1. Introduction
The rat lungworm
Angiostrongylus cantonensis (Chen, 1935) was first described in the bronchi of the rodents
Rattus rattus (Linnaeus, 1758) and
Rattus norvegicus (Berkenhout, 1769) in Guangzhou (formerly Canton), China [
1]. This nematode is the etiological agent of neuroangiostrongyliasis which is the main cause of eosinophilic meningitis (EM) or eosinophilic meningoencephalitis (EME) in humans, an infectious disease of the central nervous system [
2]. This disease is characterized primarily by an increase in the proportion of eosinophils in peripheral blood and cerebrospinal fluid, among other symptoms such as fever and severe headache [
3,
4].
The first documented human case of neuroangiostrongyliasis was in Taiwan in 1944, although it took nearly two decades to establish a clear link between the parasite and the disease (i.e.,
A. cantonensis as a causative agent of EME) [
5]. Since the first report, several outbreaks were reported globally as the parasite has spread from traditional endemic regions of Southern China and Southeast Asia to the Pacific islands, Japan, Australia, Africa, the Canary Islands, the Balearic Islands, and the Americas, including the USA, Caribbean islands, and Brazil [
2,
4,
6,
7]. By 2008, more than 2,800 cases of human angiostrongyliasis had already been recorded in 30 countries [
8]. The spread of parasites in different regions poses a threat not only to people living in endemic areas but to a growing number of travelers visiting these regions [
9].
In 2008,
A. cantonensis was reported for the first time in Ecuador parasitizing the giant African land snail
Achatina (
Lissachatina)
fulica Bowdich, 1822 and the rats
R. rattus and
R. norvegicus [
10,
11]. Since then, outbreaks and isolated cases have been reported to the Ministry of Public Health of Ecuador (MSP) [
12], with most clinical-epidemiological suspicion and one necropsy-confirmed case [
13]. The parasite is now considered endemic throughout most of the country [
10,
14]. The invasive pest
A. fulica is one of the main intermediate hosts for
A. cantonensis [
15]. This mollusk lives in urban and rural areas and plays an important role in the spread of the parasite [
16]. Humans may become infected by ingesting food contaminated with third-stage larvae or by eating infected raw snails [
2]. Thus,
A. fulica is an important transmitter of eosinophilic meningoencephalitis and ocular angiostrongyliasis [
17].
Different molecular biology methods have been employed to detect
A. cantonensis [
18,
19,
20,
21,
22,
23]. Furthermore, they have been applied to explore systematic and population genetic aspects of
Angiostrongylus taxa, since there is great variability within populations [
24,
25,
26,
27,
28,
29,
30,
31]. The use of mitochondrial genes, such as cytochrome c oxidase subunit I (MT-CO1), as molecular markers for specific identification of the parasite has been efficient [
32,
33,
34,
35]. The MT-CO1 gene has been used in studies on phylogeny, phylogeography, and haplotype identification [
36,
37,
38,
39]. However, some basic questions remain unanswered about the presence of
A. cantonensis in Ecuador: 1) Was the invasion of
A. cantonensis in Ecuador a single event or did it occur in different waves? 2) Was this invasion as recent as historical records suggest? 3) Did this invasion come from other regions of South America or elsewhere?
To tackle these questions, we assessed the genetic diversity of MT-CO1 gene sequences from isolates obtained in eleven of Ecuador's 24 provinces. Thus, we were able to verify how many lineages could be found in different regions of Ecuador and whether there was enough time for the lineages to diversify. We also established the phylogenetic and phylogeographic relationships of these isolates, comparing them with other sequences from South America and the rest of the world. Consequently, we could retrace the possible origin of the lineages found in Ecuador.
4. Discussion
The introduction of non-native mollusks, such as
A. fulica, plays an important role in the transmission of
A. cantonensis [
59]. Since the mid-20th century,
A. fulica has been introduced into the tropics and subtropics and has since been considered the most harmful snail pest in these regions [
16]. In Brazil, these mollusks were possibly introduced more than once, on different occasions [
60]. The first account is from the mid-1970s in the state of Minas Gerais [
61]. The second, better documented, and probably the major introduction was in the late 1980s in the state of Paraná from specimens brought from Indonesia for commercial purposes (snail farming) that were unsuccessful [
62]. The giant African snail is currently widespread in all 26 Brazilian states and the Federal District [
63,
64].
According to data from an Ecuadorian government organization, these snails were brought into the country in the mid-1990s. As in Brazil, this was for commercial purposes. Snail farms were built in some valleys of the Ecuadorian highlands, which offered an ideal temperature between 17 °C and 25 °C [
65]. However, their breeding did not provide the expected economic returns, inevitably, most of the farms were abandoned and the snails were released into the environment. The result was a widespread infestation of urban and rural areas in almost all provinces of the country [
11].
Achatina fulica was probably the vector that introduced
A. cantonensis to the country, as was the case in Brazil [
66] and China [
67].
As for the definitive hosts, it is presumed that
R. rattus arrived in Ecuador between the 16
th and 17
th centuries with the ships of the Spanish conquistadors [
68].
Rattus norvegicus probably originated in China and spread to Europe, reaching North America through shipping during the second half of the 18
th century. Both species are now widely distributed in urban areas around the planet [
69].
In 2008, the snail
A. fulica (intermediate host) and the rat
R. rattus (definitive host) were found naturally infected by
A. cantonensis in Ecuador [
10,
70]. The existence of intermediate and definitive hosts in almost the entire Ecuador has contributed to the endemic nature of angiostrongyliasis distribution, making the control of this disease even more complex [
10]. Both intermediate (
A. fulica) and definitive (
R. rattus) hosts are non-native species to Ecuador and are considered among the 100 most important invasive species in the world, according to the World Conservation Union [
71]. Invasive species in an ecosystem can affect biotic alter interactions, impacting the economy, the environment, or public and animal health [
72,
73]. Moreover, the practice of eating raw snails by elder Ecuadorians increases the risk of
A. cantonensis infection [
11].
Earlier studies using the MT-CO1 to distinguish
A. cantonensis isolates have shown different geographical isolates in determinate regions [
36,
38,
39,
58]. Tokiwa et al. [
39] reported seven different haplotypes (AC1 to AC7): five were found in Japan (AC1, AC2, AC3, AC5, and AC7), two in mainland China (AC2 and AC6) and only one in Taiwan (AC1). In Brazil, analyses from 15 geographic isolates determined the presence of three different MT-CO1 haplotypes (AC5, AC8, and AC9). Most of the sample sequences were AC5 or AC8, whereas AC9 was a new haplotype [
36]. Rodpai et al. [
58] identified different
A. cantonensis haplotypes in Cambodia, Myanmar, Thailand, and Hawaii, USA. Two of them (AC2 and AC5) had been previously reported. The AC2 haplotype, previously reported in China and Japan, was found in Myanmar. The AC5 haplotype, previously reported in Brazil and Japan, was found in Hawaii. Additionally, four new haplotypes (AC10-AC13) were reported in Southeast Asia [
58].
Such studies have shown that
A. cantonensis in Asia has greater genetic diversity [
38,
39,
58], indicating that this parasite has been circulating in these regions for a long time. Conversely, the sequence diversity of
A. cantonensis is low in several regions outside Asia [
74]. Otherwise, there is little or no genetic information on the parasite in other regions of the planet, such as the Americas, except in Brazil [
36].
In the present study, all sequences of the isolates from Ecuador were identical, the haplotype AC17a. In our phylogenetic analyses, this haplotype was nested into a polytomy with other sequences from different parts of the world. Remarkably, all samples were obtained from provinces of Ecuador in different geographic and climatic regions, and yet they did not show any genetic divergence between them.
The findings reported here represent a novelty in the study of the genetic diversity of A. cantonensis isolates. Although there is a lack of information on the genetic diversity of this parasite in other Latin American countries, except for Brazil, our results are different from previous studies due to the complete absence of genetic diversity of A. cantonensis in Ecuador. Even admitting that the low number of nucleotide base pairs obtained could make the sequence homogeneous in the isolates from Ecuador, this same region of the MT-CO1 gene showed variations in the other haplotypes compared.
The fact that only one haplotype was found in 11 different Ecuadorian provinces, is revealing. It strongly advocates a single introduction event. Furthermore, this result suggests that A. cantonensis has been recently introduced in the country, as there was no time for new haplotypes to differentiate from the original. This may justify the non-existence of genetic diversity among different circulating isolates.
Interestingly, the sequences from Ecuador shared a recent common ancestor with two Brazilian haplotypes (AC5 and AC8) [
75]. However, it is unlikely that this could indicate a historical connection between the strains from both countries. The AC5 haplotypes found in Brazil from Pirituba (state of São Paulo), Queimados, and Niterói (state of Rio de Janeiro) correspond to a haplotype found in Japan, Hawaii, and French Polynesia [
58,
74,
76], suggesting that the arrival of the parasite in Rio de Janeiro or São Paulo may have occurred from the Asian continent [
36]. This hypothesis is also considered for the AC8a haplotype, closely related to AC8b, found in Australia, the Balearics, the Canaries, Taiwan, and the United States of America (USA). This shows the possible spread of
A. cantonensis, with the giant African land snail, as a vector, from the arrival localities in Brazil to the Southeast, Northeast, and North Brazilian regions [
36].
The sequences obtained here also grouped with AC13 and AC17 haplotypes from Thailand and the USA, respectively. The haplotypes AC10, AC11, and AC13, from Thailand, and AC12, from Cambodia, were described by Rodpai et al. [
58] in phylogenetic studies using different DNA regions of
A. cantonensis and
A. malaysiensis. The haplotypes AC17, from the USA, were reported in a study to identify
A. cantonensis and determine the association between ecological characteristics and factors related to definitive hosts (
R. rattus, R. norvegicus, Sigmodon hispidus, and
Oryzomys palustris) associated with transmission risk of angiostrongyliasis in New Orleans [
77]. The haplotypes AC12, AC13, and AC17 formed a cluster in the haplotype cluster analysis, suggesting that
A. cantonensis may have arrived in Ecuador from Asia via the USA.
Author Contributions
Conceptualization, L.S.A., L.R.R. and H.H.A.; methodology, H.H.A., L.R.R., L.S.A., R.V.V.; software, C.B.P., L.M.P., R.V.V.; validation, J.F.N., M.R.O.; formal analysis, C.B.P., J.F.N., L.M.P., R.V.V.; investigation, L.S.A., H.H.A.; resources, L.S.A., R.V.V.; data curation, C.B.P., L.S.A., R.V.V.; writing—original draft preparation, C.B.P., L.S.A.; writing—review and editing, C.B.P., L.S.A., R.V.V., R.O.S.; visualization, R.V.V., R.O.S.; supervision, L.S.A.; project administration, L.S.A.; funding acquisition, L.S.A. All authors have read and agreed to the published version of the manuscript.