Swine Flu: H1N1 Influenza Explained

Swine flu, caused by the H1N1 influenza virus, has a history that dates back over a century. The story begins with the 1918 H1N1 influenza pandemic, commonly known as the Spanish Flu, which affected humans and is believed to have impacted swine. This pandemic set the stage for swine flu to become endemic in pigs. 

The virus was first isolated from diseased pigs in 1930, establishing a long-term presence in swine populations. Since then, classical swine flu viruses have circulated primarily in the Americas and Asia, maintaining relative antigenic stability.

Human interaction with these viruses has been sporadic but significant. The H1N1 subtype occasionally crosses over to humans, with one notable event being the 1976 Fort Dix outbreak. Although these infections can be severe, especially for pregnant or immunocompromised individuals, they rarely result in epidemics. 

The H3N2 influenza virus, which caused a human pandemic in 1968, also infected pigs and demonstrated less antigenic drift in swine than in humans.

A significant development occurred in 1998 when “triple reassortant” H3N2 viruses, containing genes from human, swine, and avian influenza viruses, were first isolated from pigs in the USA. These triple-reassortant viruses have since continued to evolve, generating new variants like H1N2 and H1N1.

Human infections with swine-origin influenza viruses have been reported periodically. Between 1958 and 2005, 37 such cases were documented, with a notable portion involving recent exposure to pigs. Although the overall fatality rate was 17%, widespread human-to-human transmission remained rare until the 2009 H1N1 pandemic.

The 2009 pandemic was a landmark event, as the virus displayed a unique combination of gene segments not previously seen in pigs or humans. Genetic analysis revealed that one of the swine genes derived from the 1918 human virus, making the 2009 H1N1 virus a fourth-generation descendant of the 1918 strain. This new virus was more pathogenic in mammalian models compared to seasonal H1N1 viruses, leading to significant global health concerns.

Wilson Smith, C.H. Andrewes, and P.P. Laidlaw, who discovered that the disease was caused by a virus rather than bacteria, identified the pathogen as an influenza virus in 1933. Over time, the nomenclature for these viruses has been standardized based on their hemagglutinin (H) and neuraminidase (N) proteins, with the official name for the 2009 pandemic strain being the H1N1pdm09 virus.

While no Nobel Prizes have been directly awarded for work on swine flu, significant advancements in understanding the immune response to virus-infected cells were recognized with a Nobel Prize in 1996. 

These advancements are crucial for developing new vaccines, especially given the rapid evolution of influenza viruses.

Swine flu, caused by the H1N1 influenza virus, tends to occur in pandemics every few decades. One of the most notable pandemics was in 2009-2010, during which children and young adults were the most at risk. 

The primary mode of transmission is from pigs to humans, especially for individuals with direct exposure to infected pigs, such as agricultural workers, those in the meat industry, and people attending agricultural fairs. Person-to-person transmission is rare except during widespread pandemics like that of 2009-2010.

Certain groups are at higher risk of severe disease if they contract swine flu. These include:

• Children under 5 years old
• Adults over 65
• Immunocompromised individuals
• Those with pre-existing conditions such as diabetes, heart disease, and asthma.
• People who are pregnant and people with morbid obesity are also particularly vulnerable to severe outcomes if they contract the virus.

Notable outbreaks

• 1976: Fort Dix, New Jersey, USA – 13 cases and 1 death.
• 2009-2010: Swine flu pandemic: Worldwide. 
  • US caseload: 60.8 million cases, 274,304 hospitalizations, and 12,469 deaths 
  • Worldwide caseload: 151,700-575,400 people worldwide died from (H1N1) pdm09 virus infection during the first year the virus circulated.
• 2011: Indiana, Iowa, Maine, Pennsylvania, and West Virginia – 12 cases.
• 2012: 12 states in the US – 309 cases reported.
• 2015: India – 30,000 laboratory-confirmed cases with more than 1700 deaths.

Swine flu is highly contagious and spreads in two main ways.

Airborne transmission: When an infected pig coughs or sneezes, droplets containing the flu virus can spread through the air. If these droplets land in your nose or mouth, or if you inhale them, you can get infected.

Contact transmission: You can also catch swine flu by touching surfaces contaminated with the virus and then touching your mouth or nose.

Swine flu is a viral infection that infects the body by targeting cells in the nasal passage. The virus invades these cells using two main proteins: hemagglutinin and neuraminidase.

Hemagglutinin forms spikes on the virus that help it bind to the surface of the host cell and enter it. Once inside, the virus fuses with the cell’s internal membranes, allowing it to release its genetic material into the host cell’s cytoplasm. From there, the viral components move to the cell’s nucleus, hijacking the cell’s machinery to produce more virus particles.

These newly formed viruses are assembled in the cytoplasm and then released from the host cell by budding, which can cause the host cell to die (necroptosis). The death of these cells leads to inflammation and triggers an immune response, often resulting in symptoms like a runny nose and sore throat. 

In severe cases, the immune response can be so strong that it leads to a cytokine storm, a potentially dangerous condition where the body attacks its own tissues.

Dendritic cells (DCs) play a crucial role in the immune response to H1N1 influenza. They capture viral antigens from infected cells and present them to T-cells, which helps activate a targeted immune response to fight off the infection.  

Swine flu symptoms resemble those of seasonal flu and include:

Respiratory Symptoms:

• Cough
• Sore throat
• Runny nose

Gastrointestinal Symptoms

• Diarrhea
• Vomiting

Systemic Symptoms

• Fever
• Chills
• Body aches/muscle pain (myalgia)
• Headache
• Fatigue
• Loss of appetite.

Swine flu symptoms are similar to those of seasonal influenza, making it crucial to identify any recent exposure to swine for an accurate diagnosis. 

While antigen detection tests and molecular assays can detect the virus, they cannot distinguish between different influenza subtypes. 

In the United States, samples must be tested at state public health laboratories and verified by the US CDC as confirmation of swine flu.

Key Symptoms 

• Fever
• Cough
• Sore throat
• Body aches
• Headache
• Fatigue
• Gastrointestinal symptoms like diarrhea.

Epidemiological Link

Suspect variant influenza virus infection if there is an epidemiological link to recent swine exposure within 7 days of illness onset.

Exposure Criteria

• Contact with swine or their bodily fluids.
• Contact with areas where swine live or surfaces contaminated with swine germs.
• Close contact with an ill person who had recent swine exposure or is known to be infected with a variant influenza virus.

Respiratory Samples

• Nasopharyngeal swabs, aspirate, or combined nasal and throat swabs in viral transport medium.

Testing

Contact the state or local health department for transport and request timely influenza diagnostic testing at a state public health laboratory.

Laboratory Diagnosis and Test Interpretation

Antigen Detection Tests

• Rapid influenza diagnostic tests (RIDTs) and immunofluorescence assays (e.g., direct fluorescent antibody staining (DFA)) can detect variant influenza viruses in respiratory specimens.
• These tests cannot distinguish between seasonal influenza A, variant influenza A, or other novel influenza A viruses.

Molecular Assays

• Commercial molecular assays (e.g., RT-PCR) can detect influenza viruses with higher sensitivity than RIDTs in respiratory specimens.
• These assays cannot differentiate variant viruses from seasonal influenza A, variant influenza A, or other novel influenza A viruses.

Confirmation

• Testing for variant influenza viruses in respiratory specimens is done at state public health laboratories.
• Confirmation is performed at the public health agencies like the US CDC.

Clinical Management

• Outpatients with mild symptoms: Can be managed at home with close monitoring.
• High-risk outpatients: Early antiviral treatment (within 48 hours) is recommended.
• Severe cases: Hospitalization may be necessary for severe complications.
• Hospitalized patients: Antiviral treatment should begin as soon as possible.
• Secondary infections: Antibiotics are prescribed if a secondary bacterial infection is suspected.
• Supportive care: This may include oxygen, mechanical ventilation, vasopressors, or renal replacement therapy for complications.

Antiviral Treatment

• Recommended antivirals: Neuraminidase inhibitors (oseltamivir, zanamivir, peramivir) and baloxavir.
• High-risk outpatients: Antivirals are recommended.
• Symptomatic outpatients: Antivirals can be considered.
• Severe illness: Oral or enteric oseltamivir is recommended for hospitalized patients.

Over-the-Counter Medication

• Avoid Aspirin in Children: Aspirin or aspirin-containing products should be avoided in children with flu-like symptoms due to the risk of Reye syndrome.

Here are the ways to prevent swine flu:

• Avoid contact with pigs: Stay away from pigs, especially at agricultural fairs and pig barns. If you have to be in these areas, limit your exposure time.
• Avoid contaminated items: Do not consume food or drinks, and avoid using children’s toys or pacifiers in areas where pigs are present.
• Avoid sick pigs: Stay away from pigs that appear unwell to minimize the risk of infection.
• Avoid exposure when sick: If you have influenza-like symptoms or are sick with the flu, avoid contact with pigs to prevent the virus from spreading.
• Wear protective gear: If exposure to pigs is unavoidable, wear protective clothing, gloves, and masks (such as N95, KN95, or surgical masks) to reduce the risk of infection.
• Practice good hygiene: Wash your hands frequently with soap and water, or use an alcohol-based hand sanitizer.

Note on vaccines

Human seasonal flu vaccines do not protect against the flu A variant viruses that commonly spread in pigs.
Developing vaccines for swine flu is difficult because pigs often mix human, avian, and swine viruses, creating new strains. These frequent changes make it hard for existing vaccines to stay effective. Although live attenuated vaccines show promise, they can mix with other circulating viruses, creating new challenges. 

Also, the global trade of swine spreads new swine flu viruses to herds that haven’t been exposed before, increasing the risk of spread. 

Consequently, monitoring, staying alert, and reporting suspected swine flu cases to health agencies are crucial steps to manage and reduce the spread of the virus.

As we reflect on the journey of swine flu, it’s clear that this virus has a complex and impactful history. From its initial emergence in pigs and the subsequent human infections to the major pandemic in 2009, swine flu has challenged scientists, healthcare workers, and public health systems worldwide. Its ability to mix genes from swine, avian, and human influenza viruses creates a constantly evolving threat, making vaccine development particularly difficult.

Moving forward, it’s crucial that we continue to invest in research to understand this virus better and develop more effective vaccines. Public health campaigns play a vital role in raising awareness and educating people about preventive measures, especially in high-risk areas like agricultural settings. By staying vigilant and fostering a well-informed public, we can better manage and mitigate the spread of swine flu, protecting both human and animal populations.

Effective prevention methods, continuous monitoring, and timely reporting of suspected cases to health authorities are essential steps. Together, these efforts will help us stay ahead of the evolving challenges posed by swine flu and ensure a healthier future for all.

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Learn more about more viral infections on the GIDEON platform.

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