Red deer (Cervus elaphus) have a pronounced seasonality in their physiology. The aim of this study was to assess the effect of the season on red deer responsiveness to skin testing with the phytohaemagglutinin (PHA) mitogen. Study subjects included 270 farmed adult red deer (19 stags and 251 hinds). The skin testing was carried out between January 2009 and August 2010. The animals were injected intradermally with a 0.1 ml volume containing 250 μg of PHA diluted in phosphate buffered saline. The skinfold thickness was measured immediately prior to injection and 72 h after administration, always by the same person and with three repeats per measurement. Single effects of sex and time on skin test responsiveness were significant (p < 0.001) as well as their interaction (p < 0.001). In winter (January), and considering the average of two years, the skinfold increase in response to the intradermal injection of 250 μg PHA was 2.1 times larger in stags and 1.4 times in hinds than in summer (August). While stags had 1.3 times larger responses than hinds in winter, the inverse occurred in summer, with 1.1 times larger responses in hinds. We also evidenced a limited inter-annual variation of skinfold increase in response to PHA in red deer. These findings have important consequences regarding the interpretation of skin test results in the ante-mortem diagnosis of tuberculosis and paratuberculosis, at least in deer.
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