The dynamics, transmission, and population impacts of avian malaria in native Hawaiian birds: a modeling approach

Abstract

We developed an epidemiological model of avian malaria (Plasmodium relictum) across an altitudinal gradient on the island of Hawaii that includes the dynamics of the host, vector, and parasite. This introduced mosquito‐borne disease is hypothesized to have contributed to extinctions and major shifts in the altitudinal distribution of highly susceptible native forest birds. Our goal was to better understand how biotic and abiotic factors influence the intensity of malaria transmission and impact on susceptible populations of native Hawaiian forest birds. Our model illustrates key patterns in the malaria–forest bird system: high malaria transmission in low‐elevation forests with minor seasonal or annual variation in infection; episodic transmission in mid‐elevation forests with site‐to‐site, seasonal, and annual variation depending on mosquito dynamics; and disease refugia in high‐elevation forests with only slight risk of infection during summer. These infection patterns are driven by temperature and rainfall effects on parasite incubation period and mosquito dynamics across an elevational gradient and the availability of larval habitat, especially in mid‐elevation forests. The results from our model suggest that disease is likely a key factor in causing population decline or restricting the distribution of many susceptible Hawaiian species and preventing the recovery of other vulnerable species. The model also provides a framework for the evaluation of factors influencing disease transmission and alternative disease control programs, and to evaluate the impact of climate change on disease cycles and bird populations.

Forests of opportunities and mischief: disentangling the interactions between forests, parasites and immune responses

“…The use of heavy machines during logging causes the formation of puddles on the skid trails and forest grounds. These habitat features can serve as nursery habitats for vectors, and thus their numbers will increase locally as a function of forest structure and forest management regimes (Samuel et al, 2011).…”

Is Avian Malaria Playing a Role in Native Bird Declines in New Zealand? Testing Hypotheses along an Elevational Gradient

“…Possible factors include rates of transmission, parasite fitness, and host susceptibility [11,37,38]. If differences in transmission rates caused the observed prevalence patterns, we would expect native birds to be exposed to mosquitoes less often than non-native birds.…”

Mitigating Future Avian Malaria Threats to Hawaiian Forest Birds from Climate Change

“…However, introduced avian malaria ( Plasmodium relictum ) transmitted by the southern house mosquitoes ( Culex quinquefasciatus ) is considered to be a key factor limiting present distribution and abundance of the remaining Hawaiian native birds [4–8]. Because malaria dynamics are strongly influenced by ambient temperature and precipitation patterns [9–12], predicted future climate changes (i.e., increasing temperature and altered rainfall) are expected to increase the occurrence, distribution, and intensity of avian malaria transmission to Hawaiian forest birds [8,13,14]. …”

“…A comprehensive epidemiological model of the Hawaiian forest bird-malaria system illustrates that transmission varies across altitudes on Hawai’i Island; from high year-round transmission in warm lowland forests, to seasonal episodic transmission at mid elevation, and infrequent summer infections at cool high elevations [13]. Consequently, predicted climate change will substantially alter the geographic and elevational distribution of malaria pathogens by facilitating transmission in high-elevation forests, currently with low disease transmission, and increasing transmission at lower elevations [12,15,16].…”

“…In the Hawaiian Islands, warming has already been observed at higher elevations [17], where cool seasonal temperatures previously inhibited Plasmodium development in mosquitoes [18]. Malaria prevalence may have recently increased in endemic birds at high elevations in Hawai’i [19–21], which constitute the only remaining refugia for native species that are highly susceptible to malaria, supporting the concern that native Hawaiian forest birds may suffer further population declines and/or species extinction due to climate-induced increase in disease transmission [8,11,13,14,22–25]. …”