Env. Microbiology: Environmental temperature and relative humidity shape post-emission aerosol fate and airborne influenza transmission

Photo Credit PHIL (Public Health Image Library)

#19,205

While it is no secret that winter heralds the arrival of flu season, one of the enduring mysteries about influenza is why it is predominantly a winter phenomenon – at least in temperate zones of the world.

Theories include:

• During the colder weather people tend to gather indoors, with less fresh air ventilation.
• Diminished sunlight exposure may reduced Vitamin D levels (see Study: Vitamin D And Flu-Like Illnesses)
• With schools in session, millions of children co-mingle and more efficiently share viruses
• But perhaps most importantly, at lower relative humidity (RH), evaporation of water from exhaled large droplets occurs rapidly, leading to the formation of  lighter, smaller, and more persistent droplet nuclei. 

This is a topic we've revisited often over the years (see links below), but due to using different methods (and different viruses), we've seen variable results.

• 2007's  Cold And Dry Statistics

• 2009's  It's Not So Much The Heat, It's The Humidity

• 2013's PLoS One: High Humidity Reduces Flu’s Infectivity

• 2013's NIH Study: Climate & Influenza Transmission

• 2019's PNAS: Low Ambient Humidity Impairs Barrier Function & Innate Resistance Against Influenza Infection

In general, higher RH appears to decrease transmission, but one of the caveats from a 2019 study - mSphere: Environmental Persistence of Influenza Viruses Is Dependent upon Virus Type and Host Origin - was that they found considerable variation in RH tolerance among the 6 flu strains they studied, and between droplet and aerosolized particles.

But we've also seen evidence (see 2012's  Influenza Virus Survival At Opposite Ends Of The Humidity Spectrum) that too much humidity may have the opposite effect.

Today's study tested 2 scenarios (20°C/50% RH (ambient indoor), and 7°C/73% RH (cold/high-humidity), using one flu virus (A/California/04/2009 (H1N1), in a swine model. Briefly, they found:

• Donor pigs shed comparable nasal viral loads across both conditions

• Naïve sentinel pigs housed 4 m away became infected 1 day earlier under ambient conditions

• Breath and environmental air sampling showed cold/high-humidity conditions transiently increased viral RNA in exhaled aerosols at 1 day post-infection (dpi)

• Ambient conditions supported greater and more persistent airborne viral burdens at 2–3 dpi, particularly at downrange locations.

This study argues that `. . . post-emission aerosol fate, shaped by environmental temperature-humidity conditions . . . ', heavily influences the viability and spread of the flu virus.   

This is obviously a complex study - with highly nuanced results - so you'll want to follow the link to read it in its entirety.

 I'll have a bit more after the break.

 

Environmental temperature and relative humidity shape post-emission aerosol fate and airborne influenza transmission
Authors: Xuan-Dung Nguyen https://orcid.org/0000-0002-1721-8335, Bac Tran Le, Jacob Bleich, Constanza Espada, Wei Zhang, Xiu-Feng Wan https://orcid.org/0000-0003-2629-9234 wanx@missouri.eduAuthors Info & Affiliations
https://doi.org/10.1128/jvi.00634-26
 
PDF/EPUB
 

ABSTRACT

Airborne transmission is a major route of influenza virus spread, yet how environmental conditions shape the persistence and downrange transport of infectious exhaled virions is not fully understood. Using a physiologically relevant swine model infected with A/California/04/2009 (H1N1), we investigated how temperature and relative humidity (T/RH) influence airborne influenza emission, persistence, and transmission under two environmental conditions: 20°C/50% RH (ambient indoor), and 7°C/73% RH (cold/high-humidity).

Donor pigs shed comparable nasal viral loads across conditions, but naïve sentinel pigs housed 4 m away became infected 1 day earlier under ambient conditions. Breath and environmental air sampling showed that cold/high-humidity conditions transiently increased viral RNA in exhaled aerosols at 1 day post-infection (dpi), whereas ambient conditions supported greater and more persistent airborne viral burdens at 2–3 dpi, particularly at downrange locations.

Controlled aerosol generation experiments further showed that ambient conditions enabled substantially greater recovery of infectious virus with distance, even though RNA-containing particles were transported under both T/RH states. Together, these results demonstrate that, under the tested environmental conditions, infectious influenza aerosols persisted longer and transmitted farther under the ambient indoor environment than in the cold/high-humidity environment. These findings establish that environmental temperature-humidity conditions shape post-emission aerosol fate, and thereby constrain the airborne transmission range of the influenza virus.

IMPORTANCE

Influenza viruses spread efficiently through the air, yet the environmental conditions that determine whether exhaled virions remain infectious long enough to initiate new infections remain poorly defined. Using a swine model that closely replicates human expiratory aerosol output, we identify environmental temperature-humidity conditions as a critical determinant of airborne infectious range.

Cold/high-humidity conditions increased early viral RNA levels near the host but failed to sustain infectious particles at a distance. In contrast, ambient conditions supported prolonged airborne suspension and rapid transmission to distant recipients.

Controlled aerosolization experiments showed that infectious virus is transported far more effectively under ambient indoor conditions than in cold/high-humidity air despite similar RNA dispersal. These results reveal post-emission aerosol fate as the critical bottleneck in determining airborne influenza transmissibility. This mechanistic insight is essential for refining predictive models of influenza spread and developing environmental and public health strategies that more effectively limit airborne infection.

        (SNIP) 

In summary, environmental temperature-humidity conditions shape airborne influenza virus transmission under conditions where donor shedding magnitude is comparable. Ambient conditions supported persistence of infectious airborne virus and effective transmission, whereas cold/high-humidity conditions limited recovery of infectious virus at distance despite detectable viral RNA. These findings indicate that post-emission aerosol fate, shaped by environmental temperature-humidity conditions, influences whether exhaled virus remains airborne and infectious over distance.

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Over the past decade there has been growing interest in the idea of raising the humidity inside homes, offices, schools, and health care facilities during times of heightened flu activity (see PLoS One Humidity as a non-pharmaceutical intervention for influenza A).

This is not exactly a new idea, as the Chinese have boiled vinegar for centuries in their homes to `disinfect the air' during epidemics (including SARS). While vinegar is unproven to add any beneficial effect, vinegar is 95% water, and boiling it undoubtedly raises the humidity inside their homes.

The irony here is that hospitals are normally kept cool and dry in order to inhibit the growth of mold and bacteria, but may be unintentionally providing an environment conducive to the spread of respiratory viruses like influenza, SARS & MERS.

Since today's study is based on a single influenza subtype, restricted to a swine model, and tested across only 2 scenarios (where the effects of temperature vs humidity were not fully isolated), it can't tell us the optimum environmental conditions to strive for to limit flu transmission. 

But it does provide additional reasons to believe that environmental controls could be useful NPIs (Non-Pharmaceutical Interventions), that could help slow the spread of a virus in an indoor setting.

As for determining where the `sweet spot' is, additional research across a wider range of viruses will be needed. 

HK CHP: Follow Up Report on H9 Case in Hong Kong

 


#19,204

Three days ago (June 13th) we looked at a report (Hong Kong CHP: Locally Acquired Case of H9 Infection - Notification Letter For Doctors) on the first locally acquired case of H9 infection in Hong Kong in more than 6 years.

While not the most dangerous novel flu virus on the CDC's short list of zoonotic influenza viruses (see CDC IRAT SCORE), the CDC does rank two H9N2 lineages has having some pandemic potential.

And in terms of likelihood of sparking a pandemic, the H9N2 Y280 lineage is ranked higher than H5N1, while the G1 lineage is ranked only slightly lower.

Yesterday the CHP released a follow up on their investigation where they reveal the subtype to be LPAI H9N2 (fully expected), and that so far, the patient remains stable and no other cases have been identified. 

Although additional genetic analysis is ongoing, the CHP reports `no significant genetic variations were detected.'

While the toddler's exact exposure is unknown, the CHP believes the boy was most likely exposed by touching a contaminated surface at a local live market. A not uncommon scenario, as the following past blogs attest:

B&E: Assessing The Airborne Spread Of Avian Influenza From LPMs

J. Virol: H9N2 Virus Isolated From Air Samples In LPMs In Jiangxi, China

J. Infection: Aerosolized H5N6 At A Chinese LBM (Live Bird Market)

Detection Of Airborne H9 Nucleic Acid In Chinese Live Poultry Market 

While this appears to be a one-off infection, Hong Kong's CHP is obviously taking this case seriously.  I've reproduced a large portion of their update below. Follow the link to read it in its entirety.

CHP continues to actively follow up on a case of low-pathogenic avian influenza A (H9) infection and reminds the public of the possibility of "twin-peaks" for seasonal influenza and COVID-19 during summer
 

The Centre for Health Protection (CHP) of the Department of Health (DH) today (June 15) continued to actively follow up on a case of human infection with influenza A (H9) in collaboration with the relevant government departments. Following whole genome sequencing and analysis of the patient's clinical specimens, the virus strain was confirmed to be a low-pathogenic avian influenza A (H9N2) virus. All of the virus genes were avian in origin and no significant genetic variations were detected. The patient is currently in stable condition and all six of his household contacts have remained asymptomatic. As the H9N2 avian influenza virus has long been present in local poultry with low mortality rate for birds, and that the H9N2 avian influenza virus involved in this case has not shown evidence of human-to-human transmission or significant genetic variation, the CHP currently assessed the risk of a local avian influenza pandemic as low. Nevertheless, the CHP once again strongly urged the public to maintain good personal and environmental hygiene at all times, avoid contact with live poultry, birds or their droppings, thoroughly cook poultry meat and eggs before consumption, and wash hands thoroughly after visiting places where live poultry is sold, so as to reduce the risk of avian influenza infection.

In addition, with the recent rise in the activity of seasonal influenza and COVID-19, the CHP does not rule out the possibility that the activity of these two respiratory diseases will continue to rise in the coming months, leading to a "twin-peaks" phenomenon. Members of the public, particularly high-risk individuals, are advised to receive vaccination in a timely manner to reduce the risk of severe disease and death.

Human infection with influenza A (H9) virus
 
In relation to the recent influenza A (H9) infection in a two-year-old boy, the Public Health Laboratory Services Branch of the CHP conducted whole genome sequencing and analysis of the virus, confirming that the virus strain is a low-pathogenic H9N2 avian influenza virus and that no significant genetic variations were detected.

The CHP has collected 17 environmental samples from the residence of the patient, the fresh provision shop at Wo Che Market he had visited, as well as a park in Fung Wo Estate. One sample collected from a metal tray placed at the bottom of a live chicken cage inside the shop that was used to collect chicken droppings was tested positive for the H9 avian influenza virus. The remaining 16 samples tested negative.

The CHP will conduct further analysis on the positive environmental sample. The CHP believed that it is more likely for the boy to have contracted H9 avian influenza by touching a contaminated surface at the fresh food shop selling live poultry in Wo Che Market. Thorough disinfection and cleaning will be conducted at the fresh food shop in question.

The patient remains hospitalised in stable condition. His symptoms remain mild. Neither his family members nor the staff at the fresh provision shop concerned have developed any symptoms. The CHP has provided them with preventive medication and will continue to put them under medical surveillance.

Based on the above epidemiological and virological evidence, the CHP assessed that the recent local case of infection has not changed the current risk level. The risk of an influenza pandemic due to local avian influenza remains low. The Government's response level under the "Preparedness Plan for Influenza Pandemic" remains at "Alert" level.

Avian influenza viruses are generally classified as highly pathogenic or low pathogenic, and they mainly affect birds and poultry. Birds are also natural hosts for avian influenza viruses. In occasional circumstances, cross-species transmission may occur when human come into close contact with infected poultry or contaminated environments. However, there is currently no scientific evidence to suggest that the existing avian influenza viruses are capable of sustained and efficient human-to-human transmission. No novel influenza virus arising from genetic reassortment between human seasonal influenza viruses and animal influenza viruses has been found either.

Since 1999, a total of 11 cases of human influenza A (H9N2) have been recorded in Hong Kong, including five local cases and six imported cases. No fatal case has been recorded so far.

According to data published by the World Health Organization (WHO), more than 160 cases of human infection with influenza A (H9) have been recorded globally in the past decade. The vast majority of patients presented with mild symptoms. As poultry is a natural host of the virus in many regions, sporadic human infections caused by contact with infected poultry or contaminated environments are expected to continue occurring worldwide.

The CHP will continue to strengthen public education and publicity efforts to reduce the risk of avian influenza infection among the general public. A letter has been issued to all doctors in Hong Kong to update them on the latest situation regarding influenza A (H9), urging them to heighten vigilance and report any suspected cases.

Seasonal influenza and COVID-19
 
Influenza activity in Hong Kong has increased in recent weeks but remains below the baseline level. Based on past experience, Hong Kong may experience two influenza seasons each year. The onset of summer influenza season began at a later time than usual last year, and sustained a longer period, extending from early September last year to early January this year, resulting in the absence of the winter influenza season that traditionally occurs in the first quarter of each year. Since the summer influenza season typically occurs between July and August, it cannot be ruled out that it may begin earlier than usual this year.

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FAO calls for stronger prevention and global action as transboundary animal diseases spread across regions

 

How ASF Spreads

#19,203

Although this blog tends to focus more on animal diseases with major zoonotic potential (avian flu, Nipah, Ebola, Mpox, Q Fever, etc.) - due to their enormous impact on local and regional economies and on global food security - we've also spent a good deal of time discussing non-zoonotic animal disease threats like African Swine Fever, Classical Swine Fever, FMD, Porcine Epidemic Diarrhea (PED), etc.

There are also animal diseases that inhabit the middle ground, in that they can infect humans - but do so only rarely or have little or no epidemic or pandemic potential - such as the NWS screwworm, Newcastle disease, and Orf Virus (to name a few). 

But the line between zoonotic and non-zoonotic is not set in stone; there are some pathogens (canine influenza, Influenza D, bovine or porcine coronaviruses, henipaviruses, etc.) that appear to be near the cusp, and could someday acquire the ability to spillover into humans. 

Our health, and in many ways our fate, is inexorably intertwined with the global biosphere - making what happens in a Fruit bat in Bangladesh, or a Shrew in China - something that could change our world overnight.

Six months ago the FAO announced the creation of a new program ( Global Partnership Programme for Transboundary Animal Diseases (GPP-TAD)) focused on `. . . prevention, early warning, preparedness, anticipatory action and rapid response'.

Over the past 6 months a global consultation process has worked to finalize the program's design. 

Late last week the FAO published the following update.

FAO calls for stronger prevention and global action as transboundary animal diseases spread across regions

Animal diseases, including avian influenza, African swine fever, foot-and-mouth disease and New World screwworm, pose growing risks to food security, trade and livelihoods

Cattle in Italy.

©FAO/Alessandro Penso

11/06/2026

Rome – Countries around the world are facing a growing threat from transboundary animal diseases (TADs), including New World Screwworm, African Swine Fever, Avian Influenza, Foot-and-Mouth Disease, and Peste des Petits Ruminants, as well as other emerging zoonotic threats such as Andes hantavirus, Ebola, and Nipah. As diseases and pests move more rapidly across borders, countries are facing increasing pressure to strengthen prevention, preparedness and response systems.

The stakes are high. Livestock sectors support more than one billion livelihoods and contribute trillions of dollars in economic value each year. Protecting animal health is therefore critical not only for farmers and livestock keepers, but also for food security, trade, economic stability, and rural prosperity.

The factors driving disease spread are becoming increasingly complex. Increased movement of animals, people and products, changing production systems, environmental pressures and uneven veterinary and surveillance capacity are creating new opportunities for diseases and pests to spread across regions. Addressing these threats requires stronger surveillance, earlier detection, greater information sharing and closer international cooperation.

“The impacts of these outbreaks extend far beyond animal health. They disrupt agricultural production, trade, and tourism, threaten livelihoods, increase food security risks, and in some cases pose direct risks to human health,” said Dr Tiensin Thanawat, FAO Assistant Director-General, Director of the Animal Production and Health Division, and Chief Veterinarian.

The economic impacts of transboundary animal diseases are substantial. Avian influenza has resulted in the loss of more than 633 million poultry and threatens a $48 billion market, while foot-and-mouth disease causes $11.3 billion in annual losses and African swine fever has reduced pig herds by over 40 percent in some parts of Asia. These impacts underscore the importance of stronger surveillance, faster detection and coordinated international action.

Recent developments highlight the urgency of the challenge. New World Screwworm has now re-emerged in the United States of America after decades of successful containment, following its northward spread through Central America and Mexico. At the same time, the emergence and international spread of the SAT1 serotype of Foot-and-Mouth disease beyond its historical range in Africa has raised concern across parts of Asia, the Middle East and other regions. These developments demonstrate how quickly animal health threats can cross borders and create significant risks for food production, trade, and livelihoods.

Strong prevention and preparedness remain the most effective and least costly tools for reducing the impact of animal disease outbreaks. When outbreaks exceed national capacity, FAO acts as the Provider of Last Resort, rapidly deploying expertise, coordinating the response, and mobilizing resources to sustain action when national systems are overwhelmed or other actors cannot operate.

"Experience consistently shows us that prevention and preparedness are more effective—and less costly—than responding after an outbreak has taken hold,” said Beth Bechdol, FAO Deputy Director-General. “Investing in animal health systems is one of the most effective ways to protect livelihoods, support trade, strengthen food security and improve resilience across agrifood systems."

Building on more than 80 years of experience and programmes such as the Emergency Prevention System for Animal Health (EMPRES), the Emergency Centre for Transboundary Animal Diseases (ECTAD) and the Emergency Management Centre for Animal Health (EMC-AH), FAO is working with Members and partners to strengthen global capacity for prevention, preparedness and response to transboundary animal diseases.

As part of this effort, FAO is working with Members to develop the Global Partnership Programme for Transboundary Animal Diseases (GPP-TAD), a long-term, country-owned platform focused on prevention, early warning, preparedness, anticipatory action and rapid response. The objective is to help countries detect and contain outbreaks earlier, reduce disruptions to food production and trade, and minimize the need for costly emergency interventions.

The programme is being developed as a collaborative, multi-partner initiative—including with organizations such as the World Organisation for Animal Health (WOAH)—with a focus on strengthening national capacities, supporting sustainable financing approaches, and better linking country-level investments with regional and global support. Building on decades of experience, the initiative aims to strengthen existing international efforts and help countries move from reactive responses toward more durable systems for prevention and preparedness.

Whether it is possible we can learn to look beyond the profit margin for the next accounting period - and begin to make rational decisions based on a long-view - remains to be seen. 

But it is nice to see that some people are still trying. 

NJ & RI Both Report H5N1 in Live Markets

#19,202

Yesterday we looked at an H9 avian flu infection in a Hong Kong toddler who's only known risk exposure was visiting a live market. While not exactly a smoking gun, the CHP's epidemiological investigation stated `.  .  . It cannot be ruled out that the patient was infected through indirect contact with a contaminated environment at the wet market.'

The link between live bird markets and the spread (and potential reassortment of) avian flu strains has been long established. LBMs typically bring together birds of varying species (chickens, ducks, geese, quail, and others) - often imported from different farms - which are housed in cramped quarters.

Twelve years ago, in CDC: Risk Factors Involved With H7N9 Infection we looked at a case-control study conducted by an international group of scientists, including researchers from both the Chinese and the US CDC that concluded.

Exposures to poultry in markets were associated with A(H7N9) virus infection, even without poultry contact. China should consider permanently closing live poultry markets or aggressively pursuing control measures to prevent spread of this emerging pathogen. 

In 2016's Interventions in live poultry markets for the control of avian influenza: A systematic review Vittoria Offeddu , Benjamin J. Cowling, and J.S. Malik Peiris laid out the risks of avian influenza from live bird markets, reviewed some of the possible interventions, and concluded:

Highlights

• Avian influenza viruses (AIVs) can infect humans. Bird-to-human transmission is particularly intense in live poultry markets.
• Periodic rest days, overnight depopulation or sale bans of certain species significantly reduce AIV-circulation in the markets.
• Market closure would lastingly reduce the risk of animal and human infection.

In 2022 we looked at Zoonoses & Public Health: Aerosol Exposure of Live Bird Market Workers to Viable Influenza A/H5N1 and A/H9N2 Viruses, Cambodia, and we've seen cases whose likely exposures were cited as simply living near, or walking past an LBM (see J. Infection: Aerosolized H5N6 At A Chinese LBM (Live Bird Market)).

Two years ago 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;

Despite this tarnished reputation, and repeated calls to close them, LBMs flourish around the world. 

While most common in Asia and the Middle East, live are also found in Europe and the United States. Last March, in USDA Report 9 More Live Bird Markets Infected With HPAI H5 we looked at outbreaks in 3 states (New York, Florida, Pennsylvania).

Earlier this week the USDA reported a new outbreak at a live Market in Passaic, New Jersey.

Interestingly, Passaic was one of 3 wastewater monitoring sites in the U.S. that reported HPAI H5 positives in the latest CDC report.

All of which brings us to the second report this week, this time from Rhode Island, where a routine quarterly inspection found asymptomatic H5-positive poultry.  This press release from the Rhode Island Department of Environmental Management.

Avian Flu Confirmed at Live Bird Market in Providence

Published on Saturday, June 13, 2026

The Rhode Island Department of Health (RIDOH) and Rhode Island Department of Environmental Management (DEM) want to alert consumers that birds at Antonelli Poultry in Providence tested positive for the H5N1 strain of avian influenza during routine quarterly testing by the US Department of Agriculture (USDA). The infected birds, which included live chickens and ducks, did not come from Rhode Island farms. They were from out-of-state dealers.

Earlier today (June 13), the State Veterinarian oversaw the USDA-required humane euthanasia of about 445 asymptomatic birds at the market to prevent the spread of the disease to other birds. Per USDA regulations, Antonelli Poultry will be closed until 5 days after they have disposed of infected birds and have cleaned and sanitized all areas of the business. Antonelli Poultry is closely cooperating with DEM and RIDOH.

Because staff at Antonelli Poultry may have been exposed to avian influenza, and out of an abundance of caution, RIDOH is monitoring all staff for 10 days for symptoms of avian influenza. The overall risk of humans getting H5N1 remains low.

“Cooking poultry to the proper internal temperature of 165° kills bacteria and viruses, including avian influenza A viruses,” says Director of Health Jerry Larkin, MD. “RIDOH recommends that if anyone still has poultry they bought between June 9 and June 12 that was killed and dressed by Antonelli Poultry, they should double bag the poultry and dispose of it in their regular trash. If you have properly cooked and eaten chicken from Antonelli Poultry, the risk of becoming ill is very low; however, if you develop symptoms of avian influenza, you should seek medical care.”

Symptoms of avian influenza include eye redness, fever, cough, sore throat, runny nose, muscle or body aches, fatigue, shortness of breath or difficulty breathing, or pneumonia that requires hospitalization. People who get avian influenza can be treated with antivirals.

To prevent any foodborne illness, RIDOH recommends:

• Wash hands, utensils, and cutting boards before and after contact with raw poultry, meat, seafood, and eggs.
• Keep raw poultry and meat away from food that won’t be cooked—like fruits and vegetables.
• Cook food to the proper temperature and use a food thermometer to check the food’s internal temperature. You cannot tell by looking at food if it is cooked to the proper temperature.

Avian influenza infections in humans are rare. The best way to prevent avian influenza in humans is for people to avoid exposure.

• Avoid direct contact with birds or other animals infected with, or suspected to be infected with, avian influenza.
• Avoid direct contact with sick or dead wild birds, poultry, or other animals.
• Do not touch surfaces or materials contaminated with saliva, mucous, or animal feces from wild or domestic birds or other animals with confirmed or suspected avian influenza.
• Do not touch or drink raw milk (unpasteurized milk), especially from animals with confirmed or suspected avian influenza
• Do not handle any sick or dead wild birds or other animals without wearing personal protective equipment (PPE).

“DEM works closely with federal and State veterinary and public health officials to respond quickly to confirmed H5N1 cases in domestic birds,” said State Veterinarian Scott Marshall, DVM. “The USDA performs quarterly testing at live bird markets to ensure the public’s safety.”

This is Rhode Island’s first confirmed domestic bird case of avian influenza in 2026. Rhode Island has previously confirmed infections in noncommercial flocks in 2022 and in 2025.

To learn more about avian influenza in humans, visit RIDOH’s website. To learn more about avian influenza in animals, visit DEM’s website.
Date Sat, 06/13/2026 - 16:02

By my tally, this is the 20th Market outbreak in the United States in 2026, and 77th since HPAI H5 returned in early 2022. 

Notably, the press release stated that the birds had been imported from another (unnamed) state. Worth noting, we've not seen any commercial flocks reported by the USDA as H5 positive east of Indiana since early May.

Government agencies are quick to reassure the public that the risk of contracting avian influenza remains low in the United States, but exposure to live birds (via LBMs or raising poultry) is a known risk factor (see CDC graphic below).

And as we discussed three weeks ago in MMWR: Knowledge, Attitudes, and Practices Regarding Avian Influenza Among Owners of Backyard Flocks, many backyard poultry producers still have limited knowledge of avian flu symptoms and risks, and their biosecurity measures often fall short of recommendations. 

Over the past few years the threat from H5N1 has grown markedly in the Western Hemisphere. Things we used to do without much thought - like raising a few chickens the backyard or frequenting live markets - carry more risks today.

While those risks can be largely mitigated through improved biosecurity practices, four years after its arrival, we still seem to be tempting fate.  

USDA Screwworm Update (n=11) - FDA EUA For Drug to Treat Dogs & Cats

 

#19,201

The number of confirmed screwworm cases (NWS) continues to mount, and a 5th Texas county (Tom Green) has been added, in the latest update to the USDA's dashboard. 

Thus far, all confirmed detections have been in either livestock (cattle or goats), or in pets. While unknown, there are likely a far larger number of wildlife infections, as the NWS will infect just about any warm-blooded animal (including birds).

The CDC describes its life cycle as:

New World screwworm infestations begin when a female fly lays eggs on open wounds or other parts of the body in live, warm-blooded animals. Most infestations occur in animals, but they also occur in people. The smell of a wound or body opening such as the nose, mouth, eyes, ears, or genitals can attract female flies. Wounds as small as a tick bite may attract a female fly to lay her eggs. One female can lay 200 – 300 eggs at a time and may lay up to 3,000 eggs during her 10- to 30-day lifespan.

Eggs hatch into maggots that burrow into the wound to feed on the living flesh. After feeding for about 7 days, larvae drop to the ground, burrow into the soil, and emerge as adult screwworm flies.

Due to its recent return to U.S. soil, the FDA this week granted an Emergency Use Authorization (EUA) for a generic OTC drug to treat NWS infection in dogs and cats. 

FDA Issues Emergency Use Authorization for Generic Over-the-Counter Drug to Treat New World Screwworm in Dogs and Cats

Most dogs and cats in the U.S. are at low risk of NWS; risk is elevated for pets recently in areas with confirmed cases of NWS

For Immediate Release:June 11, 202

The U.S. Food and Drug Administration today issued an Emergency Use Authorization (EUA) for generic Nitenpyram Tablets (nitenpyram) for the treatment of New World screwworm (NWS) infestations (myiasis) in dogs, puppies, cats, and kittens that weigh at least two pounds and are at least four weeks old. This is the first generic animal drug authorized for use against NWS.

       (Continue . . . )

While this provides a faster, and cheaper way for people to treat their pets, it also makes it easier for cases to go unreported.  Hopefully pet owners will still contact their veterinarian, and have their pet examined, after treatment has been initiated.

As the CDC warns, proper disposal of maggots is imperative in order to interrupt the life cycle of these parasites. 

Hong Kong CHP: Locally Acquired Case of H9 Infection - Notification Letter For Doctors

 

#19,200

While Mainland China leads the world in reported H9N2 infections, Hong Kong has only reported 10 since 1999, with just 4 of those locally acquired. The last non-imported HK case I can find (see FluTrackers H9N2 case list) was in early 2020. 

And that makes sense, given the nearly 200-fold difference in population pools, and the fact that H9N2 (and other) avian flu subtypes are poorly controlled in mainland poultry. 

Yesterday Hong Kong's CHP announced the detection of a locally acquired H9 case (likely H9N2) in a child without recent travel, and only limited indirect contact with poultry at a live market.  The epidemiological investigation is ongoing. 

Generally mild, H9N2 is closely monitored because it continues to evolve and accrue mammalian adaptations (see CCDC Weekly: Epidemiological and Genetic Characterization of Three H9N2 Viruses . . .).  Our CDC has designated 2 different lineages (A(H9N2) G1 and A(H9N2) Y280) as having at least some pandemic potential (see CDC IRAT SCORE).

While this is likely a one-off infection, Hong Kong's CHP is treating this seriously, and has published a detailed report (below), and has sent notification letters to local doctors. 

CHP investigates case of influenza A (H9) infection (with photo) 

     The Centre for Health Protection (CHP) of the Department of Health (DH) is today (June 12) investigating a case of human infection with influenza A (H9) in collaboration with relevant departments. The patient is a two-year-old boy. His condition has remained mild and he is currently in stable condition. The CHP will send a letter to all doctors in Hong Kong to update them on the latest developments regarding avian influenza A and to urge them to remain vigilant and report any suspected cases. 

Case information 

     The boy lives in Sha Tin District. He developed a fever and mild diarrhoea on June 9. On the following day (June 10), he was brought to Prince of Wales Hospital and was admitted for treatment. His clinical specimen tested positive for the influenza A (H9) virus by the Public Health Laboratory Services Branch (PHLSB) of the CHP. The subtyping result is pending. His clinical diagnosis was novel influenza. He is currently in stable condition and has been admitted to an isolation ward at Princess Margaret Hospital for treatment.      

     The CHP's preliminary investigation revealed that the patient had no travel history during the incubation period. The case has been classified as a locally acquired case. The patient does not attend school or receive daycare services. He is primarily cared for by his family members and spends most of his time at home or nearby. His household does not keep poultry. According to information provided by his family members, he has neither consumed undercooked poultry nor come into contact with any patients.

In early June, one of his family members took him to Wo Che Market on two occasions. During these visits, the patient stayed at a fresh provision shop in the market that sells live chickens to watch the poultry and touched the surroundings of the fresh provision shop. The CHP conducted an investigation with the Food and Environmental Hygiene Department (FEHD) and collected environmental samples from the shop concerned. The shop staff remained asymptomatic. The patient has six household contacts and they remain asymptomatic so far. The CHP has provided them with preventive medication and put them under medical surveillance.      

     The CHP is continuing to investigate the source of infection of the case and is conducting whole genome sequencing of the virus sample. The CHP will also report the case to the World Health Organization (WHO).      

     Humans are primarily infected with the influenza A virus through direct contact with infected poultry or through indirect contact with environments contaminated by their droppings. The CHP's epidemiological investigation indicated that the patient had visited a location where live poultry was sold. It cannot be ruled out that the patient was infected through indirect contact with a contaminated environment at the wet market. As young children have weaker immune systems and are incapable of maintaining good hand hygiene, the CHP advised parents to avoid taking young children to places where live poultry is sold. Transporting poultry may contaminate the ground and the surrounding environment. As young children are shorter in height and easy to be in contact with the surrounding environment, they are at greater risk of coming into contact with poultry droppings or contaminated areas.      

     In the past ten years, the WHO has received reports of a total of over 160 cases of human infection with influenza A (H9) worldwide. To date, most case of human infection with influenza A (H9) have presented with only mild clinical illness. According to the WHO's risk assessment, the influenza A (H9) virus has not acquired the ability for sustained human-to-human transmissions. 

Government's comprehensive follow-up actions

Novel influenza A infection, including influenza A (H9), is a notifiable infectious disease in Hong Kong. Compared to other highly pathogenic avian influenza strains such as H5N1 and H7N9, influenza A (H9) is a low-pathogenic avian influenza strain that causes milder illness. Excluding the aforementioned case, 10 cases of influenza A (H9N2) have been reported since 1999, including four locally acquired cases and six imported cases. No deaths have been recorded so far. In response to the latest local case, the CHP will issue a letter to all doctors in Hong Kong, reminding them of the latest situation of influenza A (H9), and urging them to remain vigilant and report any suspected cases.

Sporadic cases of human infection with avian influenza occur from time to time internationally. Although the current risk of an outbreak is low, the Hong Kong Special Administrative Region Government has consistently implemented preventive measures, including a disease surveillance system, the implementation of livestock control measures at farms, markets and ports, in order to prevent avian influenza.

The PHLSB of the CHP comprises laboratories with high biosafety standards, capable of conducting, testing for high-risk pathogens, and which also possess sufficient testing and genetic analysis capabilities and facilities. Hong Kong currently has sufficient reserve of antiviral medications.

Preventive measures to be taken by the public

Humans are primarily infected with the avian influenza A virus through contact with infected birds, poultry or other animals (whether alive or dead), or through surfaces or environments contaminated with saliva, mucous and animal faeces (such as wet markets and live poultry markets). The virus has very low transmissibility among humans. People who have close contact with live poultry are more susceptible to contracting avian influenza. The elderly, children and people with chronic illnesses have a higher risk of developing complications such as bronchitis and pneumonia, if infected. Members of the public should remain vigilant and take the following measures to prevent avian influenza: 

• Avoid contact with poultry, birds or their droppings. If contact has been made, thoroughly wash hands with soap and water;
• Poultry and eggs should be thoroughly cooked before eating;
• Perform hand hygiene at all times, especially before touching the mouth, nose or eyes; after contact with animals or their living environments; after touching public installations such as handrails or doorknobs; or when hands are contaminated with respiratory secretions, such as after coughing or sneezing;
• Cover the mouth and nose with tissue paper when sneezing or coughing. Dispose of soiled tissues into a lidded rubbish bin, then wash hands thoroughly;
• When having respiratory symptoms, wear a surgical mask, do not go to work or school, avoid crowded places and seek medical advice promptly;
• Avoid crowded public places or areas with poorly ventilated; high-risk individuals may consider putting on a surgical mask when staying in such places; and
• Travellers returning to Hong Kong from areas affected by avian influenza outbreaks should consult doctors promptly if they have flu-like symptoms, and inform the doctor of the recent travel history and wear a surgical mask to help prevent spreading of the disease.

     ​The public may visit the CHP's webpages for more information: Avian Influenza Webpage, Avian Influenza Report, Avian influenza statistics and affected areas around the world, Facebook page and Youtube channel. 

Ends/Friday, June 12, 2026

Issued at HKT 22:19

The CHP has also sent the following letter to doctors reminding them to remain vigilant, and how to report suspected cases. 

Complicating matters, Hong Kong is known for having a biphasic or `double peaked’ flu season, with most activity reported between February and April, but often seeing a less severe season in mid to late summer (see Seasonality of Influenza A(H3N2) Virus: A Hong Kong Perspective (1997–2006).

And, as the latest HK flu report indicates, they are beginning to see signs of a summer uptick in ILI. 

While this is likely an isolated incident, H9N2 is a legitimate zoonotic threat, and so we'll be watching Hong Kong carefully for any additional reports.