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 (P < 0.001) whereas that of C57BL/6 mice decreased only on day 21 post-infection (P < 0.01). In addition, the rate in the BALB/c mice was significantly lower than that of C57BL/6 mice on days 14 (P < 0.001) and 21 (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 (P > 0.05). From day 14, the weight of the infected group significantly decreased (P < 0.05) (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 (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 (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 (P > 0.05) and the weight of the infected ones significantly decreased since day 12 (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 (P > 0.05). Since day 13, the food intake in the infected group significantly decreased (P < 0.05) (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 (P > 0.05) and that in the infected ones became significantly decreased since day 13 (P < 0.05) (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 (P > 0.05). Since day 12, the water intake in the infected group significantly decreased (P < 0.05) (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 (P > 0.05) and that in the infected ones became significantly decreased since day 14 (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 (P < 0.001) (Fig. 6a). No significant difference was found in the immobility time in the infected and uninfected C57BL/6 mice on day 7 (P > 0.05). Moreover, the immobility time in the infected mice was found to be significantly shorter than the uninfected ones on day 14 (P < 0.05) and day 21 (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 (Fig. 7a) and longer walking distance (Fig. 7b) the uninfected ones on day 9 post-infection (P < 0.01), 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 (Fig. 8a). Since day 12, the latency in the infected mice was found to be significantly higher than that of the uninfected ones (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 (P > 0.05) (Fig. 8d), the latency in the infected ones became significantly higher after day 14 (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 (Fig. 9a) and the time for the infected mice moving to the target quadrant was significantly longer than that of the uninfected ones (Fig. 9c) on days 7, 14, and 21. In the infected C57BL/6 mice, these phenomena only occurred on days 7 and 21 (Fig. 9b 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.