#18,820
It is the oft-stated position of the CDC, and the FAO/WHO/WOAH that the global public health risk from influenza A(H5) viruses remains low, but the risk of infection for occupationally exposed persons may rise to moderate (or even high).
Last November, in MMWR: Serologic Evidence of Recent Infection with HPAI A(H5) Virus Among Dairy Workers - we learned that in a sampling of 115 farm workers (from Michigan & Colorado) Eight (7%) showed antibodies for H5N1, yet half of them could recall no symptoms, while four others reported `very mild' symptoms.
Last February, in MMWR: Seroprevalence of Highly Pathogenic Avian Influenza A(H5) Infections among Bovine Veterinary Practitioners – United States, September 2024, we learned that 3 veterinarians who volunteered for antibody testing at a veterinary conference last September tested positive for H5.
None of these veterinarians suspected they had ever contracted the virus, and two deny working with infected cattle (
including one who worked in two states that have never reported bovine H5). A finding the authors suggest may indicate
`. . . . there could be U.S. states with A(H5)-positive people and animals that have not yet been identified.'Of the 77 confirmed and probable H5N1 cases in the United States (see chart below), 92% are linked to infected cattle or poultry exposure. There are 4 cases where the exposure source remains unknown.
As we've
seen in recently in Cambodia, and frequently in the past in places like China, Indonesia, and Egypt, the raising of backyard poultry also entails heightened risks.
Last year the WHO published Interim Guidance to Reduce the Risk of Infection in People Exposed to Avian Influenza Viruses, which lists a number of `risk factors', including:
- keep live poultry in their backyards or homes, or who purchase live birds at markets;
- slaughter, de-feather and/or butcher poultry or other animals at home;
- handle and prepare raw poultry for further cooking and consumption;
All of which brings us to a lengthy review on HPAI exposure patterns and risks among `occupational populations', which is well worth reading in its entirety.
I'll return with a postscript after the break.
A Review of Avian Influenza Virus Exposure Patterns and Risks Among Occupational Populations
Huimin Li ,Ruiqi Ren , Wenqing Bai , Zhaohe Li , Jiayi Zhang , Yao Liu, Rui Sun ,Fei Wang , Dan Li ,Chao Li ,Guoqing Shi , and Lei Zhou
Vet. Sci. 2025, 12(8), 704; https://doi.org/10.3390/vetsci12080704
Published: 28 July 2025
Simple Summary
Avian influenza virus (AIV) remains a significant threat to public health. Occupational groups with frequent exposure to poultry, livestock, or contaminated environments face a higher risk of AIV infection. This review outlines the key exposure pathways and risk factors for AIV in these populations, including viral characteristics, animal host factors, environmental conditions, and host susceptibility. By synthesizing current evidence, we aim to support the development of targeted prevention strategies to mitigate infection risks and protect occupational workers’ health.
Abstract
Avian influenza viruses (AIVs) pose significant risks to occupational populations engaged in poultry farming, livestock handling, and live poultry market operations due to frequent exposure to infected animals and contaminated environments.
This review synthesizes evidence on AIV exposure patterns and risk factors through a comprehensive analysis of viral characteristics, host dynamics, environmental influences, and human behaviors. The main routes of transmission include direct animal contact, respiratory contact during slaughter/milking, and environmental contamination (aerosols, raw milk, shared equipment).
Risks increase as the virus adapts between species, survives longer in cold/wet conditions, and spreads through wild bird migration (long-distance transmission) and live bird trade (local transmission).
Recommended control measures include integrated animal–human–environment surveillance, stringent biosecurity measures, vaccination, and education. These findings underscore the urgent need for global ‘One Health’ collaboration to assess risk and implement preventive measures against potentially pandemic strains of influenza A viruses, especially in light of undetected mild/asymptomatic cases and incomplete knowledge of viral evolution.
(SNIP)
Conclusions and Prospects
Since the first reported human case of H5N1 infection in Hong Kong in 1997, the risk of cross-species transmission of avian influenza viruses has continued to escalate. The recent confirmation of a novel transmission pathway from ruminants to occupational populations has further highlighted the significant health threats faced by exposed workers.
This study comprehensively analyzes the primary exposure routes of AIV among occupational groups, demonstrating that infections originate from direct contact with infected poultry, dairy cattle, or contaminated environments, followed by viral entry through mucosal (ocular, nasal, or oral) contact or respiratory routes.
Guided by the “One Health” concept, we innovatively developed a four-dimensional occupational exposure risk assessment framework incorporating viral molecular characteristics, host animals, human susceptibility, and environmental factors, confirming that avian influenza epidemics result from multifactorial interactions.
Regarding prevention strategies, this study proposes tiered recommendations. The primary objective is establishing a coordinated surveillance network spanning animal-human–environment interfaces for early outbreak detection, followed by enhanced implementation of biosecurity measures, particularly targeting high-risk exposure scenarios. These findings provide critical scientific foundations for improving avian influenza control systems.
(SNIP)
Future research should focus on fundamental studies elucidating molecular mechanisms and evolutionary patterns of cross-species transmission, applied research developing occupational exposure risk assessment tools and protective technologies for high-risk occupations, and implementation science conducting cost-benefit analyses and intervention optimization across resource settings.
(Continue . . . )
Even though we are now more than 15 months, and
over 1,075 infected dairy herds, since the discovery of HPAI H5N1 in dairy cows in the United States, there is still much we don't know about how the virus is spreading.
Testing of cattle has been limited, is generally voluntary, and is focused almost exclusively on lactating dairy cows.
A few months ago we saw the
average time to submit sequences to GISAID was 7 months, with some countries taking more than twice that. Many
countries are slow to share outbreak information, and some some appear to be burying `bad news' completely (see
From Here To Impunity).
While the reporting of new cases, and outbreaks, has plummeted over the past 6 months - and the situation looks better `on paper' - the HPAI problem has not gone away
Like so many other reviews we've seen, today's calls for enhanced surveillance and increased biosecurity, and the rapid sharing of clinical samples, viral isolates, and genetic sequences of novel strains; all under a coordinated `One Health' approach.
But despite these repeated pleas, there are relatively few signs that they are being taken seriously by governments, agencies, or stakeholders.
The world continues to treat the spread of HPAI as more of an economic or political concern, than a public health threat. And while that optimistic assessment may be true today, it could abruptly change with little warning.
