21. influenza viruses (don't need to memorize strain numbers)

what type of genome do influenza viruses have?

segmented RNA → reassortment possible

what is hemagglutinin (HA or H)?

• surface protein involved in virus attachment/entry

• major antibody response

• influences host range & pathogenicity

what is neuraminidase (NA or N)?

• surface protein involved in virus budding/release

• antibody response

what determines influenza virus subtype?

• combination of the HA and NA surface proteins

  • ex. H5N1, H1N1, H3N2, etc.

• antigenically distinct

  • cross-reactivity of antibodies is rare/weak

what is the primary reservoir for avian influenza?

waterfowl

• maintain large diversity of avian HA subtypes

• shed in feces 

what are the major mechanisms of influenza virus evolution?

• antigenic drift

  • RdRp = error prone

  • point mutations → amino acid changes

• genetic reassortment (genetic shift)

  • infection of same host & same cell

how do influenza viruses interact with host cells to establish infection?

• influenza HA protein → viral attachment factor

• host sialic acid (SA) → cell surface receptor

what host receptors do avian influenzas recognize? human influenzas?

• avian virus HA proteins recognize SA alpha 2-3

• human virus HA proteins recognize SA alpha 2-6

why are pigs a “mixing vessel” for influenza viruses?

• express both SA alpha 2-3 and alpha 2-6 receptors equally

• can easily be infected by avian, swine, and human influenza viruses

clinical signs of swine influenza

• lethargy, lack of appetite

• fever, weight loss, poor growth

• nasopharynx: sneezing, ocular/nasal mucus

• respiratory: coughing, sore throat

• intestinal: diarrhea

where did canine influenza originate?

• H3N8 (now extinct) → equine origin

• H3N2 → transmitted/jumped from bird to dog

  • circulating in china and korea

  • introduction to US → dogs rescued from korea and re-housed in US

clinical signs of CIV (H3N2)

• mild upper respiratory disease

• frequent coughing and fever

• can lead to secondary bacterial infection

• highly contagious

CIV diagnostic test

rapid antigen test (RAT)

• lateral flow tests

where did feline influenza virus originate?

avian origin (spread from birds → cats)

clinical signs of feline influenza

mild disease with coughing, sneezing, and runny nose

where did equine influenza originate?

avian influenza origin

clinical signs of equine influenza

• high fever

• nasal discharge

• cough

• swollen lymph nodes under jaw

• muscle pain & weakness

• secondary bacterial pneumonia

1-2 weeks to resolve

how is equine influenza virus diagnosed?

RT-PCR or RAT

how does avian influenza present in different bird species?

• waterfowl: resistant to disease; shed virus

  • predominantly GI tract

  • fecal-oral

  • waterborne transmission

• gulls & shore birds: subclinical infection

• chickens, turkeys, swans, geese, raptors: lethal, systemic disease

what affects pathogenicity in low path avian influenza (LPAI) vs. high path avian influenza (HPAI)?

LPAI

• HA cleavage site → monobasic amino acid motif

  • host proteases restricted to respiratory/GI tissues → localized infection + minimal disease

HPAI

• HA cleavage site → multibasic amino acid motif

  • host proteases found in every tissue → systemic infection + severe disease

clinical signs of low path avian influenza (LPAI) in poultry

• mild respiratory distress

• nasal discharge

• fever & lethargy

• reduced egg production

clinical signs of high path avian influenza (HPAI) in poultry

• systemic disease, high fever

• hemorrhage (subcutaneous)

• necrotic tissue

• neurological signs (stargazing)

• high mortality; death in 1-7 days after infection

how was high path avian influenza introduced to the US?

• reassortment event produced novel HPAI virus

• European ducks migrated and brought virus to US

how can HPAI infect other animals (mammals)?

• spillover: dead birds as food source

• shedding in feces → contaminate water → infect marine mammals

• spillover into cattle

clinical signs of bovine influenza (H5N1)

• loss of appetite

• reduced milk production (NOT restored after recovery)

• abnormal appearance of milk (thick & discolored)

• infection isolated in the mammary tissues

• mainly affects lactating cows

how can cows transmit influenza virus?

infected raw milk