n 1.4.1 PATHOLOGY DUE TO HOST RESPONSE
There are only few diseases caused by the types of parasites already outlined that result
in the death of the host. That is: not many parasites can be considered to be ‘killers’.
All multicellular organisms (the host) react against internalised non-self material (the
parasite). The host and the parasite have different genomes and hence each possesses
a cell membrane with unique molecular structures. Recognition of this difference leads
to specialised host cells attacking the invader. This first reaction leads to non-specific
inflammation which is an innate immune response.
If the infection is localised near to the surface, skin inflammation results in localised
redness, swelling and pain. However this also occurs when the infection is internalised
and, although not visible, the pain will make the host aware of its presence.
If the immediate innate immune response does resolve the infection, this leads to the
development of an adaptive immune response. One of the end products of an immune
response is the formation of a granuloma. This basically consists of a cellular reaction to
the presence of the parasite (see Chapter 5 for details). The parasite is eventually sur-
rounded by fibrous cells and as it dies off, calcium is deposited within the cells and the
physical presence of such a structure may block fluid flow and lead to necrosis of sur-
rounding tissues.
n 1.4.2 PATHOLOGY DUE TO PARASITE NUMBERS
Protozoan parasites undergo a multiplicative phase at some stage. This exaggerates all of
the pathological effects already described.
The number of adult helminth parasites is generally dependent upon the number of
infective stages that actually invade the host. The greater the number of infective stages
that enter the host the greater will be the pathological damage.
The majority of helminths that reach sexual maturity within the definitive host repro-
duce, producing either numerous eggs or larvae. These later stages can also become
pathological but have a different effect upon the host compared to that of the adult para-
site. Hence it is possible that a single or pair of adult parasites releasing numerous repro-
ductive stages such as eggs or motile larvae will have a considerable effect upon the
health of the host.
There are relatively few, but significant, parasitic helminths that invade the host as eggs
and once inside the host hatch into larval stages and then undergo a multiplicative phase
eg Echinoccus granulosus. This results in the larval stages remaining for considerable
lengths of time within the definitive host. A host reaction builds up around the parasites
and the physical presence of such a structure can cause blockages and necrosis of surrounding
tissues leading to a slow deterioration of the host.
n 1.5 PARASITE LIFE-CYCLES
Two reproductive phases can occur during a parasite’s life history:
n An asexual phase which results in the accumulation of numerous individuals of a par-
ticular species in one host. In some species, particularly among the trematodes, part
of this process is considered to be parthenogenic rather than asexual.
n A sexual phase: the mature phase of the parasite’s life history.
There are certain parasites which can undergo the processes of both sexual and
asexual multiplication. Very seldom do both processes occur at the same time and they
INTRODUCTION TO PARASITOLOGYIt is of no real advantage
for the parasite to kill
the host.