**3. The vectors**

*Chrysops dimidiata* and *C. silacea (*Figure 3) (family *Tabanidae*) live in the canopy, and are particularly attracted by smoke and blue tissue. They lay eggs on mud or leaves overhanging water, and the larvae develop in detritus, taking a year before they pupate, with probably seven moults. The pupa is partially buried and the adult emerges after 1-3 weeks. Chrysops is a good intermediate host or vector for *Loa loa*, and it is not unusual to find more than one hundred infective larvae in one fly. For vector control, William (1963) used water emulsions containing various concentrations of DDT, dieldrin, aldrin or gamma-BHC, and found that *Tabanus* larvae were more susceptible than *Chrysops* larvae to all the insecticides tested. Dieldrin emulsion can keep breeding sites free of tabanid larvae for at least eight months (Crewe & Williams, 1964), and has been proposed for the control of chrysops larvae. However, vector control is difficult due to the scale and remote location of breeding sites.

Encephalitis Due to *Loa loa* 345

Calabar swelling commonly appears on the arm, elbow, face, or chest, and is often accompanied by localized pruritus and discomfort. Another diagnostic sign is ocular passage (Figure 4) of the adult worm (eye worm). These two signs are the most frequent and specific among autochtones and visitors. However, eye worm is more frequent in autochtones than in visitors, while calabar swelling is more common among visitors (82%) (Churchill et al., 1996). Visitors may developed a syndrome of immunological hyperresponsiveness (Nutman et al., 1986) characterized by high titers of antifilarial antibodies, elevated IgE levels (Klion et al., 1991), hypereosinophilia, frequent pruritus around the angiodema, and complications such as endomyocardial fibrosis, renal disease, lymphoma (Gerd et al., 1996), and subcutaneous nodules. Calabar swelling and eye worm may appear alone or simultaneously. Retinal hemorrhage can occur in case of high microfilaremia (Toussaint & Danis 1965), especially after treatment with DEC or ivermectin. Examination of the fundus is therefore necessary, even in the absence of antifilarial treatment. Onset of calabar swelling and eye worm may be followed by cardiac (Andi et al., 1981), renal (Pilay et al., 1973; Bariety et al., 1967) or neurological disorders. Non specific symptoms include

Direct specific diagnosis is based on detection of *Loa loa* microfilaria in peripheral blood or of the migrating adult worm. In wet films of blood samples taken around noon, *Loa loa* microfilaria are highly mobile, with a snake-like movement among red blood cells, enveloped in a translucid membrane (sheath). Body size is 231-300 µm by 5-7 µm. A concentration technique on fresh blood can be used: 1 ml of blood is diluted 10X in PBS (Akue et al., 1996), then red cells are lyzed with 2% saponin and the solution is centrifuge at 2000 rpm for 10 minutes. The pellet is smeared on a slide and examined under a microscope. Microfilariae appear as described above. Microfilariae can also be stained with Giemsa in a

**4. Diagnosis of loaisis 4.1 Clinical diagnosis** 

pruritus, fever, urticaria, rash, myalgia and arthralgia.

Fig. 4. Ocular passage of *Loa loa* adult worm

**4.2 Biological diagnosis**  *Direct specific diagnosis* 

Fig. 3. Chrysop silacea, vector of Loa loa

Fig. 3. Chrysop silacea, vector of Loa loa
