Are intestinal parasites of great apes and humans as closely related as their hosts are?
The genus Entamoeba (phylum Amoebozoa) regroups various animal internal parasites and commensals (i.e., species benefiting from interactions with other organisms that gain nothing but also suffer no harm from the relationship).
The Entamoeba life cycle is simple. It includes a vegetative (active, feeding) stage, a trophozoite, which colonizes the large intestine of its host, and a cyst, which is a stage able to resist the conditions outside the host in order to be transmitted to another host. Seven Entamoeba species are found in non-human primates. Two of them, E. nuttalli and E. histolytica are considered pathogenic. E. histolytica is even recognized as the cause of the third most lethal human parasitic disease (100,000 deaths per year, 35–50 million cases of symptomatic disease). Its closest relative, E. nuttalli, is found in macaques, colobus monkeys and spider monkeys.
In general, the presence of more Entamoeba species or lineages in one host is common. Clinical diagnosis of Entamoeba organisms is often performed with light microscope and is based on cyst morphology. However, this can be very complicated, as commonly, in one host, more Entamoeba species and lineages coexist, and their cysts may look very similar. The use of more effective molecular methods therefore appears to be a more convenient approach to identification.
In this study, the team developed a new approach based on high throughput sequencing methods (also called next generation sequencing), which permits several species and lineages, including rare examples, to be detected in several hundreds of samples at once. Specifically, the authors used the Illumina MiSeq platform (for small genomes and targeted sequencing) to sequence part of the 18S rDNA gene.
The DNA was obtained from faecal samples of wild central chimpanzees, western lowland gorillas and humans inhabiting closely neighbouring areas at the periphery of the Dja Faunal Reserve in Cameroon. The method detected 36 18S rDNA haplotypes (i.e., variants of the analysed DNA fragment) belonging to six haplotype groups (i.e., groups of similar haplotypes: E. hartmanni, E. dispar, E. nuttalli, E. histolytica, E. invadens and E. coli), with the most numerous groups being E. hartmanni and E. coli. The E. invadens-like haplotype group may even be a new species, as the divergence was quite high, and so far, the most closely related E. invadens has only been detected in reptiles. The presence of more than one haplotype was detected in 63 % of cases, usually with two co-occurring haplotypes, and in 43 % of samples the haplotypes belonged to more than one haplotype group. The most haplotypes (33 haplotypes, 6 groups) were found in humans, 14 (6 groups) in gorillas and 8 (4 groups) in chimpanzees. The haplotype diversity in humans was significantly higher than in chimpanzees or gorillas. Almost half of all detected haplotypes were found in two or all three host species.
To answer the question from the title of this article: the analyses show that all three host species have different Entamoeba communities. However, the compositions of human and chimpanzee Entamoeba communities overlap, while the Entamoeba communities in gorillas include a clearly separate group, which corresponds to the phylogenetic relationships of the hosts. The fact that the haplotypes overlap among the studied host species probably means that certain haplotypes are potentially zoonotic (i.e., transmissible from animals to humans).
Some of the Entamoeba haplotypes showed high specificity to the host, while some were present in more hosts. Most of the detected haplotypes belonged to rather commensal Entamoeba species. E. histolytica was detected in all host species, while E. nuttalli only in humans and chimpanzees. In this study, E. histolytica was confirmed for the first time in wild great apes through the use of molecular methods.
The authors developed a new method for identifying Entamoeba, including rare and unknown species and even mixed infections, which is important not only from the Entamoeba epidemiology point of view but also for the protection of wild great apes.
Vlčková, K., Kreisinger, J., Pafčo, B., Čížková, D., Tagg, N., Hehl, A. B., & Modrý, D. (2018). Diversity of Entamoeba spp. in African great apes and humans: an insight from Illumina MiSeq high-throughput sequencing. International Journal for Parasitology, 48(7), 519–530.