Mem Inst Oswaldo Cruz, Rio de Janeiro, 108(2) April 2013
Certifying the interruption of Chagas disease transmission by native vectors: cui bono?
1Instituto Leônidas e Maria Deane-Fiocruz, Manaus, AM, Brasil
2Centro de Pesquisas René Rachou-Fiocruz, Belo Horizonte, MG, Brasil
3Universidade de Brasília, Brasília, DF, Brasil
4Universidad de Buenos Aires, Buenos Aires, Argentina
Certifying the absence of Chagas disease transmission by native vectors lacks scientific grounds and weakens long-term control-surveillance systems to the detriment of people living under risk conditions. Instead, a regular u201ccertification of good practiceu201d (including vector control-surveillance, case detection/patient care and blood safety) could help achieve sustained disease control.
Chagas disease vector control is among the greatest public health achievements ever. The impact of the Inter-Government Initiatives coordinated by the Pan American Health Organization/World Health Organization (PAHO/WHO) has been enormous, with large reductions of incidence, prevalence and disease burden (Schofield et al. 2006). Yet, when entomological surveillance breaks down, native vectors re-invade and re-infest human residences, leading to the re-emergence of transmission (Gürtler et al. 2007). In fact, and as with other zoonoses, eradication of Chagas disease is impossible: sporadic Trypanosoma cruzi transmission to humans will always occur, even in the presence of the best conceivable control program (Schofield et al. 2006, Tarleton 2011).
Since the late 1990s, PAHO/WHO issue certifications of "interruption of T. cruzi transmission" by non-native, accidentally introduced vectors. This approach was based on evidence that such vectors, and particularly Triatoma infestans and Rhodnius prolixus, were strictly domestic and did not occupy natural ecotopes in their non-native ranges. Therefore, it was argued, those introduced populations could be eliminated through insecticide-spraying campaigns. Empirical data show that this idea was basically right. T. cruzi transmission by domestic T. infestans populations was virtually or effectively interrupted in Uruguay (certified in 1997 and 2012), Chile (in 1999), Brazil (2006), eastern Paraguay (2008), some parts of Argentina (2001, 2004, 2011, 2012) and southern Peru and transmission by domestic R. prolixus populations in southern Mexico (2009), Guatemala (2008), Honduras (2010), Nicaragua (2010), El Salvador (2010) and Costa Rica (2011) (Table I) (PAHO 2011, 2012, Hashimoto & Schofield 2012).
Certification was intended to recognise the huge efforts made by Governments and international agencies to achieve the specific goal of interrupting transmission by these non-native vectors. Native vector populations, which occur in natural ecotopes across their geographic ranges, pose a radically different problem; here we argue that certification of interruption of transmission has no role to play in this case and question who is likely to benefit from such a certification policy.
We first wish to note that, on purely logical grounds, certifying the absence of an event is absurd - the absence of evidence cannot be taken as evidence of absence. In addition, native triatomines are nearly ubiquitous in the Americas; many species are very efficient vectors of T. cruzi and some are also capable on invading and colonising artificial environments (Lent & Wygodzinsky 1979). Among these species are, of course, also T. infestans and R. prolixus, one native to the Chaco and the western Bolivian Andes and the other to the vast Orinoco plains (Table I). We also know, on empirical grounds, that sporadic vector-borne transmission does take place from the United States of America to Argentina, even in areas under tight surveillance - and we know that, unfortunately, most surveillance systems are far too weak to confidently rely on the data they produce (Abad-Franch et al. 2011).
What, then, would a "certification of interruption of transmission by native vectors" mean? Perhaps reflecting on a few recent examples (PAHO 2012) could help outline an answer. For what does it mean that T. cruzi transmission has been interrupted in, say, the Bolivian Department of La Paz or some localities of the Chaco in Argentina - where native T. infestans populations are common, surveillance is weak and most rural houses are still substandard? And what would it mean that transmission by native vectors no longer takes place in non-Amazonian Brazil - where over 50 triatomine species occur, a dozen of which are routinely collected inside houses?
One hardly controversial fact is that all available entomological data are extremely fragmentary and become even more so as funds decline. Furthermore, we must remember that acute Chagas disease is "typically" asymp-tomatic or oligosymptomatic (Rassi et al. 2010) and goes often undiagnosed, and that vector detection methods all have very low sensitivity (Abad-Franch et al. 2011). Will the absence of detection be interpreted as true absence of events? (and would a single new case result in "de-certification"?). Note also that even the best possible serological survey of children born in the "post-control era" would not be truly informative about the occurrence of "sporadic" transmission. The last large survey was conducted in Brazil about seven years ago and it seems unlikely that many of similar size (and costs) will be conducted in the future, because no political priority can be attached to a disease whose transmission by its main domestic vectors has been (officially) "interrupted". Would a decision-maker devote funds to monitoring the possible existence of events that have been declared inexistent by the most authoritative international health agency? Again, we think this highly unlikely; on the contrary, "certification" has usually had conspicuous "fund-freezing", priority-lessening effects. This, of course, weakens surveillance and a weaker surveillance system produces fewer records regardless of epidemiological reality, reinforcing the impression that transmission does not occur. Such a positive feed-back loop generates a false feeling of safety, with transmission remaining overlooked until the pre-control situation is restored - and control investment wasted for good. This has been quite rightly called "the punishment of success" (Schofield et al. 2006), a phenomenon whose grave effects are, unfortunately, well documented (Gürtler et al. 2007).
Should then PAHO/WHO stop certifying altogether? We do not think so: publicly acknowledging the efforts and achievements of health authorities is both fair and important. But we need to ask and answer the question we chose for our title: cui bono - who benefits from certification? As it currently stands, certification effectively discourages sustained action rather than promoting it and this surely does not benefit people living under risk conditions: they deserve better. Perhaps the simplest strategy would be to certify "good practice" instead of unverifiable facts about the non-occurrence of transmission. This would have several advantages. First, "good practice" is a dynamic concept - it does not focus on a "state" to be reached, but on better ways of running long-term programs, encouraging decision-makers to improve performance and secure adequate funds. Second, "good practice" can be assessed periodically based on quantitative indicators (Table II); in defining these indicators, issues of validity, reliability, comparability and ease of ascertainment must be taken into account (Murray 2007). Third and perhaps most important, "good practice" can be defined as encompassing the several key aspects of Chagas disease control, including not only vector control-surveillance, but also case detection/patient care and blood safety - it might even include a technical assessment of financial management. Thus, "good practice" evaluations can generate a more comprehensive picture of how control programs are performing, providing the means to dynamically detect and correct malfunctions. In the long run, "good practice" certification could be crucial to the strengthening of control programs and the prevention of new Chagas disease cases. It would thus directly benefit those at risk, whereas certifying interruption of transmission, we suspect, primarily benefits authorities.
In conclusion, we strongly advocate the idea that PAHO/WHO, the relevant public health officials and the scientific community should engage in a serious debate to define better ways to monitor the performance of Chagas disease control programs. Certifying interruption of transmission has many conceptual drawbacks and effectively weakens long-term control-surveillance systems; in contrast, periodically assessing "good practice" would stimulate sustained action and could therefore represent a major step in the right direction. It may well be, in fact, the best way forward.
To the memory of Antônio Carlos Silveira.
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