Methods of escape:n The most common method of escape of eggs or larvae is via the faeces or urine.
However escape can occur through any of the orifices.
n Via a vector, while feeding an arthropod could also passively consume a stage of the
parasite.
n The parasite may actively break through the skin to escape.n 1.1.2 LIFE-CYCLES
1.1.2.1 Life-cycles using a single host (Direct life-cycles)
Only one host is invaded during a single cycle of the parasite’s life history. The parasite
enters the host either as egg or an infectious larva. It grows, develops, matures and
reproduces and then deposits eggs or larvae which escape to invade another host.
Development usually proceeds only after the invasive stage begins to migrate to a selected
or predetermined site. Reproduction within the host can be sexual, asexual, or both.1.1.2.2 Life-cycles using two or more hosts (Indirect
life-cycles)
At least two hosts are invaded during a single life-cycle of the parasite. The intermedi-
ate hosts are invaded by the eggs or early larval stages. Development of the larval stage
occurs within the intermediate host. There may or may not be an asexual multiplicative
phase within the intermediate host.
Larval stages escape from the intermediate hosts in one of two ways. Either they
actively escape from the intermediate host and undergo a free-living stage before invad-
ing the next host; or they remain within the intermediate host, encyst and wait until the
intermediate host is eaten by the next or definitive host.
Parasites that have evolved the requirement for more than one host will only sur-
vive if the two hosts share the same habitat and have some close association with each
other. One of the most common associations is that of a predator–prey relationship.
Another frequently encountered connection is when the intermediate host feeds upon
the body fluids of the definitive host (as in the case of the arthropod vector). It can be
said that in such situations the intermediate host is itself a temporary parasite of the
definitive host. In those parasites whose life-cycles involve more than one intermediate
host, normally no multiplicative phase occurs within the second or even third inter-
mediate host.n 1.1. 3MULTIPLICATIVE PHASES
The probability of a single, free-living stage reaching a new host in most situations is rel-
atively low. In most instances, contact with a new host occurs by chance. In order to increase
the odds of reaching a new host the majority of parasites are very fecund, ie they can
produce large numbers of eggs. In addition many can undergo multiplication during
their developmental stages. Parasites have evolved many different methods of increasing
their numbers and some of them are listed below:n Eggs are swallowed from which larvae hatch out. The juveniles develop/mature into
adult male and female forms. The female after copulation produces fertile embryon-
ated eggs which pass out of the host eg Ascarisspp, Toxocara canis.
n Larvae are swallowed by the host and mature into adult male and female forms. The
female produces numerous live larvae which remain within the host eg Trichinella spiralis.PARASITOLOGY