Académie royale de Médecine de Belgique

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Vidéo et résumé de Pierre Buekens, membre étranger


THE NEGLECTED EPIDEMIOLOGY OF TROPICAL DISEASES

par Pierre BUEKENS (Tulane University, New Orleans - USA), membre étranger.                 

Determining if someone has an infectious disease is not as easy as it seems, especially if it is a neglected tropical disease. For more than 100 years, isolating and cultivating the infectious agent has been the gold standard in proving an infection. This is the foundation of Koch’s postulates which still influence the way we think today. Laboratory techniques, however, have been improving quickly, and diagnosis is now often based on identifying small fragments of DNA or RNA. These techniques are so powerful that the results are often not easy to interpret. For instance, weak results could merely indicate background noise. Such results could also be revealing the fragment of a dead microorganism. A small variation in the technique could influence the results, and the interpretation might vary from one observer to another. The answer, then, is often not yes or no, but rather a probability or a likelihood of infection.

The difficulty in diagnosing infectious diseases is even stronger for tropical diseases and especially challenging for neglected tropical diseases. Neglect is widespread for diseases that affect only the very poor in low-resource areas and countries. The biotech industry and, even, governments have little incentive to develop new diagnostic tools for such diseases, and the most recent molecular techniques are often not used to their fullest potential. In many cases the molecular tools available have not been adapted to specific geographic variations. For example, the laboratory techniques available to diagnose Chagas disease have mostly been developed in South America and are suitable for the strains of parasite prevalent there. The same techniques, however, might not perform as well when used in Mexico or the United States, where the parasite might be different.

It is not only difficult to determine if an individual is infected or not; it can be even more challenging to know how many people are infected at the population level. Epidemiology is the discipline that specializes in studying diseases in groups of people at the local, regional, country, or global level. Epidemiologists use mathematical methods to estimate how frequently diseases occur, and to predict their spread. Epidemiologic methods are well suited to address the new challenges of using sensitive molecular techniques to diagnose infectious diseases because they deal with probabilities rather than certainties.

It is surprising, then, that epidemiologists are often exclusively focused on chronic diseases, and that tropical medicine is sometimes guilty of underusing modern epidemiologic methods. This paradox is deeply rooted in history. Epidemiologists are distant successors of hygienists, who believed in the miasma theory and never regained the prestige lost after the success of the germ theory. During the nineteenth century hygienists played a central role in promoting sanitation and showing the social roots of diseases. The solutions they proposed were generally the right ones, but the hypothetical mechanisms they presumed to cause diseases were often the wrong ones. Epidemiologists regained prestige by focusing on chronic, non-communicable diseases. Cardiovascular epidemiology is one such example of an extremely successful discipline which has had a strong influence on behavioral change. But epidemiologists’ success with chronic diseases seems to have decreased their interest in infectious diseases.

Tropical medicine has its roots in the colonial period and has long seemed to be a discipline of the past to many epidemiologists. In Europe tropical medicine institutes were created more than a century ago to address health issues in the colonies. They were often founded and, even today remain, in port cities such as Antwerp, Amsterdam, Bordeaux, and Liverpool. In the United States the Tulane University School of Hygiene and Tropical Medicine was launched in 1912 in New Orleans, also a major port city often affected by yellow fever. Its founding also occurred immediately after the Spanish-American war and the United States taking control over Cuba, Puerto Rico, Guam, and the Philippines. It is only recently that tropical medicine’s colonial roots have been progressively forgotten. The discipline regained prestige, in part, due to the widely recognized impact of global warming on health, especially on vector-borne diseases such as dengue, Zika, and chikungunya. Advocating for the elimination of neglected tropical diseases has also recently increased attention to the discipline.

Parasitologists play a key role in tropical medicine and often continue to follow Koch’s postulates, seeking to find the truth by isolating organisms, while epidemiologists provide only probabilities. Epidemiologists often conduct large, population-based surveys and focus on reducing selection biases, while, in most cases, tropical medicine practitioners must quickly isolate organisms from available samples in an attempt to rapidly find clinical solutions to new emerging diseases. The recent epidemics of Ebola virus and Zika virus have shown again the need to very quickly characterize the infectious agents to develop new diagnostics and to initiate the process of vaccine development. Epidemiologists do play a key role in the acute phase of an epidemic by identifying the causes and the transmission mechanisms. It is only in a second phase, however, that they are able to design large population-based studies to quantify the issue, and they are often late to launch these very large studies. This scenario again creates a disconnect between tropical medicine experts and epidemiologists.

Both tropical medicine and epidemiology suffer from unconscious historical biases in the design of the studies they conduct. There is an urgent need to reconcile epidemiology and tropical medicine so that they may find solutions together. It is of utmost importance to take a multidisciplinary approach to diagnose infectious diseases accurately, to measure their frequency, and to evaluate the effectiveness of vaccines and treatments.