Obesity facilitates spread of influenza A

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A study recently published in The Journal of Infectious Diseases reveals that obesity can help spread the influenza A virus by increasing the virus shedding time in infected adults.

The global obesity epidemic has been linked to an increasing number of chronic diseases. Obesity may also influence the progression of infectious diseases, such as the influenza virus. The risk of developing severe complications associated with the flu virus tends to be higher in obese individuals. This risk increases with age, possibly because obesity places an enormous strain on the immune system, and over time, this leads to chronic inflammation. Compared with non-obese people, obese individuals are also more likely to spread the flu virus to other people through a process called viral shedding.

What is viral shedding?

Viruses rely on human cells to help them multiply. After the virus has replicated, the new copies are released from the cells. These new copies can then infect other cells, other parts of the human body, or even be released from the body in different ways, such as sneezing and coughing. If the virus is released, or shed, from the body, there is a chance that the virus may infect other individuals. People who are infected with a virus but have few or no obvious symptoms can also shed the virus and infect other individuals.

Various factors can influence the duration of viral shedding, for example, children tend to shed the flu virus over a longer period compared with adults. A team of researchers in Britain and Nicaragua also believed that chronic inflammation caused by obesity may influence the duration of viral shedding. They speculated that obese individuals would shed flu virus over a longer time period than non-obese individuals. They published their results in the Journal of Infectious Diseases.

Monitoring the flu virus in Nicaraguan households

The team observed 1783 Nicaraguan individuals from 320 households over three flu seasons (2015-2017). The cohort included 340 children aged 0-4 years, 631 children aged 5-17 years, and 812 adults aged 18-92 years. The prevalence of obesity (BMI > 30 kg/m2) was relatively low in children (2-9%) but increased dramatically to 42% in adults.

When an individual developed symptoms of the flu virus, such as acute respiratory illness, a 100°F (37.8°C) fever, sore throat, runny nose or a cough, all household members were monitored for 10-13 days even if they did not present with any symptoms. Nose and throat swabs results revealed that 87 of the participants had influenza A and 58 had influenza B. The participants who experienced more symptoms or more severe symptoms tended to be obese.

Viral shedding occurs over a longer period in obese adults

The results from the nose and throat swabs were also used to work out the duration of viral shedding. The impact of obesity on shedding duration could not be assessed in children aged 0-4 years due to the low prevalence of obesity in this age group. Obesity did not affect shedding duration in children aged 5-17 years, perhaps because they had not yet developed chronic inflammation associated with obesity.

In contrast, viral shedding was extended by 42% from 3.7 days in non-obese adults to 5.2 days in obese adults who had at least two symptoms of influenza A. This increase was even higher in obese adults with either one or no symptoms of influenza A (excluding fever). The duration of shedding was extended by 104% from 1.6 days in non-obese adults to 3.2 days in obese adults. Of note, obesity had no effect on the shedding duration of influenza B.

The duration of viral shedding is longer in children

Although there were no associations between obesity and duration of viral shedding in children, there were vast differences in the duration of viral shedding in the overall cohort. Shedding was 40% longer in young children (0-4 year old) and 30% longer in the older children (5-17 years old) compared with adults.

Future directions

The authors emphasized that more studies are needed to understand the effect of obesity on the spread of Influenza A in the wider community rather than within the confines of a household. Ideally, these studies should incorporate methods to precisely determine the end of the shedding period in addition to the amount of virus released during this period. These studies may also help unravel the connections between obesity, chronic inflammation, and infectious diseases.

Despite these limitations, this study clearly demonstrated that obesity increases the duration of influenza A shedding in obese adults, even in the absence of visible symptoms. Due to the global impact of obesity, even in low-income countries, interventions and prevention strategies to combat obesity must be prioritized.

Written by Natasha Tetlow, PhD

References:

  1. Maier HE, et al. Obesity increases the duration of influenza A virus shedding in adults. J Infect Dis. 2018. Available at doi:10.1093/infdis/jiy370.
  2. Obesity extends duration of influenza A virus shedding. Retrieved from https://www.nih.gov/news-events/news-releases/obesity-extends-duration-influenza-virus-shedding, accessed 20 August 2018.

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