PARASITOLOGY

represent different phases of the life history. It is often referred to as ‘alternation of

generations’ but unlike a similar phenomenon in plants there is no change in the chro-

mosome number. Both phases or generations are normally diploid, the only haploid

individuals being the gametes. To avoid confusion, all non-sexual reproduction will be

referred to as asexual. If there is an increase in individuals within a single host, this will

referred to as internal accumulation.

There are parasites that have a free-living phase outside the host. Some of these phases

are active and actually reproduce, while others are quiescent.

The different types of parasite life-cycles can then be classified together according to

the following criteria:

n The presence or absence of sexual reproduction.

n The presence or absence of asexual multiplication.

n Change and number of hosts.

n Internal accumulation of individuals due to either sexual or asexual reproduction.

n Whether or not there is an active or quiescent free-living phase.

n 1.5.1 ONE HOST ONLY, NO ASEXUAL MULTIPLICATION AND

NO INTERNAL ACCUMULATION

When the host is infected, the parasite grows and develops. Once it has mature re-

productive organs, sexual reproduction takes place. The resultant eggs, or if it is

ovaviparous the larvae, escape into the external environment for dispersal and a ‘chance’

encounter with a new host.

PARASITOLOGY

Entry DH Dispersal

DH = definitive host

• Figure 1.3A simple
  direct life-cycle involves
  only one host. Within the
  definitive host (the host in
  which sexual reproduction
  occurs (DH) ) are both
  male and female parasites.
  The eggs/larvae escape
  from the host to infect a
  new host.

All the Monogenoidea, trematodes and many of the nematodes eg Ascarisspp and the

cestode Hymenolepis nanabelong to this category.

H. nanais thought to have a simplification of a more complicated life-cycle. The stage

normally associated with the intermediate host (the metacestode) develops within the

definitive host. Eggs are swallowed, the oncosphere (the embryo) hatches out and penet-

rates the intestinal villi and grows into a diminutive cysticercoid (the metacestode

stage). Within 4–5 days the cysticercoids emerge into the intestinal lumen and proceed

to develop into adult tapeworms. The eggs pass out with the faeces.

Eugregarinida are parasitic protozoans in which sexual reproduction leads to the

development of encysted zygotes, known as oocytes. Within each oocyte sporozoites

develop. The function of the oocyte is to ensure dispersal.

n 1.5.2 LIFE-CYCLES IN WHICH NO SEXUAL REPRODUCTION HAS

BEEN OBSERVED OR RECORDED BUT THERE IS ASEXUAL

MULTIPLICATION AND INTERNAL ACCUMULATION

Entamoeba: the infective stage is a cyst which is accidentally swallowed. Within the gut

lumen the cyst capsule is digested releasing ‘juveniles’ which invade cells and which then