Sick child girl sitting on couch covered with blanket blowing nose, sneezing in tissues spending day at home with mother.

Everything You Need To Know About Influenza A (H1N1)

 

Influenza A (H1N1) is an acute respiratory infectious disease caused by a novel influenza A (H1N1) virus that spreads among human populations. Unlike previous or seasonal influenza viruses, this viral strain contains genetic segments from swine, avian, and human influenza viruses.

The general population is universally susceptible to the influenza A (H1N1) virus, which can be transmitted between humans. Early symptoms of H1N1 infection are similar to common flu, including fever, cough, sore throat, body aches, headache, chills, and fatigue. Some patients may also experience diarrhea, vomiting, muscle pain, fatigue, and red eyes.

In 2009, influenza A (H1N1) became a global pandemic. In August 2010, WHO declared the end of the H1N1 pandemic. According to data released by India's Ministry of Health and Family Welfare in September 2017, India reported 34,009 cases of H1N1 influenza nationwide, with 1,741 deaths.

 

I. Basic Information

Transmission Route: Primarily through respiratory droplets

Infectiousness: Yes

Common Symptoms: Flu-like symptoms, signs include pharyngeal congestion and tonsil enlargement

Common Cause: Infection with influenza A (H1N1) virus

Susceptible Population: Universal susceptibility

Medical Departments: Respiratory, Infectious Diseases

 

II. Disease Causes

Source of Infection: The main source of infection is H1N1 influenza patients. Asymptomatic carriers are also infectious. Currently, there is no evidence of animal-to-human transmission.

Transmission Routes: Primary transmission occurs through respiratory droplets, and can also spread through direct or indirect contact with mucous membranes of the mouth, nose, and eyes. Infection may also occur through contact with respiratory secretions, body fluids, and virus-contaminated objects.

Susceptible Population: The general population is universally susceptible.

High-Risk Groups:

(1) Pregnant women

(2) Individuals with the following conditions:

    • - Chronic respiratory diseases
    • - Cardiovascular diseases (excluding hypertension)
    • - Kidney diseases
    • - Liver diseases
    • - Blood disorders
    • - Neurological and neuromuscular diseases
    • - Metabolic and endocrine disorders
    • - Immunosuppression (including those on immunosuppressive therapy or with HIV infection)
    • - Individuals under 19 years old on long-term aspirin therapy

(3) Obese individuals (BMI ≥40 high risk, BMI 30-39 potential risk factor)

(4) Children under 5 years (those under 2 years are at higher risk for severe complications)

(5) Adults aged 65 and above

 

III. Clinical Manifestations

The incubation period for H1N1 influenza is longer than that of seasonal flu and avian flu, ranging from 1-7 days, typically 1-3 days. Some patients may experience rapid disease progression with sudden high fever exceeding 38°C, potentially developing severe complications such as pneumonia, acute respiratory distress syndrome, pulmonary hemorrhage, pleural effusion, pancytopenia, renal failure, sepsis, shock, Reye's syndrome, respiratory failure, and multiple organ damage, which can be fatal. Pre-existing conditions may also worsen.

Typical symptoms include flu-like manifestations: fever, sore throat, rhinorrhea, nasal congestion, cough, sputum production, headache, body aches, and fatigue. Some cases present with vomiting and diarrhea. A few cases may only show mild upper respiratory symptoms without fever. Physical examination typically reveals pharyngeal congestion and enlarged tonsils.

 

IV. Disease Examination

1. Laboratory Tests

(1) Peripheral blood count: White blood cell count is typically normal or decreased

(2) Blood biochemistry: Some cases show hypokalemia; few cases show elevated creatine kinase, AST, ALT, and LDH

(3) Pathogen detection:

  • - Viral nucleic acid testing using RT-PCR (preferably real-time RT-PCR) of respiratory specimens (throat swabs, nasal swabs, nasopharyngeal or tracheal aspirates, sputum)
  • - Virus isolation from respiratory specimens
  • - Serological testing showing four-fold or greater rise in H1N1-specific antibody titers between paired sera

2. Chest Imaging

Patchy shadows may be visible in cases with pneumonia.

 

V. Diagnosis

Diagnosis primarily combines epidemiological history, clinical manifestations, and pathogen detection. Early detection and diagnosis are key to prevention and effective treatment.

1. Suspected Cases

A suspected case meets one of the following criteria:

(1) Close contact with a confirmed H1N1 case within 7 days before onset of flu-like symptoms

(2) Travel history to H1N1-affected areas within 7 days before symptom onset

(3) Positive for influenza A virus without subtype determination

2. Clinically Diagnosed Cases

Limited to cases during confirmed H1N1 outbreaks showing flu-like symptoms after excluding other causes.

3. Confirmed Cases

Cases with flu-like symptoms and one of the following laboratory findings:

(1) Positive H1N1 nucleic acid test

(2) Isolation of H1N1 virus

(3) Four-fold or greater rise in H1N1-specific antibody titers

 

VI. Treatment Methods

1. General Treatment

Rest, adequate hydration, close monitoring of condition; antipyretic treatment for high fever cases.

2. Antiviral Treatment

Studies show the virus is sensitive to neuraminidase inhibitors (oseltamivir, zanamivir) but resistant to amantadine and rimantadine. Mild cases without complications may not require neuraminidase inhibitors. Severe cases and high-risk individuals should receive prompt antiviral treatment, ideally within 48 hours of onset (optimal within 36 hours).

 

VII. Prevention

  1. 1. Frequent hand washing and good personal hygiene
  2. 2. Adequate sleep, hydration, and maintaining good health
  3. 3. Proper indoor ventilation, avoiding crowded places
  4. 4. Proper food handling, cooking pork above 71°C
  5. 5. Avoiding contact with live pigs or pig facilities
  6. 6. Covering mouth and nose when coughing or sneezing
  7. 7. Maintaining cold medicine supplies
  8. 8. Avoiding contact with symptomatic individuals
  9. 9. Specific H1N1 vaccines are available; seasonal flu vaccines don't provide protection
  10. 10. Regular household disinfection with alcohol

 

VIII. Epidemiological Data

As of September 2017, India reported 34,009 H1N1 cases nationwide with 1,741 deaths, according to the Ministry of Health and Family Welfare.

 

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