Laos Study Finds 12 Bird Flu Genotypes, Including Genetic Patterns Linked to Human Infections

A changing virus across three genetic branches

Scientists studying bird flu in Laos have identified 12 distinct genetic combinations across three branches of H5 avian influenza viruses, revealing years of circulation and repeated exchanges of genetic material. The investigation sequenced 37 viruses collected between 2019 and 2024 and compared them with publicly available Lao sequences from 2015 through 2024.

One finding carries particular importance for the region: a genotype called E7 was detected continuously from 2022 to 2024 and had the same arrangement of genes as viruses associated with human and other mammalian infections in Cambodia and Vietnam. Another genotype, H2, was genetically similar to H5N6 viruses linked to a human infection in Laos and found in poultry and human patients in China.

The research, published in Frontiers in Veterinary Science in 2026, fills a gap in genetic information about H5 viruses circulating in Laos after 2018. Its central finding is that several related virus families have persisted while exchanging genes, producing new combinations that animal and human health services need to track together.

Evidence from neighboring Cambodia makes that task urgent. A separate academic investigation documented 16 human H5N1 infections from February 2023 through August 2024, with mortality of 38%. All 16 patients reported contact with sick or dead poultry. Together, the Lao and Cambodian findings connect changes in poultry viruses with the circumstances in which people become infected.

What the 12 genotypes mean

The Lao analysis identified seven genotypes, E1 through E7, within clade 2.3.2.1e; three, B1 through B3, within clade 2.3.4.4b; and two, H1 and H2, within clade 2.3.4.4h. A clade is a branch on a virus family tree, grouping viruses with shared ancestry. A genotype describes the particular combination of genetic segments carried by a virus.

Influenza A viruses have eight genetic segments. If two influenza viruses infect the same cell, their descendants can inherit a mixture of those segments. Scientists call this reassortment. It works somewhat like shuffling two sets of instructions and producing a new set containing parts of each.

The familiar names H5N1 and H5N6 describe two proteins on the virus surface: hemagglutinin, or H, and neuraminidase, or N. Viruses with the same H and N labels can still carry different internal genes. Sequencing the whole genome allows researchers to follow those differences, rather than examining just the surface proteins.

Seven of the 12 identified genotypes belonged to clade 2.3.2.1e, compared with five across the other two clades combined. Those numbers describe the diversity found within each branch. The researchers separately found that H5N1 viruses in clade 2.3.2.1e predominated during the study period, continuing a pattern seen in Laos since 2015.

The dominant branch produced new genotypes by exchanging segments with clade 2.3.4.4b H5N1 viruses and Eurasian avian influenza viruses of low pathogenicity. Low pathogenicity describes their disease effects in poultry. Such viruses can still contribute genes to new combinations. H5Nx viruses in clade 2.3.4.4b and H5N6 viruses in clade 2.3.4.4h appeared only sporadically in the Lao analysis.

How Laos connects to a regional virus network

Genetic comparisons placed related clade 2.3.2.1e H5N1 genotypes in Laos, Cambodia and Vietnam during overlapping periods. The finding places Lao poultry infections within a wider regional pattern, where a change detected in one country can become relevant to surveillance in its neighbors.

Lao clade 2.3.4.4b H5N1 viruses detected in 2023 also grouped closely with viruses documented in East Asia and North America since 2021. Researchers establish these relationships by comparing genetic sequences and constructing family trees that show which viruses share recent ancestors.

The precise movement routes behind the recent Lao detections remain unresolved. Genetic relationships help identify connected virus populations, while investigations of poultry movements, markets and wild birds are needed to reconstruct how infections traveled between places.

E7 deserves attention because it appeared across three successive years, from 2022 through 2024, and carried a gene constellation also found in viruses associated with infections in people and other mammals in Cambodia and Vietnam. H2 provides another connection between poultry surveillance and human health through its similarity to H5N6 viruses associated with human infections in Laos and China.

Cambodia shows the human cost of poultry exposure

Researchers including Erik A. Karlsson and Vijaykrishna Dhanasekaran described Cambodia's resurgence in an H5N1 research letter published in the New England Journal of Medicine on October 22, 2025. Most patients were children or adolescents. National surveillance systems for influenza symptoms and severe respiratory infections detected the 16 cases.

Laboratory capacity expanded during the COVID-19 pandemic helped strengthen those surveillance systems. Investigators also found contemporary, genetically similar viruses in poultry around affected households or through continuing surveillance of markets selling live birds. Sequencing traced the human infections to avian viruses.

Multiple infections occurred in four households. A father and daughter initially drew concern about possible transmission between people, but investigations by Cambodia's Ministry of Health and the World Health Organization found simultaneous symptom onset and direct contact with infected backyard poultry. Investigators identified poultry exposure as the most likely driver.

Karlsson, Dhanasekaran and their fellow researchers described the exposure shared by every patient:

All 16 patients reported exposure to sick or dead poultry, and outbreak investigations identified contemporary, genetically similar viruses in poultry collected in or around case households or from active, longitudinal surveillance of live-bird markets.

For households keeping poultry, the connection is immediate: disease among backyard birds can put family members in contact with an animal source of infection. Animal outbreak investigations and human case investigations therefore need to exchange information quickly.

A reassortant spread and replaced earlier strains

Cambodia's first cases in February 2023 involved the regionally established clade 2.3.2.1e, previously classified as 2.3.2.1c under World Health Organization terminology. Later cases involved a novel reassortant that had acquired genes from clade 2.3.4.4b and avian influenza viruses of low pathogenicity.

The Cambodian researchers reported that this reassortant spread across the Greater Mekong Subregion and replaced earlier strains. They identified wild birds or undetected transmission in poultry as possible pathways through which the genetic mixing occurred, with limited sequence data leaving its exact origin uncertain.

The reassortant also carried PB2:E627K, a genetic change associated with increased activity of the machinery influenza uses to copy its genome, as well as greater virulence and replication capacity in birds and mammals. PB2 is part of that copying machinery. A change affecting how it works can influence how efficiently a virus multiplies inside an infected host.

The Cambodian findings add a human health dimension to the Lao discovery of persistent reassortment. Both investigations describe an established regional H5 lineage acquiring genes from other avian influenza viruses. Following the entire genome helps surveillance teams recognize when familiar virus names conceal a changed genetic combination.

A 2014 outbreak traced a likely poultry trade route

Laos had already experienced a separate H5N6 emergence in March 2014, five years before the new study's sampling period began. Frank YK Wong and colleagues documented that episode in their H5N6 outbreak study. Four virus isolates from chickens and ducks in Luang Prabang and Xayabouly represented a single genotype.

The investigation reconstructed a likely route through poultry trade. Chicks and ducklings aged two to three days were imported from Jinghong in China's Yunnan Province to a small distributor in Luang Prabang on March 1. Consignments reached the affected villages about a week later. Birds became ill and died suddenly within 24 hours of arrival.

Provincial agriculture officials collected samples from sick and dead birds during March 12 through March 14. Results were reported to the World Organisation for Animal Health on March 31. The combination of delivery records, disease timing and genetic findings pointed to an introduction through the imported poultry.

Sequencing identified a triple reassortant, meaning the virus combined genetic ancestry from three virus groups. Its genes were linked to H5 clades 2.3.2.1b and 2.3.4.6 and to H6N6 viruses associated with ducks in China. Corresponding genes were closely related to those of an H5N6 virus independently identified in Guangdong in March 2014.

Laboratory work also found that the Lao virus differed in its antigenic properties from reference H5N1 viruses. Antigenic properties determine how the immune system recognizes a virus. Antibodies against several reference viruses generally failed to block the Lao virus in a laboratory test, prompting concern about poultry vaccine effectiveness and the selection of candidate vaccines for pandemic preparedness.

Key dates in a continuing pattern

Laos has faced outbreaks involving several H5 branches since 2004. The investigations across different periods show both repeated introductions and continued evolution within the region.

• 2004: H5N1 disease was reported in Lao poultry, involving clade 1.
• March 2014: H5N6 was detected in poultry in Luang Prabang and Xayabouly, with imported chicks and ducklings identified as the likely introduction route.
• Since 2015: Clade 2.3.2.1e H5N1 viruses have predominated in Laos.
• 2019 through 2024: Researchers collected the 37 Lao isolates sequenced for the new investigation.
• 2022 through 2024: Genotype E7 was detected continuously in Laos.
• February 2023 through August 2024: Cambodia detected 16 human H5N1 infections, with reported mortality of 38%.
• 2023: Lao clade 2.3.4.4b H5N1 viruses grouped with viruses documented in East Asia and North America since 2021.
• 2026: The Lao genetic diversity study appeared in Frontiers in Veterinary Science, volume 13, article 2002252.

The older outbreak supplies a concrete example of how poultry movements can carry a newly mixed virus into villages. The newer investigations show why surveillance must continue after an outbreak ends: related viruses can persist for years while their genetic contents change.

What surveillance needs to follow

The Lao researchers called for sustained genomic surveillance to track transmission across borders and risks of infection passing from animals to people. Sequencing can identify which genotypes are circulating, where they recur and whether they share genetic patterns with viruses found in human cases.

The Cambodian researchers called for One Health investment, an approach that connects human, animal and environmental health work. In practical terms, that means linking poultry testing, human illness surveillance, outbreak investigations and the sharing of sequence data. Cambodia's detection of similar viruses in patients, nearby poultry and markets illustrates how those activities can support one another.

Laos's 12 genotypes, Cambodia's human infections and the documented 2014 poultry trade route each capture a different part of the same problem. Viruses move with infected hosts, exchange genes and sometimes infect people exposed to poultry. Tracking those processes together gives health authorities a more useful picture than counting poultry outbreaks or human cases separately.

Key Points

• Sequencing of 37 Lao H5 viruses collected from 2019 through 2024, combined with earlier public sequences, identified 12 genotypes across three clades.
• Clade 2.3.2.1e H5N1 predominated and acquired genes from other avian influenza viruses.
• Lao genotype E7 persisted from 2022 through 2024 and shared a gene constellation with viruses associated with human and mammalian infections in Cambodia and Vietnam.
• Cambodia documented 16 human H5N1 infections from February 2023 through August 2024. All patients reported exposure to sick or dead poultry.
• A separate Lao H5N6 outbreak in 2014 was linked to the likely importation of infected chicks and ducklings from China.
• The findings support coordinated poultry surveillance, human case investigations and genome sequencing across borders.