Ascaris lumbricoides, Ascaris suum, or “Ascaris lumbrisuum”?

To the Editor—For decades, there has been a discussion about the zoonotic potential of the 2 species Ascaris lumbricoides and Ascaris suum or whether these should be a single species [1–3]. It has been demonstrated that the biological cycle of Ascaris organisms originating from humans or from pigs can be completed in both hosts, that cross-transmission occurs, that gene flow and hybridization events occur, that there are genotypes of Ascaris that are common to both hosts, and that, when found, there are few differences in their nucleotides. As more Ascaris organisms are isolated from different hosts and geographic localities and with different methods, increased evidence will be generated to clarify this question. Progress regarding this matter was recently demonstrated by Betson et al in this journal [4].

We previously demonstrated that, in developing countries where there is a close relationship between these hosts, it is also not uncommon to find Ascaris genotypes typical of pigs in humans [5]. This might be interpreted as an indication that cross-transmission also occurs in these regions and that, therefore, there is zoonotic potential among these organisms. We recently conducted a study in the municipality of Guapimirim in the state of Rio de Janeiro and found that, among the subjects with ascariasis, most of the worms genotyped for the target cox1 had the genotype of Ascaris organisms typical in pigs, known as P3 [6].

This result might lead us to think that the infection was acquired from a pig. However, in this population, the majority of the individuals had not had contact with pigs. Therefore, this suggests that the infection was acquired from another human and that it was this other human who had had contact with a pig. Will we reach this so-called mitochondrial Eve someday in the future?

Are we faced with a single species, a more troublesome notion for scientists? Studies have provided new evidence that this is a single species, through comparing the complete mitochondrial genome and, most recently, through analysis of the microRNA of A. suum and A. lumbricoides [7, 8]. It seems that is it not possible to differentiate them even by using additional molecular targets for genotyping. Perhaps this is simply because these differences do not exist.

In most of the isolates that we have genotyped, independent of the molecular target used, the differences in the nucleotides (when they exist) are small and almost always result in synonymous mutations in the translation to amino acids. This also applies to other Ascaris sequences that have been deposited in GenBank by other research groups. This is despite what is seen with so many other parasites that are zoonotic or have genotypes that are considered to be zoonotic, yet are not classified as distinct species, as exemplified by Giardia duodenalis.

Notes

Financial support. This work was supported by the Fundação de Amparo à Pesquisa do Estado do Rio de Janeiro, Universidade Federal Fluminense and Laboratório de Doenças Parasitárias, Instituto Oswaldo Cruz-Fiocruz. The authors thank André Barbosa for their technical support and UFF-Platform sequencing.

Potential conflicts of interest. All authors: No reported conflicts. All authors have submitted the ICMJE Form for Disclosure of Potential Conflicts of Interest. Conflicts that the editors consider relevant to the content of the manuscript have been disclosed.

References