Study: Vets show signs of past avian flu infections

Jun 21, 2007 (CIDRAP News) – A study from the University of Iowa suggests that veterinarians who work with poultry have a significantly higher risk of infection with avian influenza, compared with nonveterinarians.

The researchers studied blood samples from 42 veterinarians and 66 healthy nonveterinarians and found that the vets were significantly more likely to have antibodies to avian flu subtypes H5, H6, and H7, indicating previous infection with the viruses. The scientists did not ascertain whether the vets had had any clinical illness caused by their infections.

"These data suggest that occupational exposure to avian species may increase veterinarians' risk of avian influenza virus infection," says the report by Kendall P. Myers and colleagues in the Jul 1 issue of Clinical Infectious Diseases. "Veterinarians should be considered for priority access to vaccines and antiviral drugs in pandemic planning."

They say their study appears to be the first search for serologic signs of a wide variety of avian flu subtypes in US veterinarians.

The scientists enrolled vets who attended a conference in 2004 and recruited control volunteers in the spring of 2006. Vets who had no exposure to birds and controls who reported such exposure were excluded, leaving 42 vets and 66 controls. Varying numbers of the vets had worked with chickens, turkeys, ducks, geese, and quail. Bloods samples were taken from both groups and analyzed for antibodies to avian flu virus subtypes H4 through H12 and human flu subtypes H1N1 and H3N2.

Compared with the controls, some of the vets were found to have increased levels of antibodies to subtypes H5, H6, H7, and H9. Further analysis of the data revealed no evidence that the elevated avian flu antibodies were accidental results of seasonal flu infection or vaccination (cross-reactions).

After adjusting for possible confounding variables and doing further statistical analysis, the researchers determined that the vets had significantly higher odds of having increased antibodies to three avian flu subtypes: H5 (adjusted odds ratio [OR], 16.7; 95% confidence interval [CI], 2.1 to infinity), H6 (OR 12.2; 95% CI, 2.0 to 138.2), and H7 (OR, 17.7; 95% CI, 2.3 to infinity).

"Veterinarians who reported having examined birds known to be infected with avian influenza presented an increasing trend of being seropositive, compared with veterinarians without this exposure and with control subjects," the report adds.

Acknowledging limitations of the study, the authors said their sample was too small to allow an assessment of whether any of the avian flu subtype findings were a result of cross-reactions against other avian subtypes.

They also said the study design didn't allow them to assess whether seropositivity was linked with any clinical signs of illness. "The nature, frequency, and severity of clinical illness caused by infection due to avian influenza remains unknown," they write.

The researchers note that when different flu virus subtypes infect the same host, there is a risk they will trade genetic material and give rise to a dangerous new virus. The authors say that increasing vets' rate of immunization against seasonal flu would limit the chances of such reassortment events and should be considered. Thirty-six of 75 vets (48%) in the study reported having had a flu shot in the preceding year, lead author Myers told CIDRAP News.

The researchers also say their findings suggest that vets who work with birds have an increased risk of zoonotic flu infection. "We posit that they should be considered for inclusion on priority access lists for pandemic vaccines and antiviral drugs," they conclude.

David A. Halvorson, DVM, a veterinary pathologist and avian flu expert at the University of Minnesota in St. Paul, said the study suggests that humans may be infected with avian flu viruses more commonly than was previously thought. It seems that, as has happened in Asia with the deadly H5N1 virus, "Humans intimately exposed to birds may get infected," he said.

He added that the findings reinforce doubts about the older view that pigs were the "mixing vessel" for new flu strains because they could be simultaneously infected with avian and human strains. "Rather, an avian virus may infect people directly and result in a recombinant with no other mixing vessel involved," he said.

However, Halvorson expressed doubt about the authors' idea that, in view of their findings, vets should have priority access to pandemic flu vaccines

"The only way that I see a need to rank veterinarians high for pandemic flu vaccines would be to possibly prevent them from infecting poultry," he said. "Once a virus is a human pandemic strain, it is difficult to imagine that poultry will play a significant role in transmission to people."

However, Myers asserted that the H5N1 strain could reach birds in the United States, putting those who work with poultry at risk. "The geographic range of the virus is rapidly expanding and efforts to contain it have been unsuccessful. Many feel that the spread of H5N1 into birds in this country is inevitable," she told CIDRAP News by e-mail.

She pointed to a recent commentary in Vaccine by her colleague Gregory C. Gray, the senior author of the veterinarian study, and two other authors. They make a case for promoting annual flu shots for those who work with swine and poultry and including those workers in the priority groups for pandemic flu vaccines and antiviral drugs. Among other reasons, they suggest that H5N1 infections in domestic ducks, geese, or pigs might not be readily identified, posing a threat to humans who work with them.

Myers KP, Setterquist SF, Capuano AW, et al. Infection due to 3 avian influenza subtypes in United States veterinarians. Clin Infect Dis 2007 Jul 1;45(1):4-9 [Full text]

See also:

Gray GC, Trampel DW, Roth JA. Pandemic influenza planning: Shouldn't swine and poultry workers be included? Vaccine 2007 May 30;25(22):4376-81 [Abstract]

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