Positive Pigeons: Dealing with questionable results in avian influenza surveillance
In the spring of 2022, North America’s wild bird populations experienced an outbreak of highly pathogenic avian influenza (HPAI). We at CWHC Western/Northern tested hundreds of birds submitted to us during the outbreak, which allowed us to see firsthand which species were being affected by HPAI. Not all species were impacted by the outbreak, but there were many regulars – namely waterfowl and those that eat them: geese, hawks, owls, eagles, corvids, vultures, and even foxes and skunks appeared to be very susceptible to HPAI. Other species – mostly songbirds – seemed virtually unaffected by the outbreak and we saw no positive cases during the outbreak. One species stood out as a big question mark: the Rock Pigeon. Out of nearly one hundred pigeons that we tested during the 2022 outbreak, two tested positive for H5 avian influenza.
Data on highly pathogenic avian influenza in pigeons is sparse. A study looking at experimental infection in pigeons suggested that although pigeons can become infected with HPAI, they are more resistant than most species and rarely become significantly ill from the virus. Another review on the topic found that pigeons are ineffective spreaders of the virus in that they do shed small amounts of virus through their digestive system, but the amount is not enough to infect other birds. Despite these studies, or perhaps because of these studies, we felt it was important to investigate these positive pigeons. Were these birds sick from HPAI or merely carriers of the virus? Or were these results in fact false-positives caused by contamination during sample collection or the testing process?
You might wonder why we would care about pigeons and whether they can catch HPAI. Rock Pigeons don’t have a great reputation as far as wild birds go. They are a non-native, invasive species that was introduced to North America from Europe during the 17th century. Many people think of them as pests as they are highly adaptable and able to colonize both urban and agricultural areas, living closely with humans in cities and on farms. For this reason, they are uniquely positioned as a bridge species between wild birds, human populations, and agriculture. If they were susceptible to HPAI they could play a major role in disease spread across urban and agricultural settings.
The two H5 positive pigeons that we received separately through our diagnostic lab were both found in the City of Saskatoon and submitted by local wildlife rehabilitators. One died after possibly striking a window, and the other was found sick and unable to walk or fly. Necropsies were performed on both birds to determine whether they showed signs of infection and to collect tissues to examine microscopically. No signs of infection were seen in either bird and no significant lesions were seen microscopically, leading us to suspect that the birds weren’t sick with the disease. We decided to run additional tests and found that we weren’t able to detect HPAI in the tissues of either birds.
Interestingly, both birds had recently eaten whole kernel corn which is often used in Avitrol bait, a product commonly used in pigeon removal. Avitrol causes seizures and abnormal behavior in birds, which is supposed to scare away other members of a flock and deter groups of pigeons from nesting in an area. Because Avitrol often kills the affected bird, it seemed a likely cause of death in these two cases. We tested the stomach contents for Avitrol and both tests came back positive, confirming that these two Rock Pigeons died from a cause other than HPAI.
We may never know with 100% certainty whether the two pigeons were false positives or whether they were infected but not sick with HPAI. Either way, these two cases were a reminder that false positives (and false negatives) do occur in disease surveillance and we must continue to ask questions and take steps to pursue answers when uncertainties arise.
Submitted by Erin Moffatt – CWHC Western/Northern