Seasonality of presentation of imported Plasmodium vivax malaria in Birmingham, UK.

Residents of the UK returning from northern Pakistan with Plasmodium vivax infection tend to develop symptoms and present to hospital in the summer months, irrespective of the month of return. Thus, infections acquired in the cooler months of November to April appear to have a longer latency before presentation. Experiments suggest that more hypnozoites arise from the liver when ambient temperatures fall, somehow 'programming' parasites within biting mosquitoes.