PHRM 541 Influenza Presentation, Diagnosis, and Testing

What influenza virus types can infect humans?

Influenza A and B

Describe influenza A

Regular, seasonal flu epidemics

Categorized into different subtypes based on changes in two surface antigens (HA and NA)

Describe influenza B

Associated with sporadic outbreaks

Describe the envelope virus of the influenza virus

Located inside the envelope

Single stranded RNA genome

What are the two surface antigens of the influenza virus?

Hemagglutinin (HA) = 80%

Neuraminidase (NA) = 20%

What is the function of HA?

Allows the influenza virus to enter host cell and replicate

What is the function of NA?

Releases newly produced virus particles from host to infect other cells

What are the three circulating human subtypes of HA associated with influenza A?

H1, H2, and H3

What are the two circulating human subtypes of NA associated with influenza A?

N1, N2

What are the two lineages of influenza B?

B/Yamagata

B/Victoria

How does immunity to the influenza virus occur?

Via the development of antibodies directed at the surface antigens

Describe antigenic drift

Gradual continuous change

Antigenic variants created by point mutations in surface antigens resulting in small changes in the HA and/or NA molecules

Causes seasonal epidemics and changes in annual flu vaccines

Describe antigenic shift

Abrupt, major change

Occurs when the influenza virus acquires a new HA and/or NA via genetic combination rather than point mutations

Novel influenza virus that carries potential to cause a pandemic

How is influenza transmitted?

Person-to-person via inhalation of respiratory droplets

Touching a contaminated object, then touching mucus membranes

What is the incubation period of influenza?

1-7 days (average 2 days)

When does influenza transmission occur in adults?

1 day before until 7 days after illness onset

Can occur as long as the infected person is shedding virus

When do symptoms of influenza begin?

About 2 (1-4) days after virus enters body

When do influenza symptoms typically resolve?

In 3-7 days, although cough and malaise may persist for > 2 weeks

What are s/sx of influenza?

Fever, chills, malaise, fatigue, headache, nasal congestion, rhinorrhea, sore throat, myalgias/arthralgias, weakness, abdominal pain, N/V/D, non-productive cough, pleuritic chest pain

What laboratory tests/imaging are performed to diagnose the flu?

CBC, BMP

CXR

Cultures of potential sites of coinfection

What is the gold-standard diagnostic test for influenza?

Reverse-transcription polymerase chain reaction (RT-PCR)

How long does it take for PT-PCR results to be available?

1-8 hours

Describe rapid influenza molecular assays (RIMAs)

Influenza viral RNA or nucleic acid detection with high sensitivity and specificity

How long does it take to get results from a RIMA?

15-30 minutes

Describe rapid influenza diagnostic tests (RIDTs)

Antigen detection with differentiation of influenza A/B

Low sensitivity, high specificity (false negatives)

When are RIDTs not recommended?

Not recommended for hospitalized patients

How long does it take to get results from RIDTs?

10-15 minutes

Which influenza diagnostic tests are point of care tests?

RIMAs

RIDTs

Describe a viral culture

Extensive antigenic and genetic characterization

How long does it take to get results from a viral culture?

1-10 days

When should you test a patient for influenza?

Inpatients with s/sx consistent with influenza

If testing will influence clinical management

What specimens should be collected when testing someone for influenza?

Collect upper respiratory tract specimens as soon after illness onset (preferably within 4 days of symptoms)

What tests are used outpatient to diagnose influenza?

Rapid molecular assays (RIMA) > rapid influenza diagnostic tests (RIDTs)

What tests are used inpatient to diagnose influenza?

Reverse-transcription polymerase chain reaction (RT-PCR) preferred

Do NOT use viral cultures for initial diagnosis (takes too long)