Parasites and their role in infectious diseases

Parasites live on or in a host and can cause various diseases.

Parasites
Parasites are organisms that live wholly or partially at the expense of a host. In German-speaking countries, the term mainly refers to animal parasites such as protozoa (single-celled organisms), worms (helminths), and arthropods.

These parasites can cause numerous infectious diseases, including malaria, sleeping sickness, leishmaniasis, bilharzia, toxoplasmosis, and Chagas disease.

A distinction is made between:

• Obligate parasites: Live permanently as parasites.
• Facultative parasites: Live parasitically only under certain conditions.
• Stationary parasites: Remain permanently with the host.
• Temporary parasites: Live only temporarily on or in the host.

Parasites can live as ectoparasites on the surface of the body or as endoparasites inside the host. Some require a host change, whereby a distinction is made between intermediate hosts, final hosts and transport hosts.

Parasites are particularly common in tropical and subtropical regions, where poverty and poor hygiene provide favorable conditions.

Protozoa
Protozoa are single-celled eukaryotes that have structural similarities to animal cells, e.g., through cell organelles such as mitochondria or flagella. Their DNA is stored in a cell nucleus, which distinguishes them from prokaryotes.

They adapt flexibly to different environmental conditions and reproduce by division or the formation of gametes. Some form resistant cysts for transmission, while others require an intermediate host, usually an insect.

Protozoa are divided into:

• Flagellates (flagellated animals) – e.g. Trichomonas vaginalis, Trypanosoma (sleeping sickness)
• Rhizopoda (root-footed animals) – e.g. Entamoeba histolytica (amebic dysentery)
• Sporozoa (spore-forming organisms) – e.g. Plasmodium (malaria), Toxoplasma gondii
• Ciliates (organism with cilia) – e.g. Balantidium coli (balantidiasis)

Helminths (worms)
Helminths are multicellular parasites with complex organ systems. Their life cycle usually involves intermediate hosts, with reproduction mostly taking place outside the human body.

The symptoms of the disease depend heavily on the number of worms ingested. Minor infestations may go unnoticed.

There are two main types:

• Flatworms (phylum Plathelminthes)
• Roundworms (phylum Nematoda)

Arthropods (joint-legged animals)
Many parasitic arthropods live permanently or temporarily on a host. They are of great medical importance as:

• Nuisances (e.g., through itching or skin reactions)
• Causes of skin diseases
• Vectors for viruses, bacteria, protozoa, and helminths

Important examples are:

• Ticks – e.g., transmission of TBE and Lyme disease
• Mites – e.g., pathogens of scabies
• Mosquitoes – e.g., Anopheles mosquitoes as carriers of malaria
• Lice and fleas – e.g. pests and disease carriers