Abundance, distribution, diversity, resistance...: key words for vectors of infectious microorganisms
Many microorganisms have established million-year old associations with certain insect genera and one of the consequences to humans obliges us to invest precious resources in the study of this relationship: the transmission of infectious diseases. If left uncontrolled, insect vectors and their transported microorganisms can represent a deadly threat to human populations. When only the infectious agent is considered in this association, features like distribution, diversity and resistance, though still conveying technical and conceptual challenges, can be dealt with in simpler terms. But a scenario analysis that looks at the simultaneous presence of an infectious agent and its vector elevates the research endeavor to a higher degree of complexity.
Most researchers (and society as well!) would be happy for a solution to the potential complex problem surrounding both these key words and the vector-infectious agent: what would happen if the most drug resistant strain of a given infectious microorganism meets the most insecticide resistant population of the respective vector?
Certainly, this question is a nightmare for public health strategists and decision-makers, but it is also a formidable scientific challenge for entomologists, evolutionary biologists and epidemiologists. The mechanisms of nature operate under rules that we do not fully understand, and though this meeting is possible, the probability that such an encounter would result in devastating effects for humans remains paradoxically low. Fortunately, this phenomenon has not been unequivocally described, but information on drug resistance of infectious organisms such as Plasmodium sp and insecticide resistance of its vector Anopheles sp raises concern about their spread (WHO 2014, 2015). This pair is one example of a possible association with drastic consequences.
The adaptation of an organism to a particular condition results in the increase of fitness, that is, an improvement in the odds of survival and reproduction. But the biological trade-off (fitness cost) that the success of a trait (let us say resistance to insecticide) poses might incur costs to other biological functions (e.g. growth rate, immune response to invading microorganisms etc.). Thus, if a certain vector population acquires the trait for insecticide resistance, the vector capacity to resist infection by microorganisms (virus, protozoa, bacteria) might be reduced. A trade-off between vector population survival and parasite resistance is attained at some point in this relationship, as has been suggested by a study of DDT-resistant Anopheles gambiae and Plasmodium berghei (Saddler et al. 2015).
Four articles published in the May issue of Mem Inst Oswaldo Cruz, 111(5) bring information about vector populations:
1) Pessoa et al. "Assessing the mitochondrial DNA diversity of the Chagas disease vector Triatoma sordida (Hemiptera: Reduviidae)";
2) Cornel et al. "Anopheles darlingi polytene chromosomes: revised maps including newly described inversions and evidence for population structure in Manaus";
3) Chediak et al. "Spatial and temporal country-wide survey of temephos resistance in Brazilian populations of Aedes aegypti"
4) Vianna et al. "Abundance of Lutzomyia longipalpis in urban households as risk factor of transmission of visceral leishmaniasis".
We hope this dreaded meeting of simultaneous resistance by vector and parasite never occurs, but, if it does, we expect that manuscripts detaling the meeting emphatically convey a key word we purposely left out in the title of this text: efficient control.
Ideally, from these future reports we will learn how to prevent the transformation of this interaction into a self-sustained phenomenon!
Adeilton Alves Brandão | Editor
Saddler A, Burda P-C, Koella JC. Resisting infection by Plasmodium berghei increases the sensitivity of the malaria vector Anopheles gambiae to DDT. Malar J. 2015; 14: 134.
WHO - World Health Organization. Geneva: WHO. 2014. Available from: http://www.who.int/malaria/areas/drug_resistance/overview/en/.
WHO - World Health Organization. Geneva: WHO. 2015. Available from: http://www.who.int/malaria/areas/vector_control/insecticide_resistance/en/.