Disease is influenced by environment and can be precipitated by the environment. Obvious cases include yellow fever due to Aedes aegypti and malaria associated with various Anopheles spp. mosquito. Both diseases are only endemic within the natural range of the mosquito species that can transmit them. Location, soil and water are among the factors that may change morbidity and mortality patterns. Thus Central China has problems with goitre due to iodine deficiency and Keshan disease due to viral infection in the context of selenium deficiency. The wasting disease in all livestock due to combined selenium and cobalt deficiencies of the central volcanic plateau in the North Island of New Zealand had human consequencies too.
In fact most diseases have geographical correlations that can lead to hypothesis generation. More than one environmental toxin has been discovered in this way. Sometimes disease distribution can be difficult to explain on first principles, such as the higher incidence of osteoporotic fracture in the Mid West of the USA. Others such as Multiple sclerosis which is more common far away from the equator, have been postulated to be due to HLA subtypes of the respective human populations or conifer distribution. The p53 gene which protects us from many cancers by triggering apoptosis in runaway cells requires Selenium which as mentioned is more common in some parts of the world than others and so may be correlated with cancer incidence.
- ↑ BMJ 2005 330 1017
- World maps