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‘Tis the Season for… Gonorrhea? The Seasonality of Infectious Diseases

Illustration: Micaela Martinez (PLOS Pathogens)
A few examples of seasonal infections. Credit: Mailman School of Public Health.
A few examples of seasonal infections. Credit: Mailman School of Public Health.

From African Sleeping Sickness to Zika, a new journal article presents the evidence for seasonality in 69 infectious diseases.

Micaela Martine, Columbia University’s Mailman School of Public Health Professor of Environmental Health Sciences, gathered data for the diseases from the U.S. Centers for Disease Control and Prevention (CDC), the World Health Organization and peer-reviewed journal articles. She then mapped the time of year when outbreaks tended to occur, ranging from common infections to rare tropical diseases.

Martine found that in a given year, flu outbreaks occur in the winter, chickenpox in the spring and polio and gonorrhea in the summer.

She found seasonality occurs not just in acute infectious diseases like flu but also chronic infectious diseases like Hepatitis B, which depending on geography, flares up with greater regularity certain times of the year. Preliminary work has shown that even HIV-AIDS has a seasonal element, thought to be driven by seasonal changes in malnutrition in agricultural settings.

The paper describes four main drivers of seasonality in infectious diseases.

  • Environmental factors like temperature and humidity regulate seasonal flu; in vector-borne diseases like Zika too, the environment plays a role in the proliferation of mosquitos.
  • Host behaviors such as children coming into close proximity with each other during the school year are a factor in measles.
  • Ecological factors such as algae play a role the outbreak of cholera.
  • Seasonal biological rhythms, similar to those that govern migration and hibernation in animals, may also be a factor in diseases like polio, although more research is needed.

Identifying the drivers of seasonal outbreaks is not always straightforward, but can pay dividends. For instance, the bacteria that cause cholera, which spread to humans by fecal-oral transmission, can be maintained in water supported by algae. Public health officials might undertake an intervention to prevent the transmission of cholera from infected individuals and/or target the bacteria surviving in algal-filled waterbodies; importantly, the key season to undertake each of these interventions would likely differ.

“Seasonality is a powerful and universal feature of infectious diseases, although the scientific community has largely ignored it for the majority of infections. Much work is needed to understand the forces driving disease seasonality and understand how we can leverage seasonality to design interventions to prevent outbreaks and treat chronic infections.”

Micaela Martine, Columbia University’s Mailman School of Public Health Professor of Environmental Health Sciences

In the case of polio, public health researchers once thought summer outbreaks were driven by the seasonal mixing of children in swimming pools or theaters or by the climate, but neither of these factors could explain the summertime outbreaks around the world. In 2001, a researcher at the CDC hypothesized that seasonal changes in the hormone melatonin may play a role in modulating the immune system. In an ongoing study funded by a prestigious National Institutes of Health Director’s Grant, Martinez is investigating this possibility by comparing levels of immune molecules in blood drawn from patients at different times of the year.

Read more:

The calendar of epidemics: Seasonal cycles of infectious diseases  Martinez ME (2018) The calendar of epidemics: Seasonal cycles of infectious diseases. PLOS Pathogens 14(11): e1007327.

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Across Africa, diseases transmitted by bloodsucking insects – including ticks, fleas and sandflies as well as mosquitoes – are emerging and re-emerging, and epidemics are reported more frequently than before. Evidence is mounting that climate change – in addition to population movements and deficient urban planning – is contributing to these public health crises which place huge socio-economic burdens on vulnerable populations in low-capacity countries. Gabriel Mabikina, an 80-year-old retired businessman in the port city of Pointe-Noire in the Republic of Congo commented on the many changes in his city. “In the neighborhoods there are a lot of mosquitoes. A lot, lot, lot of mosquitoes,” said Mabikina. Mabikina’s observations about the profusion of mosquitoes are supported by surveys conducted in the affected area which reveal high densities of Aedes aegypti, the mosquito that spreads Yellow Fever. Following the declaration of a Yellow Fever outbreak in the Republic of Congo in August 2018, Mabikina was among the more than 1 million people who turned up to be immunized during a vaccination campaign against the disease in September. According to The 1.5 Health Report, a synthesis of the health content of the Intergovernmental Panel on Climate Change (IPCC) special report on global warming of 1.5C, climate change may put more people at risk of other infectious diseases including other insect-borne diseases such as malaria and Dengue Fever and waterborne disease such as cholera, typhoid and hepatitis. In addition to the impact of infectious diseases, climate change poses other health risks from increased flooding and sea-level rise, reduced air quality and food insecurity. In response to the increasing climate-related health threats, member states in African region in 2016 adopted the Regional Strategy for Health Security and Emergencies which, along with the International Health Regulations, aims at strengthening countries’ capacities to detect, prevent and response to public health emergencies. Source: WHO

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