[2] In 2022 and 2023, three people in China were infected with H3N8,[3] with one fatality, marking the first time a human has died from this strain of flu.

Transmission of the equine influenza virus (EIV) to humans has not occurred during outbreaks of the disease in horses.

Since 1963, the H3N8 virus has drifted along a single lineage at a rate of 0.8 amino acid substitutions per year.

It can also be spread by direct contact between horses, or indirectly via a person's hands or clothing, or on inanimate objects (e.g. buckets, tack, twitches).

Spread of the disease has been associated with the movement of people, pets, horse equipment and tack where proper biosecurity procedures have not been followed[16] Subclinical infection with virus shedding can occur in vaccinated horses, particularly where there is a mismatch between the vaccine strains and the virus strains circulating in the field.

The virus disperses throughout the trachea and bronchial tree within 3 days, causing hyperemia, edema, necrosis, desquamation, and focal erosion.

Viremia is rare, but is possible if the virus crosses the basement membrane and enters the circulation, potentially causing inflammation of skeletal and cardiac muscle (myositis and myocarditis), encephalitic signs, and limb edema[6] Fever of 102.5–105.0 °F (39.2–40.6 °C), frequent dry cough for several weeks, ‘drippy’ nose with discharge and secondary bacterial infection are some of the clinical signs of Equine influenza virus infection.

isolation of influenza virus from nasopharyngeal and or large rise in antibody titer in equine-1 or 2 serum can be used as diagnosis in horses.

Other clinical findings may include a serous or light mucoid nasal discharge, epiphora, tender but rarely swollen submandibular lymph nodes, hyperemia of nasal and conjunctival mucosa, tachypnea, tachycardia, limb edema, muscle soreness and stiffness.