Human Health and Global Warming

Earth climate warming and globalization have had a direct impact on human health. Infections involve pathogens, parasites, hosts, and the environment. Some pathogens are carried by vectors or require intermediate hosts to complete their life cycle. The climate can influence pathogens, vectors, host defenses, and habitat. A range of vector-borne diseases are geographically and temporarily limited by variations in climatic variables, such as temperature, humidity, and rainfall. The direct impact of the climate on infectious diseases can occur in three principal ways: effects on human behavior that can have a significant impact on disease transmission patterns; direct effects on disease pathogens; and effects on disease vectors.

For infectious diseases caused by pathogens that have part of their life in the environment or in an intermediate host or vector, climatic factors can have a direct impact on the developmental rate. Most viruses, bacteria, and parasites do not complete their life cycles if the temperature is below a certain threshold (e.g., 18°C for the malaria pathogen Plasmodium falciparum). Increases in ambient temperature above this threshold shorten the time needed for the development of the pathogen and thus increase reproduction rates, whereas temperatures in excess of the tolerance range of the pathogen may increase mortality rates.

The geographical distribution and population dynamics of insect vectors are closely related to patterns of the temperature, rainfall, and humidity. A rise in temperature accelerates the metabolic rate of insects and increases egg production, along with the frequency of blood feeds. Rainfall establishes the relatively wet conditions that create favorable insect habitats and in this way increases the geographical distribution and seasonal abundance of disease vectors.

Diseases carried by mosquito vectors are particularly sensitive to meteorological conditions. Excessive heat kills mosquitoes; but within their survivable range, warmer temperatures increase their reproduction rate and biting activity and the rate at which pathogens mature within them. Freezing kills Aedes eggs, larvae, and adults, whereas warm winters favor insect survival; thus, expanding tropical conditions can therefore enlarge the ranges and extend the season, with conditions allowing the transmission of pathogens. For some disease vectors, an evident northward shift and population increase related to global warming was demonstrated and simulated with statistical models for ticks belonging to Ixodes genus in Neartic (e.g., Ixodes scapularis) and Palaearctic (e.g., Ixodes ricinus) regions (Brownstein et al. 2005; Lindgren and Gustafson 2001). These species, vectors of several pathogens (e.g., Lyme borreliosis, human babesiosis, anaplasmosis) could become a public health challenge for states in the Northern Hemisphere, such as Canada and the Scandinavian countries.

0 0

Post a comment