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Avian Influenza A(H5N1) -
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Situation at a Glance
The World Health Organization (WHO) was notified about a
laboratory-confirmed case of human infection with an influenza A(H5N1) virus on
1 April 2024 by the United States of America IHR National Focal Point (NFP).
The patient developed symptoms on 27 March and had a history of exposure to dairy
cattle (cows) presumed to be infected with influenza A (H5N1) virus. This is
the first human infection with A(H5N1) acquired from contact with infected
cattle and the second confirmed human case of influenza A(H5N1) detected in the
country. No additional associated cases of human infection with influenza
A(H5N1) have been identified. Since the virus has not acquired mutations that
facilitate transmission among humans and based on available information, WHO
assesses the public health risk to the general population posed by this virus
to be low and for occupationally exposed persons, the risk of infection is
considered low-to-moderate.
Description of the Situation
On
The case is aged over 18 years. On 27 March, the case
developed conjunctivitis, was reported to be while working at a commercial
dairy cattle farm and had a history of exposure to dairy cattle (cows) presumed
to be infected with influenza A(H5N1) virus (1).
On 28 March, respiratory and conjunctival specimens were
collected from the case and tested at the
On 28 March, the patient was advised to isolate and given
antiviral treatment (oseltamivir) following
Household contacts of the patient have not reported illness
and have been provided influenza antiviral prophylaxis as per US CDC
recommendations. No additional cases of
human infection with influenza A(H5N1) associated with this case have been
identified (1).
In the
Influenza A virus infection is exceptionally rare in bovine
species (3). This human case had exposure to dairy cattle in
The USDA has publicly posted the genetic sequences of
several recently detected HPAI A(H5N1) viruses found in US wild birds, poultry,
wild mammals and cattle in
Epidemiology
Animal influenza A viruses normally circulate in animals but
can also infect humans. Infections in humans have primarily been acquired
through direct contact with infected animals or contaminated environments.
Depending on the original host, influenza A viruses can be classified as avian
influenza, swine influenza, or other types of animal influenza viruses.
Avian influenza virus infections in humans may cause
diseases ranging from mild upper respiratory tract infection to more severe
diseases and can be fatal. Conjunctivitis, gastrointestinal symptoms,
encephalitis and encephalopathy have also been reported in previous human
infections with A (H5N1) viruses. There have also been a few detections of
A(H5N1) virus in asymptomatic persons who had exposure to infected birds.
Laboratory tests are required to diagnose human infection
with influenza. WHO periodically updates technical guidance protocols for the
detection of zoonotic influenza using molecular methods, e.g. RT-PCR. Evidence
suggests that some antiviral drugs, notably neuraminidase inhibitors
(oseltamivir, zanamivir), can reduce the duration of viral replication and
improve prospects of survival in some cases.
Avian influenza A(H5N1) virus belonging to Gs/GD lineage,
clade 2.3.4.4b Eurasian-origin infections have been reported in birds in the
This is the first human infection caused by avian influenza
A(H5N1) virus in 2024 in the
In the past, small clusters of A(H5) virus infections were
reported, including some involving health care workers, where limited
human-to-human transmission could not be excluded; however, sustained
human-to-human transmission has not been detected. In this event, the case had
direct contact with cows presumed to be infected with A(H5N1) viruses and no
further human cases have been detected associated with this event.
Public Health Response
Local and national health authorities implemented the
following public health measures (1,10):
Public health officials are conducting surveillance
activities in
US CDC is working with state health departments to monitor
workers who may have been in contact with infected or potentially infected
birds/animals and test those people who develop symptoms.
US CDC has issued recommendations for the public regarding
avoiding unprotected contact with sick or dead animals, including wild birds,
poultry, other domestic fowl, and other wild or domestic animals, as well as
animal droppings, litter, or materials contaminated by birds or other animals
with suspected or confirmed HPAI A (H5N1) virus infection.
The
WHO Risk Assessment
This human case was reportedly exposed to dairy cattle in
From 2003 to
Whenever avian influenza viruses are circulating in birds,
there is a risk for sporadic infections in mammals and humans due to exposure
to infected animals (including livestock), or contaminated environments and
thus, further human cases are not unexpected.
Influenza A infection has been rarely reported in bovine species and
spread among dairy cattle herds in four
Since the virus has not acquired mutations that facilitate
transmission among humans and based on available information, WHO assesses the
public health risk to the general population posed by this virus to be low and
for occupationally exposed persons the risk of infection is considered
low-to-moderate.
There are no specific vaccines for preventing influenza
A(H5N1) virus infection in humans. Candidate vaccines to prevent H5 infection
in humans have been developed for pandemic preparedness purposes. Close
analysis of the epidemiological situation, further characterization of the most
recent viruses (from human cases and animal) and comprehensive investigations
around human cases are critical to assess associated risk and to adjust risk
management measures in a timely manner.
If needed, the risk assessment will be reviewed should
further epidemiological or virological information become available.
WHO Advice
This case does not change the current WHO recommendations on
public health measures and surveillance of influenza. Establishing and
monitoring the extent of influenza A(H5N1) virus in dairy herds in the affected
localities of the country should inform changes in the ongoing risk assessment.
Due to the constantly evolving nature of influenza viruses,
WHO continues to stress the importance of global surveillance to detect and
monitor virological, epidemiological and clinical changes associated with
emerging or circulating influenza viruses that may affect human (or animal)
health and timely virus sharing for risk assessment.
When there has been human exposure to a known outbreak of an
influenza A virus in or when there has been an identified human case of
infection with such a virus, enhanced surveillance in potentially exposed human
populations becomes necessary. Enhanced surveillance should consider the health
care seeking behaviour of the population, and could include a range of active
and passive health care and/or community-based approaches, including: enhanced
surveillance in local influenza-like illness (ILI)/severe acute respiratory
infection (SARI) systems, active screening in hospitals and of groups that may
be at higher occupational risk of exposure, and inclusion of other sources such
as traditional healers, private practitioners and private diagnostic
laboratories.
In the case of a confirmed or suspected human infection caused
by a novel influenza A virus with pandemic potential, including avian influenza
virus, a thorough epidemiologic investigation (even while awaiting the
confirmatory laboratory results) of history of exposure to animals, of travel,
and contact tracing should be conducted. The epidemiologic investigation should
include early identification of unusual events that could signal
person-to-person transmission of the novel virus and clinical samples collected
from the time and place that the case occurred should be tested and sent to a
WHO Collaboration Center for further characterization.
Travelers to countries with known outbreaks of animal
influenza should avoid farms, contact with animals in live animal markets,
entering areas where animals may be slaughtered, or contact with any surfaces
that appear to be contaminated with animal feces. Travelers should also wash
their hands often with soap and water. Travelers should follow good food safety
and good food hygiene practices. Should infected individuals from affected
areas travel internationally, their infection may be detected in another
country during travel or after arrival. If this were to occur, further
community-level spread is considered unlikely as this virus has not acquired
the ability to transmit easily among humans.
All human infections caused by a novel influenza A virus
subtype are notifiable under the International Health Regulations (IHR) and
State Parties to the IHR (2005) are required to immediately notify WHO of any
laboratory-confirmed case of a recent human infection caused by an influenza A
virus with the potential to cause a pandemic. Evidence of illness is not
required for this report.
WHO does not advise special traveler screening at points of entry or restrictions regarding the current situation of influenza viruses at the human-animal interface.