4. Discussion
In Morocco, ACL due to L. tropica is transmitted by P.
sergenti which has a large geographic distribution probably related to
the wide ecological plasticity of this vector (Rioux et al.,1986; Ramaoui et al., 2008; Boussaa et al., 2009). CL due
to L. tropica is an emerging disease even though the geographical
extension of the vector is greater than that of the parasitic protozoan,
and the identification of factors for parasite expansion is essential
for effective disease control. P. sergenti density and genetic
characteristics were investigated as determining factors for the
existence of ACL transmission. Comparative intradomiciliary and
peridomiciliary sand fly captures in the ACL endemic locality of El
Borouj and the undamaged locality of Sidi Hajjaj were made using CDC
light traps and sticky papers.
P. sergenti density in the ACL free locality was lower than that
of the endemic locality, both peridomiciliary and within households.
Interestingly, P. sergenti was the most abundant and densest
species within households in Sidi Hajjaj, however it was the fourth
species outdoors, after S. minuta and the L. infantumvectors, P. perniciosus and P. longicuspis (Table
1). The relative abundance of P. sergenti males and females
varied between the trapping methods as males were more abundant in the
sticky papers. In both localities, the female P. sergenti density
was higher in the intradomiciliary June captures and all gonotrophic
cycle categories, non-fed, fed and gravid females, were found (Table 2).
Although to date no ACL cases have been diagnosed in Sidi Hajjaj, these
sand fly density figures seem sufficient for the maintenance of L.
tropica transmission (Rioux et al., 1986; Ramaoui et al.,2008; Barón et al., 2013) and would make this locality
susceptible to the establishment of an ACL transmission cycle. Over the
last few decades, L. tropica foci have spread to several regions
of Morocco including those where CL caused by L. major orL. infantum has been reported, which shows the changing
geographical patterns of this species (Baghad et al., 2020). The
growing mobility of humans from endemic to non-endemic cities raises the
possibility of emerging foci in areas where P. sergentipopulations are well established. Kholoud et al. in 2020 suggested that
ACL dissemination in Morocco is associated to an increase in human
travel and local tourism linked to economic expansion and infrastructure
development as shown by the synchronized occurrence of new ACL foci with
the construction of new motorways. However, the factors underlying the
spatio-temporal transmission dynamics of leishmaniasis are not well
understood, and the epidemiological picture is not as simple as deduced
from the previous statement.
The molecular characterization of P. sergenti populations in both
localities using the PCR-RFLP technique of the cytochrome b
mitochondrial gene, has allowed us to find that the main P.
sergenti mitochondrial lineage in El Borouj is Lineage IV (97.6%)
while the remaining 2.4% belongs to lineage II. In contrast, lineage II
was the most abundant in Sidi Hajjaj (80%) followed by 20% lineage I
specimens. Therefore, a different main P. sergenti mitochondrial
lineage has been highlighted in each one of the 2 localities under
study. Phlebotomus sergenti is characterised by high genetic
diversity and classified in at least twenty haplotypes in four
mitochondrial lineages (Yahia et al., 2004; Barón et al.,2008).
In El Borouj, L. tropica DNA was detected in 5 out of 184 (2.7%)
female P. sergenti (Gijón-Robles et al., 2018) in captures
made throughout 2014 (4+/72: 5.6% and up to 18,000 parasites/µg DNA;
data not shown) and 2015 (1+/112: 0.9%). The five positive L.
tropica females belonged to the most prevalent P. sergentilineage in El Borouj, lineage IV. This is the first time that the
mitochondrial lineage of a P. sergenti specimen that is acting as
a vector for L. tropica has been identified. A local increase in
the abundance of this P. sergenti lineage that seem transmitsL. tropica more efficiently, could explain the emergence of ACL
in El Borouj and its absence in Sidi Hajjaj.
Lineage I is over-represented in southwestern Europe (Merino-Espinosaet al., 2016) and this is the first time that its presence in
Morocco is reported. No autochthonous ACL cases have been detected in
the Iberian Peninsula, despite P. sergenti being commonly found
at sufficient densities to act as a vector, and the existence of 2
mitochondrial lineages, one of them, held in common with Morocco
(Lineage III) (Barón et al., 2008, 2013; Merino-Espinosa et
al., 2016).
The existence of differential ecological traits between P.
sergenti mitochondrial lineages has been pointed out: Merino-Espinosa
et al. in 2016 found that Lineage I appear to have adaptive advantages
represented by a wider tolerance to temperature and altitude changes,
that would make it better suited to leading geographical expansion into
the rest of Europe. Similarly, there are bioclimatic differences between
El Borouj and Sidi Hajjaj (Table 4) that could explain the over
representation of P. sergenti lineage IV in El Borouj which is
warmer and drier, and its absence in Sidi Hajjaj.