Abstract:
Biomphalaria pfeifferi, a predominantly self-fertilizing freshwater snail, is the
world’s most important intermediate host for Schistosoma mansoni, one of the
causative agents of schistosomiasis, which affects about 200 million people in
sub-Saharan Africa. Understanding the genetic variation, selfing rates, and
refractory traits of B. pfeifferi is important for improving snail-based strategies to
interrupt S. mansoni transmission. The use of multilocus genotypes (MLGs) is
important in identifying unique lineages within the snail population. This study
sought to determine: (i) if lineages of B. pfeifferi within Asao stream could persist
through different seasons of the year, (ii) if infections by S. mansoni or other
trematodes occur within the persistent lineages, (iii) if B. pfeifferi from the Asao
stream are selfers and finally (iv) if there are refractory genotypes among snails
from Asao stream. B. pfeifferi snails were collected from Asao Stream in six bi
monthly (every other month) sampling times between November 2018 and
September 2019. Snails were isolated and screened for S. mansoni and other
trematode infections using the shedding method, cercarial morphology was
examined microscopically. Successive infection and breeding experiments was
done to determine refractory genotypes. Using 14 microsatellite markers in a
multiplex Polymerase Chain Reaction (PCR), B. pfeifferi snails were genotyped.
Data analysis was done using GENALEX software and R studio software; poppr
and coin packages. For the first three objectives, a total of 502 B. pfeifferi snails
were collected. Genetic differentiation among the snails was low, ranging from
FST = 0.019, p = <0.05 (November and March) to FST = 0.372, p = <0.001) (July
and March). Analysis of molecular variance (AMOVA) showed a small variation
(8%) among sampling times. A total of 319 MLGs were identified, 26 of which
were present at two or more sampling times. Four MLGs persisted across the 10
month sampling period, one of which was represented by 17 individuals. Of the
trematode species S. mansoni was the most common at 64.79%, other trematodes
included; Echinostomes (3.29%), Xiphidiocercariae (6.10%), Paramphistomoids
(23.47), Sangunicolids (0.94%) and Strigeids (1.41%). The persistent MLGs had
more trematode infections compared to those found only at a single sampling
time. There was high inbreeding coefficients (Mean FIS =0.639) and low
heterozygosity (Mean Ho=0.145), with selfing rates estimated at 77.77%.
Objective four began with 300 non shedding snails, the breeding experiment
generated 10 Fillial 3(F3) B. pfeifferi non-shedding snails. These findings suggest
that B. pfeifferi in Asao stream is predominantly self-fertilizing, with persistent
genotypes that may sustain S. mansoni transmission across seasons. The
identification of 10 F3 non-shedding snails suggests the possible presence of
refractory snails. This study highlights the need for year-round snail control and
monitoring of potentially refractory snail genotypes should be considered in
control strategies.
