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PH-IM10

Parasitic infection

How protozoa and worms live at the host’s expense: how malaria parasites destroy red cells and block small vessels, how gut parasites damage the mucosa, and why worms provoke eosinophils, IgE and, around trapped eggs, fibrosis.

By the end, you can

  • Distinguish protozoa from helminths and explain why the difference shapes the immune response.
  • Explain how the malaria life cycle causes fever and anaemia and, in falciparum malaria, blocked small vessels.
  • Describe how intestinal protozoa damage the gut, with and without invading it.
  • Explain the type 2 response to worms: eosinophils, IgE, mucus and expulsion.
  • Explain how schistosome eggs trapped in tissue cause granulomas, fibrosis and portal hypertension.
  • Recognise the classic blood-film and tissue appearances and the tests of parasitic infection, and explain how the main antiprotozoal and antihelminthic drug classes act on the parasite.

What it is

A parasite lives on or in a host and benefits at the host’s expense. The parasites of medicine are protozoa, single-celled organisms that multiply inside the body (such as Plasmodium, Entamoeba and Giardia); helminths, multicellular worms that mostly do not multiply inside the host (roundworms, tapeworms and flukes); and ectoparasites such as lice and mites. Disease comes from the parasite’s own activity, from the space it occupies and from the host’s response to it.

4 types to tell apart
Protozoa
Single cells that multiply in the host, many of them inside cells: Plasmodium (malaria), Entamoeba histolytica, Giardia, Toxoplasma, Leishmania and Trypanosoma.
Nematodes (roundworms)
Ascaris, hookworms, Strongyloides, threadworm (pinworm) and the filarial worms.
Cestodes (tapeworms)
Taenia and Echinococcus; their larval cysts in tissue (cysticercosis, hydatid disease) often do more harm than the adult worms.
Trematodes (flukes)
Schistosoma and the liver flukes, which use freshwater snails as intermediate hosts.

Normal function

The gut lining and its two kinds of defence

Many parasites enter by mouth or live in the gut. The small intestine is lined by covered in absorptive cells and mucus-secreting ; new cells come from the , and the lining is replaced every few days. The beneath holds lymphocytes, macrophages, mast cells and eosinophils.

Microbes that live inside cells, including many protozoa, are controlled by type 1 immunity: Th1 cells release interferon-gamma, which activates macrophages to kill what they contain, and cytotoxic T cells kill infected cells. Antibodies act on parasites outside cells.

Worms are far too large to be engulfed. Damaged epithelium releases alarm signals (IL-25, IL-33 and TSLP) that start type 2 immunity: Th2 cells and innate lymphoid cells make IL-4, IL-5 and IL-13. These switch B cells to making IgE, bring in eosinophils, make goblet cells pour out mucus and make gut muscle contract, the "weep and sweep" that helps expel worms.

Point to it on the model