Influenza A virus: not just for the birds
by H. Cody Meissner, M.D., FAAP
Influenza A viruses circulate in many different animals, including humans, ducks, geese, pheasants,
swans, chickens, pigs, dogs, horses, seals, whales, ferrets and cats. All known subtypes of influenza A can
infect birds, except subtypes A(H17N10) and A(H18N11), which only infect bats (bats are mammals).
Influenza B strains circulate widely only among humans.
Match the type of influenza A in Column A with the best answer in Column B.
1. Bat influenza
2. Highly pathogenic avian influenza virus
3. Avian influenza
4. Variant influenza virus
5. Canine influenza
A ns w er: 1-c, 2-a, 3-e, 4-d, 5-b
Avian influenza viruses
a. caused severe disease in the United States last winter
and spring among egg-laying hens
b. also isolated from cats
c. first identified in 2009 in Central America among
mammals not found in the United States
d. the name given to influenza viruses that normally cir-
culate in pigs when isolated from humans
e. classified as either high or low pathogenicity
Avian influenza type A viruses circulate among
wild aquatic birds worldwide and can infect domestic poultry. Infected wild birds generally do not
develop illness. Susceptible birds become infected
when they are in contact with surfaces contaminated by infectious stool or respiratory secretions from
an infected bird. Avian influenza A viruses can be
highly contagious in flocks of poultry and cause severe disease among domesticated species, including
chickens, ducks and turkeys.
should be considered in a symptomatic person with
history of contact (particularly in a closed space)
with infected birds through handling, slaughtering,
defeathering, butchering or culling.
Variant influenza virus
Avian influenza A viruses are classified as either
low pathogenic avian influenza virus (LPAI) or highly pathogenic avian influenza virus (HPAI) based
on the ability to cause mortality in chickens in a
laboratory setting and on certain molecular characteristics. Among domesticated species, LPAI viruses
cause mild signs of disease such as ruffled feathers
and a reduction in egg production.
Swine influenza is a respiratory disease of pigs
caused by type A strains. Swine influenza viruses
normally do not infect humans. Human infection
caused by an influenza virus that normally circulates in pigs is called a variant (v) virus. In 2009,
A(H1N1)v influenza caused a worldwide pandemic,
although disease severity was modest. Spread of other swine influenza viruses to humans has occurred
in the United States most commonly following prolonged exposure to pigs at agricultural fairs.
To reduce the risk of infection, contact with pigs
should be minimized, especially by people at risk of
severe disease, including children younger than 5
years of age and pregnant women.
Influenza A viruses circulate in many animals, including turkeys, dog and pigs. All known subtypes
of influenza A can infect birds, except subtypes
A(H17N10) and A(H18N11), which only infect bats
(bats are mammals).
A severe avian influenza (HPAI) outbreak occurred in the United States during the winter and
spring of 2014-’15 and was caused by A(H5) strains.
More than 40 million egg-laying hens died or were
exterminated. These hens accounted for about 10%
of the U.S. egg-laying flock and resulted in a noticeable increase in the wholesale price of large shell eggs.
A reduction of approximately 2% in the nation’s
production of turkeys is anticipated to occur because
of this outbreak.
Bat influenza viruses
hounds). The canine H3N2 virus first was detected
in 2007 in South Korea and subsequently was re-
ported in China and Thailand. The canine H3N2
strain was isolated from cats in 2015.
From December 2003 through February 2015,
a total of 777 human cases with 428 deaths have
been confirmed in 16 countries due to A(H5). The
mortality rate is about 60% when humans become
infected with avian A(H5) strains.
Bat influenza A viruses were first described in
2009 in Guatemala and subsequently described in
other countries in Central and South America. The
species of bat currently known to be infected,
Sturni-ra lilium (little yellow-shouldered bats), is not native
to the continental United States. Human cells do
not appear to support the growth of bat influenza
viruses, although reassortment with human strains
is possible. These bat influenza viruses are sufficiently distinct to justify classification as new subtypes,
A(H17) and A(H18).
An influenza pandemic will occur in humans
when a non-human influenza virus acquires three
characteristics. First, the novel strain must acquire
mutations that enable transmissibility among humans. Second, large segments of the population
must lack protective immunity to the novel strain.
Third, if the novel strain has the ability to cause
severe disease in humans, illness similar to the devastating 1918-’19 worldwide pandemic may occur.
Between February 2013 and February 2014, a
total of 602 human cases and 227 deaths due to
another avian strain A(H7) have been reported.
Globally, no evidence of sustained human-to-hu-man transmission has been reported.
Canine influenza viruses
Human infection with HPAI virus had not been
detected in the United States as of May 2015. How-
ever, the possibility of avian influenza infection
Two different canine influenza viruses have been
identified: A(H3N8) and A(H3N2). No human infections attributable to canine influenza viruses have
been reported. The A(H3N8) strain may have been
an equine influenza virus that jumped the species
barrier to cause disease in dogs (initially in grey-
Dr. Meissner is professor of pediatrics
at Floating Hospital for Children, Tufts
Medical Center. He also is an ex officio
member of the AAP Committee on Infectious Diseases and associate editor of
the AAP Visual Red Book.