Seasonal Health

Yesterday I was on a stretch of highway on which I frequently drive, and noticed a lot of road kill, much more than normal, it seemed.  I began to wonder if there might be an explanation.  Is there something about mid-fall that might bring more animals out crossing roads?  Are animals more restless due to a change in mating behavior, or has the food supply changed, requiring them to range farther to find enough to eat?  Maybe animals are slower moving in cold weather, making it harder for them to avoid a vehicle crossing their paths.  Of course it could be that the night weather was foggy, cutting down on driver visibility enough to put more animals at risk.  At least one more possibility occurring to me was that there may have been more traffic on the road, either because of some event occurring in the area, or perhaps there was something happening in trade and commerce, such as trucks delivering goods as stores gear up for holiday sales.  What we need to answer these questions is a veterninary epidemiologist.

I don't know if there is such a field, but human epidemiologists have taught us the factors related to human injury and deaths on our roads, and there are seasonal patterns.  You will expect to see more highway accidents in winter months, because road conditions tend to be worse.  On the other hand, the rate of bicycle and motorcycle accidents goes up in the summer months because there are more riders in warm weather, fewer in the winter.  One study found more child pedestrian accidents in summer, perhaps because children are more likely to be out in warm weather, but more adult pedestrian injuries, in winter.  This last observation is harder to explain.

There are seasonal variations with many other health concerns.  For example, colds and flu spike in the late fall and winter months, but generally are lowest in incidence in the summer months. The explanation is that as weather turns colder, indoor air quality declines.  People spend more time indoors in close quarters with others.  Heating systems dry out skin and mucous membrane barriers, making people more vulnerable to those types of infections.

Violent death and injury increase in the summer, presumably because heat is a stressor, and people are involved in more social interactions for potential conflict.  In the winter people tend to cocoon more, with less social contact.  With youth, summer represents more unsupervised time, providing opportunities for trouble.

Allergies usually spike in the spring and summer, because of the plant constituents in the air, and people spending more time out-of-doors.

In far northern or southern regions that experience extremes of frigid conditions, we expect to see seasonal variations in cold-related illness and injury.  Likewise, there are seasonal variations in heat stroke and exhaustion.

Cardiovascular disease also increases in the winter, with several reasons suggested.  Smoking and secondhand smoke exposure increase in winter months.  People are more sedentary, and body weight tends to increase in the winter.  In addition, it is proposed that because of reduced sun exposure, Vitamin D levels drop, having an impact on cardiovascular system dynamics, including an increased cholesterol level.  Finally,  cold temperatures impact the components of blood, particularly the tendency of blood to clot.

As is apparent from the various types of seasonal patterns in health, causes are a combination of biology, ecology, social organization and behaviors related to annual events and cycles.  As far as we know, wild animals are oblivious to these changing risks, and are incapable of taking timely precautions.  Humans on the other hand can learn from these patterns, applying appropriate steps to avoid things putting them at higher risk.  While there are seasonal variations that effect us in this way, there should not be seasonal variation in intelligence and thoughtfulness about it.