Factors affecting the relapse of Haemoproteus belopolskyi infections and the parasitaemia of Trypanosoma spp. in a naturally infected European songbird, the blackcap, Sylvia atricapilla.

The effects of photoperiod and stress on the relapse of Haemoproteus belopolskyi infections and the parasitaemia of Trypanosoma spp. were studied in naturally infected blackcaps, Sylvia atricapilla. Twenty-one birds (12 infected with H. belopolskyi, 5 with Trypanosoma spp.) were kept indoors and investigated by microscopic examination of stained blood films. All infections were latent before the experiments started on 7 February 2002. The relapse of H. belopolskyi and the parasitaemia of Trypanosoma spp. after the latent stage of infection were induced by increased day length and stress due to a change of room. At 44 days after the beginning of the experiment, all latent infections were patent in the experimental birds, while no parasites were recorded in the control birds. The relapse of avian Haemoproteus spp. infection and of the parasitaemia of avian Trypanosoma spp. may be related to gonadal hormones, corticosterone and melatonin.