Potential occupational risk factors included working with birds known to be infected with influenza, number of years of exposure
to birds and the use of protective personal equipment, such as gloves and masks.
Veterinarians who reported having examined birds known to be infected with influenza presented an increasing trend of being
seropositive, compared with veterinarians without this exposure and with control subjects.
No other risk factors showed a statistically significant association with elevated antibody titers.
DVM: How many veterinarians participated in the study, and where and how did you select them?
MYERS: We enrolled a total of 75 veterinarians who were attending a conference of the Iowa Veterinary Medical Association in the
spring of 2004. For this study, we included only veterinarians who reported having exposure to birds, resulting in 42 veterinarians.
Most of them were from Iowa. Control subjects were volunteers associated with the University of Iowa (Iowa City) who
were enrolled in the study during the spring of 2006.
DVM: Did the study look at any other risks besides avian influenza?
MYERS: We also performed a seroprevalence study of swine influenza viruses. From the same group of veterinarians described above,
a total of 65 reported exposure to pigs. We found that these veterinarians also had increased odds of being seropositive for
the swine H1N1 and H1N2 viruses. This was published as a separate study and is available at
DVM: What is the significance of the higher levels of antibodies in the blood of DVMs against the milder H5, H6 and H7 strains,
indicating previous infections? Might that result in some immunity to H5N1?
MYERS: There are many different strains of each influenza subtype, but we tested for antibodies to only one strain of each of H4
through H12 subtypes.
Avian influenza can be categorized as either highly pathogenic avian influenza (HPAI) or low pathogenic avian influenza (LPAI).
All strains tested were low pathogenic avian influenza (LPAI). To our knowledge, only H5 and H7 subtypes are capable of becoming
Based on laboratory data, there is some cross-reactivity between strains, so having antibodies to a different H5 strain may
theoretically confer some partial immunity to H5N1.
On the other hand, there may be enough difference between the HPAI H5N1 strain and an LPAI strain that any partial immunity
is useless. Generally, antibody titers of at least 1:40 are necessary to provide protection, and most of the veterinarians
had titers lower than this. So, while the titers were high enough to indicate previous infection, they aren't considered
high enough to provide immunity.
The study is unique in that we found evidence of infection with these viruses when surveillance had not detected LPAI in Iowa
in many years. Distribution of hemagglutinin subtypes in birds varies geographically, temporally, and by species. However,
the H6 subtype appears to be commonly found in birds, whereas H5 and H7 are less common.
Why these veterinarians have elevated risk for antibodies to subtypes not frequently found in birds is unknown. Different
subtypes are likely to vary in their ability to infect and produce an antibody response in humans. In addition, surveillance
data for avian species are limited, and it is possible that outbreaks of these subtypes have occurred but have gone unnoticed.
DVM: Please elaborate on the statement in your paper, "Birds are the source of all influenza viruses in all other species." What
are the implications of that?
MYERS: Phylogenetic studies have shown that aquatic birds are the original source of all influenza viruses. Waterfowl are the natural
reservoir for all 16 hemagglutinin (HA) and 9 neuraminidase (NA) subtypes, which occur in many different combinations (H5N1,