Invasion Biology Meets Parasitology: A Case Study of
Parasite Spill-Back with Egyptian Fasciola gigantica in
the Invasive Snail Pseudosuccinea columella
Daniel S. Grabner1*, Faten A. M. M. Mohamed2, Milen Nachev1, Eman M. H. Me ´abed2,
Abdel Hameed A. Sabry2, Bernd Sures1
1Aquatic Ecology and Centre for Water and Environmental Research, University of Duisburg-Essen, Essen, Germany, 2Parasitology Department, Faculty of Medicine,
Fayoum University, Fayoum, Egypt
The liver fluke Fasciola gigantica is a trematode parasite of ruminants and humans that occurs naturally in Africa and Asia.
Cases of human fascioliasis, attributable at least in part to F. gigantica, are significantly increasing in the last decades. The
introduced snail species Galba truncatula was already identified to be an important intermediate host for this parasite and
the efficient invader Pseudosuccinea columella is another suspect in this case. Therefore, we investigated snails collected in
irrigation canals in Fayoum governorate in Egypt for prevalence of trematodes with focus on P. columella and its role for the
transmission of F. gigantica. Species were identified morphologically and by partial sequencing of the cytochrome oxidase
subunit I gene (COI). Among all 689 snails found at the 21 sampling sites, P. columella was the most abundant snail with 296
individuals (42.96%) and it was also the most dominant species at 10 sites. It was not found at 8 sites. Molecular detection
by PCR and sequencing of the ITS1-5.8S-ITS2 region of the ribosomal DNA (rDNA) revealed infections with F. gigantica
(3.38%), Echinostoma caproni (2.36%) and another echinostome (7.09%) that could not be identified further according to its
sequence. No dependency of snail size and trematode infection was found. Both high abundance of P. columella in the
Fayoum irrigation system and common infection with F. gigantica might be a case of parasite spill-back (increased
prevalence in local final hosts due to highly susceptible introduced intermediate host species) from the introduced P.
columella to the human population, explaining at least partly the observed increase of reported fascioliasis-cases in Egypt.
Eichhornia crassipes, the invasive water hyacinth, which covers huge areas of the irrigation canals, offers safe refuges for the
amphibious P. columella during molluscicide application. As a consequence, this snail dominates snail communities and
efficiently transmits F. gigantica.
Citation: Grabner DS, Mohamed FAMM, Nachev M, Me ´abed EMH, Sabry AHA, et al. (2014) Invasion Biology Meets Parasitology: A Case Study of Parasite Spill-Back
with Egyptian Fasciola gigantica in the Invasive Snail Pseudosuccinea columella. PLoS ONE 9(2): e88537. doi:10.1371/journal.pone.0088537
Editor: Shoba Ranganathan, Macquarie University, Australia
Received October 22, 2013; Accepted January 6, 2014; Published February 11, 2014
Copyright: ? 2014 Grabner et al. This is an open-access article distributed under the terms of the Creative Commons Attribution License, which permits
unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited.
Funding: This study was conducted in the framework of the Integrated Water Technologies (IWaTec) joint project between the Centre for Water and
Environmental Research (ZWU) at the University of Duisburg-Essen and the Fayoum University in Egypt. IWaTec is funded by the German Academic Exchange
Service (DAAD) and financed by the Federal Foreign Office. This publication has been supported by the Open Access program of the German Research
Foundation (Deutsche Forschungsgemeinschaft). The funders had no role in study design, data collection and analysis, decision to publish, or preparation of the
Competing Interests: The authors have declared that no competing interests exist.
* E-mail: email@example.com
The liver fluke Fasciola gigantica is a trematode parasite native to
Africa and Asia and infects ruminants, but also humans as final
hosts. Adult F. gigantica live in the bile ducts of the liver, where they
can reach a length of up to 76 mm. Infection with this parasite can
cause severe disease symptoms referred to as fascioliasis .
Intermediate hosts are various lymnaeid snails , inside of which
the parasite proliferates asexually and produces free swimming
cercariae that will attach to submerged surfaces, mostly plants.
These cercariae develop into encysted and durable metacercariae
that are transmitted when the final host ingests the metacercariae
together with plants, or by consumption of water contaminated
with metacercariae . Due to the durability of the metacercariae,
transmission can also be mediated by ingestion of terrestrial plants
and crops that were submerged in water containing infected snails
for a couple of weeks [1,4], which is a common irrigation
technique in Fayoum area. Especially in Egypt, fascioliasis is an
increasing problem, reaching prevalences in animals of sometimes
more than 50% and up to 19% in humans [5–7]. According to
estimations of the World Health Organization , at least 830,000
people are infected with either the introduced Fasciola hepatica or F.
gigantica in the Nile delta. Presence of both of these closely related
species has been confirmed for Egypt [1,9,10], but they are usually
not distinguished diagnostically .
The natural first intermediate host of F. gigantica in Egypt is the
snail Radix natalensis, but the trematode was also found commonly
in the introduced species Galba truncatula . Additionally, single
cases of F. gigantica infections were reported from Biomphalaria
alexandrina and Pseudosuccinea columella in Egypt [13–15]. For the
latter species, release of F. gigantica cercariae was proven in
laboratory infections . The highly invasive lymnaeid snail P.
columella was introduced from North America to many countries
worldwide and was reported from Africa for the first time in the
PLOS ONE | www.plosone.org1February 2014 | Volume 9 | Issue 2 | e88537
middle of the 20thcentury, where it is now widely distributed [17–
20]. P. columella is well known as a suitable host for F. hepatica ,
but its importance for the maintenance of the natural life cycle of
F. gigantica and the transmission to humans is uncertain.
According to the parasite spill back hypothesis, invasive species
are colonized by local parasites. If the invader becomes abundant
and the parasite can develop successfully, a high number of
transmitting stages will develop and increase local parasite
abundance and prevalence . In this way, free living invasive
species may help native parasites to increase their population size
and extend their distribution range . P. columella is a very
efficient invader and became an important snail host for F. hepatica
in many countries . We hypothesize that this might be the case
for the P. columella/F. gigantica–system as well. As a result, infected
P. columella might be responsible for the observed increase in
infection intensity and prevalence in livestock and in the human
population in Egypt. Therefore, we investigated the trematode
species occurring in the invasive snail P. columella, collected from
irrigation channels in the Fayoum governorate where cases of
fascioliasis are commonly reported [23,24]. We assessed the
potential of this invasive species as a host for trematodes, especially
for Fasciola spp. to estimate the effect of P. columella for the spread
of fascioliasis in the area and found evidence for a possible spill-
back effect on animals and the human population.
Materials and Methods
Snails were collected from July to September 2012, 2 month
after the last molluscicide treatment, from water plants (mainly
water hyacinths Eichhornia crassipes) and with dip nets in irrigation
channels at 21 different sites in Fayoum governorate (surrounding
Markaz El-Fayoum, Itsa, and Ibshway cities), Egypt. Samples were
taken with permission of the local farmers owning the land
adjacent to the irrigation channels. No endangered or protected
species were sampled. Sampled snails were fixed in 99% ethanol
for molecular analysis. Collected specimens from the different
sampling sites were identified morphologically according to the
key of Brown  and by molecular biology. After identification,
P. columella individuals were measured (shell length) and crushed to
check visually for trematode infections. Samples of the soft tissue
(for molecular analysis of parasites) and the foot muscle (for
molecular species identification of all snails; presumed to be free of
parasites) were taken and frozen at 220uC for molecular analyses.
Snail tissue samples were homogenized in 1.5 ml reaction tubes
with micropestles (Eppendorf) and DNA was extracted with a
JETQUICK DNA Clean-Up Spin Kit (Genomed) according to
manufacturer’s instructions. Molecular species identification of the
snails was done by sequencing of the Folmer-region of the
cytochrome oxidase subunit 1 (COI) with the primers LCO1490
and HC02198  (about 700 bp). At least two individuals of each
species were sequenced to confirm morphological identification.
For molecular detection of trematode infections in the soft tissue
of P. columella, the universal trematode primers Trem1 F/Trem1 R
were designed that amplify a short part of the internal transcribed
spacer 2 (ITS2) and the beginning of the 28S ribosomal DNA
(rDNA) (about 200 bp). For sequencing, an additional PCR with
the primers Trem2 F (end of 18S rDNA) and Trem1 R was
conducted to obtain a longer sequence, including almost the whole
internal transcribed spacer 1 (ITS1) – 5.8S rDNA– ITS2 region
of the ribosomal DNA (about 1300 bp). The primer pair Fasc-
ITS1 F/R was designed for specific amplification of a 716 bp
ITS1-segment of both F. gigantica and F. hepatica, to distinguish
Fasciola spp. from host and infections with other trematodes.
Sequences and additional information for primers designed for the
present study are given in table 1. One 20 ml PCR reaction mix
contained 4 ml of 56Crimson Taq buffer (New England Biolabs),
0.2 mM dNTP mix (New England Biolabs), 0.5 mM of each
primer 0.5 U Crimson Taq (New England Biolabs) and 1 ml
template DNA. The mix was topped up to 20 ml with PCR grade
water. The DNA was amplified by a Labcycler (SensoQuest)
under the following conditions: initial denaturation at 95uC for
5 min, 40 cycles of 95uC for 30 s, annealing (temperatures see
table 1) for 30 s and elongation at 72uC for 30 s followed by a final
elongation of 72uC for 5 min.
Sensitivity of the Trem2 F/Trem1 R primer pair was lower and
not all samples were amplified successfully with the Crimson Taq,
although tested positive before with Trem1 F/Trem1 R primers.
Therefore, the more robust Phire Animal Tissue Direct PCR Kit
(Thermo Scientific) was used for amplification with those primers.
Reactions contained 10 ml of 26 Phire PCR Buffer, 0.5 mM of
each primer, 0.4 ml Phire Hot Start II DNA Polymerase and 1 ml
DNA. Water was added to 20 ml. PCR conditions were 98uC for
5 min, 40 cycles of 98uC for 10 s, annealing at 54uC for 10 s,
elongation at 72uC for 20 s and a final elongation at 72uC for
1 min. PCR conditions for the LCO1490/HC02198 primers were
as described in Folmer et al. . PCR products were checked by
standard agarose gel electrophoresis, purified with a JETQUICK
PCR Product Purification Spin Kit (Genomed) according to
manufacturer’s instructions and sent for sequencing (GATC).
Sequences were checked for homology with database entries by
BLAST searches (http://blast.ncbi.nlm.nih.gov/Blast.cgi).
Mean prevalences and confidence intervals of parasite infections
were calculated with the program Quantitative Parasitology v.3.0
. Dependency of snail size and infection was analyzed by
logistic regression using R v.3.0.1 .
In total, 9 different snail species were identified morphologically
and genetically. The sequences of different isolates of each species
were identical, so one representative COI-sequence of each species
was deposited in GenBank. Sorted by their abundance, the
following species were found: Pseudosuccinea columella (accession
no. KF412765), Physa heterostropha (accession no. KF412768),
Cleopatra bulimoides (accession no. KF412769), Bulinus truncatus
no. KF412770), Biomphalaria alexandrina (accession no. KF412766),
Succinea sp. (accession no. KF412772), Bellamya sp. (accession
no. KF412773) and Theodoxus anatolicus (accession no. KF412771).
The five COI sequences obtained from P. columella individuals were
between 97%–100% similar to the different isolates of P. columella
Numbers of snails and species diversity varied greatly between
sampling sites. The highest number of snails was found at Disya
with 89 individuals (83 P. heterostropha and six B. truncates), only two
snails of one species (C. bulimoides) were found at the Qalamshah
village site. The most diverse snail community was present at the
Zawyet El-Karadsah-site with 6 different snail species, while only a
single snail species was found at two sites (P. heterostropha in El-Girb
village and C. bulimoides in Qalamshah village). P. columella was not
only the most abundant snail in total numbers (296 of 689,
42.96%; see fig. 1), but it was also the most dominant species at 10
Fasciola gigantica in Pseudosuccinea columella
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sites and the second most dominant at one site. It was not found at
eight of the 21 sampling sites. The second and third most
abundant snails were P. heterostropha (19.59%) and C. bulimoides
(9.43%), respectively (fig. 1). The latter species was found mostly in
the locality of Izbat Ashur and Hawwarat Al-Maqta. Proportions
of the other snails found in the area were 8.56% for B. truncatus,
6.53% for M. tuberculata, 4.35% for B. alexandrina, 3.92% for
Succinea sp., 2.76% for Bellamya sp. and 1.89% for Theodoxus
anatolicus. Detailed results on the number of snails and snail species
at each site are shown in table 2. Size range of P. columella
individuals varied between 0.15 and 1.18 cm. Mean and median
size were 0.54 and 0.50 cm, respectively.
No trematode stages were found in P. columella specimens by
visual inspection, as ethanol fixation made differentiation between
host and parasite tissue impossible. By PCR with the Trem1 F/
Trem1 R primers, 38 of the 296 P. columella specimens collected in
total at all sampling sites were positive for trematode infection
(12.84% [confidence interval (CI) 9.42–17.19%]). Sequencing of
the ITS1-5.8S-ITS2 rDNA regions (with the Trem2 F/Trem1 R
primers) revealed that 7 (2.36% [CI 1.12–4.83%]) of those snails
were infected with Echinostoma caproni (accession no. KF425322)
according to 98% sequence identity with AJ564382 (isolate from
Cairo). F. gigantica was detected in 10 P. columella individuals
(3.38% [CI 1.81–6.18%]) by PCR with Fasc ITS1 F/R primers
and sequencingwith Trem2
no. KF425321; distinguished from F. hepatica according to variable
positions listed in Mas-Coma et al. 2009). An infection with an
unknown echinostome trematode was found in 21 P. columella
individuals (7.09% [CI 4.60–10.60%]). The closest match for this
sequence in the GenBank was the echinostome Philophthalmus
lucipetus with only 85% similarity, therefore a more detailed
identification was not possible. Figure 2 illustrates the overall
prevalences of the three trematodes found. The highest prevalence
for trematode infections was found in P. columella from El-
Misharrak site, where 9 of 26 snails (34.62%) were infected, while
the lowest infection rate was detected in Itsa (Bahr Arus) with one
infected snail among 18 (5.56%). E. caproni was found at three and
the unknown echinostomid at 8 out of 10 sites. F. gigantica infected
snails were present at 5 of the 21 sampling sites with a prevalence
of up to 11.11% (Al Hadeer village), but with only one infected
snail out of nine. The lowest prevalence of F. gigantica was detected
in Sayyidna Al-Khidr village with 4.76% (2 out of 42 P. columella).
At three sites, no trematode infection was detected in P. columella,
but in these cases snail numbers were low (8 in Tutun village, 2 in
Izbat Ezbat El-Eslah El-Zraei and 1 in Zawyet El-Karadsah
Table 1. Sequences of primers designed for the present study and annealing temperatures.
NameSequence Target regionAnnealing Temp. Approximate length of product
Trem1 FTAG CCT YGG ATC AGW CGT GAITS2 54uC 200 bp with Trem1 R
Trem2 FCAA GTC ATA AGC TTG CGC TGA18S rDNA54uC1300 bp with Trem1 R
Trem1 R ACC YAA ACA CCA CAT TGC CTA28S rDNA54uC
Fasc ITS1 FTCT ACT CTT ACA CAA GCG ATA CAC ITS155uC 716 bp
Fasc ITS1 R GGC TTT CTG CCA AGA CAA GITS1
Figure 1. Total numbers of the snail species found at all sites.
Fasciola gigantica in Pseudosuccinea columella
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Table 2. Number and species of snails collected at the different sites. Sorted by number of indidiviuals per site.
No. of individuals
(no. of species)
Itsa (Bahr El Ghaba)
Al Amiriyah village
Sayyidna Al-Khidr village
Ahmed Afandi village
Itsa (Bahr Arus)
Izbat Hamada Dahman
El-Atamna Itsa village
Al Hadeer village
Izbat Ezbat El Eslah
Hanna Habib village
Proportion of total no.
Fasciola gigantica in Pseudosuccinea columella
PLOS ONE | www.plosone.org4 February 2014 | Volume 9 | Issue 2 | e88537
village). The infection rates per sampling site are summarized in
Although trematode infected snails were slightly smaller (mean
0.48 cm, median 0.46 cm) than uninfected snails (mean 0.54 cm,
median 0.50 cm), no significant relationship between snail size and
trematode infection was found (p=0.26). P. columella infected with
F. gigantica were on average 0.40 cm long (median 0.46 cm). Also
for this parasite, no significant relationship between infection and
size was found (p=0.99).
The results of the present study show that the invasive snail P.
columella is found frequently in irrigation channels in the Fayoum
governorate (at 61.90% of sites investigated in the present study)
and it even turned out to be the most abundant snail species at
most sites. Additionally, our findings imply that P. columella
probably became one of the major snail intermediate host species
for F. gigantica. Although F. gigantica infections in P. columella have
been reported before , this is the first study to provide
molecular data for this host-parasite relationship. Possibly, the
spread of P. columella as an additional intermediate host explains
partly the increase of human fascioliasis cases in Egypt in the last
50 years , as well as the occurrence of hyperendemic outbreaks
in the Nile delta [3,6]. Both Fasciola species have been identified
from parts of Egypt [9,29,30], but until now, no information was
available for Fayoum governorate. Only F. gigantica was found in
the irrigation system of the Fayoum oasis in the present study,
indicating that this parasite is the major cause of fascioliasis-cases
reported in humans  and animals [7,24] in that area. The host
snail, P. columella, is well known to be a suitable host for F. hepatica
, but apparently it can maintain the life cycle of F. gigantica as
well. Surprisingly, Radix natalensis, the indigenous host for F.
gigantica was not found at all within the present study. This might
be explained by seasonal peaks in abundance of the two species
reported by Ahmed & Ramzy . These authors observed that P.
columella was predominant in autumn and R. natalensis from
December to February. If this is the case in the area investigated in
the present study, it might come to an increased infection pressure
due to the presence of infected intermediate hosts throughout the
year. Another reason for the absence of R. natalensis in the samples
of the present study might also be the high requirements of this
species on water quality and oxygen levels [14,31] that can be
limiting in the eutrophic irrigation channels, especially at high
temperature. Compared to P. columella, R. natalensis might also be
more sensitive to the molluscicide treatment in the channels, which
gives the invader an additional selective advantage. In Brazil, P.
columella was reported to be the only available host snail in some
areas where F. hepatica-infections were reported , therefore this
snail might also be able to maintain the life cycle of F. gigantica in
Egypt. During a parasitological examination of lymnaid snails in
Dakahlia governorate (sampling date was not reported), El-Shazly
et al.  found mainly R. natalensis (68.4%) as well as G. truncatula
(16.0%), but only few P. columella (3.4%). Also, no infection with
Fasciola sp. was detected in P. columella in their study, indicating
that F. gigantica is transmitted mainly by the natural intermediate
host if it is present. According to previous experimental studies, P.
columella is less susceptible to Cuban F. hepatica and produced lower
numbers of rediae than the indigenous Galba cubensis . This
indicates that P. columella might not be a relevant host for F. hepatica
(and possibly F. gigantica as well), if the natural snail host is present.
Figure 2. Total prevalence of the trematodes detected in P.
columella. Error bars: 95% CI.
Table 3. Number of infected P. columella and prevalences for each trematode and site (total no. of tested snails given in brackets).
Site Fasciola giganticaEchinostoma caproni unknown Echinostomidall trematodes
El-Misharrak village2 (26)/7.69%2 (26)/7.69%5 (26)/19.23% 9 (26)/34.62%
Sayyidna Al-Khidr village2 (42)/4.76%2 (42)/4.76% 3 (42)/7.14%7 (42)/16.67%
Itsa (Bahr El Ghaba)3 (68)/4.41%7 (68)/10.29%10 (68)/14.71%
El–Khawagat village2 (14)/14.29%2 (14)/14.29%
Al Hadeer village1 (9)/11.11% 1 (9)/11.11%
Ahmed Afandi village 2 (24)/8.33%2 (24)/8.33%
Al Amiriyah village 3 (56)/5.36% 1 (56)/1.79%4 (56)/7.14%
Abu Ish 1 (14)/7.14%1 (14)/7.14%
Izbat El-Bank 1 (14)/7.14%1 (14)/7.14%
Itsa (Bahr Arus)1 (18)/5.56% 1 (18)/5.56%
Total no. and mean prevalence
with 95% CI
10 (296) 3.38% [1.81–6.18%] 7 (296) 2.36% [1.12–4.83%]21 (296) 7.09% [4.60–10.60%]38 (296) 12.84% [9.42–17.19%]
Results sorted in decreasing order by prevalence of all trematodes.
Fasciola gigantica in Pseudosuccinea columella
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Several studies reported a rather high prevalence of liver flukes
in their snail hosts. Caron et al.  found 6.25% of G. truncatula
infected with Fasciola sp. in Algeria. In Dakahlia governorate,
similar prevalences were reported in R. natalensis (5.50%) and G.
truncatula (3.10%) . Prevalences of F. gigantica observed in the
present study (mean 3.83%) were within the range that was
reported from P. columella in a previous study in Giza governorate
, although F. gigantica infected snails in the present study were
smaller (average of 0.40 cm) than reported by Ahmed & Ramzy
, who noticed that most infected snails were larger than 1 cm.
This might be due to the generally rather small size of snails in the
present study with only a few individuals larger than 1 cm.
Apparently, the infection is not restricted to large snails and can
also occur in populations where only smaller snails are present.
One important factor that might lead to increased abundance of
P. columella, but also other amphibious snails, is the water hyacinth
(Eichhornia crassipes) that is present in most surface waters in Egypt.
Like P. columella, E. crassipes is an invasive species in Egypt. It was
introduced in Africa by the end of the 19thcentury and spread
throughout the continent . According to recent estimations,
the total area infested with water hyacinths in Egypt is as large as
487 km2, covering large parts of irrigation channels all over the
country . The resulting problematic link of aquatic vegetation,
snail abundance and increased infection rates with Schistosoma spp.
and Fasciola spp. was already recognized long ago and has been
further studied since then, mainly with focus on vectors of
schistosomiasis [37,38]. Depending on focal outbreaks of diseases,
the irrigation channels in Fayoum governorate are treated in loose
intervals with molluscicides by the Ministry of Health, especially
for prevention of schistosomiasis. In this context, water hyacinths
will not only provide a habitat for P. columella, but might also be a
refuge for the snail, when molluscicides are present in the water.
Amphibious snails like P. columella might just move on the plants
above water level and endure until the waves of molluscicides have
passed. P. columella can also withstand detrimental circumstances
by digging into moist mud where the snails survive even
centimeters away from water . Their ability to survive adverse
conditions might be one reason for the success of this snail as an
invader and explains the dominance of P. columella at most
sampling sites in the present study.
We believe that the reported scenario of invasive P. columella as
efficient snail host for F. gigantica represents a case of parasite spill-
back, resulting in a drastic increase of infections in humans. This
situation might be aggravated by the presence of water hyacinths,
another invader that provides habitat for the snail intermediate
host, altogether giving a good example how invasive species can
alter biotic conditions and influence parasite life cycles, in this case
of a human pathogen. A similar situation is likely to occur in many
other agricultural areas in Africa.
Besides F. gigantica, E. caproni and an unidentified echinostome
were also detected in the snails tested in the present study. To the
best of our knowledge, this is the first report of E. caproni in P.
columella. Echinostome cercariae are released from the first
intermediate snail host, infect other snails and use them as second
intermediate host where they form metacercariae. The respective
final host becomes infected by ingestion of snails containing
metacercariae. Detection of trematodes by molecular methods in
the present study does not allow for distinction of sporocysts/
rediae and metacercariae in the snails, but most likely P. columella is
used as second intermediate host for E. caproni that normally infects
Biomphalaria spp. as first intermediate hosts . In cases of co-
infections with Schistosoma mansoni and echinostome trematodes in
the same snail, the latter were found to impair infection,
development and infectivity of schistosome cercariae or even to
consume larvae of other trematodes in the same snail (reviewed by
Fried & Huffman and Loker & Adema [39,40]). In case P. columella
would prove to be first intermediate host for echinostomes, these
parasites might have to be considered as regulating factor for F.
gigantica. Therefore, the presence of echinostomes in P. columella in
the Fayoum irrigation system might reduce the level of F. gigantica-
infections in the area.
A recent study revealed the presence of different morpholog-
ically indistinguishable lineages F. gigantica from Africa and India
that might in fact be two separate species, as well as an African
highland lineage of F. hepatica-like flukes that use Galba truncatula as
snail host and can be clearly differentiated genetically from
European F. hepatica . Various other studies have shown that
species boundaries seem to be not clear for F. hepatica and F.
gigantica (see review in Mas-Coma et al. ). Also, intermediate
Fasciola individuals were described from Iran and Egypt, sharing
characteristics of F. hepatica and F. gigantica [30,43,44]. The ITS1-
5.8S-ITS2 rDNA sequences of F. gigantica from isolates obtained in
the present study did not show the polymorphic sites characteristic
for the intermediate forms reported by Amer et al. .
Therefore, we consider the infection in P. columella as ‘‘regular’’
F. gigantica. Nevertheless it is of great importance to clarify the
identity of intermediate forms as well as lineages of F. gigantica and
F. hepatica, their host spectrum and significance for human
The large Fayoum oasis is a fertile agricultural area with a tight
network of irrigation channels that provide ideal habitat for vector
snails especially for Schistosoma and Fasciola spp. The invasive snail
P. columella will contribute to the increased prevalence of fascioliasis
in the human population in that area and put the population of
about 2.9 million people (http://www.geohive.com/) at risk.
Many of them live in agricultural areas and are exposed to
parasitological problems linked to the irrigation system directly,
but fascioliasis, in contrast to schistosomiasis, might even affect the
urban populations, if crops bearing metacercariae are transported
to the cities . The present study showed that F. gigantica seems to
be the major cause of fascioliasis in the Fayoum governorate and
that the invasive snail P. columella is responsible for the
maintenance of the infection in spite of snail eradication programs.
Further studies are required to evaluate the effectiveness of
molluscicide treatments on P. columella and to evaluate alternative
approaches of snail control.
Conceived and designed the experiments: DSG FAMMM MN EMHM
AHAS BS. Performed the experiments: DSG FAMMM MN. Analyzed the
data: DSG FAMMM BS. Contributed reagents/materials/analysis tools:
DSG FAMMM EMHM AHAS BS. Wrote the paper: DSG.
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