Organism There are two forms of human sleeping sickness in Africa - one due to Trypanosoma brucei gambiense and the other caused by T. b. rhodesiense. A third form T. b. brucei is found in cattle, causing considerable economic loss.
The trypanosome exists in several different forms during its life cycle (Fig. 15.18). When seen in human blood, the trypomasti-gotes are long and slender, short and stumpy, or intermediate between the two, probably representing a cycle of antigenic variation (Fig. 15.18). They are introduced into the blood by the bite of the tsetse fly and multiply locally. After being disseminated round the body, they continue to multiply, rapidly in T. b. rhodesiense, less so in T. b. gambiense. They are infective to any tsetse fly when the fly bites, being taken up into the mid-gut. They multiply, migrate into the space between the peritrophic membrane and the gut wall and pass forward to the salivary glands. The epimastigote developmental form changes into a trypomastigote to infect the next person who is bitten.
Clinical features The bite of a tsetse fly generally causes a local reaction, but 7-10 days after it has subsided, it can become red and inflamed, which is the first sign of infection. Trypanosomes multiply at the bite site and aspirated fluid will contain the dividing forms. In T. b. gambiense, an enlargement of the lymph glands takes place, especially those in the cervical region. This rarely occurs in T. b. rhode-siense, the disease progressing rapidly to involve the CNS, with invariably a fatal outcome. The main clinical signs are fever and protracted headaches. In T. b. gambiense, the course is much more prolonged and personality changes may be the indication of infection, but inevitably the disease leads to progressive lethargy, emaciation, coma and death.
Diagnosis is by finding trypomastigotes in the blood, CSF or gland puncture. The blood smear should be repeated several times before a negative diagnosis is made. Parasite concentration techniques, such as capillary tube centrifugation or minianion exchange centrifugation, are valuable. Antibodies may be detected by serological techniques, while circulating antigen using the Card Agglutination Test (CATT) in which a drop of finger prick blood is mixed with a suspension of trypanosomes has revolutionized diagnosis. This technique is particularly useful for surveys in the field.
Transmission is from the bite of the tsetse fly in which the parasite goes through a devel opmental stage. However, as the infective form for the fly and the human is the same (trypomastigote), mechanical transfer can occasionally happen if a contaminated fly bites another person within a short space of time.
The tsetse fly (Glossina) is easily recognized by its characteristic stance and behaviour. It is a large, powerful fly and rests on a surface with wings folded like a pair of scissors. Within the venation of these wings, a characteristic hatchet cell (Fig. 15.14) can be defined which helps in identification. However, when passing through 'fly' country, there is normally no doubt about its presence, as tsetse flies attack any moving object in large numbers, rendering the most painful bite. They are attracted by movement and will cling to the side of a vehicle travelling at 30-40 km/h without being dislodged. They prefer dark colours and if there is a large object, they will fly to that in preference. They are more abundant near their preferred breeding place in the sandy soil beside rivers.
Distribution of tsetse flies is shown in Fig. 15.15, where it will be noticed that distinct species are often related to particular sleeping sickness areas (compare with Fig. 15.16). Table 15.5 is a simplified guide for assistance in identifying the species of Glossina, but professional confirmation should always be obtained.
Incubation period 3 days to 3 weeks in T. b. rhodesiense, months to years for T. b. gam-biense.
Period of communicability The trypanosome takes 12-30 days to complete its developmental stage in the tsetse fly, and depending on temperature, the fly then remains infected for life. Humans can be infected for many years with T. b. gambiense, but due to the shorter history with T. b. rhode-siense, the animal reservoir is probably more important.
Occurrence and distribution T. b. gambiense mostly occurs to the west of the Central Rift Valley of Africa, containing the lakes of Tanganyika, Kivu, Edward and Albert,
while T. b. rhodesiense is to the east (Fig. 15.16). T. b. gambiense infection is particularly prevalent in Congo (formerly Zaire) and T. b. rhodesiense in Tanzania. These two diseases differ markedly in their epidemiology and control.
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