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The Application of Immunological Tests of Sera from Captured Wild Animals to the Study of Yellow Fever Epidemiology¹

A study has been made of the response of some of the more common vertebrates of Brazil to yellow fever virus introduced by the bites of infected mosquitoes. The results obtained are summarized in Table 11.

Akodon arviculoides, Orzomys and Didelphis paraguayensis failed to circulate virus following such exposure; Metachirops opossum, Proechimys dimidiatus, Dasypus novemcinctus and Euphractus sexcinctus either circulated virus only occasionally or in too low concentrations to be epidemiologically significant. Metachirus nudicaudatus circulated virus with sufficient regularity and abundance to be a potential source of infection for blood-sucking arthropod vectors.

A. arviculoides, Oryzomys and D. paraguayensis fail to develop neutralizing antibodies following the bites of infected mosquitoes; in M. opossum, P. dimidiatus, D. novemcinctus and E. sexcinctus these antibodies appear irregularly or in barely demonstrable concentrations; furthermore, in the first two species mentioned they are not permanent. M. nudicaudatus produces neutralizing antibodies with regularity but they tend to disappear within a few months. Consequently, negative neutralization test results obtained in immunity surveys based on sera from any of these animals are of no value. Positive results obtained with sera from M. opossum or M. nudicaudatus would, if specific, indicate recent infections; positive results with sera from E. sexcinctus or D. novemcinctus might indicate either a recent or an old infection. Complement fixation tests are applicable only to sera from P. dimidiatus; a positive result with sera from this species indicates a recent infection.

Immunity surveys based on the immunological examination of sera from captured wild animals are valid only when it is well established that animals of the species under consideration are capable of forming antibodies with regularity following the bites of infected insect vectors. Consideration must also be given to the length of time during which these antibodies, if formed, persist in a demonstrable concentration in the serum of the infected animal.

¹ The studies and observations on which this paper is based were conducted with the support and under the auspices of the Servico de Estudos e Pesquisas sôbre a Febre Amarela of the Ministry of Education and Health of Brazil in co- operation with the International Health Division of The Rockefeller Foundation.