Seasonal variations in temperature, rainfall and resource availability are ubiquitous and can exert strong pressures on population dynamics. Infectious diseases provide some of the best-studied examples of the role of seasonality in shaping population fluctuations. In this paper, we review examples from human and wildlife disease systems to illustrate the challenges inherent in understanding the mechanisms and impacts of seasonal environmental drivers. Empirical evidence points to several biologically distinct mechanisms by which seasonality can impact host-pathogen interactions, including seasonal changes in host social behaviour and contact rates, variation in encounters with infective stages in the environment, annual pulses of host births and deaths and changes in host immune defences. Mathematical models and field observations show that the strength and mechanisms of seasonality can alter the spread and persistence of infectious diseases, and that population-level responses can range from simple annual cycles to more complex multiyear fluctuations. From an applied perspective, understanding the timing and causes of seasonality offers important insights into how parasite-host systems operate, how and when parasite control measures should be applied, and how disease risks will respond to anthropogenic climate change and altered patterns of seasonality. Finally, by focusing on well-studied examples of infectious diseases, we hope to highlight general insights that are relevant to other ecological interactions.

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Seasonality and the dynamics of infectious diseases.

Seasonal cycles of infectious diseases have been variously attributed to changes in atmospheric conditions, the prevalence or virulence of the pathogen, or the behavior of the host. Some observations about seasonality are difficult to reconcile with these explanations. These include the simultaneous appearance of outbreaks across widespread geographic regions of the same latitude; the detection of pathogens in the off-season without epidemic spread; and the consistency of seasonal changes, despite wide variations in weather and human behavior. In contrast, an increase in susceptibility of the host population, perhaps linked to the annual light/dark cycle and mediated by the pattern of melatonin secretion, might account for many heretofore unexplained features of infectious disease seasonality. Ample evidence indicates that photoperiod-driven physiologic changes are typical in mammalian species, including some in humans. If such physiologic changes underlie human resistance to infectious diseases for large portions of the year and the changes can be identified and modified, the therapeutic and preventive implications may be considerable.

/pdf/seasonal-variation-in-host-susceptibility-and-cycles-of-1lzz9otayj.pdf

Seasonal variation in host susceptibility and cycles of certain infectious diseases.

Seasonal change in the incidence of infectious diseases is a common phenomenon in both temperate and tropical climates. However, the mechanisms responsible for seasonal disease incidence, and the e...

/pdf/seasonal-infectious-disease-epidemiology-9ah8q0vztg.pdf

Seasonal infectious disease epidemiology

https://www.thelancet.com/pdfs/journals/lancet/PIIS0140673606688156.pdf

Rotavirus vaccines: current prospects and future challenges

The decision to develop rotavirus vaccines was predicated on the extensive burden of rotavirus disease among children worldwide. US reports on nationwide hospitalizations (1979-1992) and deaths (1968-1991) due to diarrhea and weekly reports of rotavirus infection by 74 laboratories were reviewed to estimate the burden of rotavirus disease, identify epidemiologic trends, and consider methods for evaluating an immunization program when a vaccine becomes available. From 1968 to 1985, diarrhea-related deaths among US children <5 years old declined from 1100 to 300/year. This decline was associated with the disappearance of winter peaks for diarrhea-related deaths previously associated with rotavirus infection among children 4-23 months old. From 1979 to 1992, however, hospitalizations for diarrhea averaged 186,000/year and retained their winter peaks, which have been linked to rotavirus infections. Each year an estimated 54,000-55,000 US children are hospitalized for diarrhea, but <40 die with rotavirus. A rotavirus vaccine program will require improved surveillance, including the timely collection of data from sentinel hospitals, in which a diagnosis of rotavirus can be established or ruled out for all children hospitalized for diarrhea.

https://www.jstor.org/stable/info/30129612

The epidemiology of rotavirus diarrhea in the United States: surveillance and estimates of disease burden.

Data from 34 studies of the etiology of childhood diarrhoea were compiled in order to investigate the seasonal patterns of rotavirus gastroenteritis and consider their implications for transmission of the virus. Rotavirus was detected in 11-71% of children with diarrhoea, and the median rate of detection (33%) was independent of the level of economic development or geographical region of the study area, as well as of the method of detection used. While rotavirus infections have been called a winter disease in the temperate zones, we found that their incidence peaked in winter primarily in the Americas and that peaks in the autumn or spring are common in other parts of the world. In the tropics, the seasonality of such infections is less distinct and within 10 degrees latitude (north or south) of the equator, eight of the ten locations exhibited no seasonal trend. Throughout most of the world, rotavirus is present all the year round, which suggests that low-level transmission could maintain the chain of infection. The virus is spread by the faecal-oral route but airborne or droplet transmission has also been postulated. The epidemiology of rotavirus--its seasonality in the cooler months, its universal spread in temperate and tropical zones in developed and less developed settings--more closely resembles that of childhood viruses that are spread by the respiratory route (such as measles) than that of common enteric pathogens that are spread predominantly by the faecal-oral route.

Global seasonality of rotavirus infections.

While rotavirus infections have been called a winter disease in the temperature zones, we found that their incidence peaked in winter primarily in the Americas and that peaks in the fall or spring are common in other parts of the world. In the tropics, the seasonality of such infections is less distinct and within 10˚ latitude (north or south) of the equator, eight of the ten locations exhibited no seasonal trend

S. M. Cook

Papers

Global seasonality of rotavirus infections.

Variabilidad estacional mundial de las infecciones por rotavirus