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The Kinetoplastida is a large diverse group of protozoa that includes both plant and animal parasites (Table 4.1). Some authors consider the Kinetoplastida to be a phylum, while others refer to it as a class or an order. They are commonly known as the trypanosomes from the genus Trypanosoma that includes the causative agents of Human African Trypanosomiasis (HAT) and several other parasites of medical and veterinary importance. The genus name Trypanosoma derives from the Greek words trypano ( τρύπανο ) = an auger [a device for boring holes in wood] and soma ( σώμα ) = body that refer to their corkscrew‐like locomotion. Magez and Radwanska (2014) provide a comprehensive review of all aspects of trypanosome biology and their transmission.

Table 4.1 Examples of kinetoplastid parasites of medical, veterinary, and agricultural importance and the diseases they cause.

Genus	Example	Host	Vector/transmission	Disease
Leishmania	Leishmania donovani	Humans, dogs, rats	Phlebotomus sandflies	Kala‐azar (visceral leishmaniasis)
	Leishmania major	Humans, monkeys, dogs, rodents	Phlebotomus sandflies	Cutaneous leishmaniasis
	Leishmania tropica	Humans, monkeys, dogs, rodents	Phlebotomus sandflies	Cutaneous leishmaniasis
	Leishmania braziliensis	Humans, sloths, monkeys, opossums, and many others	Lutzomyia sandflies	Cutaneous/mucocutaneous leishmaniasis
Trypanosoma	Trypanosoma brucei gambesiense	Humans	Tsetse flies (Glossina spp.)	African trypanosomiasis (sleeping sickness)
	Trypanosoma congolense	Cattle	Tsetse flies (Glossina spp.)	African trypanosomiasis (nagana)
	Trypanosoma equiperdum	Horses	venereal	Dourine
	Trypanosoma cruzi	Humans, dogs, cats, rats, and many others	Triatomid bugs	Chagas disease
Phytomonas	Phytomonas staheli	Coconut palms	Lincus lobuliger (Pentatomid bug)	Hartroot

The Kinetoplastida are characterised by their possession of a flagellum and a unique intracellular structure called a kinetoplast (Figure 4.1). The kinetoplast is a disk of interlocking DNA circles (kDNA) located within a large mitochondrion. The structure of kinetoplast DNA is unlike that found in any other organism and its complex replication involves special proteins (da Silva et al. 2017). It may therefore be possible to design drugs to interfere with the replication of kinetoplast DNA. The position of the mitochondrion is such that the kinetoplast is just underneath the kinetosome that is itself situated underneath the base of the flagellum. The kinetosome (sometimes called the basal body), is a structure found in many organisms and is homologous with the centriole; it is involved in the formation of the flagellum. The Kinetoplastida always have a flagellum that may be long and free, incorporated into the cell surface to form an undulating membrane, or small and enclosed within a pocket. The inner core of the flagellum is the axoneme. Alongside this, and connecting to it, is the paraxial rod that consists of a lattice‐like crystalline array of structural proteins. In the promastigote, epimastigote, and trypomastigote stages, the flagellum emerges at the anterior end of the cell and therefore acts as a propeller that pulls the cell along rather than pushing it from behind (Figure 4.2).

Figure 4.1 Diagram of a typical trypanosome. Kp: kinetoplast; Ks: kinetosome; F: flagellum; Mit: mitochondrion; Gly: glycosome; N: nucleus; Lys: lysosome; Um: undulating membrane.

Figure 4.2 Morphological forms of trypanosomes. (a) Trypomastigote. (b) Epimastigote. (c) Promastigote. (d) Amastigote. The flagellum pulls the cell forwards rather than pushes it from behind. Arrows: direction of movement.

The Kinetoplastida have a unique organelle called the glycosome. This may be related to the peroxisomes (which they do not have) found in other organisms. The glycosomes are about 0.25 μm in diameter and contain glycolytic enzymes that are normally present in the cytoplasm of other organisms. The bloodstream forms of trypanosomes are extremely metabolically active and soon die if they run out of glucose to metabolise. The glycosomes are the site of glucose metabolism, and therefore, like the kinetoplast DNA, they are a potential target for antiparasitic drug design.

Many species within the Kinetoplastida are only parasitic in insects. For example, members of the genus Leptomonas live in the gut of various insects including blood‐feeding reduviid bugs (Kaufer et al. 2017). Because they have only a single host, these species are referred to as monoxenous although there are rare case reports of human infections in patients who are HIV+ve. There are also accounts of Leptomonas co‐infections with visceral leishmaniasis and post kala‐azar dermal leishmaniasis caused by Leishmania donovani (Thakur et al. 2020).

Many of the Kinetoplastida alternate between an invertebrate host such as a blood‐feeding insect or leech, and a vertebrate host with development occurring in both. Parasites that have more than one type of host are called heteroxenous. Heteroxenous Kinetoplastida species often express two or more morphological forms with one form present in the invertebrate and the other in the vertebrate (Table 4.2). Some members of the Kinetoplastida exhibit sexual reproduction, or something similar (Berry et al. 2019; Gibson and Peacock 2019), but it is uncertain whether it is a widespread phenomenon in the group.