Do veterinarians catch influenza from their patients?

Most influenza subtypes are said to be diseases of birds, so it is somewhat surprising there hasn't been more study of poultry workers or veterinarians exposed to infected birds in the course of their work. A study by Gregory Gray and his team at , an epidemiologist at the Center for Emerging Infectious Diseases, at the University of Iowa's College of Public Health has just been published. The results cast into doubt the mantra that transmission of avian influenza virus to humans is difficult and uncommon.

Graduate student Kendall Myers in Gray's team studied the blood sera obtained from veterinarian volunteers who attended the Iowa Veterinary Medical Association meeting in spring 2004. They looked for evidence of immune reaction to a panel of avian influenza viruses and compared it to the sera of volunteers from the University of Iowa (Iowa City). The comparison sera were obtained in spring 2006. After removing vets who gave no history of occupational exposure to birds and excluding eleven control subjects who did report exposure, the two groups had data from 41 vets and 66 control subjects. The two groups were similar with the exception the vets had a higher proportion of males. Subtypes with hemagglutinin proteins H4 to H12 were examined.

This is called a cross-sectional design in epidemiology. It is comparing the proportion of antibodies to specific subtypes of avian flu viruses between people occupationally exposed to infected birds versus the unexposed, general population. If there is no transmission to humans, one would expect there to be no difference in antibody prevalence between the two groups. It is possible that both groups could have antibodies based on the non-specificity of the test (e.g., it measures things that are not related to either the specific subtypes or not related to avian influenza viruses) or because antibodies to these viruses is much more common in the general population that anyone thought.

The results, however, were consistent with the proposition that veterinarians who handle birds sick with various diseases that include bird influenza infections have much higher odds of having antibodies to specific subtypes than the general population. In particular, the odds ratios (a type of effect measure; ahem) were significantly elevated for neutralizing antibodies to subtypes H5, H6 and H7 (subtypes H5N2, H6N2 and H7N2). [For the geeks: H5 (adjusted OR, 16.7; 95% CI, 2.1 - ?), avian H6 (adjusted OR, 12.2; 95% CI, 2.0 -138.2), and avian H7 (adjusted OR, 17.7; 95% CI, 2.3 - ?, adjustment via logistic regression]. Subtypes H1 and H3 are known human subtypes and the authors looked to see if there was cross-reactivity between avian and human subtype reactors but didn't find an association. Sample size was not large enough to see if there were correlations between reaction to the various avian subtypes.

What do we make of this preliminary study? First, that veterinarians, poultry workers and those employed in culling infected flocks are at occupational risk of being infected with avian influenza viruses of various kinds, and that this risk is neither small nor unusual. This kind of study wasn't designed to see if clinical illness was associated with infection, so we don't know if humans infected this way get sick or not or whether they transmit it to others. Experience with the H7N2 outbreak in Wales and the H7N7 outbreak in The Netherlands suggests than answer to both questions is "yes." Second, as the authors point out, the subtypes of influenza subtypes in birds varies with locality, but H6 subtypes are relatively common, H5 and H7 much less so. But all three were implicated in this study, suggesting humans may be more susceptible to H5 and H7 subtypes. Since both are known to produce illness, this finding is consistent with experience. Third, the opportunity for reassortment and/or recombination from co-infection with human subtypes seems quite evident right here in the US. It is something to keep an eye on.

You can find the article here.


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Would checking our duck hunters find similar results?
(Or wild pig hunters?)

By crfullmoon (not verified) on 04 Jun 2007 #permalink

Fourth. How do we then explain the lack of positive seroprevalence data for H5N1?

crf: Possibly. But if the risk is much smaller (because their exposure is much less for two reasons: fewer of the birds they are in close contact with are infected, since they are more likely to be healthy, flying around birds with no signs or symptoms and hunters do not do it full time), then you need an awful lot of them to do the study. So it may not be feasible, practically speaking.

Your title contains 'influenaz' . Please transpose the last two characters.

RE duck hunters

serologic evidence of avian influenza infection in 1 duck hunter and 2 wildlife professionals with extensive histories of wild waterfowl and game bird exposure showed evidence of past infection with influenza A/H11N9

FROM Gill et al (2006) Avian Influenza among Waterfowl Hunters and Wildlife Professionals

llewely: Thanks. Have been busy at work and just saw your comment.

(Thanks ZoKun) Since really healthy animals might get away, and sometimes infected animals can be asymptomatic, and, no matter how infrequently they do it,
many hunters touch their faces, eat, drink, smoke, out there without infection control; I'd expect there to be chances of infection. People who prepare the game in the kitchen might also be infected; same as in other nations.

By crfullmoon (not verified) on 04 Jun 2007 #permalink

I always enjoy your essays and the many comments that follow, and I learn much from both. I post on avian influenza and sometimes other diseases like TB on a very conservative nursing forum with strict Terms of Service. Sometimes I am unable to link to this site because the comments violate that TOS. This is not to say that I would suggest any censorship or that folks should not post as they have been doing here.
What I would like to do is get permission to just post your essays alone, saying of course, who wrote them and some background on who you are. Nurses could really benefit from these essays, and many comments would be useful also, but alas some are problematic due to the TOS I have to work with. So how about it, Revere? You have my email address if you would like to discuss this further. This nursing forum has over 200,00 members. They could really use your expertise. Thanks for reading this.

By indigo girl (not verified) on 04 Jun 2007 #permalink

Unfortunately, this article was published, and comments are from people who are not influenza experts. Did it ever occur to anyne that these purported results might be all false? Check the article: titers of 1:10 and 1:20 are NEGATIVE and not indicative of infection!!! Yes, titers can decline over time and we do not understand the immune response to avian influenza virus infection over time, but showing an association with titers of 1:10 or 1:20 means nothing. Recall an article published in Arch of Int Medicine about purportedly "mild" H5N1 virus infections in northern Vietnam - there was no serology done. It is so sad that bad science is published as in the Vietnam article, and in this article. Few laboratories in the world have experience with and know how to interpret the microneutralization assay for human antibodies to avian influenza viruses. Careful labs will try to re-absorb appropriate viruses to present cross-reactivity. I don't believe any of these results (I have conducted these studies in several countries).

Fred: How do you explain the difference in >1:10 titers between the exposed and comparison group? The ORs are quite large. Are you explaining them by measurement bias? If so, where does the bias come from? Or are you saying this is a chance difference? The issue of chance is evaluated with statistical tests, so unless you are claiming systematic error -- the difference is not an interpretive issue -- I don't think you have a valid criticism. But I'm open to opposing arguments.

Fred, who would you consider to have that know-how re: microneuts? I work with Greg, and as noted in the acknowledgements of the paper, collaborators/consultants on this and other projects (including the Gill paper mentioned above, which also came out of our center) include Robert Webster and the influenza group at the CDC, among others.

Okay guys... You are all talking WAY over us morons who didnt take microbiology or became doctors. Anyone want to take the first stab at bringing it down to the level of say tenth grade. I think we all have the titers thing down pat, but Revere I think that third para got quite a few. Then Fred and Tom muddied it a bit.

Tara was clear that she thought that these guys should know what they are talking about. Okay, but I didnt understand it and I read a LOT!

BTW Tara/Revere others. Ducks Unlimited would likely open their membership rolls via an email for people to participate in a study for people with antibodies and samplings of their kills this fall. Probably could do it in a 20 state area if anyones interested.

By M. Randolph Kruger (not verified) on 04 Jun 2007 #permalink

In influenza vaccination, a resulting titer of > 1:40 would indicate only a threshold of success, so these numbers that Fred are quoting are pretty low. That doesn't invalidate the comparison, but it does mean that the strains probably gave rise to sub-clinical infections, bearing in mind that titers decrease over long periods of time.

Titers of >1:40 are associated with protection from repeated infection. Titers less than that indicate that an immune response occurred, but that there are insufficient antibodies to provide protection.

why should they have been sick ? They should have noticed...
The trick is to just slowly get enough exposure so to build
antibodies but without becoming sick.
Also, vets should have Tamiflu, Vitamin D, elderberries
and such, I assume.

Thousands of H5N1 Infected chicken cullers, wild bird and poultry markets sales persons not to mention poultry slaughter houses in multiple areas of Asia, Central/Middle East and Africa are readily available for testing.

Why not settle this question with the viral agent that is known higly pathogenic now? Go over there and do a cooperative study to settle the issue of "degree of infectivity" (at least for this virus) and once it is determined that the infectivity is high in a specific group, perform genetic comparisons on those individuals to determine if and why they are more prone to infection than others.

If the results demonstrate, as many "experts" believe, that the H5N1 highly pathogenic virus is actually quite difficult to become infected with and requires a special genetic predisposition, should not the world be significantly relieved?

Pandemic preparedness has taken on a very expensive and labor intensive life of its own worldwide, somewhat like WMDs in Iraq. Its time to bring true science into this entire phenomenon.

By John Triplett (not verified) on 05 Jun 2007 #permalink