Ascaris lumbricoides or Ascaris suum: What's in a Name?

To the Editor—We thank da Silva Alves et al [1] for their comments. As they rightly point out, the species status of Ascaris lumbricoides and Ascaris suum has been a matter of debate for some years [2]. We welcome widening the discussion and draw attention to another important milestone in the study of this parasite, the publication of a book about Ascaris, edited by Celia Holland [3]. In this volume, we elaborated on this question, concluding that it all depends on the species definition used [4]. For example, based on a phenetic species concept, A. lumbricoides and A. suum would belong to the same species. In contrast, based on a biological species concept, it could be argued that they comprise 2 separate species. Confirmation of this would entail experimental crosses, but what experimental infections, hosts, and parental isolates should be used?

As discussed by a number of authors, including da Silva Alves et al and ourselves, the use of mitochondrial markers or DNA barcoding approaches to infer species relationships and transmission dynamics for Ascaris is controversial [5]. We have also found pig-associated haplotypes among Ascaris worms collected from humans who live in areas where there are no pigs, suggesting retention of ancestral haplotypes. In contrast, based on nuclear markers (microsatellites), these parasites looked like human-associated Ascaris [6]. This is an interesting puzzle in Zanzibar, where porcine transmission could have taken place, although hundreds of years ago. We firmly believe that our African-focused sampling has unveiled an important genetic legacy and diversity of ascarids in this region, where it might have first parasitized early hominids. Hopefully, future archeopaleontological studies of parasites will expand and elaborate on this.

This debate on Ascaris can, of course, be expanded into the zoonotic transmission of other soil-transmitted helminthiases, with a new spotlight on Trichuris trichiura. Although it is generally accepted that T. trichiura (in humans) is a separate species from Trichuris suis (in pigs) [7], until recently it was thought that Trichuris in humans and nonhuman primates composed a single species (T. trichiura). However, ongoing molecular studies of Trichuris in samples obtained from humans and nonhuman primates have revealed the evolutionary history of T. trichiura to be more complicated than originally thought. It may comprise a number of species or subspecies, some of which are specific to particular host species and others that are shared between humans and nonhuman primates [8].

To play devil′s advocate, does the species status of soil-transmitted helminths really matter? From a public health perspective, it does. With the drive to control and eliminate these parasites from humans, as exemplified by the new initiative of Deworm3 [9], uncovering any zoonotic potential or other natural environmental refugia is important for alternative intervention strategies that may be required. Additionally, any gene flow between worms infecting different hosts could favor the spread of anthelmintic resistance, and, as yet, it is not clear whether the newly described (sub)species of T. trichiura shows any significant biological differences in factors such as pathogenesis or response to treatment. This calls for further research.

Our current understanding is lacking. Even though future molecular appraisals will continue to shed new light on parts of the problem, a bottleneck will remain in obtaining sufficient worm material from humans and animals at a level truly representative of natural transmission cycles. We are sure that da Silva Alves et al would agree that better integration of studies in humans and animals and adoption of a One Health approach is a sensible way forward.