Background

Avian influenza is a type of influenza which is found in birds, potential to human health concern due to its enveloped, segmented, and negative-sense RNA. It is a part of the orthomyxovirus family that has three strains A, B, and C. The influenza A virus is characterized by 8 RNA segments that poses a potential and unpredictable threat to humans. The virus has 16 H and 9 N serotypes along with H3N2 being the most prevalent strain. 

The H5N1 virus strain which known as a significant risk due to its high mortality rate has shown decline in recent years. H7N9 strain first documented in China in 2013 and is another concern. These serotypic play a crucial role in species specificity mainly through variations in receptor usage particularly sialic acid. 

The immune response against these antigens determines host protection but the viral RNA polymerase lacks error checking mechanisms that leading to antigenic drift from year to year. 

The segmented genome also presents the potential for reassortment where different influenza strains co-infecting a host can exchange genome segments leading to new viral combinations. 

Epidemiology

The 2021-2022 US season saw 9 million cases and 4 million medical visits in 100,000 hospitalizations over 5,000 deaths due to seasonal influenza with a decrease compared to pre-COVID-19 levels. 

The first H5N1 case in the US occurred in April 2022. As of 2015 844 cases were reported worldwide which caused 449 deaths. Underreporting in China is a concern. H7N9 cases are primarily in China. 

Global travel and recent studies on low pathogenicity of H1N1 influenza have highlighted the risk of wide spread. Assessing the successful recombination between H1N1 and pathogenic avian influenza strains is a bit challenging. 

Anatomy

Pathophysiology

Avian influenza differs from typical influenza in several key aspects primarily affecting lower airways more than human influenza due to its differences in the hemagglutinin protein and specific sialic acid residues despite both being respiratory infections. 

This virus prefers sialic acid alpha (2-3) galactose in human terminal bronchi and alveoli while human viruses prefer sialic acid alpha (2-6) galactose on upper respiratory tract epithelial cells supporting avian influenza replication in laboratory settings. 

Avian influenza, despite severe respiratory infections has limited human-to-human transmission due to lower airway infection. Concerns arise over potential mutations in hemagglutinin protein potentially enabling virus binding to both respiratory epithelium. 

Etiology

Genetics

Prognostic Factors

The mortality rate of confirmed human cases of avian influenza is 60% with largely attributed to respiratory illness rather than bacterial complications. There is limited evidence about the disease lasting effects among survivors. 

Clinical History

Avian influenza is a lethal disease that can be identified through close contact with infected ailing birds or humans. Its incubation period is longer than human influenza and severe distress usually occurs 5 days after symptoms appear. 

Factors contributing to the susceptibility includes visiting regions with documented cases and underlying health conditions also exposure to individuals with avian influenza. Diagnostic complexities can arise when initial cases are mistakenly diagnosed as SARS. 

Physical Examination

Rapid breathing and crackling sounds 

Occasional wheezing 

Conjunctival suffusion or conjunctivitis are common signs of viral pneumonia often accompanied by bleeding gums in some cases. 

Age group

Associated comorbidity

Associated activity

Acuity of presentation

Differential Diagnoses

Laboratory Studies

Imaging Studies

Procedures

Histologic Findings

Staging

Treatment Paradigm

Treatment involves antiviral medication, supportive care, and ventilatory support. Antiviral therapy should be customized based on the patient’s age and virus resistance. Treatment should be initiated promptly even if presentation is delayed. Antibiotics may be necessary for treating bacterial pneumonia but not always empirically required. 

Steroids have limited benefits potentially only in cases of sepsis with adrenal insufficiency. Baloxavir acid and its prodrug baloxavir marboxil have shown effectiveness against H7N9 influenza in vitro and in vivo. In a mouse model BXM administration provided complete protection against lethal A/Anhui/1/2013 (H7N9) challenge suggesting potential for human trials. 

Infection control is crucial to prevent transmission of influenza to patients and healthcare workers. Droplet precautions including eye protection should be implemented. If fine aerosols are expected particulate respirators should be used properly. Adults and children over 12 years should follow infection-control precautions for 1 week from symptoms onset. For children under 12 years WHO recommends the same duration for avian influenza precautions. 

by Stage

by Modality

Chemotherapy

Radiation Therapy

Surgical Interventions

Hormone Therapy

Immunotherapy

Hyperthermia

Photodynamic Therapy

Stem Cell Transplant

Targeted Therapy

Palliative Care

Use of Antivirals for Influenza

Compounds that hinder the function of neuraminidase may provide some benefits. 

Oseltamivir: 

It inhibits the neuraminidase glycoprotein on the influenza virus surface thereby limiting viral dissemination. 

It is effective against both influenza A and B and is recommended for hospitalized patients and outpatients with severe or progressive illnesses like pneumonia. 

It is available in capsule form and as an oral suspension and should be initiated within 40 hours of symptom onset. 

Zanamivir: 

It inhibits the viral glycoprotein neuraminidase which destroys infected cell receptors for viral hemagglutinin. 

It reduces virus release from infected cells reducing viral spread. 

It is effective against influenza A and B and can be used for outpatients without underlying asthma or COPD. 

It is administered via inhalation using the Diskhaler device and is under investigation for intravenous use in severe influenza in hospitalized adults or children. 

Peramivir: 

It is a neuraminidase inhibitor which is prescribed for acute uncomplicated influenza in patients aged 6 months and older that is symptomatic for less than 2 days and is administered intravenously in hospitalized patients. 

Medication

Future Trends

Media Gallary

References

Avian Influenza – StatPearls – NCBI Bookshelf (nih.gov)