Seasonal infections of humans range from childhood diseases, such as measles, diphtheria and chickenpox, to faecal–oral infections, such as cholera and rotavirus, vector-borne diseases including malaria and even sexually transmitted gonorrhoea. Despite the near ubiquity of this phenomenon, the causes and consequences of seasonal patterns of incidence are poorly understood.
Some of these include;
Foodborne Disease
Technology has provided the means for mass production and distribution of food. Therefore, foodborne disease often occurs on a massive scale, whereby hundreds or thousands are exposed and may become ill. Mobility and travel have resulted in exposure to foods abroad, where regulation of food safety and food products for sale may vary. When traveling, the axiom “boil it, peel it, cook it, or forget it” remains true in many areas of the world. Travelers bringing home unique foods as gifts may unwittingly expose family members and friends to unexpected illness. International ships discharging their bilge in ports are another possible means of disseminating pathogens.Food importation has steadily increased to meet the demand for seasonal and nonseasonal foods. Conditions of production and harvest may be unsupervised or uncontrolled, with resultant importation of contaminated foods. Raw manure is frequently used as fertilizer, causing contamination of fresh produce. If improperly cleaned, the fertilized produce may cause illness when consumed.
Unique ethnic food preferences and preparation have been associated with several food-related illnesses. One example is the African American tradition of eating chitterlings (cooked swine intestines) during the Christmas holiday season. This food has been associated with an outbreak of Yersinia enterocolitica infection in infants. Fresh cheese made from unpasteurized milk has been associated with episodes of listeriosis in Hispanic neighborhoods.
Foodborne disease is more likely to affect the extremes of age as well as immunocompromised patients and pregnant women. These groups suffer higher incidence, morbidity, and mortality. The effect of foodborne disease may extend beyond the immediate illness. This has been shown by a Danish study, which demonstrated a greater than threefold risk of dying in the year after contracting a foodborne illness.
Most foodborne disease has a short duration of illness and a self-limited course. Others may cause a more protracted illness, such as Cryptosporidium and Cyclospora. However, some foodborne diseases are associated with long-term chronic sequelae. Salmonella, Shigella, Yersinia, and Campylobacter spp. are linked to reactive arthritis; Campylobacter has also been associated with the Guillain-Barré syndrome, and STEC O157:H7 has been linked to renal failure.
The most commonly identified pathogens are Campylobacter spp., Salmonella spp., Shigella spp., and STEC O157:H7. These organisms have evolved and now have greater cold, heat, and acid tolerance, as well as resistance to multiple antibiotics. Increased drug resistance has been associated with prolonged illness and a greater risk of hospitalization.
Almost any food can be a source of foodborne disease. Some foods are more commonly associated with particular organisms. Salmonella has traditionally been associated with poultry and eggs, Campylobacter with chicken and unpasteurized milk, and STEC O157:H7 with ground beef. An outbreak of STEC O157:H7 was associated with steak that had been needle-tenderized, thereby exposing the center of the meat to surface organisms. When the steak was not thoroughly cooked to an adequate internal temperature, the microorganisms survived and illness occurred after consumption.
Water may be the vector of illness when contaminated with viruses, bacteria, parasites, or chemicals. Crowding, poor sanitation, disruption of water supplies, and natural disasters are closely linked to waterborne illness. Viruses are the most common cause of waterborne illness and include rotaviruses, enteric adenovirus, astrovirus, caliciviruses and hepatitis A virus. Outbreaks of gastroenteritis aboard cruise ships in recent years were a result of noroviruses. Salmonella spp., Shigella spp., E. coli, and Vibrio spp. are the predominant bacterial pathogens involved. Cryptosporidium spp. and Giardia lamblia are the parasitic pathogens most commonly encountered in water-borne illness. Immunocompromised hosts, particularly organ transplant recipients and HIV-infected patients, should exercise extra precaution in situations of potential waterborne illness.
Influenza
Influenza reaches peak prevalence in winter, and because the Northern and Southern Hemispheres have winter at different times of the year, there are actually two different flu seasons each year. This is why the World Health Organization (assisted by the National Influenza Centers) makes recommendations for two different vaccine formulations every year; one for the Northern, and one for the Southern Hemisphere.A long-standing puzzle has been why outbreaks of the flu occur seasonally rather than uniformly throughout the year. One possible explanation is that, because people are indoors more often during the winter, they are in close contact more often, and this promotes transmission from person to person. Increased travel due to the Northern Hemisphere winter holiday season may also play a role. Another factor is that cold temperatures lead to drier air, which may dehydrate mucus, preventing the body from effectively expelling virus particles. The virus also survives longer on surfaces at colder temperatures and aerosol transmission of the virus is highest in cold environments (less than 5 °C) with low relative humidity. Indeed, the lower air humidity in winter seems to be the main cause of seasonal influenza transmission in temperate regions.
However, seasonal changes in infection rates also occur in tropical regions, and in some countries these peaks of infection are seen mainly during the rainy season. Seasonal changes in contact rates from school terms, which are a major factor in other childhood diseases such as measles and pertussis, may also play a role in the flu. A combination of these small seasonal effects may be amplified by dynamical resonance with the endogenous disease cycles. H5N1 exhibits seasonality in both humans and birds.
An alternative hypothesis to explain seasonality in influenza infections is an effect of vitamin D levels on immunity to the virus.[146] This idea was first proposed by Robert Edgar Hope-Simpson in 1965.[147] He proposed that the cause of influenza epidemics during winter may be connected to seasonal fluctuations of vitamin D, which is produced in the skin under the influence of solar (or artificial) UV radiation. This could explain why influenza occurs mostly in winter and during the tropical rainy season, when people stay indoors, away from the sun, and their vitamin D levels fall.
Yersinia
Yersiniosis is an uncommon cause of foodborne disease in the United States. It has been associated with the consumption of raw pork, unpasteurized milk and dairy products, and contaminated water. The preparation, handling, or consumption of chitterlings (see earlier) has been associated with Yersinia infections. Young African American children are disproportionately affected by this holiday food tradition.The incubation period is 4 to 7 days and is followed by fever, abdominal pain, and bloody diarrhea. The abdominal pain may mimic that of appendicitis. Additional manifestations include carditis, joint pain, and sepsis. Although most cases resolve spontaneously, severe cases may require therapy with doxycycline, a fluoroquinolone, aminoglycoside, or trimethoprim-sulfamethoxazole.
Source: Wikipedia Cleveland Clinic Hethcote & Yorke 1984
