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Pavlovsky revisited. New clothes for ecology in epidemiology. A commentary on An Ecosystem Approach to Health and its Applications to Tropical and Emerging Diseases, by David Waltner-Toews.

The ecological approach to understanding disease dynamics in populations is at least as old as Hippocrates (Hippocrates, s.d.). From Hippocrates until the dawn of the bacteriological era, disease theory was mostly ecological in nature (Galen, s.d.). In fact, not truly ecological according to the current scientific definition, but ecological in nature. According to the Hippocratic definition, an endemic was a disease determined by the nature of a certain place. Demos had a broad meaning, and could be understood as people or population, but also as place, home. Under this definition, climatic, hydrological, and behavioral determinants were seen as the main forces. This view of disease occurrence and distribution persisted for centuries. Hippocrates' writings were recycled by Galen (Galen, s.d.) in the early Christian era and resisted even the Galilean modernization of science during the Renaissance, surviving as the mainstay of medical and public health science until the late 18th century. Hippocrates and Galen are not the main subject of discussion here, but it must be noted that their concept of disease was ecological. Disease was a consequence of local conditions, which had to be favorable for a particular disease to occur. Diseases were named after the respective scenarios in which they occurred, such that different scenarios gave rise to different diseases.

Pavlovsky (Pavlovsky, s.d.), a little-known parasitologist from what was then the Soviet Union, can be considered one of the first to propose a reasonably well-structured theory of infectious disease ecology. Unfortunately, in the late 1930s Soviet epidemiology (and indeed Soviet science as a whole) were not very well known in the West, and Pavlovsky's theory of the natural nidality of transmissible diseases had a very restricted circulation (Audy, 1958; Marr, 1995). Nonetheless, Pavlovsky succeeded in furthering the understanding of disease occurrence and the consequences of ecosystem modification. Unfortunately Pavlovsky did venture too far forward, since he appeared to be satisfied with understanding leishmaniasis and the then-emerging tick-borne encephalitis.

We can, with reasonable confidence, accept that the ecosystem approach to infectious diseases began with Pavlovsky. Waltner-Toews' article presents ecosystem analysis as a recent invention, which it is not. A large number of researchers have employed one or another method of ecosystem analysis for understanding and controlling infectious diseases (Audy, 1958; May, 1958; Burnet et al., 1972; Croll et al., 1983; Blower et al., 1991). In fact, the extensive use of the term "tropical" to define a broad array of diseases is inappropriate. Globalization has erased boundaries between endemic and disease-free areas; these diseases are not a prerogative of tropical ecosystems, albeit the latter are doubtless extremely favorable to their occurrence.

What distinguishes contemporary ecosystem analysis of diseases is the ecological paradigm adopted (Science et Vie, 1996; Earn et al., 1998). Virchow, cited by Waltner-Toews, concluded the obvious. Appalling living conditions in mid-19th century Silesia made the association obvious; it was not the result of scientific reasoning, but only of sound common sense. The failure of post-War economic development projects in the Third World and of large disease-control programs preceded the late 1970s debt crisis. The Marshall Plan had been a success in Europe; malaria eradication was also a success in Southern Europe and the Southeastern United States, but fell short of its objectives in the Third World. Worse yet, many development projects, like irrigation schemes and hydroelectric dams, caused diseases to spread (Hughes et al., 1970; Gordon-Smith, 1975). Cultural traits of target groups conflicted with behavioral changes needed for disease control. It gradually became obvious that infectious disease control programs demanded a more comprehensive approach than the prevailing linear reasoning.

The ecosystem approach proposed by Waltner-Toews is no doubt interesting, but the theoretical basis is not clearly presented. Modern ecological theory differs from Pavlovsky's. Modern science has abandoned the passion for precision to embrace a love of the imprecise, the uncertain. Chaos theory is finding its place in virtually all scientific disciplines (Hénon, 1989; Casti, 1995; Science Illustrée, 1996; Earn et al., 1998). Waltner-Toews commits a Freudian slip when he discusses attractors and catastrophic changes, which are an integral part of chaos theory. Waltner-Toews' ecosystem approach is more than a cookbook recipe for understanding and controlling infectious diseases, but this is not made clear as he expounds on its theoretical base. Public health needs a sounder theoretical base, and ecosystem analysis is a major step towards raising epidemiology to the same level of philosophical soundness as physics. Public health must be wary of "new" approaches. Infectious disease control has advanced enormously in its technical aspects, but the latter have still proven to be insufficient. A more appropriate understanding of disease is needed, and Waltner-Toews comes close. However, he appears to be distracted by the siren's song of the biased world view still prevailing in many scientific circles in industrialized countries. Infectious diseases must be seen as a whole, not divided into tropical and non-tropical, meaning in fact underdeveloped and developed. Waltner-Toews proposes a distinction between tropical and non-tropical diseases, according to which the former frequently "occur in settings where the links between local ecosystems and local communities are intense and obvious."[quoted from a preliminary version of the article]. Links between ecosystems and communities are hard to understand, as communities are integral parts of ecosystems. Here lies another Freudian slip, since ecosystem is seen as merely the natural ecosystem, detached from human presence.

The ecosystem approach presented by Waltner-Toews is appealing, but unfortunately infected by a biased view of the developing world.

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