Further Studies on Parasitic Obesity in Mice, Deer Mice, and Hamsters

Abstract

Accelerated weight gain caused by infection with spargana of Spirometra mansonoides occurs in male mice as well as females, and in older mice approximating 40 g starting weight, as well as in younger animals. Below a starting weight of 10 g the effect is greatly reduced and delayed if seven spargana per mouse are used. Deer mice, Peromyscus, show the same response. The oriental worm, S. ranarum, has an even greater stimulating effect than mansonoides, but is more antigenic, becoming encapsulated in the first 2 months, after which the effect is lost. The weight gain effect is additive, increasing with the number of spargana, but reaches a plateau at about 12 worms per 20-g mouse, after which either no further gain occurs, or gains are offset by increased mortality of the mice. Hamsters show an even more pronounced effect. Both males and females respond. In this case the infected animals grow at a very similar accelerated rate, while the controls show wide variation. True growth as well as obesity is involved, since infected hamsters are not only heavier but larger as shown by inspection and by X-ray. Small numbers of spargana appear to have no effect on longevity; a group of the original experiments allowed to die off naturally showed similar mortality rates for both experimental and control mice. Nor is there any effect on reproduction. Infected mice and hamsters breed as well as controls if given the opportunity. Mueller (1963a) described accelerated weight gain in mice infected with small numbers of plerocercoids of Spirometra mansonoides. In the original experiments only female mice were used at an average starting weight that varied from 11.7 to 21.4 g. The numbers of spargana per mouse varied from seven to eight. For smaller numbers of worms the effect was shown to be additive. The obesity effect was not dependent upon the strain of mouse since white mice from various suppliers reacted similarly. Gains of experimental mice over controls at 20 weeks averaged 17.60%, the larger gains tending to occur in mice of greater starting weight. Mice infected at a starting weight of 11.68 g showed an initial lag, but in the 2nd week overtook and passed the controls, thereafter increasing their advantage. ADDITIONAL EXPERIMENTS ON MICE This phenomenon has now been examined in greater detail, with preliminary results published in abstract (Mueller, 1963b, 1965). In five experiments on male mice (21 experimental, 20 control), at starting weights ranging from 9.5 to 38.75 g, and continuing for periods of 2 months or more, four experiments showed marked acceleration of the infected animals over the controls (Fig. 1), the maximum occurring in one experiment (E-93) where starting weight was 22.5 g. In male Received for publication 23 February 1965. mice with starting weight of 9.5 g (E-90) the experimentals lagged behind the controls for the first 4 weeks and then surpassed them, achieving an advantage in weight increase of 11% at the end of 12 weeks, but with the differential diminishing from 14 to 29 weeks (Fig. 2). Twenty-two further experiments have been done on the effect in female mice (185 experimental, 114 control animals1) ranging in weight from 10 to 39 g. With females of 10 and 11 g starting weight (E-95 AB, CD) there occurred only minimal gains over controls (5 and 12% respectively at 11 weeks) and then only after an initial period of several weeks in which the experimental animals lagged behind the controls (Fig. 3), as in the case of young male mice. It is thus clear that any advantage in weight gain bestowed on mice by this type of parasitism is minimal if the animals are infected at or below 10 g in weight, and it seems probable that below this weight the effect disappears altogether, or could be demonstrated only with a smaller number of worms. Contrary to expectation, a weight gain was also produced in older female mice (E-117, E-105) of 33.5 and 39 g starting weight, respectively (Fig. 4). The gains over controls The numerical asymmetry results from the fact that some of the experiments had multiple groups of mice, infected with different numbers of worms, with only a single set of controls.