PARASITOLOGY

PARASITOLOGY

n BOX 6.2 A SUMMARY OF THE BASIC TRANSMISSION

SYSTEMS

nThe egg or infective larval stage is eaten by the potential definitive host.

nThe egg or larval stage is eaten by the intermediate host. The larval stage that develops

within the intermediate host is shed and eaten by the definitive host. The intermediate

host is the prey of the definitive host.

nFree-living larval stages shed by the intermediate host then actively penetrate the

definitive host.

nTransmission from one host to another via vectors.

nTransmission can be affected by circadian rhythms. A circadian rhythm is based on a

24 h cycle and certain parasites have become adapted to this system:

nRhythms associated with synchronous cell division, eg the malaria parasites.

nRhythms associated with the discharge of infective forms either from the definitive host,

eg Coccidia, Enterobiusand schistosomes or from an intermediate host, eg Schistosomes.

nRhythms associated with migration of the parasite, eg Trypanosomes, malaria para-

sites and nematode microfilaria.

nNo rhythms of any sort have been observed among the cestodes.

n Effects on membrane permeability.

n Initiation of activity of the larva/metacestode.

n Lytic effect on parasite surfaces.

n Synergistic action with host digestive enzymes.

n Effects on the metabolism of the establishing parasite.

A bile salt molecule is composed of a hydrophilic and a hydrophobic domain and

reacts with both the lipids and proteins in the membrane thereby affecting the perme-

ability. This allows water and enzymes to assist the hatching of eggs or excystation of cysts.

The deoxycholic acid in bile salts may influence the host specificity of certain ces-

todes. Deoxycholic acid has been shown to be able to lyse protoscoleces. Rabbits and sheep

are rich in deoxycholic acid and hence the protoscoleces of E. granulosusdo not survive

in those hosts. Dogs, a permissive host for E. granulosus, have much less deoxchyolate acid

in their bile salts.

n 6.4.2 FACTORS AFFECTING DEVELOPMENT

Most parasites require a signal from the host to begin their development. A clear ex-

ample of this is in the case of hydatid cysts of E. granulosus. If in the intermediate host the

cyst is damaged in any way and protoscoleces escape, but remain within the intermedi-

ate host, they just form secondary cysts.

The numbers of worms per host or the biomass of worms can influence the effect

of the parasite upon the host but also can affect the parasite. In laboratory infections of

rats with Hymenolepis diminutait has been demonstrated that the mean mass of worms

decreases as the number of worms per host increases. Relatively recently it has been shown

that the greater the number of adults present in the gut the greater is the effect upon the

length and morphology of proglottids (Stradowski 1996).