Influenza

Influenza

1. Essentials for Diagnosis

   • Epidemic Pattern
     • Epidemic pattern: annual epidemics usually appear in the fall or winter in temperate climates 
     • Influenza epidemics affect 10-20% of the global population on average each year and are typically the result of minor antigenic variations of the virus, or antigenic drift, which occur often in influenza A virus
   • Symptoms
     • Onset with fever, chills, malaise, cough, coryza, and myalgia; aching, fever, and prostrations out of proportion to catarrhal symptoms
   • Leukopenia
     • Low WBCs

2. Pathophysiology

   • Orthomyxovirus (RNA virus)
     • Type A
       • Infects a variety of mammals
       • Subtypes
         • Divided into subtypes based on hemagglutinin (H) and neuraminidase (N) surface expression
     • Type B
     • Type C
   • Transmission
     • Primarily droplet nuclei
   • Predispose pts. to secondary bacterial infections
     • Influenza causes necrosis of the respiratory epithelium, increased adherence of bacteria to infected cells, and ciliary dysfunction, which predispose to secondary bacterial infections
     • Pneumococcal pneumonia
       • most common: Pneumococcal pneumonia

3. Risk Factors

   • Elderly (>65 yo), children, immunocompromised, pregnant women

4. Presentation/Physical Exam

   • Incubation Period
     • 1-4 days
     • In unvaccinated persons, uncomplicated influenza has an abrupt onset
   • Symptoms
     • Nearly asymptomatic
     • Systemic Symptoms
       • fever (can last 1-7 days, but usually lasts around 3-5 days), chills, HA, malaise, myalgias
     • Respiratory Symptoms
       • rhinorrhea, congestion, pharyngitis, hoarseness, nonproductive cough, substernal soreness
     • Children
       • GI symptoms may be seen with influenza B
     • Elderly
       • May present with lassitude (ie. lethargy, fatigue) and confusion, often WITHOUT fever or respiratory symptoms
   • Signs
     • Mild pharyngeal injection, flushed face, conjunctival redness
     • Moderate enlargement of cervical lymph nodes and tracheal tenderness may be observed
     • The presence of fever (higher than 38.2 C) and cough during influenza season is highly predictive of influenza infection in those older than 4 years

5. Diagnostics

   • Rapid Tests
     • Nasal and throat swabs are highly widely available, highly specific, and produce fast results—however, rapid tests have low sensitivity, leading to a high number of false-negative results
   • Swabs/Cultures
     • When influenza pneumonia is suspected, lower respiratory tract specimens (swabs/cultures) should be collected and tested for influenza viruses (RT-PCR commonly used)
     • Viral culture

6. Management

   • Supportive Care
     • Treatment for healthy individuals is supportive
   • Antivirals
     • Treatment for individuals at risk for developing complications with either a suggestive clinical presentation or lab-confirmed influenza: antivirals
     • Maximum benefit of antivirals is expected with earliest initiation of therapy (within first 48 hours of onset of symptoms), although benefit has been noted up to 4-5 days into illness
     • Three neuraminidase inhibitors used for treatment of influenza A and B: oral oseltamivir, inhaled zanamivir, IV peramivir
     • Oral Oseltamivir
       • Drug of choice for patients of any age, pregnant women, and patients who are hospitalized or have complicated infection; dosage—75 mg twice daily for 5 days
     • Inhaled Zanamivir
       • Indicated for uncomplicated acute influenza in patients 7 years and older, contraindicated for individuals with asthma d/t risk of bronchospasm, lacks efficacy in presence of PNA infection; dosage—10 mg, 2 inhalations twice daily for 5 days
     • IV Peramivir
       • Used for outpatient treatment of uncomplicated infection in patients 18 years or older, recommended when there is concern about inadequate oral absorption of oseltamivir
   • When To Admit
     • -Limited availability of supporting services  - PNA or decreased O2 saturation  - Changes in mental status  - Consider with pregnancy

7. Prevention

   • Vaccines, chemoprophylaxis, hand washing and surgical masks, isolation
   • Influenza Vaccine
     • Annual administration of influenza vaccine is the most effective measure for preventing influenza and its complications
     • Who
       • Vaccination is emphasized for high-risk groups and their contacts and caregivers
       • > 18 years, pregnant or not, can receive any of the vaccines (with few exceptions)
     • Vaccine Forms
       • Three vaccine forms: inactivated influenza vaccines, recombinant vaccines (recombinant is only flu vaccine completely egg free, safe for those with egg allergy), and live attenuated influenza vaccine
     • Elderly Patients
       • Elderly patients should only receive high-dose inactivated (trivalent) vaccine, usually with either hemagglutinin or an adjuvant to enhance the immune response to the vaccine
     • Precautions
       • Take precautions if pediatric patient presents with Guillan-Barre syndrome 6 weeks after inoculation
   • Antiviral Chemoprophylaxis
     • When an antiviral chemoprophylaxis is used, it prevents 70-90% of influenza infections
     • Not Recommended
       • Not recommended: prior to viral exposure to prevent development of resistance, after 48 hours after exposure
     • Recommended
       • May be recommended: patients exposed to an infected pt within two weeks of vaccination at increased risk for complications from infection, low response to vaccine d/t immunosuppression after an exposure, those with contraindication to vaccine (severe allergic rxn to flu), prevention of infection in residents of institutions during an outbreak
   • Hand Hygiene & Surgical Masks
     • Hand hygiene and surgical masks are found to be effective in preventing the transmission of flu to un-infected individuals in the home if worn within 36 hours of onset of symptoms
   • Isolation
     • Isolation (droplet) precautions should be maintained until 7 days after symptom onset or 24 hours following symptom resolution (whichever is longer)  -N95 masks should be worn for aerosol-generating procedures

8. Prognosis

   • If fever recurs or persists > 4 days with productive cough and WBC count > 10,000/mcL, secondary bacterial infection should be suspected
   • Healthy, non-elderly adults
     • Healthy, nonelderly adults: duration of uncomplicated illness is 1-7 days, excellent prognosis
   • Hospitalized Adults
     • Mortality among adults hospitalized with influenza ranges from 4-8%, although higher mortality may be seen during pandemics and among immunocompromised individuals

9. Complications

   • Hospitalization/ICU
     • Hospitalization or ICU admission for influenza is often a consequence of diffuse viral pneumonitis with severe hypoxemia and sometimes shock
   • High Risk Groups
     • Individuals with asthma, residents of nursing homes/long-term care facilities, adults aged over 65 years, morbidly obese, and persons with underlying medical conditions are at higher risk for complications
   • Pregnancy
     • Infection during pregnancy increases the risk of hospitalization, may be associated with severe illness, sepsis, PTX/ respiratory failure, spontaneous abortion, preterm labor, fetal distress
   • Cardiovascular
     • Cardiovascular diseases are a complication of influenza infection, especially among older adults (influenza is postulated to be a significant trigger for MI, cerebrovascular disease, and sudden death)
   • Neurologic
     • Neurologic complications, including seizures and encephalopathy, may occur, although more rare
   • Reye Syndrome
     • Reye Syndrome is a rare and severe complication of influenza (usually type B) in young children that causes rapidly progressive hepatic failure and encephalopathy—there is a 30% mortality rate; this syndrome is associated with aspirin use in the management of viral infections