Hygiene and sanitation have had a profound effect on the incidence of enteric infections, both viral and bacterial. Viruses that infect the intestinal tract are shed in feces, and in many human communities recycling of feces back into the mouth following fecal contamination of food or water is common. A more voluminous and more fluid output (diarrhea) increases the environmental contamination. Hands contaminated at the time of defecation and inadequately washed may transfer viruses directly or indirectly to food, which is a particular problem if it occurs among those responsible for the preparation of meals to be eaten by others, in many densely populated parts of the world there are no reticulated sewerage systems, and sewage may seep into wells, streams, or other drinking water supplies, particularly after heavy rains. Explosive outbreaks of hepatitis E, poliomyelitis, or gastroenteritis occur from time to time even in sewered areas when sewerage mains burst or overflow to contaminate drinking water supplies.
Raw sewage contains 103-106 infectious virus particles per liter (typically ÎO^IO4 per liter in Western countries), mostly enteroviruses, caliciviruses, adenoviruses, and rotaviruses. Titers drop 100-fold (typically to 10-100 pfu per liter) following treatment in a modern activated sludge plant, because virions adsorb to the solid waste which sediments as sludge. The primary sludge is generally subjected to anaerobic digestion, which reduces the titer of virus significantly. Some countries require that the treated sludge be inactivated by pasteurization prior to being discharged into rivers and lakes or being utilized as land fill or fertilizer in agriculture.
In countries where wastewater has to be recycled for drinking and other domestic purposes, the treated effluent is further treated by coagulation with alum or ferric chloride, adsorption with activated carbon, and finally chlorina-tion. Evidence that by-products of chlorination are toxic to fish and may be carcinogenic for humans has encouraged several countries to turn instead to ozonation. Ozone is a very effective oxidative disinfectant, for viruses as well as bacteria, provided that most of the organic matter, to which viruses adsorb, is removed first.
There is also a good case for chemical disinfection of recycled wastewater used for nondrinking purposes, such as agricultural irrigation by sprinklers, public fountains, and industrial cooling towers, because such procedures disseminate viruses in aerosols and contaminate vegetables. However, the lability of viruses to heat, desiccation, and ultraviolet light ensures that the virions remaining in wastewater from which most of the solids have been removed will be inactivated without intervention within a few weeks or months, depending on environmental conditions. Even during a cold northern winter, the number of viable enteroviruses in standing water drops by about 1 log per month; during a hot dry summer the rate of decay is as high as indefinitely in a state of peaceful coexistence with vertebrate and invertebrate hosts.
Disease occurs when unusual vertebrate hosts become involved. When humans live in regions where a particular arbovirus is enzootic they are vulnerable to infection, and a proportion of those infected may suffer severe, even lethal, disease. Visitors such as tourists, soldiers, or forest workers are even more vulnerable as, unlike the indigenous population, they will not have acquired immunity from subclinical infection in childhood. In tropical countries where the arthropod vector is plentiful year-round, the risk is always present and human disease is endemic (e g., jungle yellow fever). In regions subject to monsoonal rains, epidemics of mosquito-borne diseases may occur toward the end of the wet season, for example, Japanese encephalitis in parts of Southeast Asia. In some temperate countries, and particularly in arid areas, human epidemics of mosquito-borne arbovirus disease occur following periods of exceptionally heavy rain
When arthropods are active, arboviruses replicate alternately in vertebrate and invertebrate hosts. A puzzle that has concerned many investigators has been to understand what happens to the viruses during the winter months in temperate climates, when the arthropod vectors are inactive. An important mechanism for overwintering is transovarial transmission from one generation of arthropods to the next. With arthropods such as ticks that have several larval stages, this is necessarily associated with transstadial transmission, Transovarial infection occurs in most tick-borne arbovirus infections and is often sufficient to ensure survival of the virus independently of a cycle in vertebrates; as far as virus survival is concerned, vertebrate infection is only important in amplifying the population of infected ticks.
Transovarial transmission also occurs with some mosquito-borne bun-yaviruses and flaviviruses. For example, some bunyaviruses are found in high northern latitudes where the mosquito breeding season is too short to allow virus survival by horizontal transmission cycles alone; many of the first mosquitoes to emerge each summer carry virus as a result of transovarial transmission, and the pool of virus is rapidly amplified by horizontal transmission in mosquito-mammal-mosquito cycles.
Vertical transmission in arthropods may not explain all arbovirus overwintering, but other possibilities are still unproved or speculative. For example, hibernating vertebrates have been thought to play a role in overwintering. In cold climates, bats and some small rodents, as well as snakes and frogs, hibernate during the winter months. Their low body temperature has been thought to favor persistent infection, with recrudescent viremia occurring when normal body temperature returns in the spring. Although demonstrated in the laboratory, this mechanism has never been proved to occur in nature.
Examples of the complexity of the life cycles of arboviruses are given in Chapters 25, 26, and 33. Many ecologic changes produced by human activities disturb natural arbovirus life cycles and have been incriminated in the geographic spread or increased prevalence of the diseases they cause: (1) population movements and human intrusion into new arthropod habitats, notably tropical forests, (2) deforestation, with development of new forest-farmland margins and exposure of humans to new arthropods; (3) irrigation, especially primitive irrigation systems, which pay no attention to arthropod control; (4) uncontrolled urbanization, with vector populations breeding in accumulations of water and sewage; (5) increased long-distance air travel, with potential for carriage of arthropod vectors and of persons incubating diseases such as dengue and yellow fever; (6) new routing of long-distance bird migrations brought about by new man-made water impoundments.
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