n Once the bitch becomes pregnant her hormones reactivate the larvae which then
migrate into the foetuses via the placenta. Puppies can also become infected by larvae
in the milk.
n Humans become infected by accidentally ingesting eggs. The L 2 larvae, now in the ‘wrong
host’, burrow through the gut mucosa and then begin to migrate via the circulation
round the host.
n If they migrate through the tissues the condition is known as visceral larvae migransand
if the larvae become trapped in the eye it is referred to as ocular larvae migrans.
n The larvae in man or rodents (referred to as paratenic hosts) do not develop further
after completing their migration.n 4.2 THE MONOGENEA: PARASITIC TREMATODES WITH
ONLY ONE HOST
The monogenea are mostly ectoparasites of fish and amphibia and have only one host and,
like nearly all trematodes, the adults are hermaphrodite. Examples include: Diclidophora
merlangi, an ectoparasite found on the gills of the fish Merlangius merlangus(whiting); and
Diplozoon paradoxum, an ectoparasite found on the gills of freshwater fish. The adult is com-
prised of two individuals that have become fused to each other.
n 4.2.1POLYSTOMA INTEGERRIUM
The adult lives in the excretory bladder of a mature frog (Rana temporaria).
n A mature Polystomais a flat worm, about 3 ×2 mm, with an adhesive organ called the
haptor.
n The life-cycle of P. integerriumappears to be synchronised with that of its host. The par-
asite’s genitalia are activated when the host prepares to enter the water in spring for
mating. Large amount of eggs are released by the parasite once the frog is in the
water. By the time the frog eggs hatch and reach the tadpole stage the Polystomaeggs
hatch into a ciliated (five bands of cilia) oncomiracidium larva.
n The larva has a simple gut and a posterior sucker with 16 hooks. It enters the tadpole
via the branchial pore and attaches to the host’s gills.
n When the tadpole metamorphoses into a froglet, the juvenile larvae migrate over the
ventral surface of the host to the cloaca and enter the bladder.
n Some of the larvae attach to the external gills of the tadpole and become neotenic, reach-
ing sexual maturity within 20 days. These forms have only rudimentary copulatory organs,
vaginae and vitellaria and a single testis. Cross-fertilisation occurs and the fertile eggs
produce larvae which undergo normal development.n 4. 3PARASITIC PROTOZOA WITH ONLY HOST
4.3.1 Eimeria spp
A protozoan intracellular parasite that lives in the epithelial cells of the gut mucosa (see
Fig. 2.3).
n The infectious stage is the oocysts which are swallowed. The intestinal digestive juices
dissolve the outer shell and release the sporozoites, which then invade the epithelial
cells of the gut mucosa.
n Within each epithelial cell the process of schizogony commences.
n Schizogony is an asexual multiplicative phase in which the nucleus of the original
sporozoite divides into several smaller nuclei and each daughter nucleus is surrounded
by layer of cytoplasm producing numerous uninucleate merozoites.PARASITE EXAMPLES GROUPED ACCORDING TO LIFE-CYCLE