Tsetses are able to find their hosts by their sense of smell, and by sight.
Particular host species are used by tsetse probably for the following reasons:
The hosts occupy the same habitat as the tsetse.
The smell and sight of the host is attractive to the tsetse.
The hosts remain fairly quiet and still when fed upon by tsetse.
The flies do not necessarily feed on hosts that happen to be in the same habitat. For instance, the common animals, zebra and wildebeest, are not fed upon by tsetse (see 188.8.131.52). The reasons for this are not fully understood.
6.1.1 Dependence of tsetse species on various hostsThe following list gives for each of thirteen tsetse species, the most favoured hosts in order of importance. An asterisk (*) against the name of a host indicates that this host accounts for more than half the total feeds for that species of tsetse.
For example, for Glossina longipalpis, the bushbuck is the most important host and provides more than half the total feeds. The buffalo is the next most important, and the red river hog is the next after that. For G. morsitans, the four hosts, ox, buffalo, kudu and man, are grouped together because they are about equally important in providing food for this species, after the warthog.
This information comes from averaging out blood meal analysis data from studies made in many parts of Africa (see 8.6). The order of importance shown for a tsetse species may not be true for a particular limited area. The information is therefore only a rough guide.
The list shows the great importance of the pig family and of the bushbuck as hosts for tsetse flies. Only three out of the thirteen species listed here do not have one of these at the head of the list of preferred hosts.
This section is only a summary. More detailed information on the importance of different host groups as a source of tsetse meals is given in section 6.1.2.
|Glossina longipalpis||Glossina morsitans|
|(a) Bushbuck *||(a) Warthog|
|(b) Buffalo||(b) Ox, buffalo, kudu and man|
|(c) Red river hog|
|Glossina pallidipes||Glossina swynnertoni|
|(a) Bushbuck *||(a) Warthog *|
|(b) Warthog||(b) Buffalo, giraffe, rhinoceros|
|Glossina austeni||Glossina palpalis|
|(a) Bushpig *||(a) Man, reptiles|
|(b) Ox||(b) Bushbuck, ox|
|Glossina fuscipes||Glossina tachinoides|
|(a) Reptiles, bushbuck||(a) Man|
|(b) Man||(b) Ox|
|Glossina fusca||Glossina fuscipleuris|
|(a) Bushbuck *||(a) Bush pig|
|(b) Red river hog,||(b) Giant forest hog aardvark|
|(c) Ox, hippopotamus|
|Glossina tabaniformis||Glossina brevipalpis|
|(a) Red river hog *||(a) Bush pig, hippopotamus|
|(b) Porcupine||(b) Bushbuck, buffalo|
|(a) Rhinoceros *|
|(b) Buffalo, elephant|
6.1.2 Host groups and their relative importanceSome species of wild animals are very important as hosts
for tsetse, and other are less important. The type of host that is important may depend on the availability of the host animal, the species of tsetse, and on the season.
184.108.40.206 The pig family (Suidae) The pig family contains the warthog, bushpig, red river hog and the giant forest hog, as well as the domestic pig.
Members of the pig family are very important as hosts of tsetse .
feeds mainly on warthog (60–70%).
feeds mainly on bushpig (50–60%).
feeds mainly on bushpig and giant forest hog (65%).
Glossina tabaniformis feeds mainly on the red river hog (70%).
Glossina morsitans gets from one third to nearly half (30–45%) of its food from warthog.
Glossina fusca may take 15% of its meals from the red river hog.
Glossina brevipalpis may take as much as 40% of its meals from the bushpig, but this varies considerably according to the area.
Glossina palpalis, G. fuscipes and G. tachinoides generally do not feed on wild pigs very much (about 3%); but G. tachinoides and G. palpalis near to villages in southern parts of their range may get much of their food from the domestic pig.
220.127.116.11 The antelope, buffalo and cattle (ox) family (Bovidae) The bovids include a large number of important species, especially antelopes such as bushbuck, kudu and eland, as well as buffalo and cattle. In general, the larger members may be important hosts, but the smaller ones are much less important.
Glossina pallidipes, G. longipalpis and G. fusca get most of their blood meals (55–90%) from the bushbuck.
Glossina morsitans takes 25–40% of all its feeds from bovids, especially kudu, buffalo, bushbuck and eland. Cattle are very readily fed upon.
Glossina brevipalpis takes about one quarter of all its meals from bovids (buffalo and bushbuck).
Glossina austeni takes about one third of its meals from bovids; duiker (various species) may be important to this fly (10%).
Glossina palpalis, G. fuscipes and G. tachinoides take about 20–40% of their meals from bovids. The exact percentage, and the species fed upon, varies greatly according to local conditions. Cattle are fed upon if they come into infested areas.
Glossina longipennis takes about 20% of its feeds from bovids.
18.104.22.168 Primates, inclining man The primates are a group of mammals into which scientists place man for the purpose of comparison with other animals. The monkey and the baboon come into the same group.
Glossina palpalis, G. fuscipes and G. tachinoides all feed on man; the percentage of feeds may be between 8–40% / according to local conditions. Flies take a high proportion of their meals from people using watering places and river points.
Glossina morsitans may take 7–18% of its meals from primates (mainly man).
Other tsetse species take 5% or less of their meals from primates.
22.214.171.124 Other mammals sometimes used as hosts
Glossina longipennis is unusual in that about 60% of its feeds come from rhinoceros. Elephant is an important but variable host (up to 12% of all feeds).
Glossina brevipalpis may take up to 36% of its food from hippopotamus.
Glossina fuscipleuris may take up to 20% of its food from hippopotamus.
Glossina fusca takes about 12% of its meals from the aardvark.
Glossina swynnertoni feeds on the giraffe to some extent (about 8%).
Glossina tabaniformis takes up to one quarter of its food from the porcupine.
Glossina tachinoides may take over 7% of its food from the porcupine.
126.96.36.199 Other manuals seldom used as hosts The following common wild game animals are not fed upon by tse tse under natural conditions:
Many small antelopes
The following common wild game animals are fed upon only quite rarely:
Hartebeest (except that Glossina morsitanshas been recorded as using hartebeest for about 2% of its meals).
In general, animals smaller than a porcupine or a duiker are not fed upon by tsetse flies in the wild. Very active animals such as monkeys are not much fed upon.
188.8.131.52 Birds Birds are not used very much by tsetse flies as a rule.
Glossina longipennis may take up to 7% of its meals from the ostrich.
Glossina morsitans submorsitans may take about 6% of its meals from birds other than the ostrich.
Glossina palpalis and G. fuscipes may take about 10% of their meals from waterside birds such as cormorants.
184.108.40.206 Reptiles The main reptiles fed upon are monitor lizards (Varanus) and crocodile. These animals are very important as hosts for palpalis group flies.
Glossina palpalis and G. fuscipes may take about a quarter to a third of their food from these hosts, possibly more in quiet places uninhabited by man.
Glossina tachinoides takes about 10% of its food from these reptiles.
These figures are variable according to local circumstances; e.g. as much as 50% of feeds of G. palpalis may be taken from the crocodile, locally.
There are several kinds of trypanosomes transmitted by Glossina. Most of these are dangerous; some are harmless. The trypanosomes that cause diseases in man and domestic animals are called pathogenic trypanosomes.
Only an outline of the subject is given here, as it is the field of the veterinarian and the medical doctor.
6.2.1 Morphology A typical pathogenic trypanosome is a very small organism living in the blood stream of its vertebrate host. The shape is rather like that of a fish. For some of the smaller species, more than 1000 could be put end to end to measure 1 centimetre. The trypanosomes are studied with the aid of a microscope, with or without staining.
The trypanosome has the following main parts: (Fig. 6.1)
Fig. 6.1 Diagram of a trypanosome to show the main parts. Arrow indicates the direction of movement.
All these parts can be seen in well stained, highly magnified microscopical preparations. There are other important structures in the trypanosome that can be revealed by other techniques, but it is not necessary to describe them here.
Size can be a help in identifying different species (see 220.127.116.11), but the variation in size and appearance of Trypanosoma brucei, T. rhodesiense and T. gambiense are so similar that these three cannot be told apart by microscopic examination. Their chief difference is in the effect they have on their mammalian hosts. The large variation in size and appearance in each of these three species has led to their being called the polymorphic trypanosomes (polymorphic = of many shapes) (see 18.104.22.168).
6.2.2 Movement The trypanosome moves by twisting its body into S-shapes. The main power for this movement comes from the flagellum. The direction of movement is towards the free end of the flagellum (Figure 6.1, arrow). The speed of movement is variable. Trypanosoma vivax moves very quickly across the field of a microscope. Stained preparations are of course dead, so that they will show no movement.
6.2.3 Trypanosome species There follows a list of the trypanosome species that will be dealt with in this Manual, together with some others (marked *) that will not otherwise be mentioned.
Note: Ruminants include antelope, camel, giraffe, bovines.
Bovinesinclude cattle, buffalo or bushcow.
Equines include horse, donkey, mule and zebra.
1 μn = 1 micrometre = 1 thousandth of a millimetre.
22.214.171.124 Species list
|Species||Economic Importance||Main hosts||Vector||Size|
|* Trypanosoma theileri||Not important||Bovines, antelopes||Tabanid flies||60–100 μm|
i.e. it is unusually large
|Trypanosoma vivax||Very important|
(see Table 6.1)
development in proboscis
|Trypanosoma uniforms||Moderately important||Ruminants||Glossina;|
development in proboscis
|13–17 μm. Except for size, this species closely resembles T.vivax with which it may be confused.|
|Trypanosoma congolense||Very important|
(see Table 6.1)
|Ruminants, equines, pig, dog||Glossina;|
development in midgut and proboscis
|Trypanosoma simiae||Very dangerous to domestic pigs||Bushpig, warthog, camel||Glossina;|
development in midgut and proboscis. In disease outbreaks may be transmitted mechanically
(see Table 6.1)
|Wild and domestic mammals||Glossina;|
development in midgut, proboscis and salivary glands
|Trypanosoma rhodesiense||Very important, dangerous to man||Mammals including man||Glossina;|
development in midgut, proboscis and salivary glands
|Trypanosoma gambiense||Very important, dangerous to man||Man||Glossina;|
development in midgut, proboscis and salivary glands
|* Trypanosoma evansi||Important; causes the disease surra of various domestic mammals, especially camels, horses and dogs. Found outside the tsetse belts||Camels, equines, bovines||Mechanically transmitted, especially by tabanids||15–34 μm|
|Trypanosoma equiperdum||Important; causes the disease dourine of horses and donkeys, but is found only outside the tsetse belts||Equines (only)||None; transmitted by direct contact between hosts||15–36 m|
(see Table 6.1)
|Pigs||Glossina; development in midgut, proboscis and salivary glands. Not a great deal is known about this trypanosome, especially concerning its distribution.||13–19 μm|
126.96.36.199 Group names
The term 'vivax group' is used to include Trypanosoma vivax and T. uniforms. Duttonella is another term for the same group.
The term 'congolense group' is used to include T. congolense and T. simiae. Nannomonas is another term for the same group.
The term 'brucei subgroup' is used to include T. brucei, T. gambiense and T. rhodesiense. They are often regarded as three subspecies of the same species T.brucei, in which case they are termed T. brucei brucei, T. brucei gambiense and T. brucei rhodesiense.
The term 'brucei group' is used to include T. brucei, T. gambiense and T. rhodesiense, together with some other related trypanosomes. Trypanozoon is another term for the same group.
188.8.131.52 Stocks Within a species of trypanosome, there may be varieties or stocks which behave differently from the usual type. For example, within Trypanosoma gam biense there are stocks that cause the patient to suffer a more acute disease then usual, resembling the illness caused by T. rhodesiense.
In the same way, Trypanosoma vivax may cause a more severe illness in cattle or a less severe one, according to the stock involved.
6.2.4 Life history
184.108.40.206 In the mammalian host Trypanosomes get into the tissues and blood system of the mammal host by becoming injected along with the saliva of an infected tsetse fly as the fly takes its blood meal.
The trypanosomes multiply in the tissues and the blood stream. However, the number of trypanosomes to be found in the blood may vary from time to time, sometimes increasing, sometimes decreasing.
Some trypanosomes in the blood vessels in the skin of an infected mammal host may be taken up by a tsetse fly when it feeds. In this way the infection is passed on.
220.127.116.11 In the tsetse fly The tsetse fly when it emerges from the pupa is always uninfected ('clean') and cannot transmit disease.
When a 'clean' tsetse fly takes a blood meal from a trypanosome-infected animal, there is a chance that it may become infected itself.
A small proportion of the individual trypanosomes in the blood of an infected animal are in a suitable condition to infect the tsetse fly. For instance, the shorter 'stumpy' forms of Trypanosoma brucei are the only ones that can develop further in the fly.
The trypanosomes go through a cycle of development in the fly, lasting several days or weeks before the infection is mature and the fly becomes infective to new hosts. After that period, the fly remains infective for the rest of its life.
There are three types of life cycle undergone by the economically important trypanosomes in tsetse flies (Figure 6.2).
Vivax type (T. vivax, T. uniforms) Some trypanosomes taken up by the fly attach themselves to the walls of the food canal in the proboscis. They form colonies and multiply. In about 10 days infective forms are produced that migrate to the hypopharynx and can infect new hosts when the tsetse feeds.
Fig. 6.2 Diagram to show the places in the body of the tsetse fly in which the various types of trypanosomes carry out their development:
the vivax type develops within the proboscis
the congolense type develops first in the midgut and then in the proboscis
the brucei type develops first in the midgut, then in the proboscis, then finally becomes mature in the salivary glands.
Congolense type (T. congolense, T. simiae)Some trypanosomes that are carried to the gut are able to survive (most do not). They then migrate forward to the food canal of the proboscis where they multi ply. Later infective forms reach the hypopharynx, and new hosts can be infected when the fly feeds. The cycle takes 12–14 days.
Brucei type (T. brucei, T. rhodesiense, T. gambiense, T. suis) The trypanosomes are carried to the gut, later passing for ward to the proboscis. From here they enter the hypopharynx and reach the salivary glands, where finally the infective forms are produced. This cycle takes 20–30 days.
The number of days given for the cycle to be completed are average ranges only. Individual cases may take a shorter or longer time than the number of days shown.
Within the tsetse belts, the usual method for pathogenic trypanosomes to be transmitted is by this cyclical transmission. But it is also possible for pathogenic trypanosomes to be transmitted by other means. One such method is mechanical transmission (see 6.5.3).
N.B. The non-pathogenic T. grayi develops in the mid-and hind-gut. This must be taken into account when dissecting species of palpalis group.
6.3.1 Trypanosomiasis in man (sleeping sickness)The symptoms (outward signs) of the disease may vary a great deal from one case to another and from one outbreak to another. In general, there is an early fever stage (trypanosomal fever) and a later stage that affects the nervous system. The variable symptoms make it a difficult disease for the doctor to diagnose (identify). Also, because the nervous symptoms are varied and may cause the patient apparently to alter his personality (he may become more aggressive or moody, or lazy), the ordinary villager may not understand that the patient is suffering from an identifiable disease.
There are two main forms to the disease.
Gambian sleeping sickness (see 18.104.22.168) is found in different areas of West and Central Africa (see Map 6.1) It is transmitted mainly by Glossina palpalis, G. fuscipes, and G. tachinoides (all palpalis group flies). The trypanosome parasite is called Trypanosoma gambiense.
Rhodesian sleeping sickness (see 22.214.171.124) is found mainly in East Africa and in the Zambezi river basin. It is transmitted mainly by Glossina morsitans, G. swynnertoni and G. pallidipes (morsitans group), but G. fuscipes and G. tachinoides (palpalis group) may also be important sometimes. Where it occurs, G. pallidipes may be a particularly efficient vector of Rhodesian sleeping sickness. The trypanosome parasite is called Trypanosoma rhodesiense.
126.96.36.199 Gambian sleeping sickness There have been very large and serious outbreaks (epidemics) of this disease in the past. A major epidemic occurred in Uganda in 1902–7, in which it is believed that as many as 200,000 people died (two out of every three in the affected population) . Medical authorities realised the full seriousness of the disease from this time. The disease develops slowly in man.
Often the disease occurs in communities at river crossings and watering points, where man and palpalis group flies come into close contact. Such places may form the starting points (foci, singular focus) for epidemics, if conditions are suitable.
It has been found that Gambian sleeping sickness is particularly likely to occur at the extreme northern boundaries of the distribution of Glossina palpalis and G. tachinoides. It is thought that is because the hot, dry, climatic conditions here force the tsetse flies to remain around river courses and water holes, and so into close contact with man. Man-fly contact is close and repeated, so Trypanosoma gambiense can build up in numbers and can easily be transmitted from infected to uninfected people (Figure 6.3), even though the total lumber of flies present may be quite small.
Map 6.1 Present day distribution of sleeping sickness in Africa. The solid line shows the approximate boundary between the western Gambian sleeping sickness, and the eastern and southern Rhodesian sleeping sickness. In the past, major epidemics of Gambian sleeping sickness have occurred around L. Victoria.
188.8.131.52 Rhodesian sleeping sickness Epidemics of this disease have not been as large as for Gambian sleeping sickness, but can be very serious locally. The disease develops more quickly in man than does Gambian sleeping sickness.
This is a disease mainly of remote villages, and of people whose occupations take them far into the bush, for example hunters and honey gatherers. It is thought that a man may pick up an infection by being bitten by an infected tsetse, whose normal hosts are wild game animals (Figure 6.4). A minor epidemic may occur when the infected man returns to his hone village, if this village is tsetse infested, by more people being bitten by infested flies.
6.3.2 Animal trypanosomiasis The trypanosome diseases of animals cone under the general term animal trypanosomiasis. The one affecting cattle is sometimes called nagana.
The main tsetse-borne trypanosome parasites in cattle are Trypanosoma vivax, T. congolense and T. brucei.Also pathogenic to domestic animals are T. uniforme, T. simiae and T. suis, but the last three are less often important in causing livestock disease than are the first three (see 6.2.3). There are also one or two other trypanosome species that can live in the blood of live-stock without doing any harm. We are not concerned with these.
The effect of animal trypanosomiasis varies according to the species of livestock (see liable 6.1), and of the breed. For instance, Trypanosoma simiae has little or no effect on cattle or horses, but can cause very severe disease in pigs. Zebu, the commonest breed of cattle in tropical Africa, is severely affected by nagana. The breed is highly susceptible to trypanosomiasis and cannot live for long in contact with a heavy fly infestation, even with the help of drugs. But some cattle breeds (e.g. Muturu, N'Dama) are less susceptible, although affected to sane degree: they are said to be trypanotolerant. These cattle can live in closer contact with tsetse fly than Zebu cattle can.
TABLE 6.1 Severity of disease caused by pathogenic tsetse-borne trypanosomes in men and various domestic animals
|Trypanosome||Cattle||Horses Donkeys||Goats Sheep||Pigs||Man|
|T. rhodesiense||-||-||-||-||+++ more acute|
|T. gambiense||-||-||-||-||+++ more chronic|
Key: - Hosts do not normally pick up the disease
(although experimental infections may be possible).
± slight disease
+ moderate disease
++ serious disease
+++ very serious disease
By the term acute disease is meant one that develops in months rather than years;
By the term chronic disease is meant one that develops slowly over years.
Fig. 6.3. Current ideas on the transmission of Gambian sleeping sickness. The arrow indicates that the sick man (seated) is acting as a reservoir for the infection; trypanosomes are taken in by a tsetse fly when it feeds on this infected person, and the fly itself becomes infected. After 20–30 days the fly is able to transmit the disease to uninfected people it later feeds on.
Fig. 6.4. Current ideas on the transmission of Rhodesian sleeping sickness. The arrow indicates that the animal (bushbuck, for instance) is acting as a reservoir for the infection. Trypanosoma rhodesiense is taken in by a tsetse fly when it feeds on the animal and the fly itself becomes infected. After 20–30 days the fly is able to transmit the disease to uninfected people it may feed on.
The seriousness of trypanosomiasis can depend on the general state of health of the domestic animal that has become infected with the disease. An animal that is well fed and rested has a much better resistance to the disease than one that is poorly fed, worked hard, pregnant, suffering from other diseases, or under any other kind of stress.
For tsetse flies to become infected they must feed on infected hosts having trypanosomes in their blood streams. These hosts form a reservoir for the trypanosomes, which continually infect 'clean' tsetse flies that feed on them.
Where uninfected men and animals are living in contact with fly,the chance of their becoming infected will depend on:
the infection rate in the tsetse flies
the ability and opportunity of the infected fly to pass on the infection
6.5.1 Infection rates in tsetse flies These can vary according to the area, to the species of tsetse fly, and to the species of trypanosome. The figures given in this section provide only a very rough guide.
The following table gives infection rates that the three important trypanosome species causing disease in cattle might have in morsitans group flies.
|Trypanosome species||Infection rates in morsitans group|
|1. T. vivax||Approximately 20% or more|
|2. T. congolense||Approximately 10%|
|3. T. brucei||Less than 1%|
Usually vivax infections are much commoner than congolense infections, as shown in the above table, but occasionally congolense may be commoner than vivax.
The infection rate may vary according to tsetse species:
|Glossina species||Overall infection rates (all trypanosome species combined)|
|Often 20% or more|
|Usually less than 10%|
In a particular area the infection rates in flies may depend on a variety of factors:
The infection rates in preferred hosts in the area
The ability of the tsetse fly to pick up an infection from the hosts. This ability may vary according to the species of fly.
The average age of the fly population. Older flies are more likely to have a mature infection than younger ones.
The temperatures experienced by the pupae and young adults. By laboratory studies it has been found that pupae and young adults of G. morsitans kept at a high temperature give rise to mature flies that can develop unusually high infection rates.
6.5.2 The ability and opportunity of infected flies to pass on an infection
184.108.40.206 Ecological opportunity In order to pass on the infection to man or domestic animals, tsetse flies must live in the same habitat as these hosts.
Most fusca group flies are for this reason not much danger to cattle. Most fuscagroup flies live in rain forest and other heavily wooded areas.
Cattle do not normally live close to such places. But the two species G. brevipalpisand G. longipennis are important exceptions to this rule. Although they belong to the fusca group, they do live in places where they can come into close contact with livestock (see Volume II, Ch.4). Glossina fusca itself often lives at the fringes of forest and may therefore come into contact With cattle.
The palpalis group flies G. palpalis, G. fuscipes and G. tachinoides can also transmit cattle trypanosomiasis, but the disease transmitted is usually less severe. Cattle can live closer to these species than they can to morsitans group species. It may be necessary for cattle to live in rather close contact with palpalis group tsetse, in order to get water and grazing. The resulting disease probably accounts for a large part of the total trypanosomiasis of cattle in West Africa.
The morsitans group. The main species of this group (G. morsitans, G. swynnertoni, G. longipalpis and G. pallidipes) are so efficient at transmitting disease that infested areas are avoided by cattle owners as far as possible. Cattle being trekked for fresh grazing or for marketing may have to pass through morsitans country, and suffer very heavily. Locally, populations of morsitans group flies may live mainly on cattle, and cause high levels of trypanosomiasis.
220.127.116.11 Susceptibility of host to trypanosomiasisThis has also been dealt with under 6.3.2 (see table 6.1). An animal may be quite resistant to a trypanosome species and fail to suffer any disease when exposed to it. For example, man does not suffer any disease from exposure to Trypanosoma vivax or T. congolense, nor to ordinary strains of T. brucei.
Some hosts may pick up a trypanosome parasite without apparently suffering any harm. Many game animals appear to be in this group.
Some breeds of African cattle can tolerate a trypanosome challenge.
6.5.3 Mechanical transmission (see also 18.104.22.168) If a fly such as Stomoxys or a tabanid feeds on an infected animal and is interrupted in its meal, it may fly off to an uninfected animal to finish its meal. In this way some infected blood can be carried while still fresh from the first animal to be injected into the second. Transmission of trypanosomiasis in this way is called mechanical transmission.
In the case of trypanosomiasis outbreaks at the edge of tsetse belts it is difficult to be certain whether they are caused by mechanical transmission, or by very small numbers of scattered tsetse spreading the disease from the main belt. Inside the main tsetse belts cyclical transmission is probably much more important than mechanical transmission.
Trypanosoma vivax appears to be more easily transmitted by the mechanical method, than T. congolense.