Ao Editor: A Carta ao Editor, transcrita a seguir, foi enviada ao Dr. Carlos M. Morel, Diretor da TRD – WHO, em Geneva, sugerindo pequena correção no relatório da WHO/TRD nº 811.
Dear Dr. Morel:
The above mentioned technical report, wrote in 1991, constitutes one of the most important and useful documents concerning Chagas Disease Control (WHO 1991)5.
Very recently, when we were analysing an international article about the matter, a small mistake based on entomological indicators was found and the same mistake exists in the WHO document, concerning the indexes of density and crowding, as follows:
a) Density index: Number of triatomines captured/ Number of houses examined x100. IT MUST BE CHANGED to: “Number of triatomines captured/ Number of houses examined”;
b) Crowding index: Number of triatomines captured/ Number of houses with triatomines x100. IT MUS BE CHANGED to: “Number of triatomines captured/ Number of houses with triatomines”.
Comments: the mistake is the final “relativization” (percentage), since these are mere absolute correlations and not conventional indexes. In the classical Brazilian Dictionary “Aurélio”, density means: ‘the quality of that is dense, compact; correlation between the mass and the volume of a body. …and, sociologically, “grade of concentration of the collective life”, or “number of inhabitants of a determined area, for example, by square kilometre”1. In practice, the bulk we need to measure is the number of triatomines in a determined space, in order to estimate the epidemiological risk of Chagas’ disease transmission. Our basic denominator (the “determined area” where density is being evaluated), in programme control, will be the unit “house”, generally corresponding to different ensembles such as “localities”, “municipalities”, “districts”, “provinces”, “states” or “countries”, according to the extension and/or the purpose of the measure. Multiplication for 100 does not make sense, at this time: what we need to know is “how much is the amount of existent (or collected) triatomine bugs in determined region”, for instance, 3,2 bugs/ investigated house (general density data), or better, 44 bugs/positive house (“crowding data), instead of 320% (prior “density index”) or 4.400% (prior “crowding index”). An important example for the comprehension of the problem can be found in the book of Angel Valencia, in Bolivia, where “indice de densidad” is understood as “Triatomas capturados/Casas exploradas” and “indice de hacinamiento” as “Triatomas capturados/Casas com triatomas”4.
Both these mistakes apear is the page 52 of the English version of the mentioned report, and in page 55 of the Spanish version. In this last one, the name for “density index” is “índice de densidad”, while for “crowding index” is translated as “índice de hacinamiento”.
It must be remembered that both these indicators are generally considered of secondary importance and usually are not employed neither in current control programmes nor in the “Southern Cone Initiative for the Elimination of Triatoma infestans“2 6. Nevertheless, the density of triatomines inside determined ecotopus is very important in research projects which have the objective to measure the dynamic or the risk possibilities of T. cruzi transmission. Another important correlate subject concerns with the studies about insect population dynamic3.
We thank you very much for you attention and ask you to consider this question with Dr. Peter de Raadt, who was the chairman of the workshop which originated the document.
Yours sincerely,
1. Ferreira ABH. Novo Dicionário da Língua Portuguesa. Nova Fronteira Editora. Rio de Janeiro, 1975.
2. Fundação Nacional de Saúde. Controle da doença de Chagas. Diretrizez técnicas. Ministério da Saúde. Brasília, 1994.
3. Rabinovich JE. Ecologia poblacional de triatominos. In: Carcavallo RU, Rabinovich JE, Tonn RJ (eds) Factores biológicos y ecológicos en la enfermedad de Chagas. OPS/Ministerio de Salud y Acción Social. Buenos Aires, p. 129-147, 1985.
4. Valencia A. Investigación Epidemiológica de la Enfermedad de Chagas. Ministerio de Previsión Social y Salud Pública. La Paz, 1990.
5. World Health Organization. Control of Chagas Disease. Report of a WHO Expert Committee. WHO Technical Report Series no 811. Geneva, 1991.
6. World Health Organization. Iniciativa Cono Sur. Informe del taller sobre definición de indicadores para la certificación de la eliminación del Triatoma infestans. PAHO/HPC/HCT/. Washington, p. 94-20, 1994.
João Carlos Pinto Dias
Senior Researcher of FIOCRUZ
WHO Group of Parasitic Diseases
Lileia Gonçalves Diotaiuti
Associate Researcher of FIOCRUZ
TDR Committee (AFR/Secondary Vectors)
Recebido em 14/09/98.
