Following the pattern of previous fall months, HPAI outbreaks have emerged in turkey flocks in North and South Dakota and Minnesota since August 25^th.  Four outbreaks have been diagnosed in South Dakota  two in North Dakota and the latest in Minnesota collectively impacting 350,000 growing turkeys. Of greater concern is the geographical extent of non-commercial cases that have included a bird market in California, backyard flocks in New York, Maryland, Georgia and Montana in addition to four outbreaks during early September involving commercial flocks in Alberta and Quebec.  This indicates widespread dissemination of virus along all North American flyways by migratory waterfowl and possible shedding by resident birds.  The emergence of HPAI in late August through mid-September presages future outbreaks that hopefully will not parallel the extreme losses experienced in 2024 and extending through the first quarter of 2025. This said hope is not a strategy!

Operators of commercial turkey and egg production flocks will intensify biosecurity that may have become less stringent over the past three months of disease quiescence.  Notwithstanding strict standards of structural and operational biosecurity, protection is not absolute due to the reality of airborne transmission.  Proximity of wild birds has been shown to be a risk factor and accordingly attempts to disfavor proximity of migratory waterfowl to farms are recommended.  The use of laser bird-repellant devices is strongly advised especially in areas with wetlands and on farms with a history of HPAI.

Recent articles have relevance to the epidemiology of HPAI in high-risk areas of the U.S.  A retrospective study conducted in France¹ among foie gras production units holding ducks and geese confirmed that close proximity among farms and density of population were important factors in dissemination and perpetuation of infection.  The authors calculated a velocity of spread approaching six miles per week with spread by fomites, feed delivery or the aerogenous route. At peak incidence, the effective reproduction number (Re) attained 3.8 indicating the number of farms infected from an index flock. In the study area in southwestern France during 2020 and 2021, 475 infected farms were studied. These units yielded 432 sequenced isolates of the H5N8 virus all of which showed close similarity.  Based on transmission dynamics, it was evident that the conventional two- and six-mile control and surveillance zones respectively were ineffective in preventing transmission unless prompt depopulation was effected.  It was also established that the period from infection with initiation of dissemination of virus and the confirmation of a diagnosis could extend to nine days representing a “critical window of undetected viral spread”.  The authors identified high connectivity among farms with infection attributed to movement of personnel and equipment and feed deliveries. Additional factors contributing to a high incidence rate included prolonged rainfall resulting in water retention that attracted migratory ducks. A low recovery rate from large waterfowl including wild swans and geese was ascertained suggesting other genera were involved in dissemination of virus.

The conclusion from the study on ducks in France four years ago is consistent with the high incidence rate in Mercer and Darke counties of Ohio and contiguous Jay County in Indiana.  High density of poultry especially with diverse species and with commonality of feed sources, and the proximity of off-line egg packing and turkey processing plants requires high levels of both structural and operational biosecurity.

Vaccination subsequent to the 2022 severe annual losses among producers of foie gras in France especially in the Landes, Gers, Hautes-Pyrenees and Pyrenees-Atlantiques Departments, resulted in a dramatic reduction in incidence rate among farms in the region. Vaccination markedly reduced outbreaks ameliorating the financial impact of HPAI among ducks and chickens for both producers and the public sector.

The benefit of vaccination in Mexico was evaluated in a study² conducted over two seasons during 2022 and 2023 respectively. Two reverse genetic vaccines were evaluated (H5N8 and H5N2 and compared to an inactivated H5N1 product approved by The Ministry of Agriculture and Rural Development.  The study involved serologic assay to ascertain response to vaccination.  During the first period extending from November 2022 to June 2023, across twenty states, vaccination increased titer from four hemagglutination units (HAU) before vaccination to 64 HAU post-vaccination.  For the second vaccination period extending from November 2023 to June 2024 covering six states, circulating antibody response was higher increasing from 4 HAU pre-vaccination to 128 HAU. One of the two genetically engineered vaccines showed statically higher immune response compared to the second during the first vaccination period although there was no difference during the 2022 study period.  In contrast to the first vaccination period, more diligent application resulted in immune titers exceeding 80 percent of the flocks vaccinated.  The highest titers were obtained using the product derived from a genetically modified H5N8 isolate followed by the inactivated H5N1 of North American lineage, derived from a 2022 field isolate in Mexico.

The authors concluded that emergency administration of flocks against HPAI is a valuable adjunct to conventional control measures by increasing the level of immunity in populations.  Unfortunately, the article did not comment on clinical prevalence of HPAI and comparisons among flocks that were either protected or were susceptible to HPAI.  It is clear that sero- protection could play a role in limiting spread of HPAI as an adjunct to conventional methods of control.

1. Guinat, C. et al Poultry farm density and proximity drive highly pathogenic avian influenza spread. Communications Biology doi.org/10.1038/s42003-025-08687-4 (2025)

1. Badillo, B. et al H5N1 highly pathogenic avian influenza vaccination: Sero-response of Mexican poultry in the 2022-2024 outbreak. Vaccine X doi.org/10.1016/j.jvaca.2025.100700