15.5.4. Protists other than malaria
Some of the most important insect-borne pathogens are protists (protozoans), which affect a substantial proportion of the world’s population, particularly in subtropical and tropical areas. Malaria has been covered in detail above (section 15.5.1) and two important flagellate protists of medical significance are described below.
Trypanosoma is a large genus of parasites of vertebrate blood that are transmitted usually by blood-feeding “higher” flies. However, throughout South America blood-feeding triatomine reduviid bugs (“kissing bugs”), notably Rhodnius prolixus and Triatoma infestans, transmit trypanosomes that cause Chagas’ disease. Symptoms of the disease, also called American trypanosomiasis, are predominantly fatigue, with cardiac and intestinal problems if untreated. The disease affects 16–18 million people in the Neotropics, perhaps 350,000 in Brazil, and causes 45–50,000 deaths each year. From a public health perspective in the USA, some percentage of the millions of Latino migrants into the USA inevitably must have the disease, and localized transmission can occur. Other such diseases, termed trypanosomiasis, include sleeping sicknesses transmitted to African humans and their cattle by tsetse flies (species of Glossina) (Fig. 15.1). In this and other diseases, the development cycle of the Trypanosoma species is complex. Morphological change occurs in the protist as it migrates from the tsetse-fly gut, around the posterior free end of the peritrophic membrane, then anteriorly to the salivary gland. Transmission to human or cattle host is through injection of saliva. Within the vertebrate, symptoms depend upon the species of trypanosome: in humans, a vascular and lymphatic infection is followed by an invasion of the central nervous system that gives rise to “sleeping” symptoms, followed by death.
A second group of flagellates belong to the genus Leishmania, which includes parasites that cause internal visceral or disfiguring external ulcerating diseases of humans and dogs. The vectors are exclusively phlebotomines (Psychodidae) — small to minute sand flies that can evade mosquito netting and, in view of their usual very low biting rates, have impressive abilities to transmit disease. Most cycles cause infections in wild animals such as desert and forest rodents, canines, and hyraxes, with humans becoming involved as their homes expand into areas naturally home to these animal reservoirs. Some two million new cases are diagnosed each year, with approximately 12 million people infected at any given time. Visceral leishmaniasis (also known as kala-azar) inevitably kills if untreated; cutaneous leishmaniasis disfigures and leaves scars; mucocutaneous leishmaniasis destroys the mucous membranes of the mouth, nose, and throat.
Note that the tracheae are visible through the abdominal cuticle in (b). (After Burton & Burton 1975)