الفهرس 
SchistosomiasisOnly 14 pages are availabe for public view
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Abstract
The effects of heavy metals on survival of Biomphalaria
alexandrina the intermediate host of Schistosoma mansoni under
natural conditions has important consequences for predicting and
assessing the outcome of their use in bio-monitoring programs and
their role in S. mansoni transmission. In the present study we tested the
tolerance to Cd-toxicity in Biomphalaria alexandrina snails through
generations produced by self- fertilization from the most tolerant
parents. Cd-tolerant of the third generation (selected) have been tested
and compared with the (non-selected) laboratory breeding and field
collected snails for their tolerance to other heavy metals zinc (Zn), lead
(Pb) and copper (Cu) and molluscicides (bayluscide as chemical one
and Anagallis arvensis as plant one). Also, their alteration in the
susceptibility to S. mansoni infection, physiological responses
(haematological criteria and biochemical criteria), protein pattern
(SDS-PAGE), and the genetic structure (PCR-ISSR) were tested and
compared. Results of toxicity assessment showed a gradual increase in
the LC50 and LC90 values through successive generations than the
parents indicating Cd tolerance upgrading phenotype. Also, the
selected Cd-tolerant snail group showed higher tolerance to copper
(Cu) and A. arvensis than field collected and laboratory breeding snail
groups suggesting correlation between Cd tolerance and tolerance to
Cu, and A. arvensis plant compounds.
Cd-tolerant snails of the 3rd generation showed refractory tendency
to infection, higher pre-patent period over 72 days and higher survival percentage of 35%. On the other hand, non-selected laboratory snails
showed moderate susceptibility of 33.3% infection percentage, prepatent
period of 26 days, number of cercariae /snail of 1790 and
survival percentage of 15%.
The haematological criteria (total and differential blood cell
counts) and biochemical parameters, liver enzymes [Aspartate
aminotransferase (AST), Alanine aminotransferase (ALT), Alkaline
phosphatase (ALP)], protein contents [Total protein (TP), Albumin
(ALB)], and kidney functions (Urea and Creatinine), were used to
estimate the toxic effect of 24 hrs. exposure to Cd among the three
different snail groups under study, selected Cd-tolerant third
generation, adult negative field collected snails and laboratory
breeding (non-selected) in comparison to non-exposed groups. Results
indicated that the selected snails showed a significant increase in
haemocytes; granulocytes; which indicate that their defense mechanism
was stronger than non-selected ones. Also, results indicated that the
selected Cd-tolerant group had stable liver enzymes and protein levels
while had increase in the levels of urea (significant) and creatinine
(non-significant) indicating that the liver cells were still intact while
renal cells were affected. On the other hand, field snail group was the
most affected followed by laboratory one.
Protein electrophoretic patterns as revealed by SDS-PAGE and
ISSR analysis were used to differentiate between five snail groups;
three were under the selective pressure for Cd tolerance through
successive three generation; G1, G2 and G3; and two non-selected snail parent field, and lab B. alexandrina. Results of SDS-PAGE
revealed that G3 snail group was the most unique by excess four bands
(fractions 1, 7, 16 and 17) and the absence of two common bands
(fractions 2 and 3). Dendrogram showed that the lab breed snail sample
was the most similar to parent snail samples and the 3rd generation of
Cd-tolerant snails was the most diverge forming a distinct group. Also,
the dendrogram based on the similarity matrices of ISSR-PCR banding
patterns resulted with ten random primers, showed that the selected
groups of the G1 and G2 were the most similar and clustered together
forming a distinct group that in turn cluster with laboratory group that
in turn cluster with parent group, while G3 was the most divergent
group, indicating its genetic drift.