In vertebrates, disease susceptibility often varies with age. Older individuals may be more susceptible than younger individuals due to senescent declines in immune function. Alternatively, disease susceptibility may decrease with age if older individuals are more likely to have had prior exposures to parasites and acquired adaptive immune responses that allowed them to resist future infections. Disease susceptibility can also vary with reproductive state, and reproductive hormones have been shown to increase infection susceptibility. Here we investigated how age and experimentally elevated levels of the reproductive hormone testosterone affected haemosporidian infections in free-living adult male dark-eyed juncos Junco hyemalis. Samples were collected before and at least one year after experimental treatment with either a testosterone implant or empty control implant. The probability of being infected with haemosporidians in the Plasmodium/Haemoproteus group increased with host age but was unaffected by experimental testosterone elevation. Our longitudinal sampling design allowed us to determine that the increase in infections with host age was driven by variation within individuals, such that an individual's probability of being infected increased with age. This suggests that haemosporidian susceptibility increases with host age, and/or that haemosporidian-infected juncos sustain long-term chronic infections that are rarely completely cleared, and that the probability of being infected is higher in older juncos because they have experienced higher cumulative exposure risk.

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