1. One thousand four hundred and twenty-eight sera from as many residents of 63 localities in the Anglo-Egyptian Sudan, Kenya, Uganda, the Belgian Congo and Spanish Guinea have been studied with reference to their power to neutralize the West Nile virus. Some of the sera were also studied with reference to their capacity to neutralize the viruses of St. Louis and Japanese B encephalitis, both of which are related immunologically to West Nile virus.

  2. The results show that antibody against this group of viruses exists in the serum of residents of every country included in the survey except Spanish Guinea, for which the data are inadequate.

  3. The West Nile virus has apparently been active in the Sudan, Uganda, Kenya, and in the Congo. The St. Louis virus has apparently been active in Kenya and the Congo and possibly also in the Sudan. The Japanese B virus has apparently been active in the Sudan and the Congo.

  4. In several areas the incidence of humoral immunity to this group of viruses is compatible with recent epidemic activity, but in most of the areas there is evidence that more than 1 of the 3 viruses has been active.

  5. In the region where the West Nile virus was isolated in 1937, a survey carried out 2 years later showed that 16 per cent of the sera of residents contained neutralizing antibody, and so far as was determined this humoral immunity was entirely specific against the West Nile virus.

  6. The serum of 1 of 4 “blue monkeys” caught in the Kakamega forest in Kenya neutralized both West Nile and St. Louis viruses.

  7. The cross-neutralizing power of St. Louis antisera against West Nile virus and the capacity of Japanese B antisera to neutralize St. Louis and West Nile viruses at present seriously complicate immunological studies with the agents mentioned; but the same methods used for the differentiation of the viruses are applicable in determining the specificity of humoral antibody.

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The studies and observations here reported were conducted with the support and under the auspices of the International Health Division of the Rockefeller Foundation and the Medical Department of the Government of the Uganda Protectorate.

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