We report here studies of the behavior of male mice of the inbred strain C57BL/6 after prolonged exposure to two factors – social stress and infections with Opisthorchis, a combination which is often seen in humans. Mice of four groups were compared: 1) mice with prolonged experience of defeat in intermale confrontations (30 days) (SS); 2) those infected with O. felineus helminths (six months) (OF); 3) animals subjected to both factors (SS + OF); and 4) mice experiencing neither factor (CON). The behavior of all animals was evaluated in an open field test including a box, which was empty for the first 3 min of the test and contained an unfamiliar male of the inbred strain BALB/c during the second 3 min. Social stress had a stronger influence on the behavioral parameters evaluated in this test than infection. SS mice were more active than all others in exploring the box containing an unfamiliar male: they climbed onto it much more frequently and had longer mean durations of time spent close to the box. In addition, during the first 3 min of the test, these animals displayed elevated exploratory activity (number of rearings by the wall), and greater numbers and durations of grooming episodes. Infected mice of the OF group showed no difference in behavior from the CON group in either the first 3 min or the second 3 min of the test. In mice with the combination of factors (SS + OF), nonsocial forms of behavior were also no different from those in controls and reactions to the unfamiliar male were weaker than in SS mice. These data lead to the conclusions that prolonged experience of defeats in intermale confrontations had a stronger influence on social and nonsocial behavior in mice than chronic infection of the animals with O. felineus helminths and that the combination of these factors reduces social interest in mice.
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