Questions about Avian Influenza
What is avian influenza?
Influenza is an infectious and contagious disease caused by
viruses which mainly infects birds but can also cross
species barrier and infect some other animals such as pigs,
horse, large cats, dogs and occasionally humans. In domestic
birds the disease mainly occurs in two forms; low pathogenic
disease which results in mild symptoms and causes mortality
in complicated situations only and can easily go undetected,
while high pathogenic form can result in high mortality even
up to 100% and spreads rapidly among bird.
Where is the disease found?
disease in its low or high pathogenic form is prevalent in
many areas across the globe. Italy, Vietnam,
Germany, Korea, Iran, Thailand, Indonesia, China,
Myanmar, Bangladesh, Nepal, Bhutan, Pakistan and India have
reported outbreaks of Avian Influenza.
In India, the first
isolation of the H9N2 was done in 2003 from the Haryana and
Punjab States and since then the virus has been isolated
from 10 more States. HPAIV H5N1 outbreaks have been recorded
in Indian poultry in 2006 (Maharashtra, Gujarat and Madhya
Pradesh), 2007 (Manipur) and 2008 (West Bengal and Tripura)
and subsequently in Assam, Sikkim and West-Bengal.
How is the disease transmitted and spread?
birds especially waterfowls are thought of to be reservoirs
of AI viruses. They carry the viruses along with their
migration routes, usually in low pathogenic form however, in
some cases also in high pathogenic form. They can transmit
the virus to susceptible domestic birds on direct or
indirect contact with them. Within domestic poultry low
pathogenic virus circulates and can mutate to high
pathogenic form. It seems highly likely that H1N1 viruses
may pass readily between pigs, humans and turkeys and the
introduction of viruses of this subtype to turkey flocks
from infected pigs has been well documented. Illegal
movement of live birds along with dressed carcasses of
infected birds, eggs from infected hens, poultry waste and
fomites contaminated feces can all be a source of infection
among trans-boundary countries. When introduced in a flock
the spread of virus within flock occurs readily in suitable
climates of humidity and cold. Virus is excreted in large
quantities in feces and respiratory secretions. Spread may
be either through direct contact, via conjunctival sac,
nares or through shared and contaminated drinking water.
Airborne transmission occurs if birds are in close proximity
and with appropriate air movement. Avian influenza viruses
have been transmitted to different mammal species on several
occasions. Pigs, in particular, have been frequently
involved in such 'interclass transversions'. In addition to
swine, marine mammals and horses have been shown to acquire
influenza A viruses from avian sources. Natural infection
with H5N1 was seen in tigers and other large cats after the
animals were fed with virus-positive chicken carcasses.
What is the public health risk associated with the
cases of infection with the H9N2 virus has been reported
although with negligible mortality but crossing the species
barrier to mammals highlighted the pandemic potential of
H9N2 virus. Some avian H9 viruses have acquired receptor
binding characteristics typical of human strains, increasing
the potential for reassortment in both human and pig
respiratory tracts which are potentially capable of
infecting humans, suggesting that potential pandemic viruses
may emerge directly from birds. Human infection with H5N1
virus has occurred in many countries with a variable but
high mortality rate which depends on the condition of the
host and critical care provided. But with the possibility of
virus gaining the potentiality of human to human
transmission, it poses a great risk to whole humanity.
What are the clinical signs?
The clinical presentation of avian influenza in birds is
variable and symptoms are fairly non-speciﬁc which depends
upon the characteristics of the isolate, the dose of
inoculum, the species and age of the bird.
The symptoms following infection with low pathogenic AIV
may be slight and discrete and even go undetected such as
- Ruffled feathers
Transient reductions in egg production or weight loss
Slight respiratory disease.
In its highly pathogenic form, the illness in chickens and
turkeys is characterized by a
sudden onset of severe symptoms and a mortality
that can approach 100 % within 48 hours.
Laying ﬂocks stops egg production and individual birds
often reveal little or no mobility and produce soft
Edematous swelling visible at feather-free parts of the
head, cyanosis of comb, wattles and legs, greenish
diarrhoea and labored breathing may be inconsistently
present. Nervous symptoms including tremor, unusual
postures (torticollis), and problems with co-ordination
(ataxia), compulsively swimming in narrow circles on a
pond dominate the picture in less vulnerable species
such as ducks and geese leading to a preliminary
suspicion of HPAI. The disease in turkeys is similar to
that of chickens, but is often complicated by secondary
bacterial infections such as those due to fowl cholera,
turkey coryza or colibacillosis.
is the disease diagnosed?
Field diagnosis is based on clinical signs and gross
lesions. However, for a confirmatory diagnosis laboratory
tests are required. Specimens to be taken are nasal and
cloacal swabs, brain, lungs, trachea, spleen, intestine and
cloaca. Nasal and cloacal swabs must be taken separately. If
person is not well trained for the purpose, whole bird
should be sent bagged twice and maintained refrigerated all
the time. Lab diagnosis includes antigen detection using
rapid molecular diagnostic tests like real time PCR and RT
PCR followed by virus isolation in eggs, identification of
isolates as influenza A virus using HA and HI tests.
Serological investigations include ELISA, AGID and HI tests.
Pathogenicity testing of virus isolates by chicken
inoculation is also essential to confirm the class of virus
as HPAI or LPAI.
required at farm level?
The primary aim of an early and rapid response to any
occurrence of HPAI is to contain the disease before it
spreads and eliminate it by stamping out affected premises
and establishing proper disposal. There should be proper
culling, strict quarantine and movement controls, and safe
disposal of carcass with burial being the best method at
farm level, decontamination of the affected premises,
restocking of the bird after a safe period of destocking of
90 days. Vaccination, when applied must be in conjunction
with other disease control measures including mass slaughter
as due to continuous strain variation, vaccination is not