The sampling site was within Mwea Irrigation Scheme, Kirinyaga County which is about 1100 - 1200 m above sea level and stretching between latitudes 0˚37'S and 0˚45'S and longitudes 37˚14'E and 37˚26'E, see Figure 1.

Vector snails’ samples were taken from schistosomiasis endemic region of Mwea irrigation farmlands. Live snails were collected and put on wet cotton wool containers which were perorated and transported to aquaculture facility at Institute of Primate Research (IPR) within one day.

Field collected samples of snails were morphologically identified at National Museum of Kenya (NMK). The snails Shell morphometric, tool for identification of mollusc taxonomy and ecological studies was used to determine species of representative population of snails collected. A total of nine measurements were taken for each shell. These were total length of shell, width of shell, basal width of shell spire, length of aperture, width of aperture, total shell area, area of spire, area of aperture, and angular.

The snails samples (B. pfeifferi) collected from field were housed for two weeks in glass aquaria containing snail-conditioned water to acclimatize to laboratory conditions. At the culture facility the snails were feed on tetramine fish food, lettuce leaves, and calcium carbonate, in a conducive environmental condition, using the method described by Eveland [19].

Schistosome eggs were collected from stool of experimentally infected primate (baboons)in an on-going schistosomiasis experiments at Institute of Primate Research (IPR)colony were exposed to light for 2 - 3 hours to stimulate hatching into miracidia. In 10 ml beakers, individual snails were exposed to 8 - 10 newly hatched miracidia in direct sunlight for 3 - 4 hours. The miracidia exposed snails were kept under laboratory condition at room temperature as previously described by [19 - 21].

The miracidia exposed snails were transferred into aquaria each containing 48 snails replicated 10 times in a Complete Randomized Design. They were kept under laboratory condition at room temperature.

The snails were individually examined after four weeks of exposure to the parasite for cercarial shedding twice in direct light illumination procedure in which individual snails were put in beaker containing 10 ml of water and cercaria shedding was observed on 10 X magnification -microscope. Cercaria shedding was determined using the method direct illumination method [22] in which the exposure larvae infected snails to white light caused emergence of cercaria into the water and lastly snail infection and mortality rates were determined.

Snail Infection Rate was arrived at by dividing the cercaria shedding snails by total exposed snails. The resistant and susceptible snails’ percentage was determined as illustrated by [23] as follows:

Infection rate = Total positive and shedding crushed snails in each subgroup Total snails exposed in each subgroup × 100

Snail Mortality Rate was determined by counting the dead snails starting from the day of exposure to the parasite divided by exposed snails in each subgroup as follows: -

MR % = Total snails which dead in each subgroup Total snails exposed in each subgroup × 100

Populations of laboratory adapted B. pfeifferi snails at IPR were tested for resistance/susceptibility to S. mansoni infection. The snails were reared under suitable laboratory conditions according to by [19] where they were kept in glass aquaria with snail-conditioned water and fed on lettuce leaves, tetramine fish food and calcium carbonate. Source and maintenance of the parasite S. mansoni miracidia were collected from infected primate host (Baboons) at IPR schistosomias on-going experiments.

Snails were individually exposed to miracidia larvae and selection of susceptible and resistant snail isolates determined by actual biological shedding of cercaria. The field sample snails that remained uninfected after 4 - 6 weeks of exposure to infection were reared separately. Their progeny (F1) were selected and reared under the same conditions as the resistant group [24].

In 10 ml beakers, 480 resistant isolates snails were exposed individually to 8 - 10 newly hatched miracidia and the beakers were placed under direct sunlight for 3 - 4 hours. The exposed snails were maintained in aquaria each with 48 snails replicated 10 times in a Complete Randomized Design experimental design. The snails were kept under laboratory condition at room temperature as was illustrated by [19 - 21].

Each snail was checked for cercarial shedding two times a week for four weeks post-exposure to the miracidia and repeated for three weeks. The cercaria shedding or non-shedding from individual snail was determined using a direct light illumination procedure where individual snails were put in 10 ml water in beaker and cercaria shedding determined by observation on 10× magnification stage microscope. During the shedding period of three weeks were kept in darkness throughout [23 , 24]. The snail infection and mortality rates were determined.

The resistant F1 snails isolates that were exposed to miracidia in the above (2.6) experiment and did not shed cercaria were kept separately for propagation. Their progeny (F2), were selected for miracidial infection. The experiment set-up was repeated as described in 7section (2.6) and the same parameters determined.

Snails shell morphometric identification determined populations dominating the Mwea irrigation scheme in Kenyan highland were B. pfeifferi. This observation concurs with other previous works by Mutuku [25]. The snails had mean Shell Height of 9.9 and 10.0 mm and shell width of 9.5 mm and 9.5 mm for resistant and susceptible snails respectively. The morphological parameters of B. pfeifferi indicated that there are no significant differences in physical characteristics of resistant and susceptible populations of B.pfeifferi snails against S. mansoni from the study region.

These findings point to demonstrate that the exhibited resistant traits may not as a result of snails morphological characteristics but probably due to snail internal defense mechanisms. Similar of observations have been reported by Nacif-Pimenta and Negrão-Corrêaet [26 , 27]. Earlier, Webster and Newton had reported that susceptibility of Biomphalaria ssp. to S. mansoni infection was inherited character [28 , 29].

The snails populations collected from field exhibited significantly high rate of infection at 76% shedding cercaria while 24% of the snails showed some resistance. However, these observations significantly differed with infections rates exhibited by resistant snails’ progeny of F1 and F2. In F1 generation, susceptible snails dropped to 4% while 90.8% were resistant. In F2 generation susceptibility decreased even further to only 0.8% and that of resistant group increased further to 95.1%, see Figure 2.

The F-test statistics of B. pfeifferi in infection rates revealed highly significant differences between the Field Collected snails, F1 and F2 generations. The mean value for the Field, F1and F2 snail samples were 36.6 ± 3.72, 1.93 ± 1.46, 0.36 ± 0.049 respectively. Comparing the Field samples and F1 generation for the infected snails, F = 6.48 and P = 0.00 at 0.05 confidence level, the two populations variations were found to be significantly different. The Field and F2 snail samples for the infected snails, F = 57.59 and P = 0.00 at 0.05 confidence level, the two populations variations were found to be significantly different. Lastly, the F1 and F2 generation of infected were found to have an F = 8.88 and P = 0.00 and at 0.05 confidence level were found to be significantly different, see Table 1.

In the non-infected, the Field samples and F1 generation snails, F = 2.54 and P = 0.01. At 0.05 confidence

level, the two populations variations were found to be significantly different. The field and F2 snail samples for the non-infected snails, F = 3.13 and P = 0.00 and at 0.05 confidence level, the two populations variations were found to be significantly different. Lastly, the F1and F2 snail samples non-infected were found to have an F = 1.23 and P = 0.058 and at 0.05 confidence level were found to be significantly different. Since there was significant difference between the field F1 and F2 generations there might be a gene enhancement as one observes the generations’ field, F1, F2, and So on.

These laboratory observations of infections rates of B. pfeifferi from Mwea irrigation scheme against S. mansoni, seems to support previously reported work that suggests there are enhancement of resistant genes individual snails progressed through the F1 and F2 generations. Similar trend was noted by Paraense on determination of phenotypic resistance of F1 and F2 progeny [30]. In the work of Lewis and Iman while investigating the genic flux of the resistance traits in F1 progeny obtained by crossbreeding susceptible and resistant strain of the same species of B. glabrata, it was observed that resistant individuals predominantly occurred in all groups [31 , 32]. Previous investigations by Richards and Merritt docu­mented dominant resistance heritability in B. glabrata snails [33]. Rosa reported two dominant genes determine resistance in B. tenagophila, [34] while Negra-Correa reported that the factors that influence the susceptibility may be genetically determined by the activities of the Snails internal defense system [27]. Webster and Woolhouse found that both resistant and susceptibility to Schistosome infection were heritable [28].

Statistically there were no significant differences in mortality rates between snails samples from Field, F1 and F2. The mean values for the number of dead snails in the Field, F1 and F2 snail samples were 6.033 ± 2.025, 2.467 ± 2.53, 3.067 ± 1.96 respectively. Comparing the number of the dead snails for the Field and F1 samples, F = 1.62 and p = 0.22 at 0.05 confidence level, the two populations variations were not significantly different. In the case of Field and F2 snail samples, F = 1.06 and P = 0.87 at 0.05 confidence level, the two populations variations were not significantly similarly to comparison of F1 and F2 at F = 0.65 and

P = 0.26 were not significantly different, see Table 2.

Though not statistically significant differences were observed, the percentage mortality rates in the Field, F1 and F2 snails samples were determined as 15%, 6% and 7.6% respectively. This finding is similar mortality in snails as reported by Alvine [35] under the same conditions. The observed mortality rates may be due to natural causes, see Figure 3.

Overall the findings generate in this study under laboratory condition points to suggest that S. mansoni resistant population of B. pfeifferi can be isolated and mass reared with a view of diversifying biological control measures of the vector. In line with observations [36 , 37] posit that desirable long-term solution to control of schitosomiasis is to build up the field resistant or the less susceptible strains to infection, which is an ecologically safer means of breaking transmission cycles. As described by [38], one of promising measures of biological control is the introduction of parasite resistant snails into endemic areas to replace susceptible strains and avoid often destructive changes to the local ecosystem that accompany other control methods of snails, this current study could inform such interventions in schistosomiasis endemic areas in Kenyan highlands.