QUARTERLY BEAT
APRIL 2026
in Texas in March 2024, and by December 2025, over 1,000 cases were confirmed across 19 states. Clinical signs include decreased milk production, lethargy, inappetence, fever, and abnormal milk consistency or color, with subtle respiratory or gastrointestinal signs sometimes present. Transmission can occur cow-to-cow, cow-to-human, and cow-to-cat. National mitigation strategies, including mandatory testing before animal movement and participation in USDA programs such as Secure Our Herds and the National Milk Testing Strategy, aim to reduce spread. Although raw milk may contain transmissible H5N1, pasteurization inactivates the virus, maintaining the safety of the commercial milk supply.
does not predict severity in mammals or humans. The current global outbreak is caused by HPAI H5N1 clade 2.3.4.4b.
CLINICAL SIGNS OF HPAI IN POULTRY:
• Decreased egg production • Swollen eyelids, comb, wattle, shanks • Purple discoloration of comb, legs • Coughing • Nasal discharge • Diarrhea • Neurologic signs (torticollis, ataxia, paralysis)
CLINICAL SIGNS OF HPAI IN DAIRY CATTLE:
• Decreased milk production • Abnormal milk consistency and color • Lethargy and inappetence • Fever early in disease • Subtle respiratory or gastrointestinal signs H5N1 TRANSMISSION PATHWAYS FROM DAIRY CATTLE:
THE CURRENT H5N1 OUTBREAK: TIMELINE AND SCOPE
The current H5N1 epidemic is the longest-lasting documented avian influenza outbreak. It first emerged in Europe in early 2021, following reassortment with H5N8, and spread to Asia, Africa, and eventually the Americas. In the United States, the virus was first detected in wild birds in January 2022, followed by the first domestic poultry case in February 2022, which involved a turkey flock in Indiana. By June 2024, H5N1 had been confirmed across all four North American migratory flyways and had reached all 50 states by December 2025. In the United States alone, over 185 million commercial and backyard birds have been affected during the epidemic, including flocks representing more than one-third of the U.S. layer industry at different points in time, contributing to fluctuations in egg supply and prices.
• Cow-to-cow • Cow-to-human • Cow-to-cat • Potentially other transmissions
H5N1 IN DOMESTIC CATS
Domestic cats are highly susceptible, often developing severe disease after exposure to infected birds or dairy products. Clinical signs in cats have the potential to develop quickly and may include acute neurologic signs such as ataxia, seizures, and blindness, severe depression, oculonasal discharge, respiratory signs, and occasionally sudden death. Risk factors include proximity to dairy operations, consumption of raw milk or undercooked meat, and hunting or scavenging for birds or rodents. HPAI infection in cats can mimic rabies, so veterinarians should include H5N1 in the differential diagnosis for cats presenting with neurologic disease.
HOST RANGE EXPANSION: H5N1 IN MAMMALS
A significant development in this outbreak is the virus’s ability to infect mammals. More than 48 mammalian species, including species like seals, sea lions, mink, house mice, raccoons, bobcats, domestic and wild cats, and dairy cattle, have been affected by it. Most mammalian infections originate from contact with infected birds, though mammal-to-mammal transmission has been documented. Clinical signs vary from neurologic disease, such as encephalitis, to respiratory disease including pneumonia, sudden death, or subclinical infection. Human infections remain rare, with no evidence of sustained human-to-human transmission. After introduction into North America, H5N1 reassorted with local wild bird influenza viruses, producing a shift in the virus pathogenicity. These North American recombinant strains have been linked to more severe neurologic disease in mammals compared with Eurasian strains, emphasizing the importance of ongoing surveillance in multiple animal populations.
CLINICAL SIGNS OF H5N1 IN CATS:
• Acute neurologic disease, including ataxia, seizures, and blindness • Severe depression • Copious oculonasal discharge • Respiratory signs such as tachypnea, dyspnea, sneezing, or coughing • Sudden death or rapid progression to death
H5N1 IN DAIRY CATTLE
The detection of H5N1 in dairy cattle represented a major shift in epidemiology, prompting updates to surveillance protocols and biosecurity regulations in the United States. The first U.S. dairy case was reported
WEBINAR HIGHLIGHTS
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VETGIRL BEAT EMAGAZINE 23
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