Gordon A Chalmers, DVM
Lethbridge, Alberta, Canada
Article Published 01 October 2007

The spread of Highly Pathogenic avian influenza virus (H5N1) among domestic and wild birds, and the illness and deaths that have occurred in humans in direct contact with infected poultry, also raise concerns about the effects of this virus in pigeons and the potential role these birds might play in the spread of this disease.

A scientific paper published by German researchers in July, 2006 presented some interesting and very important information regarding a Highly Pathogenic Avian Influenza H5N1 virus (designated A / Chicken / Indonesia / 2003, meaning that it was a Type A influenza virus that was recovered from sick chickens in Indonesia in 2003) after it was inoculated into experimental pigeons. This strain was the cause of recent outbreaks of influenza in domestic birds and humans in several countries in Southeast Asia. Although several experiments in the past suggested that pigeons were inherently resistant to avian influenza viruses, there have been reports that indicated a certain level of susceptibility of pigeons to some strains of these viruses, especially to Highly Pathogenic H5 strains. Another earlier study from Germany had also concluded that pigeons could be more susceptible to some H7 strains of the virus than to H5 strains.

To test pigeons for their susceptibility to this currently ‘hot’ H5N1 strain of avian influenza virus, the researchers used 14 four-month-old pigeons that they inoculated by droplets in the eyes and nostrils with ½ cc of fluids collected from fertile chicken eggs that had been injected with the virus and incubated to increase the amount of virus present. As well, to determine whether these inoculated pigeons could shed the virus into their environment and infect other birds, 48 hours after the pigeons were inoculated, these investigators placed five 12-week-old normal chickens in the isolated compartment housing the pigeons.

Another group of five chickens was inoculated with the same fluid containing the virus and held in a separate isolated compartment. Four pigeons that were not inoculated with the virus were also held in a separate compartment. Blood samples were collected from the experimental pigeons prior to inoculation and again before they died or at the time they were killed at 19 days after inoculation.

Results showed that five of the 14 inoculated pigeons died over a period of 19 days; one pigeon survived for five days, two pigeons survived for seven days, and two pigeons survived for 19 days when they were killed because they had developed severe nervous signs (mild to severe paralysis of the wings, twisted necks, flicking eyes). None of the remaining nine inoculated pigeons developed any sign of illness; however, examination of blood samples from these nine birds showed that they had developed significant levels of antibodies against the inoculated virus (which is evidence that these birds were actually infected with the virus even though they had not developed any sign of illness).

At post mortem, the five inoculated pigeons that died had only mild inconsistent changes such as some hemorrhage under the skin and occasional excess fluid surrounding the heart. One of these five pigeons had severe yellow-brown discoloration and softening of one side of the brain. Microscopic examination of tissues from the five dead birds found that important changes of inflammation indicating infection were confined to the brain; no other tissue in the body had detectable changes caused by the virus. There was also no detectable change in any tissues of the nine inoculated pigeons that didn’t develop signs of illness.

All of the chickens inoculated with the virus became mildly depressed, lost appetite, developed slight respiratory distress, and died within an 8-12 hour period two days after being inoculated with the virus. In contrast to the pigeons, in which significant changes were confined to the brain, many changes were found grossly and microscopically in a variety of tissues in these chickens.

The normal chickens housed together with the inoculated pigeons did not become ill and did not have tissue changes that could be associated with avian influenza. As well, none of these chickens developed antibodies against avian influenza. These findings showed that the virus was not shed or transferred from the inoculated pigeons to these normal chickens.
This experiment showed clearly that pigeons are susceptible to this Highly Pathogenic strain of the H5N1 avian influenza virus and that this virus had the marked capability of infecting the brain of pigeons but not other tissues of the body. It also showed that most of the infected pigeons developed antibodies to the virus even though they did not become ill, which is evidence of the fact that although these birds became infected, they were successful in destroying this virus. As well, of great importance was the fact that normal chickens in direct contact with the infected pigeons did not become ill or develop antibodies against the virus. These findings indicate the highly important fact that pigeon-to-chicken transfer of the virus did not occur. (In this study it would also have been very interesting and important to have placed a group of normal healthy pigeons with influenza-infected chickens to determine if, by this means, pigeons could become infected with this pathogenic strain of virus.)

In a number of affected species of birds, influenza virus has been found, not only in brain, but also in many other tissues as well, so the authors of this study commented that the marked tendency of this virus to infect only the brain of pigeons appears to be unprecedented among birds. The authors did note however that the virus might be more widely found in other tissues of pigeons at earlier stages of the infection after inoculation.

In summary, for pigeon fanciers the findings of this study represent a bad news/good news story. Yes, some pigeons can become infected with and die from Highly Pathogenic H5N1 avian influenza virus; in this study, 35.7% of the inoculated birds became ill and died vs 64.3% that were also infected but remained healthy. On the other hand, at this stage of our knowledge, pigeons appear to be unique among birds in the fact that the infection seems to affect only the brain and that such pigeons don’t shed and transfer the infection to susceptible chickens. (In future, this fact might also become very important in terms of the relationship among strains of the avian influenza virus, pigeons and humans.) It is very obvious and important that further detailed studies are needed to look more fully into the subject of shedding of Highly Pathogenic avian influenza viruses by pigeons. Because events in biological systems are seldom black and white, in future it will be important to learn if influenza infections in pigeons will continue to be restricted to the brain and whether pigeons will also continue to be non-shedders of the virus. The important subject of shedding of virus by infected pigeons is currently under further investigation by the authors of this study.