risk of transmission of these viruses to humans was initially thought to be low, an outbreak investigation was launched to assess the extent of transmission of influenza A virus subtype H7N7 from chickens to humans.

Methods

All workers in poultry farms, poultry farmers, and their families were asked to report signs of conjunctivitis or influenza-like illness. People with complaints were tested for influenza virus type A subtype H7 (A/H7) infection and completed a health questionnaire about type of symptoms, duration of illness, and possible exposures to infected poultry.

Findings

453 people had health complaints—349 reported conjunctivitis, 90 had influenza-like illness, and 67 had other complaints. We detected A/H7 in conjunctival samples from 78 (26·4%) people with conjunctivitis only, in five (9·4%) with influenza-like illness and conjunctivitis, in two (5·4%) with influenza-like illness only, and in four (6%) who reported other symptoms. Most positive samples had been collected within 5 days of symptom onset. A/H7 infection was confirmed in three contacts (of 83 tested), one of whom developed influenza-like illness. Six people had influenza A/H3N2 infection. After 19 people had been diagnosed with the infection, all workers received mandatory influenza virus vaccination and prophylactic treatment with oseltamivir. More than half (56%) of A/H7 infections reported here arose before the vaccination and treatment programme.

Interpretation

We noted an unexpectedly high number of transmissions of avian influenza A virus subtype H7N7 to people directly involved in handling infected poultry, and we noted evidence for person-to-person transmission. Our data emphasise the importance of adequate surveillance, outbreak preparedness, and pandemic planning.

Introduction

On March 1, 2003, the Dutch Ministry of Agriculture announced a ban on the export of all poultry and poultry-related products. This measure was taken in response to outbreaks of a disease highly lethal to chickens on six farms in the province of Gelderland, an area with a high density of poultry farms. The infection spread to 255 farms, and the Ministry's order for the culling of all infected flocks led to the killing of around 30 million chickens—about 28% of the total chicken population in the Netherlands. The annual export value of poultry and eggs contributes €284 millionto the Dutch economy every year.

The pathogen was identified as a highly pathogenic avian influenza A virus (HPAI) subtype H7N7, and was related to viruses detected in 2000 during routine surveillance of avian influenza in ducks in the Netherlands. All internal genes of the viruses were of avian origin.¹ Epizootics and solitary infections of A/H7N7 avian influenza virus in poultry have been reported in surveillance studies, and humans were thought to be at low risk of infection, although there have been occasional reports of H7N7-associated conjunctivitis.2, 3, 4 In 1996, influenza A/H7N7 virus (A/England/ 268/96) was isolated from a 43-year-old duck owner with mild one-sided conjunctivitis.4, 5

In the week following the announcement of the avian influenza outbreak, four independent anecdotal reports suggested an increased incidence of health complaints, particularly conjunctivitis, in people involved in the control of the epizootic. Coincidentally, data from routine influenza virus surveillance suggested a late seasonal increase in the rate of human influenza viruses. With the almost simultaneous confirmation of an influenza virus A/H7N7-associated conjunctivitis and human influenza virus A/H3N2 in two different veterinarians involved in control measures for the HPAI epizootic, physical prevention measures were reinforced and we began vaccination and actively seeking out people with symptoms?ie, cases.

Here, we describe the epidemiological and virological results of our case finding and the preventive measures taken to control the outbreak in human beings.