CONTROL OF DISEASES PROPAGATED BY
ARTHROPOD VECTORS”
By JUSTIN M. ANDREWS
Scientist Director, U. X. Public Health Service
There are no less than forty diseases, each of unique etiolo,gy, that are transmitted to some extent by arthropods. From the standpoint of public health significance, these disorders range al1 the way from rare and exotic medical curiosities to such mighty killers as plague, cholera, malaria, yellow fever, and typhus, the epidemic ravages of which have spared neither prince, pope, nor pauper, have decided the outcome of major wars, and have caused the abandonment of fertile lands and large cities. In short, the more important arthropod-borne diseases have been, among other forces, crude but indisputable determinants of the course of the world’s social, political, and economic history.
The question 1 should like to put before this group is the following: Will these pestilentes continue to harass civilization-to hinder its progress-to paralyze shipping, agriculture, industry, or have we reason- able hope, or even assurance, of reducing these hazards to historical trivi- alities? 1 doubt that a categorical answer to this proposition can be given, and 1 do not intend to enter into detailed consideration of al1 its aspects; nevertheless, it may be instructive to reflect briefly upon certain of its ramifications.
In the first place, as far as 1 know, none of the essential elements of these “heterologous infection chains”-to use the apt expression of Dr. Karl F. MeyerLhas been completely obliterated from the face of the earth. In spite of remarkable accomplishments in the national eradica- tion of aedine and anopheline species, man has not been as successful in the total annihilation of microbes, viruses, their arthropod vectors or their reservoir hosts as he has been with the dodo, the passenger pigeon, the great auk and severa1 other species of birds now extinct. Until he does accomplish the global elimination of any one or more of the biologi- cal factors necessary for the existence of arthropod-borne disease, it is only logical to concede the possibility that, under favoring circumstances, these diseases could occur in epidemic outbreaks of terrible magnitude. However, this seems improbable in the more enlightened portions of
* From the Federal Security Agency, Public Health Service, Communicable Disease Center, Atlanta, Ga.
Beginning with articles appearing on page 998 to ‘1056, a series of scientific papers submitted to the VI Pan Ameriean Conference of National Directora of Health, held in Mexico City, October 4 to 7, 1948, will be published.
1 Meyer, Karl F.: The Animal Kingdom, A Reservoir of Human Disease. Ann. Intern. Med., 29: 326-347,1943.
[November 19481 ARTHROPOD VECTORS 999 the world where the depredations of diseases spread by insects, ticks and mites appear to have Iessened materially during rece& years. With the knowledge now available, it is hard to visualize the BIack Death again possessing London or other major cities of Europe or that yellow fever could return to wipe out 70 per cent of the white residents of Ameritan communities%
L much of which was fought in the tropics, where malaria seriously threat- British and Ameritan troops have returned only recently from a war, ened though it never actually determined the outcome of military opera- tions. The United States of America has contained, with scant evidente of transmission from these sources, the impact of probably a hundred thousand or more repatriated malaria-carrying service men and women, many of whom returned to traditionally endemic areas. The Allied Forces in the Mediterranean Theater of Operations looked squarely into * the face of an incipient OId World typhus epidemic in Italy-and sup-
pressed it calmly and effectively. Since the war, an epidemic of cholera in Egypt has been nullified far short of the destructive proportions it would probably have attained a few decades ago-and, what is even more impressive, there appear to have been no secondary outbreaks stemming from the Egyptian experience.
Numerous Latin Ameritan countries reported at the Fourth Interna- tional Congresses of Tropical Medicine and Malaria the extirpation of 8 Aedes aegypti and, pari passu, of dengue and urban yellow fever from
within their borders; others are undertaking similar programs with con- fidence. Chile indicates that malaria has been eliminated from that country. Anopheline eradication programs are under way in Sardinia and Cyprus-and are planned for other islands. We feel that another three years will see the end of malaria as an endemic disease in our * country.
TheSe accompIishments and prospects should not be viewed with stul- tifying complacence nor should they engender a faIse sense of security. They merely mark progress toward a distant goal. Preventive medicine has come a long way since “experts” gravely argued the bedside range of the infectivity of yellow fever patients,” expounded the so-called “Law of Malaria,“3 and seriously advocated perfume and tobacco-smoking as plague preventives.4 ’
The epoch of arthropodal epidemiology portended by Mason’s demon- stration of íYaria larvae in mosquitoes (1S77)6 and firmly established by 2 Keating, J. M.: A History of the Yellow Fever. The Yellow Fever Epidemic of 1878 in Memphis, Tenn. Memphis pp. 454 + VIII, 1879.
s Scott, H. Harold.: A History of Tropical Medicine. Vol. 1, pp. 648 + XVIII, London.
4 Ibid., Vol. II, pp. 570 + IV.
1000 PAN AMERICAN SANITARY BUREAU [November Smith and Kilbourne’s revelation of the tick transmission of Texas fever (1893)ô has been one of brilliant discovery. In rapid succession, the essential facts were elucidated concerning the transfer of malaria, yellow fever, plague, typhus, and a host of lesser ills spread by arthropods. These expositions have provided the basis for a cooperative assault by the sanitarian and the therapeutist in obstructing the transmission of these diseases.
It is the application of Ibis technical knowledge, plus administrative proficiency, that has made possible the achievements mentioned above. In the natural evolution of preventive medicine, these efforts have usu- ally been restricted at first to so-called “Control” activities, the primary intent being to halt the progress of an existing disease. A natural exten- sion of this objective has been to devise and execute measures which preclude its reappearance, that is, to “prevent” disease. To this lexicon of preventive medicine, Dr. Fred Soper and his associates have made an audacious addition which is undoubtedly here to stay. The verb “eradicate” is undoubtedly a fugitive from the jargon of journalism, but Dr. Soper and his cohorts have, by ‘their own acts, invested this superla- tive with a new dignity and verity.7, a+ g Already it has legitimate appli- cation on a national scale; extension to hemispherical inclusiveness is contemplated. It is not impossible that even in our own lifetime we may see the word used with studied seriousness to comprehend the global nullification of one or more diseases.
In theory, the eradication of arthropod-borne diseases can be accom- plished by the extermination either of the causative agent or the vector, the choice depending, presumably, upon which of the two can be more effectively and economically eliminated. The latter procedure, vector eradication, is exemplified by the well-known accomplishments of Dr. Soper and his colleagues.** g Without doubt, it is the method of choice when feasible, as it is probably easier to prevent or deal with the re-entry into clean areas of arthropod vectors than infected humans. It is par- ticularly applicable to geographic or biological islands where the problems of infiltration and reintroduction are minimal, though, as has been dem-
6 Smith, T., and Kilborne, F. L.: Investigations into the nature, causation and prevention of Texas or southern cattle fever. U. S. Dept. Agrie., Bur. Animal Industry, Bull. No. 1. Washington Gov. Print. Office, pp. 301, 1893.
r Soper, F. L., and Wilson, D. B.: Species Eradication. A Practica1 Goal of Species Reduction in the Control of Mosquito-borne Disease. J. Nat. Mal. soc., 1: 5-24, 1942.
* Soper, Fred L.; Wilson, D. Bruce; Lima, Servuio; and Antunes, Waldemar Sa: The Organization of Permanent Nation-wide Anti-Aedes Aegypti Measures in Brazil. pp. 137. New York. 1943.
19-481 ARTHROPOD VECTORS 1001 onstrated, it can be used successfully over extended continental areas.a v g * lo
L Thusfar, attempts at the eradication of arthropod-borne diseases solely by chemical or biological destruction of the etiologic agent have not been as successful as efforts at its exclusion, e.g. yellow fever from the United States. This has been due to the imperfections of chemotherapeutics, antibiotics and vaccines, and to the great difhculty of obtaining thorough application on a wide enough scale. Nevertheless, the role of microbi- cides, viricides and vaccines in reducing reservoirs of infection for arthro- pods is already considerable and is gaining steadily in importance. When used in conjunction with insecticides or acaricides, these disinfec- tive measures give promise of disease eradication. Thus, if vector density and the frequency of human carriers can be reduced simultane- ously, a point will be reached ultimately where the statistical probability v of transmission is nil, and the disease disappears. This is the principie
on which we are basing our malaria eradication program in the U. S. A. It is apparent that certain diseases spread by arthropods cannot be eradicated because they are perpetuated in arthropods or other lower animals. These include yellow fever, plague, relapsing fever, tularemia, the vira1 encephalitides, certain leishmanial infections, and severa1 of the rickettsial diseases. Their prevalence can, of course, be reduced mark- edly by attacking the vector which transmits from reservoir host to man, and, where the reservoir species is domestic, by its destruction, treatment or immunization. In countries where lower animal reservo% do not exist, the disease can be excluded by appropriate foreign quarantine measures.
Certain other diseases, such as malaria, while readily eradicable in some places present formidable difliculties in others because of the igno- rance, poverty, and vast human reservoirs of infection involved, for ex- ample, in tropical Africa, India, South China, etc. One wonders what the social and economic consequences might be if the burden of commu- nicable disease were lifted from such populations !
1002 OFICINA SANITARIA PANAMERICANA [Noviembre include the dissemination of diseases spread by arthropods), the possibili- ties for halting progress in the eradication of these infections is evident. To return to the question 1 raised earlier, my provisional answer would be about as follows: Barring world conflicts, especially those in which the end would seem to justify such ruthless means as biological warfare, there is every reason to believe that the present trend of arthropod-borne dis- eases should continue downward. None of them need ever threaten civilization again where the means and the will exist to control and pre- vent them. Some of these diseases appear to be susceptible to eradica- tive technology, certainly within the more advanced countries of the world.
It is to discuss the technical and administrative phases of t,hese pro- cedures that we have met together. May 1 thank you in advance for your contributions. 1 trust that you will enter heartily into the discus- sion of the following papers and that the occasion will be mutually stimulating and instructive to us all.
CONTROL DE LAS ENFERMEDADES TRANSMITIDAS POR ARTROPODOS (Sumario)
Existen por lo menos cuarenta enfermedades, cada una de etiologfa distinta, en cuya transmisión intervienen en algún grado los artr6podos. Desde el punto de vista de significación sanitaria, estos padecimientos varian desde raras y exóticas curiosidades m6dicas hasta los temibles azotes que representan la peste, el c6lera, el paludismo, la fiebre amarilla y el tifo, de cuyos estragos no se libra nadie y que han determinado el resultado de importantes guerras y causado el abandono de tierras fértiles y grandes ciudades. En resumen, las m& importantes enfer- medades de transmisión por artrópodos han sido, entre otras fuerzas, crudas pero indiscutibles determinantes del curso de la historia social, política y económica del mundo.
Ninguno de los elementos esenciales de estas “cadenaa heterólogas de infec- ci&“-para usar la acertada expresión del Dr. Karl F. Meyer-ha sido eliminado completamente de la faz de la tierra. A pesar de notables adelantos en la erra- dicación nacional de las especies abdicas y anofelinas, el hombre todavia no ha logrado la exterminación de los microorganismos y virus, o de sus vectores artró- podos o huéspedes reservorios, tal como ha hecho en el caso del dido, la paloma silvestre Ectopistes migratorius, el gran “auk” y otras especies de aves hoy ex- tintas. Mientras no se consiga la eliminación mundial de uno o m8s de los factores biológicos necesarios para la existencia de las enfermedades transmitidas por artrópodos, es lógico admitir la posibilidad de que, en circunstancias favorables, estas enfermedades se presenten en forma de brotes epidbmicos de terrible magni- tud. Sin embargo, esto no parece probable en las regiones más adelantadas del globo, donde los estragos producidos por las enfermedades transmitidas por insectos, garrapatas y Icaros parecen haber disminuido en años recientes.
19-4~1 ARTRÓPODOS VECTORES 1003 cien mil o más hombres y mujeres del servicio militar portadores del paludismo, muchos de los cuales regresaron a zonas tradicionalmente endemicas. En el Teatro Mediterráneo de Operaciones, las Fuerzas Aliadas confrontaron y domi- naron en Italia, una epidemia incipiente de tifo. Terminada la guerra se logro vencer una epidemia de cólera en Egipto, mucho antes de que alcanzara pro- porciones destructoras.
Numerosos países latinoamericanos comunicaron en el Cuarto Congreso In- ternacional de Medicina Tropical y Paludismo, haber extirpado de su territorio el Aëdes aegypli, yen consecuencia, el dengue y la fiebre amarilla urbana, y otros han emprendido programas similares con plena confianza de éxito. En Chile el palu- dismo ha sido eliminado y ya se han iniciado campañas de erradicación anofelina en Cerdeña y Chipre, proyectándose actividades semejantes para otras islas. Consideramos que en tres años mas veremos desaparecer de nuestro país el palu- dismo como enfermedad endémica.
La 6poca de la epidemiología artropódica vislumbrada por la demostración por Manson de larvas de filarias en mosquitos (1877), y establecida en firme con la comprobación por Smith y Kilbourne de la transmisión de fiebre de Texas por la garrapata (1893), ha sido una de descubrimientos brillantes. En sucesión rápida fueron descubiertos los hechos fundamentales sobre la transmisión del paludismo, la fiebre amarilla, la peste, el tifo y de gran número de males de menor importancia transmitidos por artrópodos. Estos descubrimientos han constituido la base de una lucha conjunta del higienista y el terapeuta para evitar la propagación de estas enfermedades.
1004 OFICINA SANITARIA PANAMERICANA [Noviembre 19.JS] estos recursos de desinfección son prometedores de éxito en la erradicación de las enfermedades infecciosas. Asf, pues, si se reducen simultáneamente la densidad del vector y la frecuencia de portadores humanos, se llega a un punto en que las probabilidades estadfsticas de transmisión son nulas, y desaparece la enfermedad. Sobre este principio hemos basado en los Estados Unidos nuestro programa de erradicación del paludismo.
Es evidente que hay ciertas enfermedades propagadas por artrópodos que no pueden ser erradicadas porque se perpetúan en ellos o en otros animales inferiores, y las que incluyen la fiebre amarilla, peste, fiebre recurrente, tularemia, las en- cefalitis virales, ciertas formas de leishmaniasis, y algunas de las rickettsiasis. Sin embargo, la incidencia de estas enfermedades puede reducirse notablemente atacando al vector intermediario entre el huesped y el hombre, o cuando la especie reservorio es doméstica, mediante su destrucción, tratamiento o inmunización.
