Seasonal Changes in the White Blood Cell Population and the Ability of Lymphocytes to Proliferate in Captive Brushtail Possums (Trichosurus vulpecula) .

Abstract

Brushtail possums in our captive colony die more frequently in winter than in summer. The present study examined whether the increased number of deaths in winter is due to a decreased immune response. The number and proportion of white blood cells, the proliferation of lymphocytes in response to the T cell mitogen, phytohaemagglutin (PHA) and the plasma cortisol concentration were determined in six male and six female possums during summer and winter. The total numbers of white blood cells, the numbers of lymphocytes, the proliferation of lymphocytes and the plasma cortisol concentration did not vary significantly between summer and winter. However, significant differences were observed in the numbers of neutrophils and the ratio of lymphocytes to neutrophils (L:N ratio) between the two seasons. The numbers of neutrophils in male and female possums (n=6, P=O.Ol) and 3.15 x 10 /ml (n=6, P=0.04) respectively during winter. The L:N ratio in male and female possums changed from 93:7 (n=6) and 87:12 (n=6) during summer to 78:20 (n=6; P=0.0004) and 65:33 (n=6; P=0.03) respectively during winter. The changes in the blood parameters in possums during winter are similar to those observed in unhealthy possums that subsequently die. These results suggest that possums face more difficult conditions in winter and this may play a role in the greater number of deaths observed at this time of the year.