Apparent effect of chronic Plasmodium infections on disease severity caused by experimental infections with Mycoplasma gallisepticum in house finches

•Mycoplasma gallisepticum caused an epidemic swept in North American house finches.•About half of house finches in Upstate New York are infected with Plasmodium spp.•Coinfection with both pathogens causes more severe M. gallisepticum induced disease.•Infection with M. gallisepticum may result in increased Plasmodium transmission.

An epidemic caused by a successful host jump of the bacterial pathogen Mycoplasma gallisepticum from poultry to house finches in the 1990s has by now spread across most of North America. M. gallisepticum causes severe conjunctivitis in house finches. We experimentally show that M. gallisepticum transmission to birds with or without chronic Plasmodium infection does not differ. However, once infected with M. gallisepticum house finches chronically infected with Plasmodium develop more severe clinical disease than birds without such infection. We speculate as to possible effects of coinfection.