The Seasonality of Hepatitis

Most of us are aware of the seasonal cycle of influenza outbreaks, which peak in the winter months. New research seems to show that all infectious diseases have a seasonal element.

Micaela Elvira Martinez, an assistant professor of Environmental Health Sciences, collected information from the World Health Organization, the U.S. Centers for Disease Control and Prevention, and peer-reviewed publications to create a calendar of epidemics for 69 infectious diseases. A given year will see outbreaks of flu in the winter, chickenpox in the spring, and gonorrhea and polio in the summer -- to name a few of the best described seasonal outbreaks.

She found seasonality occurs not just in acute infectious diseases like Influenza but also chronic infectious diseases like Hepatitis, which depending on geography, flares up with greater regularity certain times of the year[1].

The paper describes several drivers of seasonality in infectious diseases: (a) vector seasonality, (b) seasonality in nonhuman animal host (i.e., livestock, other domestic animals, or wildlife), (c) seasonal climate (e.g., temperature, precipitation, etc.), (d) seasonal nonclimatic abiotic environment (e.g., water salinity), (e) seasonal co-infection, (f) seasonal exposure and/or behavior and/or contact rate, (g) seasonal biotic environment (e.g., algal density in waterbodies), (h) seasonal flare-up/symptoms and/or remission/latency, (i) observed seasonal incidence with no hypotheses regarding drivers.

Let's see what Martinez' paper has to say about Hepatitis:
- Hepatitis A (Acute): seasonal drivers are (f) and (i) – Dry season (in Brasil)[2][3]
- Hepatitis B (Chronic): seasonal driver is (h) – Seasonality is observed with elevated levels in spring and summer and/or autumn in some parts of the world, whereas there is lack of seasonality in other parts of the world [3][4]
- Hepatitis C (Acute and Chronic): seasonal driver is (f) - Seasonality observed in some countries and absent in others; spring and/or summer peaks in Egypt, China, and Mexico while there is a winter peak in India[3]
- Hepatitis E (Acute): seasonal driver is (c) - Waterborne outbreaks occur during the rainy season or following flooding (in China)[5]

"Seasonality is a powerful and universal feature of infectious diseases, although the scientific community has largely ignored it for the majority of infections," says Martinez. "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."

[1] Micaela Elvira Martinez: The calendar of epidemics: Seasonal cycles of infectious diseases in PLoS Pathogens - 2018. See here.
[2] Bensabatb et al: Epidemiological and Serological Studies of Acute Viral Hepatitis in Brazil's Amazon Basin in Bulletin of the Pan American Health Organisation - 1987
[3] Fares: Seasonality of Hepatitis: A Review Update in Journal of Family Medicine and Primary Care - 2015. See here.
[4] Zhang et al: Effect of Seasonal Variation on the Clinical Course of Chronic Hepatitis B in Journal of Gastroenterology - 2006 
[5] Zhuang et al: Epidemiology of Hepatitis E in China in Gastroenterologia Japonica - 1991