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Why Are There More E. coli Infections in Summer?

The number of E. coli O157:H7 infections increases during the summer months in Great Britain and elsewhere, and a paper published this year in the British journal “Environmental Microbiology”  examined the links between that summertime spike, the physiology of infection, and geography.

The paper reviewed about 100 published works related to this concept to develop a list of possible causes for the seasonality of E. coli O157:H7 infections.

E. coli O157:H7, like all living organisms, prefers certain environments, the authors note.  The bacteria tend to inhabit the intestinal tracts of ruminants, such as cattle and sheep, but have also been found in animals ranging from pigeons to horses.  Cattle and other ruminants host the bacteria without becoming sick themselves, but can transmit the disease to others by contaminating food, water or soils.

Multiple studies have indicated E. coli O157:H7 can both survive and remain able to infect after long periods in both water and soil.  One study indicated the bacteria could live up to 245 days in a cattle water trough and remain viable.  In addition, E. coli can live up to 15 weeks in soil and stay infective.  It may infect microscopic, protozoan inhabitants of the soil and use them as hosts during this time.

While E. coli O157:H7 seems capable of thriving year ’round, the review compiles data to show why infection trends show a propensity for infecting humans during the summer months.

A primary reason for this appears to be an increased number of bacteria shed in fecal matter during the summer months.  Because only 100 bacteria can lead to an infection, the increased number leaving animal reservoirs could be a leading cause in the increased number of infections, although why there is increased fecal shedding of the bacteria remains elusive.

Some studies have found that it might relate to the host’s level of the hormone melatonin.  The production of this hormone is related to the length of days, with low amounts produced during the summer and high amounts in the winter.  Melatonin has also been linked to seasonal adjustments in immune function.

While this review examined only the geographic differences in infections in Great Britain, the information may provide clues to the geography of E. coli O157:H7 on a larger scale as well.

Studies have shown that areas with higher average rainfalls also had more infections, largely due to contamination of fields and drinking water by the feces of infected animals.

Human behavioral shifts may also account for some of the increase in the risk of infection–in the summertime people tend to spend more time cooking and eating outdoors with sometimes  less than adequate facilities for proper sanitation.  

Multiple factors lead to seasonal trends in illness.  For E. coli O157:H7, the behavior of the bacteria, host, human and environment lead to an increase of incidence in the British summer.  The areas with more cattle and higher rainfall see more cases, but just as importantly all places have a greater incidence of E. coli O157:H7 infections in the summer.  This points to specific behavioral actions, such as camping on contaminated fields and cooking without running water, as being important factors leading to summertime illnesses.

Source: Money, P, A.F. Kelly, S.W. Gould, J. Denholm-Price, E.J. Threlfall, M.D. Fielder. “Cattle, weather and water: mapping Escherichia coli O157:H7 infections in humans in England and Scotland.” Environmental Epidemiology 12.10 (2010) 2633-2644

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