Influenza

1. Epidemic Pattern

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   • 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
2. Symptoms

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   • Onset with fever, chills, malaise, cough, coryza, and myalgia; aching, fever, and prostrations out of proportion to catarrhal symptoms
3. Leukopenia

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   • Low WBCs

1. Orthomyxovirus (RNA virus)
   1. Type A

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      • Infects a variety of mammals
      1. Subtypes

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         • Divided into subtypes based on hemagglutinin (H) and neuraminidase (N) surface expression
   2. Type B
   3. Type C
2. Transmission

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   • Primarily droplet nuclei
3. Predispose pts. to secondary bacterial infections

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   • Influenza causes necrosis of the respiratory epithelium, increased adherence of bacteria to infected cells, and ciliary dysfunction, which predispose to secondary bacterial infections
   1. Pneumococcal pneumonia

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      • most common: Pneumococcal pneumonia

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

1. Incubation Period

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   • 1-4 days
   • In unvaccinated persons, uncomplicated influenza has an abrupt onset
2. Symptoms
   1. Nearly asymptomatic
   2. Systemic Symptoms

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      • fever (can last 1-7 days, but usually lasts around 3-5 days), chills, HA, malaise, myalgias
   3. Respiratory Symptoms

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      • rhinorrhea, congestion, pharyngitis, hoarseness, nonproductive cough, substernal soreness
   4. Children

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      • GI symptoms may be seen with influenza B
   5. Elderly

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      • May present with lassitude (ie. lethargy, fatigue) and confusion, often WITHOUT fever or respiratory symptoms
3. Signs

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   • 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

1. Rapid Tests

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   • 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
2. Swabs/Cultures

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   • 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

1. Supportive Care

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   • Treatment for healthy individuals is supportive
2. Antivirals

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   • 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
   1. Oral Oseltamivir

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      • 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
   2. Inhaled Zanamivir

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      • 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
   3. IV Peramivir

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      • Used for outpatient treatment of uncomplicated infection in patients 18 years or older, recommended when there is concern about inadequate oral absorption of oseltamivir
3. When To Admit

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   • -Limited availability of supporting services  - PNA or decreased O2 saturation  - Changes in mental status  - Consider with pregnancy

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• Vaccines, chemoprophylaxis, hand washing and surgical masks, isolation

1. Influenza Vaccine

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   • Annual administration of influenza vaccine is the most effective measure for preventing influenza and its complications
   1. Who

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      • 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)
   2. Vaccine Forms

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      • 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
   3. Elderly Patients

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      • 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
   4. Precautions

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      • Take precautions if pediatric patient presents with Guillan-Barre syndrome 6 weeks after inoculation
2. Antiviral Chemoprophylaxis

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   • When an antiviral chemoprophylaxis is used, it prevents 70-90% of influenza infections
   1. Not Recommended

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      • Not recommended: prior to viral exposure to prevent development of resistance, after 48 hours after exposure
   2. Recommended

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      • 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
3. Hand Hygiene & Surgical Masks

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   • 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
4. Isolation

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   • 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

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• If fever recurs or persists > 4 days with productive cough and WBC count > 10,000/mcL, secondary bacterial infection should be suspected

1. Healthy, non-elderly adults

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   • Healthy, nonelderly adults: duration of uncomplicated illness is 1-7 days, excellent prognosis
2. Hospitalized Adults

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   • Mortality among adults hospitalized with influenza ranges from 4-8%, although higher mortality may be seen during pandemics and among immunocompromised individuals

1. Hospitalization/ICU

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   • Hospitalization or ICU admission for influenza is often a consequence of diffuse viral pneumonitis with severe hypoxemia and sometimes shock
2. High Risk Groups

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   • 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
3. Pregnancy

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   • Infection during pregnancy increases the risk of hospitalization, may be associated with severe illness, sepsis, PTX/ respiratory failure, spontaneous abortion, preterm labor, fetal distress
4. Cardiovascular

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   • 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)
5. Neurologic

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   • Neurologic complications, including seizures and encephalopathy, may occur, although more rare
6. Reye Syndrome

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   • 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