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Immunologic analyses of sera from 47 selected individuals living in a mixed filariasis transmission zone in Gabon were carried out. Onchocerca volvulus, Loa loa, Mansonella streptocerca, and M. perstans are transmitted in this region. Based on parasitologic findings and age, the 47 individuals were stratified into four groups: microfilaria negative (Mf-) children (3-15 years old), Mf- adults (> 15 years old), microfilaria positive (Mf+) children and Mf+ adults. For descriptive purposes, the term microfilaria positive refers to individuals with skin and blood microfilariae. Antifilarial antibody titers were determined using an enzyme-linked immunosorbent assay with Dipetalonema viteae antigens. In general, children had higher titers of IgG antibodies than adults. For the IgG1, IgG2, and IgG3 subclass responses, both age and microfilarial status appeared to be important variables since Mf- children consistently had the highest titers whereas Mf- adults had the lowest titers. For the IgG4 antifilarial response, only the microfilarial status was an important variable. All Mf+ individuals had significantly higher levels of IgG4 antibody than Mf- individuals. Pooled sera of Mf- and Mf+ individuals reacted with similar O. volvulus antigens on Western blots. Control sera of individuals who did not reside in the study area, but who had single infections with L. loa or M. perstans, did not react with any O. volvulus antigens.

Using the Raji cell radioimmune assay, we found low levels of circulating immune complexes (IC) in a small percentage of patients with schistosomiasis and filariasis. C3d levels, measured by immunoprecipitation, were elevated in a large number of these patients, whereas complement levels were within normal limits. Proteinuria was not found in any of the 55 patients studied. Circulating IC or elevated C3d levels were not found in any of the 19 patients with hydatidosis. The increased C3d levels, apparently not related to circulating IC, may be due to direct complement activation by parasite antigens or to sequestered IC. The latter hypothesis appears more attractive because the highest levels of C3d were found in schistosomiasis whereas schistosome antigens were unable to activate complement in vitro.

Direct examinations of blood and serologic tests were used to assess the intensity of the filarial endemicity in Gabon. Both methods revealed that the prevalence of filariasis with microfilaremia (Loa loa and Dipetalonema perstans) is presently very elevated. The areas that are more involved are the forest regions as well as the highest ones (ogoue, Lolo, Ngounie). Women appeared to be suffering from loasis to a lesser extent than men. On the other hand, the prevalence was identical for Dipetalonema perstans. For the gabonese population one can approximately say that 3 out of 5 adults patients have filariasis, two being D. perstans and one L. loa. Consequently these affections represent a major public health problem. Finally one must note that no case of lymphatic filariasis has been identified.

Immunoelectrodiffusion was used to detect free serum antigens after treatment with diethylcarbamazine (DEC) and Suramin (S) in 30 cotton-rats infected with Litomosoides carinii. Precipitin lines between an antiserum produced by an adult worm extract and the test serum indicated the presence of free antigens in the serum. The number of precipitin lines was between one and 3. Before treatment only 13.3% of infected rats were positive compared with 83.7% after treatment. There was a difference in the delay before the appearance, the persistence and, possibly, the nature of antigens liberated by DEC and S treatment. With DEC release occurred earlier than with S and the antigens persisted for a shorter time. With combined DEC + S therapy the results were similar to those obtained with DEC and S used alone.

In examining skin snips from 1,830 Gabonese 32 were found to have microfilariae indistinguishable from Microfilaria rodhaini. The microfilaria is 300.2 microns long and 2.2 microns wide. Specific clinical signs were not identified, as the same person often was infected by other filaria.

Administration of ivermectin (Mectizan) in Loa loa filariasis induces a significant decrease of microfilaria load within 3 to 15 days. The excellent tolerability of the drug in patients infested simultaneously by O. volvulus and Loa loa allows to use ivermectin in mass treatments in geographical areas where the same patients can be affected by several filariasis. There is almost no efficacy against Mansonella perstans. A second administration one month after the first one in Loa loa affected patients has not led to a complete disappearance of microfilaria; the tolerability has still been good. Efficacy on Ascaris lumbricoides appears to be excellent; further studies are necessary in the cases of trichuriasis, ancylostomiasis and strongyloidiasis.

Mebendazole (Vermox 500) in long duration treatment 2 g/day for 3 to 4 weeks, is sometimes indicated in therapy of Loa Loa filariasis but remains the only active therapy on M. perstans filariasis. The perfect product tolerance and efficacy allow large indication in treatment of this last filariasis, the most often found in Gabon (Central Africa).

Two clinical studies were carried out in Gabon, Africa to evaluate the efficacy, safety, and tolerability of ivermectin in the treatment of patients with Loa loa infection. In the first study, 35 patients received single oral doses of ivermectin, 5-200 mcg/kg body weight. Blood microfilariae levels did not decrease after a single oral 5, 10, 30, or 50 mcg/kg dose of ivermectin, but levels did decrease after doses of 100, 150, and 200 mcg/kg. The most efficacious dose was 200 mcg/kg; mean blood microfilariae levels decreased to 12% of mean pretreatment values by day 15 and remained decreased for 28 days. A second study evaluated the safety and efficacy of ivermectin in patients with multifilarial infections. All 17 patients had concomitant Loa loa and O. volvulus infection. M. perstans affected 5 of the patients. Sixteen patients also had infections due to intestinal nematodes. The patients each received single oral doses of 200 mcg/kg ivermectin. Ten days later, the mean Loa loa blood microfilariae level had decreased to 20% of the mean pretreatment level. O. volvulus dermal microfilariae densities were reduced to 2% of the pretreatment levels. A minimal increase in blood microfilaria levels was observed on day 28. In contrast, dermal microfilariae levels remained near zero for the duration of the study. Intestinal infection due to Ascaris was eradicated in all of the affected patients by day 23; efficacy against Trichuris and hookworm infections, however, was poor. All patients tolerated ivermectin well including those with multiple infections.

The activity of diethylcarbamazine (DEC) and suramin (S) on Litomosoides carinii filariasis was assessed by the study of microfilaraemia in 30 cotton-rats (Sigmodon hispidus). DEC (per os) and S (injected subcutaneously) were administered alone or in combination, at doses of 50 mg/kg/day for five consecutive days. 15 rats were treated at day 91 after infection and 15 others at day 184. The immediate but temporary efficacy observed with DEC was in contrast to the delayed and long-lasting effect of S. With combined therapy there was neither antagonism nor synergism. The demonstration of an apparently smaller reduction of parasitaemia in the rats treated with DEC before the plateau phase of the microfilaraemia was consistent with the lack of action of this drug on microfilariae outside the blood system.