PARASITOLOGY
The invertebrate groups evolved before the vertebrates and, once the vertebrates
came into existence, invertebrates were able in many cases to take advantage of this new
potential habitat first by invading it and then adapting to it. There has been a relatively
long period for adaptation and those invertebrates destined to become parasitic have
been able to invade then adapt to each newly evolved species. The parasite appears to
have had the advantage and presumably before the final stages of adaptation the host suf-
fered and to a certain extent so did the parasite. Man and domestic animals are still being
invaded and are suffering as a consequence.
Man and domestic animals are both gregarious and the hosts’ gregariousness favours
the distribution of parasites. The closer the hosts live together the easier it becomes for
the infective stage to find a new host. Humans live in close association with one another
as well as with domestic animals and hence there are many parasites that are able to live
in both man and numerous other mammals. Zoonosis is the term applied the situation
where a parasite lives in both man and other animals.
There are very few vertebrates that are free of parasites; and those parasites affect-
ing the lives of both man and domestic animals are often referred to as ‘parasites of
economic importance’ eg Fasciola hepatica, Haemonchus contortus, Taenia solium, Eimeria tenella,
Plasmodium falciparum, Trypanosoma bruceietc. The majority of parasites of economic
importance occur in the following groups of invertebrates; the protozoa, the platy-
helminths, the nematodes and the arthropods. Most modern studies of the biology of
parasites concentrate on the first three groups; the parasitic arthropods tend be studied
mainly by entomologists.
The diagram in Fig. 1.2 demonstrates, from a theoretical point of view, how
host–parasite relationships have evolved. In long established hosts the parasites have
reached an equilibrium with their hosts. If the equilibrium is disturbed in any way the
following could occur:n If the balance moves in the direction of the parasite/s feeding then the host suffers and
the parasite/s have a pathological effect upon the host.
n On the other hand if the balance shifts in favour of the ability to inhibit the parasite,
the latter will be destroyed.
n There is no advantage to the parasite if the host dies as a result of its presence.
n The equilibrium is based on the parasite obtaining sufficient nutrients to survive, but
not to deplete the host in any way.
n If the host can contain the parasite, then it will only experience a minimal amount of
discomfort due to the parasite’s presence.n BOX 1. 3EFFECTS OF PARASITES UPON THE HOST
It is generally not in the interest of the parasite to destroy the host, and from the parasite’s
point of view it must try and avoid the host’s attempt to destroy it. Parasites are mostly
invertebrates and in terms of evolution are very much older and probably first adapted
to being parasites of larger invertebrates, followed by the first aquatic vertebrates, the
land vertebrates and finally the mammals. In general a state of equilibrium has evolved
between the host’s continuous attempts to destroy the parasite and the parasite’s avoid-
ance mechanisms.