Trypanosomiasis: Vector

Tsetse flies (Glossina sp.) are blood-sucking insects that occur only in Africa.

The name tsetse is derived from the sound that the insect’s flying makes. The insects have a prominent proboscis and a typical wing veining. When resting they fold their wings over their back like a closed pair of scissors. The genus Glossina is now divided into three subgenera: the fusca-group (subgenus Austenina), the palpalis-group (subgenus Nemorhina) and the morsitans-group (subgenus Glossina).

Like many insects they secrete a "peritrophic membrane" ["peri" + "trophe" = "around food"] in the intestine. This secretion takes place at the transition from the anterior intestine to the midgut (proventricular valve). This membrane consists of chitin and a glycoprotein matrix and functions as a very fine sieve. Small molecules can pass through it, but bacteria or large food particles cannot. It offers some mechanical protection to the middle intestinal cells (the intestine contains no lubricating mucus producing cells). An additional function is the compartmentalisation of the digestive process. A first portion of the food is found in the endoperitrophic cavity, a second portion in the ectoperitrophic cavity. Small food molecules diffuse through the peritrophic membrane. Absorption takes place through the microvilli of the intestinal cells. It was previously assumed that trypanosomes could not pass through the peritrophic membrane, but this assumption has become increasingly doubtful.

Both male and female tsetse flies take blood every 3-4 days. The females live longer than the males, usually 2-3 months, though exceptionally up to 6 months. In an endemic area usually fewer than 1% of the flies are infected. A female mates only once in her life. The reproduction is exceptional in that only 1 egg is fertilized at a time (with sperm stored in the female's spermatheca). The fertilized egg produces a larva, which further develops in the "uterus" of the fly until it is mature. This maturation takes place in the “uterus”. The larva feeds on the secretions of a number of "milk glands" that open into the “uterus”. The secretions from these glands can only continue as long as the fly feeds regularly on blood. The pregnant fly searches for a shady spot with loose soil to give “birth”. After the larva is deposited, it digs itself in and rapid pupation takes place within about 15 minutes. (Tsetse flies never lay eggs!). The pupa, like the larva, has two typical posterior projections that have a respiratory function (polypneustic lobes). Because the larva quickly digs itself in, it is less vulnerable to control measures. Control of the insect must thus be aimed at the adult animals. Some 3-13 weeks later (depending on the temperature) the new adult fly emerges. The insect is thus viviparous or live-bearing (no eggs are laid in the environment). In nature a female tsetse fly deposits only about 6 to 12 larvae during her lifetime (1 about every 10 days).

There are several different species of tsetse flies, each with its own ecological preference. Accurate knowledge of the resting places is essential for meaningful control measures. [For example, the maximum height at which the flies rest on tree trunks determines the area where insecticides should be applied. Large numbers of the insects are frequently found on the lower 30 cm, but this varies according to the season].

Category: Medicine Notes