n 1.3.4 TOXINS
Many of the toxins produced by parasites are waste products of their metabolism and are
simply excreted and deposited into the host’s fluids and tissues.
Haematin appears to be a metabolic waste product produced by Plasmodiumfeeding
on haemoglobin. The haematin is deposited in liver and spleen where it apparently does
have some harmful effects.
Amyloid deposits could be a reaction to certain parasite waste products.n 1.3.5 IMMUNOSUPPRESSION
Most parasitic infections are chronic and there is a continuous release of antigens into the
host. The host’s immune system has to either continuously react or become tolerant
(see Chapter 5).
This condition can make the host more susceptible to normally lesser infections.
Sheep with heavy infections of Fasciola hepaticaoften die because of the bacterium
Clostridium oedematiensrather than as a direct result of the parasite.
Polyparasitism is quite common in humans, but only occurs once an established
parasite has become a chronic infection. Measles epidemics are often associated with
malaria endemic regions.n 1.4 HOST–PARASITE REACTIONS
The study of host reactions examines two types of hosts:n A susceptible host is a host in which the parasite survives and the host may suffer as
result of the presence of the parasite.
n A non-susceptible host is resistant to the parasite and either the parasite does not
survive or exists only at a very low level of parasitaemia.There is a range of different host responses and reactions to parasites. Some are
simply mentioned below and are dealt with in detail in Chapter 5.
A host response may be altered after the experience of a primary infection. For ex-
ample it has been observed that children who survive cerebral malaria develop some
form of immunity. It is now thought that a low level Plasmodiumparasitaemia provides a
form of protection (non-sterile immunity). However this condition could also act as a
reservoir for the parasites and maintain a population of infected vectors.
Schistosomais also a parasite where living infection is thought to provide protection against
challenge infections (molecular mimicry).
A healthy normal host reacts to primary invasion of parasite (non-self ) material in a
non-specific manner (non-specific or innate immunity). The parasite has evolved various
ways of protection against this type of host reaction.
Plasmodium falciparumas rapidly as possible leaves the blood and invades hepatic cells
to escape the ‘phagocytes’.
Leishmaniainvades the macrophages and can avoid being digested.
The larger helminth parasites in general are able to escape phagocytosis. They usu-
ally become established before the host reaction can be effective. The majority of
parasitic infections develop into chronic infections and instigate an adaptive immune
response.
The details of how the response is initiated and develops are dealt with in Chap-
ter 5.PARASITOLOGY
Polyparasitism is a
condition where the host
is infected with more
than one type of parasite
eg different helminth
species and/or a
protozoan parasite.