Is the APHIS depopulation model for stamping out highly pathogenic avian influenza an effective approach?
January 8, 2025
As we enter 2025, the impact of highly pathogenic avian influenza (HPAI) strain H5N1 is all too evident. The U.S. Department of Agriculture’s Animal & Plant Health Inspection Service (APHIS) has been unable to control the epornitic through promoting biosecurity and the depopulation of infected egg production complexes, along with turkey and broiler growing farms. The poultry industry and U.S. population need the added protection afforded by immunization of flocks in high-risk areas.
The current H5N1 epornitic that commenced in 2022 was responsible for the loss of 43 million laying hens in two waves of infection, followed by depopulation of 15 million in 2023 and 40 million hens in 2024. In addition, during 2024 at least 2.5 million pullets were depopulated, along with 5.4 million broilers and parent flocks and 3.5 million turkeys and breeders.
Apart from the cost to USDA-APHIS and states for indemnity and logistics, producers have borne the loss of revenue and consumers have paid considerably more for their eggs. During the first week of 2025, California households were paying close to $9/doz. for state-mandated cage-free eggs. Midwest cartoned large eggs were trading at $5.75/doz. and the national loose price was $4.85/doz.
During 2022, it is estimated that with a conservative average increase of $2/doz. across the year, expenditure on eggs was approximately $15 billion more than would have been paid had H5N1 depopulation not suppressed the egg-producing flock by an average of 12 million per week. A similar situation pertained during 2024, but the cost to consumers was higher given the wider differential between actual shelf price and values that would have prevailed in the absence of HPAI.
Despite seasonal reoccurrence of HPAI since 2022, APHIS has pursued a futile program of attempting to “stamp out” the infection. Eradication would be possible if HPAI was an exotic viral infection imported into the U.S. affecting an index farm or a small number of biologically contiguous premises. In reality, HPAI is introduced seasonally and disseminated in fall and spring waves by the migration of millions of migratory waterfowl. This situation is in complete variance with the APHIS model. Implementing a stamping out policy for an endemic disease with continual introduction and spread by a wildlife reservoir is an inappropriate, outdated and unrealistic approach that defies the epidemiology of the disease.
The magnitude of successive annual depopulations confirms that the Sisyphean approach by APHIS is ineffective to control the infection. Evidence, both scientific and anecdotal, suggests that HPAI can be introduced onto farms by the aerogenous route with the virus entrained on dust particles. Power-ventilated egg production complexes are highly vulnerable, as evidenced by the number of cases involving farms with between 1 million and 4 million hens. Recognizing that the infection is both seasonally and regionally endemic and can be introduced by air movement predicates the introduction of vaccination to supplement biosecurity that cannot provide absolute protection, especially in high-risk areas.
The World Organization for Animal Health endorses the use of effective vaccines as an adjunct to biosecurity in suppressing outbreaks. A January publication (Swayne, et al., 2024) noted that vaccines homologous to the HA antigens expressed by an infective virus increase the ability of flocks to withstand exposure by a factor of 103 to 104 and will reduce viral shedding by infected birds by a magnitude of 102 to 105. If this data derived from controlled experiments is extrapolated to field conditions, incident rates would be reduced, since reproduction (R) ratios would fall below unity. Vaccination would result in fewer outbreaks and reduce the financial impact on both the public and private sectors.
APHIS has not released results of structured epidemiologic surveys other than two superficial reports of questionable value relating to outbreaks in 2022. Despite the availability of experienced epidemiologists, APHIS appears determined to pursue a control policy unchanged since the 1994 outbreaks in Pennsylvania and adjoining states.
The reluctance to allow limited and strategic immunization of flocks in the face of continuing outbreaks may be attributed to:
An institutional reluctance to admit that past policies and decisions concentrating on depopulation were and are currently inappropriate and ineffective.
Pressure by segments of the U.S. poultry industry to preserve exports at the expense of consumers and producers of eggs and turkeys.
A prevailing APHIS mindset that vaccination would be more expensive than ongoing serial depopulation. In the interests of transparency, APHIS should make public its epidemiologic and economic studies to justify this apparent intransigence to consider vaccination. Release of epidemiologic studies, if in fact they have been completed, would allow poultry health professionals and agricultural economists to evaluate the logic and assumptions underlying past and current USDA policy on control of HPAI.
Extension of HPAI to free-living mammals in contact with poultry and dead wild birds emerged as a concern during 2023. This included cases in freed-living terrestrial mammals and pinnipeds where contagion evidently occurred. Adaptation of the virus by mutation to the H5N1 B13.3 strain affecting dairy herds has created a new dimension to HPAI infection. To date, 917 herds have been diagnosed with infection, most of which have recovered. In states, including California, that have imposed mandatory testing of milk, over 700 premises – representing the bulk of the dairy industry in the state – have acquired infection over a four-month period.
Mutation of H5N1 avian or bovine viruses to become zoonotic is a growing concern. At present, the prospect for emergence of a human epidemic appears minimal, but the consequences would be potentially catastrophic. Infection of workers involved in depopulating infected flocks or in contact with contaminated milk from dairy herds suggests that the virus may become adapted to human respiratory tissue.
Severe H5N1 infection attributed to clade 2.3.4.4b genotype D1.1 H5N1 occurring in a case in British Columbia and the fatal case in Louisiana, respectively, indicate that mutations that occur in infected humans may be a source of ultimately contagious strains at some time in the future. Mutations in the HA gene of the specific isolates from the patients increase the ability of the virus to bind to α2-6-sialic acid receptors in the human respiratory tract. The case of the teenager with concurrent obesity and asthma who recovered after extensive treatment in British Columbia and the elderly decedent in Louisiana with predisposing conditions confirm the possibility of an avian-derived H5N1 strain undergoing mutation in human patients.
Dr. Jim Paulson of the Scripps Research Institute (Kupferschmidt, 2024) noted, “My rule of thumb is that 1 in 4,000 virus particles will have a mutation at the amino acid that you are interested in and specifically, the 226 L position.” Mutations in the H5 hemagglutinin gene (E627K E190 and Q226) were identified in virus isolated from tracheal aspirates from the teenager in British Columbia. Suppressing avian influenza in poultry flocks by preemptive vaccination as an adjunct to biosecurity would, therefore, appear to be a beneficial strategy to avert the emergence of a zoonotic strain.
Extension of H5N1 from poultry to humans emerged in 1997 in Hong Kong. Since this time, H5N1 infection has occurred sporadically in Asia from 2003 onwards. Although there is no evidence of person-to-person (contagion) transmission of H5N1 based on the approximately 70 human cases diagnosed clinically in the U.S. to date, extensive human infection may, in the words of Dr. Andrew Pekosz of John Hopkins Bloomberg School of Public Health, represent “only a matter of time.” A similar concern was expressed by Dr. Scott Hensley of the University of Pennsylvania, Perelman School of Medicine, who stated, “At the end of the day, I think it is a numbers game.” Large numbers of susceptible hens at high concentration on farms located in close proximity represent a risk for the occurrence of deleterious mutations that may increase the probability of an epidemic.
It is evident that the policy adopted by APHIS for the past three years is not working. Simply hoping that migratory birds will cease shedding, as in 2025, does not appear to be a viable strategy. Effective vaccines are commercially available and have been deployed in many nations, including the Netherlands, the PRC, France and Mexico. The incidence rate of avian influenza in the U.S. with costs to both private and public sectors predicates immunization on a limited and controlled basis. Priority should be extended to turkeys and commercial egg production flocks in areas of high risk. Application of vaccination should be based on benefit-to-cost studies taking into account risks of infection, the financial impact of losses and trade considerations that are generally overstated. In the immediate term, to avoid a recombinant event, all workers coming into contact with live poultry and dairy herds should receive both the trivalent seasonal influenza vaccine and an available U.S.-manufactured H5N1 product as deployed in Finland.
It is axiomatic that HPAI is effectively “the Newcastle disease of the 2020s.” This infection was as catastrophic with respect to commercial production during the late 1960s as avian influenza is at present. Newcastle disease is effectively controlled by vaccination.
The U.S. industry needs limited strategic and monitored vaccination against HPAI as an adjunct to biosecurity. Both failure to suppress the ongoing epornitic in commercial flocks and the remote risk of an emergent zoonotic strain should compel APHIS to consider application of preventive vaccination.
Kupferschmidt, K. 2024. Why Hasn’t the Bird Flu Pandemic Started, Science 386 1205-1206.
Swayne, D.E., et al. 2024. Strategic Challenges in the Global Control of High Pathogenicity Avian Influenza. International Office of Epizootics, 89-102.
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