Tsetse flies are obligate blood-sucking insects. Due to their capability of transmitting trypanosomes causing African sleeping sickness in humans and so-called Nagana in domestic and wild animals, they are of medical and veterinary importance.
Generally, they are considered one of the greatest factors affecting the course of economic and social development in Africa, as sleeping sickness with its morbidity and mortality still is significant and Nagana in livestock still stands as a major deterrent to the development of animal agriculture on the African continent. African animal trypanosomosis remains the main hindrance to development of efficient and sustainable livestock production, and the presence of tsetse is the root cause of hunger and poverty (Krinsky, 2019).
In the context here, tsetse flies as transmitters of the potential canine and feline trypanosome species Trypanosoma brucei (brucei) and Trypanosoma congolense are mentioned.
Photograph of adult Glossina morsitans
(by courtesy of D.D. Bowman, University of Cornell, Ithaca, NY, USA)
Tsetse flies compose a family of their own named Glossinidae. This family includes the single genus Glossina with 25 species (Bánki et al., 2022). The genus Glossina (meaning ‘tongue fly’, in reference to their prominent proboscis) is including three subgenera – Austenina, Nemorhina, and Glossina – that correspond roughly with groups of species found in different ecological settings. The subgenera often are cited by their group names: the fusca group (Austenina), the palpalis group (Nemorhina), and the morsitans group (Glossina) (Krinsky, 2019).
Classification of tabanids
|Diptera (i.e. two-winged insects)
|Austenia, Glossina, Nemorhina
Nearly all species and subspecies of tsetse flies occur in the tropical and subtropical regions of sub-Saharan Africa, approximately 15° N to 26° S (Krinsky, 2019). Only Glossina tachinoides (Carter, 1906), Glossina morsitans submorsitans, and Glosssina fuscipes fuscipes have been recorded on the Arabian Peninsula (Elsen et al., 1990). The northern limit of the tsetse distribution corresponds with the southern edges of the Sahara and Somali deserts, and in the south, no tsetse flies are found south of the Kalahari and the Namibian desert and in the eastern part below 29° S (Vreysen et al., 2013).
Species of the fusca group are most often found in forested habitats, such as rain, swamp and mangrove forests. Species in the palpalis group occur among vegetation around lakes and along rivers and streams (primarily along watercourses in western and central Africa). The morsitans group, with the exception of the forest-dwelling Glossina austeni, occurs in open country and is most often found in dry thickets, scrub vegetation, and areas of savannah woodland (Krinsky, 2019). The palpalis group occurs primarily along watercourses in western and central Africa. The morsitans group is primarily central and southeastern in distribution, and the fusca group is found in forested areas that overlay most of the western and central African distribution of the palpalis group (Krinsky, 2019).
Tsetse flies are found over an area estimated to be at least 10 million square kilometres, nevertheless, the distribution of the flies is discontinuous. The areas they inhabit may extend to several hundred kilometres and form what have been traditionally called fly belts. Within these belts are patches of forest and bush where environmental conditions such as shade and high humidity are suitable for tsetse survival and reproduction (Krinsky, 2019). Tsetse flies are distributed discontinuously throughout their range, and each taxon is restricted to a relatively specific habitat (Leak, 1998).
Tsetse live in areas where the annual rainfall is at least 0.4 m/year. They require temperatures between 16°C and 40°C, with optimal development occurring at 22-24°C; for this reason, the flies are not found at elevations above approximately 1,500 m (Krinsky, 2019).
All species of tsetse flies are probably capable of transmitting pathogenic trypanosomes. However, only a few species (such as Glossina morsitans morsitans, G. m. centralis, G. pallidipes, G. palpalis palpalis, G. fuscipes fuscipes, and G. tachinoides) are major vectors of trypanosomes that affect humans (causing African sleeping sickness) and domestic animals (causing Nagana) (Gooding and Krafsur, 2005). Cats and dogs can also be affected by tsetse-transmitted trypanosomes.
The species belonging to the fusca group (subgenus Austenina) of tsetse flies are of little or no economic importance as their habitat is confined to the lowland rain forests or the border areas of the forest and isolated relic forests (Haeselbarth et al., 1966).
The distribution of the palpalis group species (subgenus Nemorhina) is likewise associated with lowland rain forest but their habitat is extended along river systems in the humid savannah (Jordan, 1986). Their distribution extends from the wet mangrove and rain forests of the coastal regions of West Africa northward into drier savannah areas. They can tolerate a wide range of climatic conditions. The species belonging to this group are important vectors of African animal trypanosomosis in West Africa and human African trypanosomiasis in Central Africa, since they are opportunistic feeders and tolerate a high degree of disturbance of the landscape (Van den Bossche et al., 2010).
All species belonging to the morsitans group (subgenus Glossina sensu stricto) are restricted to savannah woodlands (Jordan, 1986), and their distribution and abundance often correspond with that of wild animals. Glossina morsitans spp. and G. pallidipes are the most important species of this group and are major vectors of African animal and human trypanosomosis in Eastern and Southern Africa (Vreysen et al., 2013).
The population density of tsetse is generally low (Mehlhorn, 2001). The potential difficulty of males and females finding each other in low-density populations is apparently overcome in some species by the attraction of both males and females to large moving animals. Mating usually occurs on or in the vicinity of a host. Once they have mated, however, females and males tend to be more attracted to stationary animals. Tsetse feed on an array of hosts with preferences in individual species and species groups. These preferences are of epidemiological significance in relation to the reservoir hosts of the pathogenic trypanosomes transmitted by the flies to humans and domestic animals (Krinsky, 2019). The tsetse-transmitted trypanosome species relevant for cats and dogs are Trypanosoma brucei (brucei) and Trypanosoma congolense (see under trypanosomosis).
Host attraction and host recognition are mediated by visual and olfactory cues. Host odours have been shown to be attractive to tsetse from distances up to 100 m. Although tsetse feed mostly in the daylight, adult feeding does occur at night. Tsetse rarely fly for more than 30 min/day and are known to disperse up to about 1 km/day. They spend most of their time resting on vegetation. Most species are found below 3 m, where they rest on wood surfaces of trees during the day and on leaves at night. Recently engorged flies mostly rest with their heads directed upward. Hungry flies often rest horizontally, with the dorsal side down. When seeking a host, Glossina species can fly rapidly, reaching speeds above 6.5 m/s (approximately 25 km/h) (Krinsky, 2019).
Much of the movement of tsetse flies is related to the need for a blood meal and hence, its host seeking behaviour. As mentioned above, hosts are located using a combination of olfactory and visual cues, and the host seeking behaviour is modulated both by exogenous (temperature, vapour pressure deficit, visual and olfactory stimuli) and endogenous (circadian rhythm of activity, level of starvation, age, sex and pregnancy status) factors (Colvin and Gibson, 1992).
All tsetse flies are viviparous, giving birth to a single larva at the time (see under Life Cycle). Mature larvae do not feed after parturition but simply burrow into the ground and pupariate; the adult emerges about 30 days later. This method of reproduction severely limits the fecundity of tsetse flies. The low fecundity of tsetse flies is said to make them ideal targets for genetic and other methods of control that target reproduction, although this has been questioned (Rogers and Randolph, 2003).