The antiparasitic drugs are used in the treatment of parasitic diseases, caused by Protozoa infestation (rhizopods, flagellates, sporozoa), Eumetazoi (helminths) and Arthropods (arachnids, insects, etc.). They are used to treat protozoal infections such as amebiasis, giardiasis, leishmaniasis, trichomoniasis, trypanosomiasis and malaria; for the treatment of infections by helminths (responsible for the so-called intestinal 'worms') and for the treatment of infections transmitted by bites and stings of Arthropods. The antiparasitic action can have both a therapeutic and preventive effect (as it happens for example in the case of malaria).
The anti-protozoal drugs act selectively on the parasite according to several mechanisms:
- Quinine, chloroquine, mefloquine, metronidazole, pentamidine (and others) inhibit the synthesis of nucleic acids;
- Trimethoprim, pyrimethamine, dapsone (and others) inhibit the synthesis of cofactors;
- Tetracycline inhibits protein synthesis;
- Amphotericin B destabilizes the cell membrane;
- Primaquine interferes with cellular energy metabolism;
The anti-protozoal drugs are relatively toxic to the host organism because some (such as quinine, chloroquine, mefloquine, pentamidine, tetracycline) are able to concentrate in the parasite; while others (metronidazole) are activated by the parasite itself; alternatively, pyrimethamine and dapsone have a target action that is triggered only in the parasite.
The anthelmintic drugs act selectively on the parasite according to several mechanisms:
- Benzimidazoles interfere with the function of microtubules;
- Ivermectin inhibits neuromuscular functions;
- The dichlorophen interfere with energy metabolism;
The anthelmintic drugs are relatively toxic for the host, since, as it happens for example in the case of benzimidazoles, they are able to discriminate between the target in the host and the target in the parasite.
How should antiparasitic drugs be taken?
The antiparasitics are available in the form of tablets, powder for oral suspension or solutions for injections. There are also creams that allow topical use of these medicines, which can guarantee a remission of any cutaneous manifestations of some parasitic diseases.
Antiparasitic agents must always be used under prescription and behind close medical monitoring by strictly following the instructions regarding dosage and the mode of application. Not doing so can reduce the absorption of the drug or render the treatment useless, in addition to stimulating the development of the drug-resistance phenomenon.
The duration of treatment varies from case to case: some infections (such as giardiasis and trichomoniasis, for example, due to flagellate protozoa), may require 5-7 days of treatment, while other types of treatment – such as for prophylaxis of malaria, protozoal infection sporozoa – provide a means of taking the drugs from 24-48 hours before arrival in a risk area until 7 days after leaving it. It is important to note that the unwise or even indiscriminate use of certain drugs has caused, over time, poly-resistant strains of parasites (see the example of Plasmodium falciparum, the protozoan responsible for the most deadly form of malaria to chloroquine, a drug that is effective in therapy for prophylaxis of malaria, well tolerated and inexpensive, compared to other ineffective drugs used today).
Contraindications and warnings associated with the use of antiparasitic agents
The antiparasitic drugs treat many different diseases in terms of severity, and their intake can create various side effects, from a "simple" headache with vomiting to repercussions on the central nervous and cardiovascular system. In general, however, the use of these drugs may cause the following side effects:
- Metabolic disorders: Liver and kidney dysfunction;
- Central disorders: fever, headache, tremors, convulsions, coma, drowsiness, numbness, toxic reactions on the cardiovascular system, neuromuscular system and the central nervous system;
- Dermatological disorders: itching, hives;
- Gastrointestinal disorders: nausea, vomiting, diarrhea, abdominal pain;
- Musculoskeletal disorders: muscle and joint aches, muscle weakness, joint stiffness;
In cases of pregnancy and nursing, administration of antiparasitics requires careful evaluation of the clinical benefits and potential risks for the mother and unborn child. Contraindications exist even in the case of patients with heart disease, renal and liver diseases.
