13. Soil as a Transmitter of Human Pathogens

Soils are the habitat of about 25% of the Earth’s species. The majority of these organisms are not of any threat to human health, but rather function to provide numerous ecosystem services which emerge through the multitude of complex interactions between the organisms and the soil itself. These ecosystem services range from those which are vital for maintaining life on Earth, such as the formation of soil, the cycling of carbon and nutrients with the result of maintaining the global cycles of C and N, soil fertility, the filtering of water, as well as provision of useful compounds such as antibiotics, the majority of which have been isolated from soil organisms (Chap. 12). However, soils also contain microorganisms which are capable of causing diseases in humans. A wide variety of soil-related infections need to be considered, particularly in the case of wound, respiratory tract, or gastrointestinal infections. Soil-borne microbes that are pathogenic for humans include protozoa, fungi, bacteria, and also viruses which require a host for their survival. Over 400 genera of bacteria have been identified with possibly as many as 10,000 species and, with the exception of viruses, they are in most cases more abundant than any other organism in soils. Helminths, belonging to the mesofauna size class, are also important as human pathogens. The number of bacteria that can be cultured in the laboratory is probably less than 1%. Their actual diversity is thus probably much greater. Of the approximately 100,000 species of fungi currently recognized, only about 300 may cause human disease.

Soil organism may potentially enter surface water and groundwater via the soil. Thus soil is often the origin of water-borne infections. However, as soils mostly provide an effective barrier against pathogens reaching the groundwater, and the die-off time of most pathogens in the sub-surface is short, the number of viable organisms reaching groundwater maybe normally low. Most cases of waterborne disease from groundwater consumption are caused by viruses and bacteria, as protozoa and helminths are too large to be transmitted far through the soil pore system. However, some occurrences of groundwater contamination by the protozoa Giardia and Cryptosporidium have been recorded. Most cases of waterborne disease from wells where there is a thick soil cover are due to the faulty construction of head works, or to the use of manure near sinkholes, abandoned wells or other features allowing surface water and contaminated material direct access to the groundwater. The main acute disease risk associated with drinking water in developing and transition countries is due to well-known viruses, bacteria, and protozoa, which spread via the fecal-oral route.

Several soil-borne diseases are capable of transmission to the air (e.g. Q fever, aspergillosis, tularemia, sporotrichosis) and may be then transported by dust (Chap. 3). These diseases are therefore likely to be those most directly affected by land management practices and land use change. Any activity which is associated with increased wind erosion seems likely to increase the incidence of such diseases in the surrounding area. Related activities could include land use change, for example by converting grassland into arable land, ploughing or tilling soil that is too dry, etc. These conditions may also increasingly occur under climate change, as that can lead to enhanced incidence of drought periods.