Infectious diseases cause external causes— bacteria, viruses, parasites or fungi. Many of them are characterized by seasonality — outbreaks happen at the same time of the year. For example, in the Northern hemisphere the flu comes every winter (some epidemiologists call the winter “flu season”), and outbreaks of chicken pox often happen in the spring.
Noncommunicable diseases cause all other causesfrom genetic issues to injury — they are not contagious. Such diseases can be massive, but the season they are strictly independent. For example, from cardiovascular diseases annually die of 17.9 million people, well-defined peaks in a particular season they have.
What affects weather
Infectious diseases can be compared with each other by three parameters, which depend on the weather:
Survivability of the pathogen. Cholera — Vibrio cholerae — able for months to survive in stagnant water, and of an influenza virus particle hitting, for example, on the banknotes, keep the infectiousness only one to three days. Although after this period, the virus particles off bills will not disappear, during this time, they have become worthless the capsid (the viral shell), and the virus can infect anyone.
Climatic factors: temperature, humidity, amount of sunlight, and not climate, pH and salinity of the water can prolong the life of pathogens, and to accelerate their death. For example, on the stability of influenza virus is affected by temperature and humidity. In countries with temperate climates, the virus survives better in winter and the spring losing ground. In tropical climates the seasonality of influenza outbreaks has been observed.
On the survival of Vibrio cholerae in water is affected and its pH, and salinity. Best of all the bacteria feels at alkaline pH 8.5 and a salinity of 15 percent. If the water becomes more acidic and less salty — for example, because the life of any algae or torrential rains — the Vibrio dies faster, and the chance to infect someone it has less.
Infectious, i.e. contagious. Estimating the rate of spread of the disease, epidemiologists use the metric R0 is the average number of people who can catch the disease from a sick person. Measles, for example, is highly contagious: one patient infect 12 to 18 people. The flu is ten times weaker, its R0 — 1,4-1,6.
R0 depends strongly on the behavior of people, which differs from season to season: why some illnesses are “seasonal” in this sense. For example, in the fall and winter the students a lot of time to spend in one room, and the number of cases of measles increases dramatically. And in the summer are leaving on vacation, no longer closely in contact, and outbreaks of this disease are reported less frequently.
Elena Burtseva, head of the laboratory of etiology and epidemiology of influenza Institute of Virology NIZAM of Gamaliel, said, in conversation with N+1that the rise in the incidence of many SARS is also associated with purely social factors: ends the holiday period, kids return to school. Therefore, the increase in the incidence of SARS from year to year from mid-September to early October.
A second human factor that theoretically may influence outbreaks — particularly of the human immune system, depending on the time of year. For example, with the onset of cold weather, we spend less and less time outdoors and wear clothes that cover the body. As a result, the skin gets less UV light, and in the body decreases vitamin D synthesis, which plays an important role in the protection against bacterial and viral infections. (However, there is empirical evidence that people who take this vitamin in pills, sick with the flu no less of those vitamins, not drinking).
The transfer method is part of the disease is transmitted directly and part indirectly. Directly from the source is passed influenza and SARS, which spreads from an infected person to a healthy one.
Indirectly transmitted West Nile virus, which “travels” from person to person in the stomach of the mosquito, African sleeping sickness, which carries a fly TSE-TSE. The latter actively reproduces during the rainy season, and also live three to five months, as opposed to one or two in the dry season: that is why in this time of year the flies becomes more and they are more likely to bite people — here is the outbreak of sleeping sickness. The same applies to tick-borne encephalitis, says Burtsev: ticks in the early spring, and in the spring, record the peak of the disease. The second wave registered in the autumn time and this is due to the life cycle of ticks.
Pandemic coronavirus disease (COVID-19) in some of its manifestations are very similar to the known respiratory disease, so many researchers use models of outbreaks of SARS or influenza to predict outbreaks COVID-19.
Coronavirus disease came to us in winter. Before you wonder about whether or not going to wait its end in the summer and a possible return after six months, it makes sense to understand the factors that turn a usual flu and ORVI in seasonal illness.
The fact of the seasonality of colds was obvious to people since ancient times, but to explain it is not so simple. For example, the Roman lucrezias assumedthat “pestilence and plague” are the atoms of disease which appear when the ground is saturated with moisture. And his compatriot Galen directly attributed to outbreaks of various diseases seasonal characteristics: excessive heat, dryness or cold. Today, we know that closer to the truth was of Lucrezia: it is not cold, and humidity.
It managed to show in a laboratory experiment on Guinea pigs.Four infected with influenza and four healthy pigs were kept in chambers, where the changed temperature and humidity: the rate of transmission increased as to reduce them. Best of all the virus was transmitted at a temperature of 5 degrees than at 20 degrees and 30 degrees. At five degrees of heat transmission frequency was 100 per cent relative humidity 20 per cent and 35 per cent, 75 per cent at a relative humidity of 65 percent, but only 25 percent at a relative humidity of 50 percent, and 0 percent relative humidity of 80 percent.
After a few years other authors have analyzed the same data and adjusted the findings. They decided to evaluate to evaluate the influence of absolute humidity and not relative. After conversion and new experiments original conclusion was confirmed, but with the difference that the virus is more dependent on humidity than temperature.
The flu virus was transmitted from pig to pig by droplet infection: when you exhale the sick pigs in the air hit the droplets of water vapor Laden with viral particles. Once free, the droplets gradually settle and evaporate. The faster they evaporate, the more slowly deposited, and the longer the virus hanging in the air. The evaporation rate of droplets depends on the humidity — more than a couple of, the slower it evaporates — and then they are deposited in a saturated moisture to the air faster, “taking” a virions. And as with the temperature, the humidity drops, then the winter, when cold and dry promotes maximum spread of viruses.
In the first study evaluated the impact on the transmission of viral particles, only the relative humidity — this parameter expresses the fraction of water vapor in relation to its maximum at a specific temperature. While at 20 degrees, this maximum is higher than 5 degrees.
There is also the second factor is purely human. When people breathe the dry air, my nose dries up mucus, moistening the respiratory tract and physically check all solid, including viral particles. Properties of mucus associated with a special polymer macromolecules — the Mucins, which not only give mucus viscosity, but also play an important role in the immune response. They form a special framethat allows you to optimally organize the space of proteins that epithelial cells secrete mucous. For example, the glycoprotein lactoferrin, which can neutralize many viruses, including the flu virus.
A dry nose leads to several problems. First, devoid of moisture, the easier the epithelium is damaged, so that the viral particles easier to enter the cells. Secondly, impaired spatial organization of mucin, lactoferrin and related proteins lose their protective properties, and resistance to the virus is reduced.
In addition to humidity, there is another important factor, because of which the possibility of the outbreak of influenza or SARS in the winter higher than in summer — behavior of people. Speaking in favor of this data on the spread of influenza in schools. Autumn and winter when students spend a lot of time in class, actively communicating with each other, outbreaks of flu and colds occur more often than in the summer when students are not in school and have less interaction with each other. The more susceptible people gathers in one place, the faster and more efficiently spreads the disease.
The annual match
Seasonal epidemics occurwhen a population in which many are deprived of immunity of people (e.g. tourists or newborn) faces seasonal “helper” of the disease — in the case of influenza is low winter humidity.
It looks like this. At the beginning of the epidemic — that is, in the fall — most people have no immunity to virus diseases, so every patient infects more than one person (R0>1).
Then begins to grow, the proportion immune to the virus of people — because I had been ill occurs the immune system (or, for example, used vaccine). People get less and less, and after some time, the epidemic reaches its peak (R0=1).
With the arrival of spring, in addition, the air is humidified, so the conditions for the proliferation of virus particles ceases to be optimal: the protective barrier of mucus most people have recovered, the number of vulnerable people falls even more — and the epidemic is extinguished (R0<1).