Climate Change

Avian Malaria and Hawaiian Forest Birds (Atkinson)

Evolution of Immunity: Climate Change
(Hahn, Reisen, Kogut, Summers)

Warming Climate Can Affect Fish Health (Winton)

`Akeke`e (Kaua`i `Akepa) is currently found only on the `Alaka`i Plateau and is threatened by the potential spread of avian malaria from lowland habitats on the island. Photo credit: USGS Image Gallery

Giemsa-stained blood smear from a canary with an experimental infection with Plasmodium relictum.    Malarial parasites (arrow) develop within nucleated blood cells of their avian hosts. Photo credit: USGS Image Gallery

Native Hawaiian forest birds, particularly the endemic Hawaiian honeycreepers (Drepanidinae), are frequently cited as outstanding examples of adaptive radiation and speciation, but currently face one of the highest rates of extinction in the world.  Introduced mosquito vectors (Culex quinquefasciatus) and avian malaria (Plasmodium relictum) are widely considered to be primary factors that have contributed to population declines and changes in the geographic and altitudinal distribution of native forest bird species in the Hawaiian Islands.  While there is recent evidence that some native species are evolving resistance to these diseases, most species remain highly susceptible and have likely survived into this century only because cool, high elevation refugia still exist on the higher mountains on Kaua`i, Maui, and Hawai`i Islands.  Predicted climatic changes associated with global warming will likely affect the long term stability of this disease system because the mosquito life cycle and development of the parasite within the mosquito vector are highly temperature dependent. Recent analyses of temperature trends in the islands indicate that warming is already occurring, and there is concern that these remaining high elevation disease refugia may be lost if malarial transmission moves to higher elevations. The Alaka’i plateau, 1200 to 1500 m in elevation, is the highest area on Kaua’i and falls within a zone where malarial transmission is dependent on increases in mosquito populations during the warmest months of the year.  This area is the last remaining refuge for threatened and endangered forest birds on Kaua`i and has seen recent dramatic declines in geographic range and distribution of two endemic honeycreepers that are found only on this island – the `Akikiki (Kaua`i Creeper) and `Akeke`e (Kaua`i `Akepa).  We are investigating whether prevalence of avian malaria has increased significantly in the `Alakai over the past decade through analysis of archived blood samples collected by USGS scientists in the mid-1990’s and recent blood samples collected by collaborators with the State of Hawaii, Department of Land and Natural Resources.  Results will help resource managers prioritize and guide specific recovery actions for the `Akikiki and `Akeke`e .

For more information view the following research summaries:

• Biocomplexity of Introduced Avian Diseases in Hawai‘i: Threats to Biodiversity of Native Forest Ecosystems (PDF, 659 KB)
• Ecology and Diagnosis of Introduced Avian Malaria in Hawaiian Forest Birds

Also contact Carter T. Atkinson, Pacific Island Ecosystems Research Center.

See also Fish and Wildlife Disease: Birds >>

The remote, rugged `Alakai Plateau is located near the center of the island of Kaua`i and is one of the last high elevation refuges from introduced avian diseases in the Hawaiian Islands. Photo credit: USGS Image Gallery

Two brown-headed cowbird nestlings in a hermit thrush nest. Photo credit: Caldwell Hahn, USGS

Neutrophil engulfing bacteria. Photo credit: Copyright Volker Brinkmann

Yellow-headed blackbird. Photo credit: Bill Castelman, used with permission

To improve ecologists’ ability to predict disease epidemics in wildlife and human populations, it is fundamental to understand the ecological conditions that have favored the evolution of strong immune systems. Avian species are effective model species for studying the evolution of immunity in a variety of habitats and ecosystem. We hypothesize that a group of parasitic birds, the New World cowbirds, are a good model for documenting the evolution of increased immunity as a result of evolutionary exposure to increasing diversity of environmental parasites. We showed that parasitic cowbirds are more resistant to infection with West Nile virus and other encephalitis viruses than are related blackbirds that are not brood parasites. We are characterizing the innate immune responses of three cowbird species (brown-headed, shiny, and bronzed) that have different degrees of exposure to parasites in different environments and investigating whether the immune responses of the species increase with increasing parasite-richness of their different niches. We are also characterizing the innate immune responses of three non-parasitic species in the same Family, Icteridae, as controls. Innate immune responses are those that occur in the earliest stage of infection to contain pathogens and serve as a critical determinant of disease resistance and susceptibility. We are measuring innate immunity using two assays that quantify leukocyte effectiveness, degranulation and oxidative burst.

Related Publication:

Reisen, William K. and Caldwell Hahn. Comparison of Immune Responses of Brown-Headed Cowbird and Related Blackbirds to West Nile and Other Mosquito-Borne Encephalitis Viruses. Journal of Wildlife Diseases, 43(3), 2007, pp. 439-449. (online abstract of article)

Collaborators: Caldwell Hahn, Patuxent Wildlife Research Center; William K. Reisen, Univ. California, Davis; Michael Kogut, ARS/USDA and Scott G. Summers, Fort Hood, DOD.

See also Fish and Wildlife Disease: Birds >>

back to top

Figure 1. Heart of adult Chinook salmon showing multiple, small, white, focal lesions typical of Ichthyophonus infections in salmonids. These cardiac lesions are believed to be the cause of reduced swimming stamina among infected fish. Photo credit: R. Kocan, University of Washington

Figure 2. Effect of temperature on the swimming performance (time-to-fatigue) of Ichthyophonus-infected and uninfected rainbow trout. The increase in stamina with increased temperature was significant (AOVA; P = 0.007) for controls, but not for infected fish (P = 0.26). Bars = 1 SD of the mean, * = significant difference in performance between infected and control fish (paired t-Test). Larger view

Ichthyophonus is a fungal-like protist that has caused disease in several species of marine fish in the Pacific and Atlantic Oceans including several waves of population-scale losses in Atlantic herring. More recently, reports from subsistence fishermen and tribal elders indicate the emergence of Ichthyophonus infections in adult Chinook salmon returning to the Yukon River in Alaska that are associated with adverse flesh quality and possible pre-spawning losses. A field study examined Chinook at multiple sites within the Yukon system to assess prevalence of Ichthyophonus infection and disease. Clinical signs of disease were minimal when fish entered the river, but increased significantly when fish reached the middle river. Among fish from the end of the run, the parasite was disseminated and clinical disease was apparent in multiple organs, especially the heart (Figure 1); however, female spawn-outs had lower levels of Ichthyophonus suggesting the most severely diseased fish had died before spawning. Elevated river temperatures in recent decades were postulated to be an important cause of the emergence and increased severity of the disease. To understand the role of temperature on the disease process infected and control groups of rainbow trout were held at 10, 15 and 20°C for 28 days to monitor mortality and disease progression. Infected fish demonstrated more rapid onset of disease, higher parasite load and a faster death rate at higher temperature. In a second experiment to determine the role of temperature on the swimming stamina of Ichthyophonus-infected fish, infected trout were reared at 15°C for 16 weeks before being subjected to forced swimming at 10, 15 and 20°C. Stamina was significantly impaired in infected fish as temperature increased (Figure 2). This study highlights the role of environmental stressors, such as climate change, on the ecology of fish diseases as well as the impact of these diseases on fitness traits important to the survival of natural populations.

For further information contact James R. Winton, Western Fisheries Research Center.

See also Fish and Wildlife Disease: Fish >>

back to top