Survival

Agents survive by finding a suitable host within a certain period of time. They have been able to improve their chances of finding a new host or prolonging this period by a number of different methods.

Reservoirs and parasite adaptability A reservoir is a storage place for water, but serves as an appropriate term to describe a suitable place for storing agents of infection. Reser voirs are, therefore, the final host if several intermediaries are used.

The relationship between the parasite and the host is one of continual challenge, or what has been termed a 'biological arms race'. When the parasite first attacks a new species, the host attempts to eliminate it, resulting in a severe reaction. In the course of time, adaptation can occur so that the reaction of the host diminishes and the adaptability of the parasite increases. The parasite is able to live in the host with few ill effects (e.g. Trichuris trichiura), forming an established population, continuing with minimal reaction from the host. The host then acts as a reservoir from which parasites attack new hosts of the same species or attempt to colonize different species. Reservoirs can be humans, animals, vectors or the inanimate environment (e.g. soil, water). However, it is always in the parasite's interest to improve its reproductive capability. If a new mutation arises, which is beneficial to this end, then the mutation will be selected, generally to the host's disadvantage so that virulence can increase as well as decrease.

The adaptability of parasites to their human hosts might even have advantages for us. Ascaris, Trichuris and the hookworms secrete substances to reduce the host immune response, which inadvertently are absorbed by the gut lining and help reduce allergy such as that due to hay fever. Our more hygienic surroundings, by decreasing these parasites, may be responsible for the increase in allergic diseases such as asthma in the developed countries. It is a strange irony to actually introduce these parasites to combat allergic reactions.

Persistence Another mechanism used by parasites to survive is the development of special stages that resist destruction in an adverse environment. Examples are the cysts of protozoa, e.g. Entamoeba histolytica and the eggs of nematodes, e.g. Ascaris. Bacteria can persist in the environment by the development of spores as with anthrax and tetanus bacilli (Fig. 1.2).

Latency A developmental stage in the environment that is not infective to a new host is called latency. This allows the parasite time for suitable conditions to develop before changing into the infective form. Ascaris, the hookworms and Strongy-loides exhibit latency.

Fig. 1.2. Persistence of pathogens in excreta. The lines represent conservative upper boundaries for pathogen death - that is, estimates of the time - temperature combinations required for pathogen inactivation. Organisms can survive for long periods at low temperatures, so a composting process must be maintained at a temperature above 43°C for at least a month to effectively kill all pathogens likely to be found in human excreta. From Feachem, R.G., Bradley, D.J., Garelick, H. and Mara D.D. (1983) Sanitation and Disease: Health Aspects of Excreta and Wastewater Management. World Bank, Washington, DC, p. 79. Reprinted by permission of John Wiley & Sons Ltd.

Time (hours)

Fig. 1.2. Persistence of pathogens in excreta. The lines represent conservative upper boundaries for pathogen death - that is, estimates of the time - temperature combinations required for pathogen inactivation. Organisms can survive for long periods at low temperatures, so a composting process must be maintained at a temperature above 43°C for at least a month to effectively kill all pathogens likely to be found in human excreta. From Feachem, R.G., Bradley, D.J., Garelick, H. and Mara D.D. (1983) Sanitation and Disease: Health Aspects of Excreta and Wastewater Management. World Bank, Washington, DC, p. 79. Reprinted by permission of John Wiley & Sons Ltd.

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