Biting flies is a non-taxonomical description of a number of insect families which are blood feeding on vertebrates. In the following, two important families are listed in more detail due to their vector capacity concerning Trypanosoma spp.

Tsetse Flies

Tsetse flies are obligate blood-sucking insects of medical and veterinary importance due to their capability of transmitting trypanosomes to man and animals. They are generally considered one of the greatest factors affecting the course of economic and social development in Africa.

General Aspects

Taxonomy

Tsetse flies compose a family of their own, Glossinidae, which is placed within the Hippoboscoidea due to the morphological and reproductive similarities of tsetse flies to keds and other hippoboscid flies (McAlpine, 1989). Glossinidae includes the single genus Glossina with 23 species, 6 of which are further divided into 14 subspecies (Gouteux, 1987; Potts, 1973). Glossina (‘tongue fly’, in reference to the prominent proboscis) species are arranged in 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).

Distribution

Tsetse flies occur in the tropical and subtropical regions of sub-Saharan Africa, approximately 15° N to 26° S. Species in the fusca group are most often found in forested habitats, such as rain, swamp and mangrove forests (forests cover most of the western and central African distribution areas of the palpalis group). 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 savanna woodland (primarily central and southeastern distribution).

Veterinary and Medical Importance

For centuries tsetse flies have had a great impact on human health in Africa, due to their function as transmitters of trypanosomes causing extreme human suffering in form of African sleeping sickness. Furthermore their position as vectors of tsetse-borne trypanosomosis in both domestic and wild animals is a major impact in preventing the development of commercial domestic animal production over about one-third of the African continent. Domestic animal disease continues to inhibit agricultural productivity and economic development (Krinsky, 2002).

General Morphology

Overview

Glossina species are tan or brown flies, ranging in length from 6 to 14 mm, excluding the proboscis. Members of the fusca group are the largest (9.5-14 mm). The palpalis and the morsitans group species are small to medium in size (6.5-11 mm). Tsetse adults are characterized by several distinctive morphological features. These include the shape of the proboscis, the position and branching of the fringe on the arista of their antenna, and the wing venation and folding pattern. When not feeding, the proboscis extends directly forward between the palps in front of the head. For further details on morphology see e.g., Krinsky (2002). Adults of both sexes feed exclusively on blood.

Once inseminated the female remains fertile for life and rarely mates more often than once in nature. Inseminated females start ovulation of single eggs, which develop up to a third-instar larva within the uterus of the female. About 9 days after ovulation, the fully developed third-instar larva is deposited on the ground by the female (larviposition). Shortly thereafter, the female ovulates again, depositing a third-instar larva about every 7-11 days, when well-nourished and in ambient temperature. The average interval for all tsetse species is 9-12 days. Life span of females is about 20-40 days in general; males apparently survive 2-3 weeks in the wild (Glasgow, 1963; Potts, 1973). The developmental cycle includes these three larval stages, a pupa and the final adult. The low reproductive rate in tsetse is compensated by the extreme protection given to each larva by the female, but it also makes the impact of any loss of female flies greater than in species with a mass production of eggs (Krinsky, 2002).

Feeding Behavior

Activity Dynamics

Although tsetse feed mostly in daylight, feeding does occur at night. Tsetse adults are most active in the morning and in the late afternoon. They rarely fly more than 30 min a day and are known to disperse up to about 1 km/day. Most of the time is spent resting on vegetation. Tsetse flies are more prone to start feeding on calm animals. They fully engorge within about 1-10 min.

The transmission of the different trypanosomes (T. brucei and T. congolense in dogs) is occurring during feeding of the tsetse flies.

Host Spectrum

Host preferences vary among tsetse species. Members of the palpalis group feed mainly on reptiles, bushbuck, oxen, and occasionally smaller mammals and humans visiting the watering spots. Members of the morsitans feed mostly on the mammals of the savanna. The one forest-dwelling species in the savanna group (G. austeni), feeds exclusively on suids. The fusca group feeds on a variety of host species, including bushbuck, buffalo, other cattle etc. (see Krinsky, 2002). Humans are not the preferred host of any of these fly species.

Control

Several approaches have been taken especially to prevent the tsetse transmitted diseases African sleeping sickness in man and Nagana in animals. These include intensive treatment and isolation of infected human and animal hosts, use of trypanotolerant animals in agriculture, research on vaccine development and chemical and ecological attacks on tsetse themselves. Dipping of cattle in an insecticidal bath and trapping of adults are two examples which have shown to possess an effect. Nevertheless tsetse and their transmitted pathogens remain a major issue of medical and veterinary health in Africa.

Stable flies (Stomoxys sp.)

Species of the genus Stomoxys can transmit Trypanosoma evansi to dogs in the subtropical areas of Africa, Asia and South America.

General Aspects

Taxonomy

The family Muscidae includes approximately 4000 species, subdivided into different genera. Of these, representatives of the genera Musca, Hydrotaea, Stomoxys and Haematobia are of main veterinary importance. The latter two genera are blood feeders forming the subfamily Stomoxyinae. Approximately 18 species are belonging into the genus Stomoxys. Species of this genus have biting mouthparts, and both sexes are blood-feeders.

Distribution

Stomoxys has a worldwide distribution with different species in the different climatic zones. The insects are good fliers and may follow their potential host animals over long distances. The most common species of importance in temperate habitats is Stomoxys calcitrans, the stable fly, which is found worldwide. S. niger and S. sitiens may replace S. calcitrans as important pests of livestock in Afrotropical and Oriental regions.

Veterinary and Medical Importance

Blood loss and annoyance during the painful feeding of the stable fly are influencing especially the food uptake of the host animals and are thus the most important negative effects, causing up to 15% weight loss and possibly high reduction in milk production in the case of high fly density. Furthermore the stable flies can also act as mechanical vectors of pathogens as T. evansi, e.g.

General Morphology

Overview

Stomoxys are about 7-8 mm in length and generally grey in color, with four longitudinal dark stripes on the thorax. For further details see Wall and Shearer (1997). The eggs are about 1 mm in length and hatch in 5-10 days, depending on temperature. The saprophagous larvae pass through three stadia, until they pupate. Each female lays up to 500 eggs in batches of 25-50. In warm weather the average life cycle (here for S. calcitrans) is 4 weeks, but it can vary from 3-7 weeks, depending on temperature.

Feeding Behavior

Activity Dynamics

S. calcitrans may be abundant in and around farm buildings and stables in late summer. Both sexes are blood-feeders and often ingest small blood meals several times a day. After feeding, the flies move to a resting site. Adults prefer strong sunlight, but will follow animals into buildings to feed.

The transmission of T. evansi in dogs is occurring mechanically.

Host Spectrum

Stable flies are one of the most annoying and destructive fly pests of cattle, horses, sheep and goats. But they do also feed on other animals such as dogs.

Control

A control of stable flies is often necessary and of economic importance in the keeping of cattle. A consequent cleaning of the stables and destruction of breeding material is often reducing the fly development significantly. Furthermore insecticide applications and traps inside the stable can also reduce the population.

References

• Glasgow, J.P.: The Distribution and Abundance of Tsetse. 1963, Macmillan, New York
• Gouteux, J.P.: [A new Glossina from the Congo: Glossina (Austenina) frezili sp. nov. (Diptera: Glossinidae).] Trop. Med. Parasitol., 38, 1987, 97-100 (in French)
• Krinsky, W.L.: Tsetse flies (Glossinidae). In: Mullen, G., Durden, L. (eds.): Medical and Veterinary Entomology. 2002, Academic Press, London, pp. 303-316
• McAlpine, J.F.: Phylogeny and classification of the Muscomorpha. In: McAlpine, J.F., Wood, D.M. (eds.): Manual of Nearctic Diptera. Vol. 3, Monograph No. 32, 1989, Agriculture Canada, Ottawa, pp. 1397-1518
• Potts, W.H.: Glossinidae (tsetse flies). In: Smith, K.G.V. (ed.): Insects and other Arthropods of Medical Importance. 1973, British Museum (Natural History), London, pp. 209-249
• Wall, R., Shearer, D.: Adult flies (Diptera). In: Wall, R., Shearer, D. (eds.): Veterinary Entomology. 1997, Chapman & Hall, London, pp. 141-196