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Efficient Human to Human Transmission of H7N7 Bird Flu
January 6, 2005
>>As at least 50 percent of the people exposed to infected poultry had H7 antibodies detectable with the modified assay, it was estimated that avian influenza A/H7N7 virus infection occurred in at least 1000, and perhaps as many as 2000 people. The seroprevalence of H7 antibodies in people without contact with infected poultry, but with close household contact to an infected poultry worker, was 59 percent. This suggests that the population at risk for avian influenza was not limited to those with direct contact to infected poultry, and that person-to-person transmission may have occurred on a large scale.<<
The report quote above indicates human to human transmission of H7N7 was quite efficient, with 59% of at risk individual testing positive for H7 antibodies, which were not detected in any of the control population, as reported earlier. Many of these contacts of infected poultry workers had mild or no symptoms.
These antibodies were detected in a modified assay that used equine red blood cells as targets instead of turkey red blood cells. The more efficient binding to the equine RBC is probably related to the observation that H7N7 virus has been isolated for infected horses since 1956, A/equine/Prague/1/56 (H7N7).
Thus, the ability for H7N7 to be efficiently passed from human to human may not be surprising. The vast majority of the patients in the Netherlands had conjunctivitis as a major symptom and there was only one reported death. Similarly, the two H7N3 infected poultry workers in Fraser Valley, British Columbia, or the two H7N2 infected cases in New York or Virginia also serological evidence of H7N2 infections.
The above instances of H7 infections would suggest that those with the most easily detected antibodies had mild cases (frequently with conjunctivitis) and only one fatality was reported, while those with lower levels (detected only with the modified test) were frequently asymptomatic.
Evidence for a similar population of people in Thailand or Vietnam infected with H5N1 but exhibiting mild or no symptoms is lacking. The recent study of patients in Thailand with respiratory disease failed to identify H5N1 positive patients with milder symptoms and a low case fatality rate. The group that had evidence of H5N1 infections had a case fatality rate of 67%, similar to the rate in officially confirmed cases on Thailand or Vietnam.