PARASITOLOGY

INTRODUCTION TO PARASITOLOGY

Methods of escape Methods of infection/invasion

Oral, eggs or larvae swallowed

Via vector — a blood

feeding arthropod

Penetration by

specialised mouthparts

that can pierce through

skin

Sputum

Vector removes
parasites while feeding

The most common method
is via faeces or urine

• Figure 1.1The survival
  of a parasite depends
  upon infecting a host and
  reproducing within the
  host environment. The
  next generation must be
  capable of escaping from
  the host to repeat the
  cycle. A parasite can enter
  and escape from a host
  via an orifice, or actively
  penetrate the host’s outer
  covering or be infected
  and removed via a
  (arthropod) vector.

The following are the most frequently encountered methods of invasion:

n Oral: eggs or infectious larvae are swallowed via food or accidentally.
n Penetration by specialised larvae that, once they make contact with the host, can

pierce through the skin or outer covering of the host.
n Via a vector: usually an arthropod that feeds upon the body fluids of a vertebrate

transmits an infectious stage of the parasite. The vector can infect the host either by
an inoculative method, ie injected via the arthropod mouth parts into the vertebrate,
or by the contaminative method, ie the infectious stage is released from the vector while
it is feeding, and the parasite then enters the host through the ‘wound’.
n Contact: Infectious stages penetrate a host during copulation.

n BOX 1.

There are several types of parasitism each referring to the variation in the habitat and life

of the particular parasites such as:

nObligate parasites; organisms that for all or most of their life-cycle are parasitic.

nTemporary parasites; parasitic for limited periods for either feeding or reproduction.

nFacultative parasites; organisms that are not normally parasitic but can survive for a lim-

ited period when they accidentally find themselves within another organism.

nAdaptive parasites; those organisms that have the capacity to live both as free-living or

parasitic organisms.

nObligate parasites have at least one host during their life-history.

nIf there is more than one host during the life-cycle, the host in which the parasite reaches

sexual maturity is known as the definitive host.

nHosts in which the larval stages are located are called the intermediate hosts.

nThe stage of the parasite that invades the host is called the invasive or infective stage.