INTRODUCTION

Influenza has been known since 1878, and is caused by a virus. There are three types of influenza viruses, namely, A, B and C. Type A viruses have been recovered from humans, swine and horses, and occasionally, from birds and other mammals. Types B and C are usually isolated only from humans. The virus currently causing problems in southeast Asia is a Type A influenza virus.

HOW  IS  IT  SPREAD?

Species of birds that become infected with the virus of Avian Influenza shed it from the respiratory tract, from the eyes, and in droppings. Spread of the virus can occur by means of droplets of liquid sneezed by infected birds, or in their droppings. Vehicles, equipment, cages, clothing and insects in contact with infective droplets or droppings are also means of spread. The high rate of infection allows for the maintenance and emergence of new and potentially highly dangerous strains, by means of mutation and/or genetic reassortment.

FORMS   OF   INFLEUNZA

Avian influenza is recognised in two forms: (a) highly pathogenic (b) low pathogenic

The highly pathogenic form spreads rapidly among flocks of poultry and is often highly fatal. It has been noted that one gram (about one-thirthieth of an ounce) of contaminated droppings from infected chickens can contain enough virus to infect one million birds!

The low pathogenic form generally causes only mild disease.

AVIAN  INFLUENZA  (H5N2)  IN  NORTH  AMERICA

The eminent Canadian Vet, Gordon A. Chalmers of Alberta, has outlined the Avian Influenza episodes in the north-eastern USA in the  1980s/1990s, and the results of tests carried out then.

During the outbreak of Avian Influenza (H5N2) in 1983-84, scientests conducted a survey of wildlife to determine the potential of wild birds to spread the disease locally. Included in the survey were (a) wild and free-flying domestic ducks and geese. (b) wild and free-flying domestic birds associated with poultry farms. (c) mice and rats found inside and around infected poultry. (d) wild birds of any species reported sick or dead in the quarantine zone.

Attempts to isolate the virus were conducted on 4,132 birds and rodents collected within the zone. Among these were 473 pigeons. None of the 4,132 samples were positive. Blood samples from 2,147 non-aquatic birds, including 383 pigeons, were negative for antibodies to Avian Influenza - an indication that infection had not occurred. It is important to note that experimental attempts to infect pigeons with this strain of Avian Influenza did not result in either multiplication of the virus in these pigeons, or evidence of antibodies in the blood. The results of these tests indicated that pigeons were not infected with Avian Influenza and did not spread it.

In late 1992, antibodies to Avian Influenza were found in blood samples from a commercial flock of turkeys, again in north-eastern USA. Again, there was no connection with pigeons.

In the 1993 outbreak, in the period from February to May, 160 pigeons were tested, and in every instance, all pigeons tested were negative for antibodies to Avian Influenza.

In yet another study published in 1996 on the susceptibility of pigeons to Avian Influenza, Dr. Chambers tells us, it was found that groups of pigeons inoculated with two strains of highly pathogenic influenza virus or two strains of non-pathogenic virus remained healthy during the twenty-one day trial period, did not shed virus, and did not develop antibodies to this disease - further evidence that pigeons were not a factor in the spread of this disease.

It might be important to note that the strain of Avian Influenza Dr. Chalmers discussed was the H5N2 strain, and not the H5N1 strain.

DR.  CHALMER'S  UPDATED  VIEW   ON   H5N1   STRAIN

However, Dr. Chalmers gave the following update at the end of 2005.

Over the past few years, H5N1 itself has undergone some changes. Just as pigeons are subject to the laws of genetics, so are viruses. Just as pigeon genes are subject to genetic mutations, so are viral genes subject to mutations. Just as there are different strains of pigeons (for example, Jansens, Buschaerts etc.) there are emerging different strains of the H5N1 virus. Thus the Eurasian strain of H5N1 has now been specifically named "Asian H5N1 HPAI" - the 'HPAI' stands for 'Highly Pathogenic Avian Influenza'. Unfortunately, it has extended its host range to now include pigeons. This does not mean that pigeons have become its natural host, but it can infect them and cause the disease in them.

Dr. Chalmer's views seem to have been confirmed by the report of tests done in the Peoples Republic of China.

SEVEN   STRAINS   OF   H5N1   VIRUS  USED   IN   TEST

Several avian influenza outbreaks occurred in Asia during 2003-2004. The following is the result of tests carried out in the Peoples Republic of China in early 2004.

Seven strains of H5N1 were isolated and named. Each strain was given a four-letter identification code.

CKDW, CKJZ, CKTM and CKXF were isolated from free-range chickens in vilages.

GOFZ, which was responsible for 50% of the cases and 30% of the related deaths in infected geese, was isolated from a farmed goose in a flock.

EWHC was isolated from a Eurasian widgeon in a large lake where many widgeons were found dead.

DKXF was isolated from an asymptomatic domestic duck on a farm where ducks sporadically died.

HOW   THE   TEST   WAS   CONDUCTED

The following were selected fot the test: six-week-old white Leghorn chickens, four-week-old quails and  four-week-old pigeons. They took sixty-four of each, all free from avian influenza (H5N1) , and divided them into eight groups of eight birds.

For each species, seven groups were intravenously inoculated with one of the seven avian influenza (H5N1) isolates, and one group received allantoic fluid as a negative control.

Birds were monitered daily, and the number of deaths was recorded until day ten post infection, at which point all surviving birds were killed.

Viral tissue tropism was analized in dead (on the day of death) and surviving birds (day ten when killed) by a double-antibody sandwich ELISA for the nucleoprotein of influenza A virus.

The mean (average) time to death was calculated, This is referred to as MTD hereafter.

THE   RESULTS

All chickens infected with the CKDW or EWHC isolates died within twenty-four hours (the MDT was less than 1)

All chickens injected with GOZF, CKXF or CKJZ died within two days (MDT less than 1.5)

CKTM caused a 75% death rate for chickens, with a MDT of 4.7.

All chickens injected with DKXF survived.

Quails were also susceptible to infection, but the MDTs in quail (2.8 to 6.3) were higher than those in chickens.

In quails CKDW, EWHC, GOFZ and CKJZ caused a 100% death rate. CKXF and CKTM caused 62.5% and 37.5% death rates respectively.  DKXF did not cause death.

Pigeons were  more resistant to these isolates than the other birds.

The highest death rate  for pigeons was 37.5% for the EWHC group, with an MDT of 4.3.

In pigeons, CKDW and GOZF caused a 25% death rate with an MDT of 5.0. CKJZ caused a 12.5% death rate with a MDT of 6.0. The strains CJTM, CKXF and DKXF exhibited low pathogenicity in pigeons, with no deaths by day ten.

TISSUE   SAMPLES   TESTED

Mort chicked tissue samples had positive ELISA results for avian influenza (H5N1) infection; no particular tissue tropism pattern was found in quail samples. In contrast, all tested pigeon glandulas stomach samples from the seven infected groups had positive results, while most other organs had negative results.

CONCLUSIONS

Early studies showed that pigeons were more resistant to the highly pathogenic avian influenza (H5N1) strain (A/chicken/Hong Kong/220/97; HK/220) than other birds. In the experiment it was demonstrated that isolates from 2004 could infect pigeons and could be isolated from pigeon tissues, especially from glandular stomachs. Sequence analysis indicated that the avian influenza isolates used in the study and the HK/220 strain share high homology at the animo acid level for the HA (95% - 97%0 but not the NA gene  (homology 79% - 88%) . This study indicates that the pigeon may be an asymptomatic carrier of avian influenza virus.