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The whole
truth about bird flu
By Dr Shahan Azeem &
Prof Dr M. A. Muneer
It's an early morning in Karachi. As a poultry farmer gets up
and goes to his flock, he sees some of the birds gasping.
Their heads are swollen, legs thave turned reddish and water
is running from their eyes.
Some have died, and many more are dying.
He gets alarmed because he has invested a fortune in the
flock. On postmortem, it is revealed that the birds had
respiratory tract infection with hemorrhage in trachea.
Blood serum tests are done on some live birds.
Serum analysis reveals
influenza virus infection. With no cure for the disease, the
whole flock dies - a huge loss for the farmer. Soon the
disease spreads to nearby farms.
Commercialization of animal-rearing enterprises has lead to
increased population density of birds per farm. Overcrowding
of farms in an area and increased means of communication and
transportation have raised the incidence of infectious
diseases to much higher level as compared to past years.
The recent outbreak of bird flu in south-east Asia and its
potential to infect human beings has raised enormous concerns
all over the globe.
Due to widespread publicity via mass media and a recent
outbreak of highly pathogenic avian influenza (HPAI) in
poultry farms of Karachi, which killed over 3.5 million layer
birds, most people are not consuming eggs and meat, the only
economical sources of proteins in the country.
A report in Dawn (Jan. 31) says the consumption of chicken
items has dropped by 25-30 per cent. According to the Pakistan
Poultry Association, daily consumption of chicken in Karachi
had gone down to 125,000-150,000 birds currently as compared
to the normal consumption of 300,000-350,000 daily.
Before delving deep into the nature of the bird flu and its
implications on public health, let us understand the nature of
influenza virus first.
The causative agent belongs to the virus family
Orthomyaxoviridae and genus influenza A. The virus infects
human beings, pigs, equines and various avian species
including poultry and wild aquatic birds. The virus is about
100 nm in diameter and spherical forms are commonly seen with
a nucleocapsid containing its genetic material.
The genome is segmented with eight segments. It is a
single-stranded RNA virus, the genome is minus sense.
A peculiar characteristic of the virus is that it contains a
rod shaped glycoprotein called haemagglutinin (H) and a
mushroom-shaped glycoprotein called neuraminidase (N). These
are called spikes or peplomers.
These are very important in subtyping of virus and hence to
identify the exact virus type involved in a particular
outbreak. Till today 15 subtypes of H (H1-H15) and 9 subtypes
of N (N1-N9) have been reported and can be found in any
combination. So there could be 276 possible combinations of H
and N. Common combinations reported in
recent outbreaks are as follows:
Species Virus Strain
Human Beings H2N8, H3N8, H1N1, H2N2, H3N2
Horses Usually H7N7 or H3N8
Pigs Predominantly H1N1 and H3N2
Birds/Chickens H5N1, H5N3, H7N3, H5N9, H7N7, H5N2, H5N8, H7N4,
H7N1
(In Pakistan H7 and H9 have been reported).
* H5 N1 (Bird flu in Hong Kong in 1997, 1999, 2003)
The virus is relatively unstable in the environment and is
killed at 56°C for three hours or 60°C for 30 minutes hence it
is easily destroyed by boiling. It is easily inactivated by
commonly used disinfectants like formalin, sodium
hypochlorite, sodium hydroxide and iodine compounds.
Influenza viruses are protected by organic material such as
nasal secretions or faeces, which increase resistance to
physical and chemical agents.
Cool and moist conditions favor long survival of influenza
viruses in the environment. A I viruses have been viable in
liquid manure for 105 days in the winter and in faeces for
30-35 days at 4°C and for 7 days at 20°C.
A I Virus Subtype Number affected
A/chicken/Pennsylvania/1370/83 H5 N2 17m birds mostly
chickens. A/chicken/Pakistan/447/95
A/chicken/Pakistan/1369-CR2/95 H7 N3 3.2m broilers & broiler
breeder chickens.
A/chicken/Hong Kong/220/97 H5 N1 1.4m chickens & various other
domestic birds. A/turkey/Italy/4580/99 H7 N1 10.5m chickens &
2.7m turkeys. A/chicken/Hong Kong/2001 H5 N1 1 million birds.
Flu in birds
Influenza is often termed as "Fowl Plague" Due to significant
morbidity and mortality across the globe. Some important
outbreaks resulting in millions of mortalities and economic
classes are given in the chart above.
The disease presents a variable picture according to the type
of virus involved and its virulence. But in typical outbreak
with H7 or H9 following symptoms may be found: respiratory
distress, labored breathing, diarrhoea, swollen head, eye
discharge, cyanosis and swelling of wattles and combs,
subcutaneous hemorrhages on unfeathered skin/cyanosis of shank
and toe. In layer flock marked decline in egg production has
also been reported.
Spread of disease
The first incidence of bird to human infection was reported in
1997 in Hong Kong where 18 people got infected and 6 of them
died.
Extensive investigation revealed that close contact with live
infected poultry was the source of human infection.
Again in February, 2003, an outbreak of H5N1 avian influenza
in Hong Kong caused 2 cases of human infection. Two other
avian influenza viruses have recently caused illness in
humans.
An outbreak of HPAI (H7N7) in The Netherlands in February,
2003, caused the death of one veterinarian and mild illnesses
in 83 other humans. Mild cases of
avian influenza strain H9N2 occurred in children in Hong Kong
in 1999 and in mid December, 2003. H9N2 is not highly
pathogenic in birds.
Avian flu in Pakistan
Avian influenza (serotypes H9 and H7) is endemic in Pakistan.
In this reference, a serious outbreak had been reported in
1994 whereby broiler breeder flocks were affected in northern
areas of the country, and about a million birds succumbed to
death.
The scientists isolated the pure virus and typed it as H7N3.
An inactivated vaccine by the isolate was prepared by various
laboratories and has been quite effective in controlling the
avian influenza problem since 1994.
However, an outbreak of avian influenza H9N2 continues to
occur particularly in Karachi and adjoining areas till today.
This virus isolate was also purified likewise and a vaccine
was prepared which provided satisfactory protection.
During March 2003, another epidemic of avian influenza
occurred and heavy mortality in commercial layers of Karachi
was reported. This virus was isolated and characterized as H7
at various public and private disease diagnostic laboratories.
This outbreak occurred in commercial layer flock at their peak
production. Despite all this, no human contract has yet been
reported from any of the 20,000 poultry
farms of the country.
Difficult to control
Influenza viruses depict some peculiar properties known as
antigenic drift and shift. Antigenic drift is due to
accumulation of point mutations in the genes resulting in
amino acid changes in the protein.
Virus does not have a proofreading mechanism and as a result
erroneous versions of virus are released with different
antigenic nature.
Since the mammalian/avian body has not been exposed to this
new subtype and hence not immune to it, it would result in
occurrence of disease as the antigenic nature is changed.
On the other hand genetic reassortment results in development
of new subtypes and is called antigenic shift. Swine can serve
as mixing vessel of avian and human viruses.
Here genes are swapped resulting in emergence of completely
new subtypes towards which there is no previous immunity.
Avian and human influenza viruses can exchange
genes when a person is simultaneously affected with viruses
from both species.
The added danger in case of birds to human transmission is
that if the virus persists in human population for some time
and acquire sufficient human genes it would be very easily
effecting human population via human to human infection
(rather than bird to human infection) and could lead to
serious epidemic resulting in million of casualties around the
world.
Sources of spread
Since the virus is shed in bird droppings, secretions from the
respiratory track and conjunctiva. Following factors could be
attributed to the perpetuation of infection:
(1) Movement of contaminated vehicles, equipment, feed, cages,
etc, from one farm to another can carry the virus;
(2) Technical personnel visiting various poultry farms in a
day consecutively without changing cloths could be a source of
spread;
(3) Droppings from infected wild birds can introduce the virus
to backyard as well as commercial poultry flocks;
(4) The places where live birds are sold are an important
source of spread of infection. In these markets, birds caught
or bought are placed in cages under stress and in close
proximity which serve as an ideal place of spread of
infection;
(5) Domestic/backyard poultry is at continuous risk as it
frequently comes in contact with wild birds some times even to
the level of sharing the water sources, etc;
(6) Migratory waterfowl are the natural reservoir of bird flu
viruses and these birds are also most resistant to it. They
can carry the virus over great distances;
(7) Scavenger animals or birds can play a part in the spread
of infection;
(8) Movement of live infected birds from one place to another
can spread the virus;
(9) Mechanical vectors like rodents, flies and other insects
can also carry the virus from one place to another;
(10) Non-effective poor quality vaccination;
(11) Ability of virus to increase its pathogenicity if allowed
to circulate in poultry populations;
(12) Climatic conditions also play a part as highly pathogenic
viruses can survive for long periods in the environment
specially when temperatures are low.
Following tests are commonly used for diagnosis of influenza
virus infection in various species of animals including human
beings and birds:
(1) Isolation of the virus;
(2) Identification of influenza isolate by hemagglutination
inhibition;
(3) Serologic diagnosis by hemagglutination inhibition;
(4) Neuraminidase assay for antibody detection;
(5) Direct detection of influenza viral proteins or nucleic
acids;
(6) Identification by Reverse Transcriptase Polymerase Chain
Reaction (RT-PCR);
(7) Intravenous Pathogenicity Test (IVPI);
(8) Agar gel precipitation test (AGPT) for the detection of
antibodies to avian influenza using type specific
Ribonucleoprotein (RNP) antigen.
Dr. Shahan Azeem is a microbiologist and assistant director
researchat the University of Veterinary and Animal Sciences,
Lahore.
Prof. Dr M. A. Muneer is a renowned avian
microbiologist and Dean, Faculty of Veterinary Science, at the
same university. .
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