8.2 Infectious disease or parasitosis

The terminology in common use in parasitology was originally developed in the field of microbial infectiology. Since directly transmitted parasites can also be controlled effectively by hygiene, there has been no motivation to establish special terms. However, the majority of eukaryotic parasites survive when a physiological balance is achieved by the specific modulation of the defensive components of the immune reaction of the host. Subversion and debilitation are mostly observed, e.g. in filarioses (see chapt. 2.3.5, page 57); subvention, i.e. the survival of the parasite by use of the specific immune reactions of the host, is still underestimated, e.g. in malaria (see chapt., page 15). However, the amplifying potential of immune reactions are particularly suitable for the transport of signals of the parasite by reason of its minute body mass in comparison with that of the host. Otherwise, the reduction of egg production (fecundity) in flukes, such as Schistosoma japonicum, which starts even when only a few fluke pairs are present, is hard to imagine (see chapt. and figs. 2.68 and 2.69, page 144). Premunition is another example for the immunological prevention of overparasitation.

Furthermore, various given facts and phenomena have to be treated precisely in scientific colloquial language. The term infection is used when the pathogen (microbial as a rule, flagellates) multiplies and propagates simultaneously, mostly by the same process. The term infestation is used when the pathogen propagates without simultaneous multiplication. For example, the host’s load of an intestinal worm does not increase when the worm sheds its eggs.

Many protozoic parasites do not fit into either of these categories and demand special consideration.

Plasmodium spec. multiplies in red blood cells (RBC) but does not propagate by the production of sexually differentiated gamonts. The cyclic reproduction in the RBC simply compensates the short-term life of the host cells; the plasmodium stages behave like a soma periodically producing propagation stages. Similar considerations can be extended to many if not all sporozoa directly or cyclically transmitted (Piroplasms, Toxoplasma, Sarcosporids). The differences from infection in the strict sense caused by microbial pathogens are obvious and should be kept in mind. The term invasion diseases has been proposed but has not been accepted.
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