You probably associate certain diseases with different times of the year. During the winter, you might get your annual cold, while during the spring, your allergies go haywire. Perhaps the cold months of winter make your arthritis worse or the summer months exacerbate your MS. There are many conditions impacted by the changing seasons, including autoimmune diseases, cardiovascular events, acute gout, type 1 diabetes, type 2 diabetes, hip fractures, mental health disorders, migraines, and emergency surgery. Even mortality is affected by the seasons, with summer and winter months associated with higher number of deaths.
There are many reasons for the seasonal variance in health and disease, and they most likely work in tandem to influence an individual's susceptibility to disease during certain times of the year. Let's look closely at some of the more influential factors.
Infectious Diseases
Infectious diseases are well-known to follow a trend correlated with the changing weather. Several viruses have a seasonal pattern, including human rhinovirus (the culprit behind the common cold), respiratory syncytial virus (a virus with symptoms similar to a cold), and influenza. Another disease that has seasonal variance is Staphylococcal aureus, one of the more common infections patients can get in hospitals that is also associated with food poisoning, skin disorders, pneumonia, meningitis, and sinusitis. In a systematic review reviewing studies that looked at the seasonality of S. aureus, the researchers found that there was evidence pointing to seasonal variance, with more infections occurring in the summer and fall, especially hospital-acquired MRSA.
The seasonal changes of infectious diseases do not just impact the number of people who contract an acute illness, deal with long-term disability directly linked to the disease, or die from complications during certain months of the year; it also can impact long-term health. Additionally, the seasonal outbreak of certain viruses might lead to a similar association between seasons and chronic illnesses, thanks to the infections acting as a trigger.
Although genetic predisposition plays an important role, many autoimmune disorders are triggered by infections. Several pathogens have a link to autoimmune disorders, such as the connection between the Epstein-Barr virus and lupus. In some cases, the autoimmunity arises due to something known as molecular mimicry, in which the proteins of the virus are similar to that of human cells or tissue. The antibodies created to target the virus or bacteria end up mistaking the self as the enemy and attack, leading to an autoimmune condition. The inflammation and immune response might also lead to other responses that lead to self-antibodies, triggering an autoimmune disorder.
There might also be a link between infections and other chronic illnesses, including diabetes, cancer, Alzheimer's disease, and cardiovascular disease. Infection can lead to inflammation and other immune responses that trigger imbalance leading to a chronic disease. Several cancers are linked to communicable diseases as well, the most well-known being the link between cervical cancer and HPV (human papillomavirus). H. pylori and hepatitis B and C viruses also are known to cause cancer. Emerging evidence links Chlamydia pneumonia to cardiovascular diseases, and periodontitis infections are linked to stroke and atherosclerosis. Acute diarrhea might lead to gastrointestinal conditions while getting the flu while in the womb might increase one's chance of psychiatric conditions. Even obesity has some links to infection, with a few studies finding a link between adenovirus 36, a virus that causes tonsillitis, upper respiratory infections, and other acute illnesses, and obesity.
Genetic Expression
Another reason for seasonal variance in your health might be due to changes in gene expression at certain times of the year. Your DNA codes for all processes in the body, and what proteins are synthesized determines on whether the gene is expressed or not. Although your actual genes play a role in disease susceptibility, the expression of the genes are what really matters. Your environment, including things like diet, exercise, and the seasons, affect the expression of the genes. In one study, the researchers found 94 DNA transcripts with significant seasonal variability. Many of these play a role in protein production, immune function, and lymphocyte markers, including coding for immune cells in the blood such as neutrophils and monocytes. This seasonality variation in gene expression impacts regulatory functions, including on the immune system. During the seasons in which the gene expressions for immune cells are down-regulated, it might lead to a higher susceptibility to disease.
Another study found that 23 percent of the human genome had some type of significant difference in the expressions in a seasonal pattern. The researchers used data from several different locations in different latitudes, including from Germany, the UK, Australia, the U.S., Ghana, and Iceland. There were two distinct patterns: 2,311 genes were upregulated in the summer while 2,826 genes were upregulated during the winter. Based on the findings, it appears that the immune system is more pro-inflammatory during the winter months for those living in temporal climates such as the UK. This genetic variance corresponded with the summer and winter months of the population, meaning that those in the southern hemisphere demonstrated summer upregulation during their summer months. Iceland had a unique set of gene expression, most likely due to its proximity to the poles and its almost 24-hour of sunlight during the summer months and near darkness during the winter. Ghana, representing a more equatorial location, also had its own distinct gene expression. Rather than changes in the gene expression corresponding to temperature patterns, it had changes corresponding to the rainy season, which is known for its increase in pathogens, similar to the winter months in other latitudes.
These studies point to something happening on a genetic level to impact the immune system from season to season, and one that is potentially correlated to the climate in which a person lives.
Vitamin D Levels
With vitamin D's status as the "sunshine vitamin," since the body synthesizes it from UV rays from the sun absorbed in the skin, it should come as no surprise that there is a seasonal variance in vitamin D status, with winter and early spring months having the lowest levels and summer months having the highest levels. Vitamin D deficiency is associated with an increased risk of contracting infections diseases, several autoimmune conditions, and other illnesses. There are also connections between vitamin D deficiency and metabolic syndrome. In addition to its role in immunity and chronic illnesses, vitamin D might also increase the risk of having a stroke. According to one study, 77 percent of patients who had an acute stroke were deficient in vitamin D.
A possible reason behind this is vitamin D's important role in immunity and health as a key regulator of both the adaptive and innate immune systems. Vitamin D assists in fighting pathogens through regulating the CAMP (human cathelicidin antimicrobial peptide) gene, which is found in several key cells to the immune response, including lymphocytes, dendritic cells, monocytes, natural killer cells, and epithelial cells in the gastrointestinal tract and respiratory tract. Vitamin D also supports innate immune responses through inducing reactive oxygen species to facilitate antibacterial autophagy. Additionally, vitamin D suppresses the immune responses by the Th1 cells, which can lead to inflammation and are associated with many of the autoimmune conditions. Thus, the seasonal changes of vitamin D status could play a role in the efficacy of the immune system, putting a person more at risk of disease when vitamin D levels are lower. The seasonality of vitamin D status affects its ability to regulate immunity, leading one susceptible to diseases.
Additionally, vitamin D plays a role in bone health thanks to regulating calcium levels in the body. During winter, not only is there a decrease in vitamin D levels, but there is also an increase in serum parathyroid hormone levels and an increase of bone reabsorption, which weaken the bones. This is associated with a higher risk of falling, as well as a higher risk of hip and wrist fractures, based on a study of ambulatory women in Australia. Therefore, lower serum levels of the vitamin during the winter months lead to a higher susceptibility to fractures. Although having a fracture can lead to problematic consequences for anyone, for the elderly, a population already at risk of vitamin D deficiency and broken bones, it can be a death sentence. There is a high mortality among those who have a hip fracture, typically due to complications of the fracture. One study found that the postoperative mortality at one year was 27.3 percent and at the end of the 9-year follow-up period, it was 79 percent. After adjusting for age, men had a higher risk with a hazard ratio of 1.55 (95% CI: 1.21-2.00). These findings point to a three-fold higher risk of death in patients with hip fractures than the general population.
Due to the important roles vitamin D plays in the body, monitoring vitamin D status has become routine. However, this should not be done on an annual basis; the seasonal variance should be taken into account to ensure sufficient levels during the winter and spring as well as the summer and fall months.
Melatonin
Melatonin plays several key functions in the body, including regulating the immune system and glucose, as well as providing antioxidant defenses against excessive oxidative stress. It is well known for being a major regulator of circadian rhythms, including the sleep-wake cycle. As such, it is highly impacted by the length of days and nights, so it has a seasonal variance as well, with higher serum levels during the winter compared to summer.
Alterations to the level of melatonin levels could impact susceptibility to a range of diseases, including autoimmune disorders, Alzheimer's disease and other neurodegenerative disorders, gastrointestinal imbalances, and bone loss. Melatonin also has cardioprotective properties; it is anti-inflammatory, acts as an antioxidant, and is anti-hypertensive. Disruption in melatonin production and the subsequent circadian rhythms has been implicated in several chronic illnesses, including obesity and metabolic syndrome. Melatonin also has a role in innate immunity and might act as a buffer, which means that it stimulates the immune system when needed or provides anti-inflammatory responses when the immune system is already overly stimulated.
With the seasonal variance of melatonin, many of the health benefits of the hormone could be affected, leaving one more susceptible during certain times of the year. For example, the seasonality associated with multiple sclerosis has been linked to the changes in melatonin levels rather than solely environmental factors. According to one study, there is a negative correlation between the activity of MS and night length, most likely due to interruptions in the expression of T-cells.
Temperature
Extremely cold and hot weather both have the potential to negatively impact the health of a person, especially in the elderly, children, and vulnerable populations. The impact of the weather and ambient temperature depends on the climate in which a person lives. However, in a multi-country study reviewing data that spanned around a decade from a total of 384 locations in thirteen countries with diverse climates, including Australia, Canada, China, Italy, Spain, Sweden, UK, and the USA, the researchers found that ambient temperature might play a role in mortality risk. Cold temperatures have the greatest impact on mortality, but temperature was related to 7.71 percent of the mortality. In this study, extreme temperatures only accounted for a very low number of deaths. This demonstrates that the effect of temperature on mortality has more to do than simply the effects of heat stroke or hypothermia.
This and similar studies focus on the association between ambient temperature and mortality; however, temperature can also affect other health conditions, apart from temperature-specific illnesses such as heat stroke or hypothermia. For example, heat is well-known for exacerbating MS. Temperature also impacts the environmental factors leading to an increase -- or decrease -- in pathogenic bacteria and viruses spreading. Therefore, temperature has the potential to act as a trigger for a wide range of acute and chronic illnesses, leading to a seasonal pattern as the temperature changes throughout the year.
The mechanisms behind the seasonal variance in human diseases are complex. There are several factors involved beyond the above, including dietary changes and pollution levels. Some of these can be modulated to reduce the impact of seasonality on your health, such as ensuring you have sufficient vitamin D and melatonin levels throughout the year. Recognizing the ways in which the changing seasons affect you -- and looking at possible underlying reasons for it -- might help you find ways in which to mitigate the impact.