Two methods of multiplication occur, sexual and asexual reproduction, which have different advantages. In asexual reproduction, a succession of exact or almost exact replicas are produced, so that any natural selection will act on batches or strains, rather than on individuals. By contrast, sexual reproduction offers great scope for variety, both within the cells of the single organism and from one organism to another. This means that natural selection acts on
Fig. 1.1. Agent, transmission, host and environment.
Seasonality individuals and variations of vigour and adaptability occur.
There are different consequences of these methods of reproduction. With asexual organisms, the strain of the organism is either successful or unsuccessful in invading the host, whereas in sexual organisms, certain individuals may succeed while others may not. In continuing its existence, only one organism of the asexual parasite requires to be transmitted, whereas in the case of the sexual parasite, both male and female adults must meet before reproduction can take place. Some parasites seem to be at a tremendous disadvantage, e.g. the filarial worm Wuchereria bancrofti, where both male and female individuals go through long migrations in the body to find an individual of the opposite sex, but despite all these problems, they are one of the most successful of all parasites.
Whether the organism reproduces sexually or asexually is relevant in treatment and control. If a treatment is successful in destroying an asexually reproducing organism, then it will also be successful against all the other individuals, unless a mutation occurs, which will also confer resistance to the treatment for all others of that strain. In contrast, sexual reproduction produces individuals of different vigour meaning that some individuals will succumb to treatment, while others will not. However, having two sexes can be a disadvantage for the organism in that methods of control can be devised to attack only one of the sexes or prevent them from meeting.
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