Understanding H3n2v Transmission To People

Introduction  

On rare occasions, influenza viruses normally found in pigs can cross over and infect humans. An example of this can be found in the H3N2 variant virus (H3N2v), which is a variant of the influenza A virus that exists in humans and can also be called ‘seasonal flu’. The ability of H3N2v to exist in swine and infect humans means it is a ‘zoonotic’ virus. The H3N2v virus consists of avian, swine, and human genetic material and the name comes from the surface of the virus, which contains haemagglutinin and neuraminidase proteins.  

H3N2v infection can have serious complications, with 309 infections seen in the US in 2012 including 16 hospitalisations and 1 fatal case of the virus.¹  

Understanding the transmission of the virus is crucially important to prevent future outbreaks and to monitor new viral strains. Additionally, knowledge of the transmission pathway can aid vaccine development and can help in risk assessment of high-risk patient groups.  

This article intends to explain the transmission of H3N2v to people, detailing the virus background, characteristics, method of transmission, surveillance, and prevention, including case studies and future considerations for the virus.  

Background on H3N2v  

The influenza family 

Influenza viruses belong to the family of Orthomyxoviridae, which are enveloped viruses with single-stranded RNA genomes. The genus of influenza viruses consists of influenza A, influenza B and influenza C. H1N1 and H3N2 are both examples of influenza A variants.  

The history of H3N2v  

Prior to 1998, only the classic swine virus H1N1 had been identified in pigs, with minimal genetic variation seen. At this point, a serious swine illness like influenza was observed and determined to be a new version of the influenza virus, named H3N2v.²  

The first case of H3N2v in humans occurred in July 2011, and many initial cases were linked with exposure to infected pigs at agricultural fairs.  

Characteristics of H3N2v 

Virology and genetic makeup 

The genetic information of the H3N2v virus is arranged in a segmented RNA genome. The haemagglutinin and neuraminidase proteins are arranged in unique sequences, allowing for rearrangement. Genetic analysis of the H3N2v virus shows that it possesses the ‘M’ or matrix gene, derived from the H1N1 virus, which may allow for greater ability of transmission.  

Source and reservoirs of the virus 

Pigs act as the main reservoirs for the virus, which allows it to incubate and mutate, increasing the possibility of transmission to humans.  

Seasonal patterns and variation 

As virus transmission is linked to direct human interaction with infected swine, specifically in agriculture, there is a seasonal element to the virus. More cases are seen in summer and autumn compared to winter and spring.  

Modes of Transmission 

Animal-to-Human transmission 

H3N2v is a zoonotic virus, as the main method of transmission is from pigs to humans. Along with swine, the virus may also transfer to other animals during the transmission cycle.  

Human-to-Human transmission 

Direct human-to-human transmission of H3N2v is rare, but cases have been seen during the winter when low temperatures and absolute humidity create favourable conditions for influenza transmission.² The virus can also live on environmental surfaces, which can result in indirect human-to-human transmission.  

Factors Influencing Transmission  

Environmental factors 

Climate and weather conditions can influence virus transmission, with studies showing that transmission occurs most frequently at 23 degrees Celsius.³ As previously explored, H3N2v appears mostly transmitted during the summer and autumn months, when direct human contact with infected swine is more likely due to agricultural events.  

Geographical considerations can also affect transmission. H3N2v has been prevalent in South and East Asia, which may be due to these regions holding tropical or subtropical weather patterns and the presence of semi-connected populations which can become infected in turn, allowing year-round persistence of the virus.⁴ H3N2v has been recorded in most continents, including Europe, the Americas, Australasia, and Africa.  

Host factors 

The severity of symptoms produced by the H3N2 virus depends upon the host immune response, with immunodeficient hosts experiencing much worse effects.  

Certain populations are more at risk of developing complications from H3N2v, such as people 65 years and older, children younger than 5 years, pregnant women, and those with long-term health conditions.⁵ 

Surveillance and Monitoring  

Current Surveillance methods 

Current surveillance methods include monitoring animal populations to ensure early detection of swine flu, and to obtain a greater understanding of the disease incubation within pigs. 

It is also important to report cases in humans, as timely communication about human transmission can help to prevent or control an outbreak of the virus.  

Challenges and Improvements 

Under reporting of human and animal cases of the virus can be a challenge for virus surveillance. Greater public health knowledge about the virus, including transmission, and associated risks, can help to create better awareness and lead to improved reporting of cases. Additionally diagnostic limitations exist within the medical population due to the scarcity of cases.    

Prevention and Control  

Vaccination strategies 

Current information suggests that seasonal vaccines against H3N2v can provide partial protection against infection amongst adults and no protection in children.⁶ It is still recommended that high risk populations receive the H3N2v vaccine. It is important that surveillance is carried out to monitor genetic changes in H3N2v to ensure that vaccines are still effective on the different new strains. Also, swine could be vaccinated to reduce the prevalence of the virus within their populations, reducing transmission to humans. High risk areas of the country, or high-risk individuals, such as people from agricultural industries, or immunodeficient patients, could be selected for vaccination.  

The possibility of future pandemic must be considered, and there should be stockpiles of vaccines ready in case of outbreak. 

Public Health measures 

Adequate hand hygiene practices can help to drastically reduce the chances of viral transmission.  

Quarantine and isolation protocols exist, with the Centre for Disease Control (CDC) recommending that all new cases of H3N2v be reported. Any identified close contacts should be observed for 10 days after the last known exposure to a confirmed or likely case of H3N2v infection.⁷  

Case Studies  

Highlighting past H3N2v outbreaks 

In 2012 a total of 306 cases of H3N2v were observed over 10 states in the US, with common symptoms reported as fever, fatigue, and cough. Out of this group, 16 patients were hospitalised, and 1 patient died. Most infected patients had previously attended an agricultural fair and/or had contact with pigs before falling ill. There were 15 cases of person-to-person transmission.⁸ 

Lessons learned and applied interventions. 

Following this outbreak, it was recommended that those at high risk from influenza related complications (such as young children, the elderly, and those with underlying medical conditions) should avoid contact with swine or swine barns when visiting agricultural fairs.  

Additionally, the clinical management of these patients has developed, with guidelines now suggesting that high risk patients with contact to H3N2v should immediately start antiviral treatment without waiting for test results to confirm infection.  

Future Considerations 

Anticipating potential changes in transmission patterns 

Research must be continued to further understand possible future changes in transmission patterns for the H3N2v virus. It is important that the genetic material of the virus is monitored, to highlight any adaptations that could increase transmission. Additionally, it is crucial to understand how the virus can adapt to different hosts, and transmit between animals and humans, so we can have insights into the patterns of transmission. In the future we could also potentially see more human-to-human transmission of the virus.  

Research gaps and areas for further investigation 

There are many avenues for future research into the human transmission of H3N2v. Research could focus on the exact mode of transmission, and the effect that different environments, such as in the agricultural sector, can have on increasing or decreasing likelihood of viral transference.  

It is important that we know who is more at risk from H3N2v infection, so research into patient demographics, behaviour, and occupation will help us to understand and prevent future outbreaks. Knowledge of viral genetic changes is essential to outline how the virus can adjust within the human host and mutate to create more transmissible variants and survive the attack of the host immune system.  

The exact immune system response must also be researched, to understand the full range of symptoms that infection can bring on, and the severity of the effects. Presence of the infection within swine populations should be under close ongoing surveillance to ensure early detection of any outbreaks, to try and monitor chances for animal-to-human transmission. 

Conclusion  

Summary 

In summary, comprehension of the transmission of H3N2v to people is a vast topic, requiring an understanding of many different fields, including public health, virology and epidemiology. We must fully understand the makeup of the virus and the transmission cycle to prevent future outbreaks and safeguard both humans and animals from the effects of this potentially fatal virus.  

Importance of continued research and vigilance 

One of the reasons that research into this field is vitally important is that the virus must be monitored to prevent possible pandemics. Additionally, due to antigenic shift, the virus could undergo genetic changes, which could create potentially more harmful variants, or reducing the effectiveness of current vaccines. Ongoing vaccine development ensures that we can protect the population against the virus. Surveillance and early detection of new variants can allow for public health interventions when needed, such as quarantine measures, antiviral treatments and vaccination campaigns, which can all prevent the further spread of the virus. Also, it is essential that the healthcare community are prepared for a potential public health incident, so healthcare guidelines, diagnostic protocols and resources must be available, and providers must remain vigilant.