Nature Comms: Trimester-dependent vertical transmission of H5N1 influenza virus through placental and mammary routes impairs offspring development

Photo Credit – CDC

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In late 2024, in EID Journal: Systematic Review of Avian Influenza Virus Infection and Outcomes during Pregnancy - among 30 pregnant women diagnosed with avian flu (H5N1 n = 16, H7N9 n = 13, and H5N6 n = 1) - researchers reported high mortality rates for both mothers (90.0%, 27/30) and their babies (86.7%, 26/30).

As we've discussed often (see Pregnancy & Flu: A Bad Combination), novel flu often exacts a heavy toll on expectant mothers and their unborn children.

Even during the relatively mild H1N1 pandemic of 2009, we saw studies (see BMJ: Perinatal Outcomes After Maternal 2009/H1N1 Infection) where pregnant women who were admitted to the hospital with an H1N1 infection experienced a 3 to 4 times higher rate of preterm birth, 4 to 5 times greater risk of stillbirth, and a 4 to 6 times higher rate of neonatal death.

We also know that during the 1918 pandemic an abnormally high number of pregnant women died from the influenza, and those that survived endured a very high miscarriage rate. Again, during the much milder 1957 Asian Flu, pregnant women reportedly suffered disproportionately higher mortality rates than non-pregnant women of the same age.

Historical reviews of both events are available in a Perspective, written by 3 CDC physicians (Sonja A. Rasmussen, Denise J. Jamieson, Joseph S. Bresee) and published in the CDC Journal of EID article, Pandemic Influenza and Pregnant Women in February of 2008.

As bad as they were, most of these effects have been attributed to the mother's immune response (fever, cytokine production, etc.) to the infection; not the vertical transmission of the virus to the fetus in utero.

A 2012 CDC study, however (Effects of influenza on pregnant women and infants) does cite H5N1 as potentially being an exception, due to its ability to cause extrapulmonary infection & viremia. Their conclusion, however:

"Therefore, the limited evidence to date suggests that vertical transmission of influenza viruses can occur but is likely to be very rare."

Last November we looked at a preprint (see Vertical Transmission of Bovine H5N1 Influenza Virus during Pregnancy and Lactation in Mice) which demonstrated that in (lab inoculated) pregnant mice (2nd Trimester), the bovine B3.13 H5N1 virus crossed the placenta (in a subset of litters) and, in a separate late-gestation experiment, spread through milk, infecting pups.

Exposed offspring were typically smaller, reached developmental milestones later, and showed lasting behavioral changes into adolescence.

Interestingly, they used two different isolates (A/bovine/Texas/98638/2024 and A/bovine/Ohio/B24OSU-439/2024), which produced different clinical courses and pathology in mice. 

The Texas isolate produced a faster, more lethal course and higher viral burdens in several tissues—including mammary, ovarian, and CNS tissues—than the Ohio isolate in mice. The authors elected to use the slower-progressing Ohio virus for their pregnancy experiments.

That (revised and renamed) preprint has now been published in Nature Comms, and is offered early (subject to additional edits) at the link below.  The full study is  lengthy, and at times, highly technical. 

Those wanting a deeper dive should download the PDF. I'll have a postscript after the break.

Trimester-dependent vertical transmission of H5N1 influenza virus through placental and mammary routes impairs offspring development

Brittany A. Seibert, Maclaine A. Parish, Sattya N. Talukdar, Tianle Zhang,

Jennifer A. Liu, Sabal Chaulagain, Jaiprasath Sachithanandham, Oscar Hernandez,

Cosette Schneider, Lynda Coughlan, Andrew S. Bowman, Richard J. Webby,

Jack R. Harkema, Andrew Pekosz & Sabra L. Klein

Nature Communications (2026) Cite this article 

We’re sharing this article early to provide faster access to peer-reviewed, accepted research. It is citable and carries a permanent DOI. This version is subject to further edits and will be replaced automatically by the final Version of Record. All legal disclaimers apply.

Abstract

Avian influenza H5N1 has pandemic potential and historically causes more severe disease in pregnant women than the general population. With increasing transmission of H5N1 detected among placental mammals, animal models are necessary for testing countermeasures, including during pregnancy. Pregnant outbred mice infected with a contemporary strain of bovine H5N1 during the second trimester equivalent causes in utero transmission, with infectious virus detected in the uterus, placenta, and fetus. Birth following third trimester infection results in offspring with decreased size, neurodevelopmental delays, and adolescent behavioral impairments, with infectious virus detected in the neonatal milk ring and lungs, as well as mammary tissues. H5N1 viral protein colocalizes with trophoblast cells in the placenta and epithelial cells in mammary tissue that spatially overlap with lectins for α2,3-linked SA. With the pandemic potential of H5N1, our vertical transmission model in placental mammals is essential for understanding viral spread and evaluating treatments during pregnancy.

       (Continue . . . )

The usual caveats apply; including a) mice have anatomical and immunological differences from humans. b) this study only looked at genotype B3.13, and c) and it is unknown how much these findings will actually translate to humans.

But even without vertical transmission, pregnant women and their offspring are presumed at higher risk during any flu outbreak.

Last February, in Viruses: Avian Influenza H5N1 Infection During Pregnancy: Preparing for the Next Flu Pandemic and Improving Perinatal Outcomes, we looked at a review article published in Viruses that looked at the limited data we have on novel flu and pregnancy, and argued that H5N1 could be particularly dangerous to pregnant women and their offspring if it should begin to spread in humans.

The authors proposed a basic algorithm for initial clinical management of suspected H5N1 in pregnancy and lobbied for prioritizing both antivirals and vaccines (when available) for high-risk patients, including those who may be pregnant.

While an H5Nx pandemic is by no means assured, pandemic history has not been kind to pregnant women and their offspring, making it imperative that we consider the risks - and how we might handle them - now, while we still have the luxury of time.

New Zealand Reports 3rd H5N1 Detection in a Wild Bird

Credit Wikipedia

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It has been just over a month since New Zealand reported their second H5N1 detection in a local bird (a swamp harrier hawk) found in the sparsely populated Wairarapa region on the north island (see map above).

Today, New Zealand has confirmed a 3rd detection - this time in a northern giant petrel, found dead on a remote Cape Palliser beach (also in the Wairarapa).

While it is likely there are other, as-yet undetected cases in New Zealand, they don't appear to have seen the same level of influx that Australia has had to deal with.   How long that will last is anyone's guess.

First the NZ announcement, then a brief update on Australia's numbers. 

Situation update: 21 August 2026

New Zealand has 3 confirmed detections of H5 high pathogenicity avian influenza (H5 bird flu).

New Zealand’s third case of H5 bird flu was confirmed on 21 August in a northern giant petrel, found dead on a remote Cape Palliser beach in the Wairarapa.

There is no evidence of any mass mortality in wildlife. There has been no detection in poultry.

Chicken and eggs remain safe to eat and bird flu is a very low risk to human health.

New Zealand is well prepared to respond to H5 bird flu and is working to protect poultry production and reduce impacts on wildlife and communities. Response and surveillance work has intensified in close coordination with industry partners and other agencies.

The Department of Conservation is implementing a vaccination programme for 300 core breeding birds from 5 of our most endangered birds – kākāpō, takahē, tūturuatu/shore plover, kakī/black stilt, and kākāriki karaka/orange-fronted parakeet.

Be alert and use good habits to limit the impact of bird flu

Keep your distance. Stay away from sick or dead wildlife. Keep pets away too.

Keep clean. Wash your hands and clean your gear after being outdoors.

Know when to report. If you see 3 or more sick or dead wild birds in a group, report it immediately to the exotic pest and disease hotline on 0800 80 99 66 or online at report.mpi.govt.nz

If you report online, select the ‘dead or dying wild birds’ option from the dropdown list and then the type of bird you have found. Don’t handle or move the birds.

Meanwhile, in Australia, the flow of information continues to be a bit confusing.

Depending on which DAFF (Department of Agriculture) page you land on (here or here), the last updated national bird flu numbers (as of this writing) are listed as being either 27 or  51 hours old (see double screenshot below).

The last avian flu update from Australia's Chief Veterinary Officer was on August 14th, and - while reporting formats are not uniform across the individual states - a quick perusal suggests that the national tallies are lagging a day or two behind local reporting. 

With the number of hotline reports of dead or sick birds now approaching 25,000 - far exceeding their ability to investigate and test - these sorts of delays are not unexpected. 

The DAFF bird flu site offers the following caveats:

Data disclaimer

Data reflects information provided by state and territory governments to the Australian Government as at 17:00 AEST daily. The Australian Government publishes this information for national reporting purposes. Responsibility for the accuracy, completeness and currency of the data remains with the relevant state or territory government. Due to differences in reporting timing, information on the national dashboard may differ from information published on state or territory government websites.

Approach to reporting

From 12 August 2026, we have changed how we report on H5 bird flu detections.

Reporting is now based on events, rather than individual bird detections.

An event may involve one or more birds. The new number of positive events captures all previously reported individual confirmed detections.

Definitions

Investigation: An event where one or more wild birds or wild mammals showing signs of H5 bird flu have been reported to state or territory authorities and samples have been collected for laboratory testing.

Positive event: An investigation where H5 bird flu has been confirmed through laboratory testing.

Hotline report: A report to relevant state or territory authorities through the Emergency Animal Disease Hotline or other official sources.

While continued surveillance is important to track the geographic spread - and increasing host-range - of H5N1 in Australia, the numbers posted each day become less significant over time.  

A reminder of how quickly an unprecedented event - like the first arrival of HPAI H5 to Oceania 2 months ago - can become the `new normal'.

Louisiana DOH Reports Naegleria fowleri infection in Louisiana resident

 Credit CDC

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Nearly every year we see a handful of rare - mostly fatal - brain infections caused by free living amoebas (Naegleria fowleri) that inhabit warm, fresh water (see A Reminder About Naegleria Season - 2019).

Dubbed the `brain eating amoeba' by the press - this infection is called PAM (Primary amebic meningoencephalitis) - and occurs when the amoeba enters the brain through the nasal passages, usually due to the forceful aspiration of contaminated water into the nose.

Not every PAM case is due to Naegleria, as Balamuthia mandrillaris and Acanthamoeba - and other non-Naegleria amoebic infections - can cause similar pathologies.

Last year the CDC's MMWR carried a report on a fatal 2024 case from Texas, which involved a previously healthy 71-year old woman who used unsterilized water to perform nasal irrigation.  

This is a common way that people around the world have become infected (see 2011's Neti Pots & Naegleria Fowleri).  The CDC and state health departments have long warned on the dangers of using tap water to perform nasal irrigation, and offer advice on safer alternatives. 

But most U.S. cases occur while swimming, often in warm stagnant freshwater lakes, rivers, canals, and ponds.  Less commonly, we've  seen infections attributed to water parks, private swimming pools, and even the public water supply. 

• In 2013, in Louisiana: St. Bernard Parish Water Supply Tests Positive For Naegleria Fowleri, we saw the tragic story of a 4-year-old Mississippi child visiting a St. Bernard Parish home, who (according to this NOLA.COM report) appears to have contracted the amoeba while playing with a water hose and a slip-and-slide.

• Five years ago we looked at an MMWR: Epidemiological Investigation Into A Case Of Primary Amebic Meningoencephalitis in California which suggested a poorly chlorinated spring-fed swimming pool was the likely source of infection and death of a 21 year old woman.

•  In 2013, in Arkansas: Naegleria fowleri Shuts Water Park, we saw the second suspected case in 3 years from the same recreational venue.

Only 3 or 4 cases are reported in the United States each summer, but in 2017 a research letter written by epidemiologists at the CDC (see EID Journal: Estimation of Undiagnosed Naegleria fowleri (PAM), United States) estimated the yearly number PAM cases in the United States probably averages closer to 16 (8 males, 8 females).

Meaning that 70%-80% likely go unrecognized. But even so, your odds of contracting PAM are exceedingly low.

 Louisiana Health Department report follows:

LDH reports Naegleria fowleri infection in Louisiana resident 

Rare infection most likely acquired while swimming in Lake Claiborne

Baton Rouge, Louisiana, Aug 19, 2026 - The Louisiana Department of Health is reporting a confirmed Naegleria fowleri infection in a Louisiana resident. The resident most likely acquired the infection while swimming in Lake Claiborne, located in Claiborne Parish, shortly before illness.

Naegleria fowleri, often referred to as a brain-eating amoeba, is commonly found in warm freshwater lakes, rivers, canals, and ponds throughout the United States. It can cause primary amebic meningoencephalitis (PAM), a brain infection that may result when water containing the amoeba rushes up the nose and reaches the brain. The infection is extremely rare, but nearly always fatal.

Naegleria fowleri cannot infect people if swallowed and is not spread from person to person. The amoeba is not found in salt water or in properly maintained and chlorinated pools. Symptoms of Naegleria fowleri infection typically start with severe headache, fever, nausea and vomiting and progress to stiff neck, seizures, and coma that can lead to death. Symptoms usually begin about five days after infection but can start within one to 12 days.

Naegleria fowleri thrives in warm, fresh water and grows best at higher temperatures up to 115°F. The amoeba is naturally occurring, and there is no routine environmental test for Naegleria fowleri in bodies of water.

A Health Alert Network (HAN) was shared to all state providers from Louisiana Department of Health on August 18, 2026, advising on symptoms and reporting.

Though the risk of infection is low, recreational water users should always assume there is a risk when they enter warm fresh water. You can reduce your risk of infection by limiting the amount of water that goes up the nose. Recommended precautions include:

• Use caution when engaging in water-related activities in warm freshwater during periods of extended extreme heat. Naegleria fowleri usually occurs when temperatures increase for prolonged periods, resulting in higher water temperatures and lower water levels.
• Swimmers can reduce their risk by keeping their heads out of the water and using nose clips or plugging their noses when going underwater.
• Avoid submersing your head in hot springs and other untreated fresh thermal water.
• Avoid digging in or stirring up the sediment while taking part in water-related activities in shallow, warm freshwater areas.

​Naegleria fowleri infections are rare, with only 180 known cases of PAM in the U.S. (between zero and eight cases annually) from 1937 through 2025. Louisiana had three Naegleria fowleri cases during that time period.

For more information regarding Naegleria fowleri, please visit Naegleria fowleri Infections | Naegleria fowleri Infection | CDC.

For more information on the Naegleria parasite, you can also visit the CDC’s Naegleria webpage.

Australia: Media Reports of Multiple Mass Mortality Events in Birds (H5N1 Suspected)

 
Data as of 4pm AEST, 19 August 2026

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While HPAI H5N1 reporting from governmental sources have slowed in recent days, overnight the Australian press reported on multiple mass mortality events involving birds in South Australia and Tasmania (see ABC News Bird flu suspected after more than 1,000 birds found dead in SA, more deaths in Tasmania).

According to multiple news sources (see here and here), South Australian Premier Peter Malinauskas addressed the SA parliament (Aug 20th) to announce that more than 1,000 crested terns had died across three mass mortality events on several remote offshore islands. 

Additionally there are reports of > 2 dozen dead penguins in Tasmania.  While H5N1 has not been confirmed, HPAI is strongly suspected.  For now, there are no reports of H5N1 in non-avian wildlife, poultry, or livestock. 

While we await further details (and/or confirmation) we've two other sources of background information; an expert commentary from Australia's National Testing laboratory CSIRO, and a series of academic analyses from The Conversation.

First this analysis from CSIRO (h/t Pathfinder on FluTrackers):

Expert Commentary: What do we know about H5 bird flu?

18 AUGUST 2026 4 MIN READ

In June 2026, Australia recorded its first cases of the highly contagious H5 bird flu.

As Australia's national reference laboratory for animal diseases, CSIRO's Australian Centre for Disease Preparedness (ACDP) plays a critical role in protecting Australia's valuable livestock and aquaculture industries and the wider community from infectious diseases such as bird flu.

Scientists at ACDP have been conducting testing and genetic analysis to learn more about exactly which strain of the virus has arrived on our shores, and how it is spreading.

All quotes below are available for use by media. Spokespeople quoted:

• Dr Frank Wong, Senior Research Scientist at CSIRO's Australian Centre for Disease Preparedness. Dr Wong is also a World Organization for Animal Health-designated reference expert on avian influenza
• Dr Matt Neave, Team Leader of Sequencing and Agent Characterisation at CSIRO's ACDP
• Dr Debbie Eagles, Director of CSIRO's ACDP.

What is H5N1 and why is it a concern?

Dr Frank Wong:

For the past 5 or 6 years the world has been affected by a bird flu panzootic. This is what we call a pandemic in animals. In this case, because the virus is an avian virus, it has mostly impacted birds around the world.

The clade 2.3.4.4b lineage of H5N1 is a highly pathogenic bird flu virus that emerged around 2020-2021, first spreading across Europe and then moving on to other continents, including Asia and North America.

Along the way it has caused mass mortalities in both domestic poultry as well as many species of wild bird.

H5N1 has been able to spread rapidly across different continents largely due its ability to be sustained in migratory wild birds, like waterfowl such as ducks and geese.

The genotype that we're talking about for Australia, known as genotype B3.2, arose from reassortment in North America and then spread to South America and the sub-Antarctic islands.

It had a great impact on South American poultry industries as well as wild bird life and marine mammals, so that's why we're concerned about the introduction of the virus to Australia.

What does ACDP's genetic testing show us?

Dr Matt Neave:

Sequencing the virus genome has become really important in disease outbreaks like this one. We're now able to do it much quicker than in the past and can get a whole influenza genome in around 15 to 26 hours.

From the viral genome we are able to determine several important things, including the exact strain of the virus. Another thing we can do is compare that viral genome to other reference sequences in our database or public databases online. This allows us to determine how closely related our Australian cases are to the sub-Antarctic ones, for example.

Our testing shows us that the sequences from the first two cases in Australia were closely related to ones from Heard Island. From the data we have at hand, it appears Heard Island is the most likely source of the Australian infections.

By comparing genetic sequences from virus samples, scientists can trace the relationships between the first Australian H5 bird flu cases and those detected on Heard Island. Credit: Matthew J. Neave et al. Show image description 

        (Follow link to read the full report). 

Although there are now roughly 2 dozen entries, going back to H5N1's arrival in Australia in mid-June, over the past few days these 4 essays have been published.

Published: August 20, 2026
The first wave of bird flu is the worst: what Australia can learn from overseas
Bethany J Hoye, University of Wollongong
The rest of the world has already had to grapple with bird flu – and there are lessons for Australia.

Published: August 18, 2026
More than 5,000 little penguins will be vaxxed against bird flu. How will this protect them?
Julie Old, Western Sydney University
Little penguins are flightless and return at night to the same burrow, making it easier to capture and vaccinate them.

Published: August 17, 2026
From the coasts to the plains, bird flu could threaten not just birds – but their homes
Penny Olsen, Australian National University
From raucous honeyeaters to top predators, Australia’s birds are vital to nature.

Published: August 16, 2026
Is bird flu infecting humans? How would we know?
Allen Cheng, Monash University
Even with bird flu circulating in Australia, the risk to humans is low. But some groups are still at increased risk.

 

WHO Influenza at the human-animal interface (Aug 7th) - Uptick in H9N2 Cases in China

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The WHO has released an update (dated Aug 7th, but posted overnight) of human novel flu infections reported between July 8th and Aug 7th. Included are 8 H9N2 cases from Mainland China, and 1 H5N1 case from Cambodia (all previously reported).

This report also confirms the H5 case reported from Bangladesh in July has been confirmed as H5N1. 

For reasons well above my pay grade, reporting from Hong Kong's CHP on H9N2 cases has been a bit chaotic this month (reporting the same 4 recent cases as `new' for the past 3 weeks running), and the WHO's weekly WPRO avian flu report hasn't been updated since Aug 7th. 

What we can say, however, is that China (along with a recent case in Hong Kong) has seen an uptick in reported H9N2 cases this summer. Whether this is due to more frequent spillovers, or better surveillance, isn't known. 

Despite being clearly zoonotic - LPAI H9N2 is considered a `non-reportable' disease in poultry or wild birds by WOAH (see Terrestrial Animal Code). Complicating matters, China's attempts to control LPAI H9N2 through vaccines have been less than successful. 

In 2025's NPJ Vaccines: Impact of Inactivated Vaccine on Transmission and Evolution of H9N2 Avian Influenza Virus in Chickens, we saw evidence that inactivated vaccines failed to prevent - or even reduce - H9N2 in China's poultry, and may have driven viral evolution (including mammalian adaptations).

Several recent Chinese studies have found signs that LPAI H9N2 is becoming better adapted to mammals, warning of its pandemic potential; including:

• Two weeks ago, in EM&I: Genetic evolution, phylodynamics, geographic spread of H9N2 avian influenza viruses in China from 2014 to 2025: an increasing potential zoonotic risk, we saw yet another Chinese study warning that H9N2 was accruing mammalian adaptations which increases its zoonotic risk.
• Two months ago, in Viruses: Epidemiological and Virological Characteristics of H9N2 Avian Influenza Virus in Jiangsu Province, China, 2024, researchers warned that that Jiangsu’s circulating H9N2 viruses have acquired human receptor preference and mammalian adaptation, posing silent infection and pandemic risks

• Last October, in China CDC Weekly: Epidemiological and Genetic Characterization of Three H9N2 Viruses Causing Human Infections, investigators reported a number of indicators of increased mammalian adaptation within the virus, including an enhanced ability to infect upper respiratory (α2,6-sialic acid) tract receptors, and a number of HA protein mutations, including; H191N, A198V, Q226L, and Q234L.

• The Asian Y280/G57 lineages have shown increasing signs of mammalian adaptation (see EM&I: Enhanced Replication of a Contemporary Avian Influenza A H9N2 Virus in Human Respiratory Organoids)), while the African and Middle Eastern Lineages (mostly European G1-like) are older and less evolved.

• Last summer, in Nature: Genetic diversity of H9N2 avian influenza viruses in poultry across China and implications for zoonotic transmission, researchers reported rising prevalence of key mutations associated with increased infectivity and pathogenicity in mammals.

While H9N2 doesn't garner the kind of headlines that H5N1 gets, our own CDC lists two lineages (A(H9N2) G1 and A(H9N2) Y280) as having at least some pandemic potential, and several candidate vaccines have been developed.

In terms of risk of emergence, the H9N2

Y280 lineage is ranked higher than H5N1

Which is why any uptick in activity is worthy of our attention. 

Australia H5N1 Update

 

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It's been a week since Australia Changed How They Report H5N1 - reporting `events' rather than individual detections in wildlife - and while the numbers have continued to rise (see below), we seem to be getting fewer (and less detailed) updates. 

In the first 10 days of August, Australia's Chief Veterinary Officer released 11 bird flu updates/statements, but has only released one in the past 9 days. Similarly, Victoria's last update was August 15th.  

Up until a week ago, DAFF was reporting both the number of positive and negative test results, along with number hotline reports (see below). This gave us some idea of the percentage of hotline reports being investigated (roughly 1 in 13).

(Aug 13th DAFF)

With the move towards only tracking `events', that metric is no longer provided.

While I'd certainly like more data, there is only so much that any country can reasonably do when they are faced with a highly aggressive pathogen like avian H5N1. 

For now, South Australia continues to post detailed daily updates, but added the following notice this week:

Thus far, H5N1 has not been detected in poultry, livestock, or non-avian wildlife. But how long that reassuring status quo can last is unknown.