Worm recovery and survival rate
The worm recovery rate in BALB/c mice tested with the Morris water maze significantly decreased with the time of infection (t(4) = 0.000169, P < 0.001) whereas that of C57BL/6 mice decreased only on day 21 post-infection (t(4) = 0.001452, P < 0.01). In addition, the rate in the BALB/c mice was significantly lower than that of C57BL/6 mice on days 14 (t(4) = 0.000331, P < 0.001) and 21 (t(4) = 0.006658, P < 0.01) (Fig. 1).
All animals in the control groups remained alive on day 42 post-infection, giving a survival rate of 100% (data not shown). The survival rate in BALB/c without undertaking the water maze test (the non-swimming group) rapidly decreased to 0% by day 25 whereas those undertaking the test (the swimming group) survived three more days. Among the C57BL/6 mice, the survival rate only became 70% on day 42 in the non-swimming group and that in the swimming remained 85.7% on the same day. The mortality rate in the swimming group decreased from 30% to 14% (Fig. 2).
Changes in weight
In the non-swimming group, there was no significant difference in weight of infected and non-infected BALB/c mice up to day 13 post-infection (t(9) = 0.320876, P > 0.05). From day 14, the weight of the infected group significantly decreased (t(9) = 0.006903, P < 0.01) (Fig. 3a). On day 21, the non-infected mice had a weight gain of 7% and the infected one had a weight loss of 24%. A similar decreasing pattern was observed in the swimming group. Significantly differences in weight of the infected animals were found since day 13 (t(9) = 0.011617, P < 0.05) (Fig. 3b). On day 21, the non-infected mice had a weight gain of 7% and the infected one had a weight loss of 21%.
In the non-swimming group, the weights of infected C57BL/6 mice were found to be significantly lower than the controls (t(9) = 0.031012, P < 0.05) (Fig. 3c). The infected mice had a weight loss of 12% on day 21 post-infection whereas the uninfected controls had a weight of 10%. In the swimming group, the C57BL/6 mice had a change pattern similar to the BALB/c counterparts. From days 1-11, no significant difference was found in the infected and uninfected mice (t(9) = 0.326197, P > 0.05) and the weight of the infected ones significantly decreased since day 12 (t(9) = 0.037645, P < 0.05) (Fig. 3d). On day 21, the uninfected mice has also a weight gain of 10% while the infect ones only had a weight loss of 10%.
Dietary consumption
In BALB/c mice, there was no significant difference in food intake between the uninfected control and infected one in the swimming group up to day 12 post-infection (t(9) = 0.113181, P > 0.05). Since day 13, the food intake in the infected group significantly decreased (t(9) = 0.004678, P < 0.01) (Fig. 4a). There was also no difference in food intake in C57BL/6 infected mice in the swimming group and the uninfected controls up to day 12 (t(9) = 0.692658, P > 0.05) and that in the infected ones became significantly decreased since day 13 (t(9) = 0.004806, P < 0.01) (Fig. 4b).
In BALB/c mice, there was no significant difference in water intake between the uninfected control and infected one in the swimming group up to day 11 post-infection (t(9) = 0.794676, P > 0.05). Since day 12, the water intake in the infected group significantly decreased (t(9) = 0.002576, P < 0.01) (Fig. 5a). There was also no difference in food intake in C57BL/6 infected mice in the swimming group and the uninfected controls up to day 13 (t(9) = 0.204049, P > 0.05) and that in the infected ones became significantly decreased since day 14 (t(9) = 0.019749, P < 0.05) (Fig. 5b).
Depression- and anxiety-like behavior
By the force swimming test, the infected BALB/c mice were determined to have longer immobility time than the uninfected ones on day 21, indicating the occurrence of depression (t(9) = 0.018582, P < 0.05) (Fig. 6a). No significant difference was found in the immobility time in the infected and uninfected C57BL/6 mice on day 7 (t(9) = 0.158094, P > 0.05). Moreover, the immobility time in the infected mice was found to be significantly shorter than the uninfected ones on day 14 (t(9) = 0.013031, P < 0.05) and day 21 (t(9) = 0.00011, P < 0.001), indicating an increase in activities and no depression occurred (Fig. 6b). Although no special findings was observed in the infected BALB/c mice by the open field test, the infected C57BL/6 mice were found to have significantly higher activity (t(9) = 0.024914, P < 0.05) (Fig. 7a) and longer walking distance (t(9) = 0.029771, P < 0.05) (Fig. 7b) the uninfected ones on day 9 post-infection, indicating that anxiety-like behavior occurred in these animals (Fig. 7).
Changes in spatial learning
After infection with A. cantonensis, BALB/c and C57BL/6 mice were continuously trained in water maze for 4 days to observe their spatial learning ability. Before day 7, no significant difference in escape latency to the platform was found in infected and uninfected BALB/c mice (t(9) = 0.561855, P > 0.05) (Fig. 8a). Since day 12, the latency in the infected mice was found to be significantly higher than that of the uninfected ones (t(9) = 0.000049, P < 0.001) (Fig. 8b and 8c), indicating that the spatial learning ability of infected mice significantly decreased. Although infected and uninfected C57BL/6 mice showed no significant difference in spatial learning ability from day 1 to day 14 (t(9) = 0.803257, P > 0.05) (Fig. 8d), the latency in the infected ones became significantly higher after day 14 (t(9) = 0.013565, P < 0.05) (Fig. 8e and 8f), indicating significant lower spatial learning ability also occurred in the infected C57BL/6 mice, although the occurring time was delayed.
Changes in spatial memory
After removing water maze platform, the infected BALB/c mice stayed in the target quadrant of the original platform significantly shorter than the uninfected group (t(9) = 0.002013, P < 0.01) (Fig. 9a) and the time for the infected mice moving to the target quadrant was significantly longer than that of the uninfected ones (Fig. 9b) on days 7 (t(9) = 0.016921, P < 0.05), 14 (t(9) = 0.015133, P < 0.05), and 21 (t(9) = 0.000047, P < 0.001). In the infected C57BL/6 mice, these phenomena only occurred on days 7 (t(9) = 0.03045, P < 0.05) and 21 (t(9) = 0.004776, P < 0.01) (Fig. 9c and 9d). Although the spatial memory ability of the two strains of mice significantly decreased, the ability remained intact in the C57BL/6 in the early stage of A. cantonensis infection.