Avian Malaria - Disease Process and Epidemiology

Disease Process and Epidemiology

Plasmodium relictum reproduces in red blood cells. If the parasite load is sufficiently high, the bird begins losing red blood cells, causing anemia (USDI and USGS 2005). Because red blood cells are critical for moving oxygen about the body, loss of these cells can lead to progressive weakness and, eventually, death (USDI and USGS 2005). Malaria mainly affects birds in the order Passeriformes (perching birds). In Hawaiʻi, this includes most of the native honeycreepers and the Hawaiian crow. Susceptibility to the disease varies between species, for example, the ʻIʻiwi is very susceptible to malaria while the ʻApapane less so (USDI and USGS 2005). Native Hawaiian birds are more susceptible than introduced birds to the disease and exhibit a higher mortality rate (Van Riper et al. 1982; Atkinson et al. 1995). This has serious implications for native bird faunas (SPREP) with P. relictum being blamed for the range restriction and extinctions of a number of bird species in Hawaii, primarily forest birds of low-land forests habitats where the mosquito vector is most common (Warner 1968; Van Riper 1991; USDI and USGS 2005).

The incidence of this disease has nearly tripled in the last 70 years. Notable among the species of birds most heavily affected were House Sparrows, Great Tits, and Blackcaps. Prior to 1990, when global temperatures were cooler than now, less than 10 percent of House Sparrows (Passer domesticus) were infected with malaria. In recent years, however, this figure has increased to nearly 30 percent. Likewise, since 1995, the percent of malaria-infected Great Tits has risen from 3 percent to 15 percent. In 1999, some 4 percent of Blackcaps — a species once unaffected by avian malaria —were infected. For Tawny Owls in the UK, the incidence had risen from two or three percent to 60%.