Carter Atkinson bends over the microscope. Quickly, and with surgical precision, he cuts open a doped mosquito and pulls out its gut.
Atkinson, a biologist with the U.S. Geological Survey’s Biological Resources Division at Hawai`i Volcanoes National Park, takes just a few seconds to remove the mosquito’s carcass and focus his high-powered binocular microscope on the still pulsing midgut.
“There they are,” he says, a note of triumph in his voice. Atkinson, who studies the strain of malaria afflicting Hawai`i’s native forest birds, has detected the infectious agents in the body of the mosquito he’s just dissected. The malarial oocysts are clearly visible as smooth, transparent circles on the stomach wall.
On this day, Atkinson has about 120 mosquitoes to examine. And each time he finds one that is infected he will make a slide, mark it, and file it for future reference.
Atkinson’s small laboratory, in an unprepossessing building off an unmarked road in the national park, is the front line in the battle to save Hawai`i’s birds from the disease that has ravaged them. By the end of the 20th century, avian malaria is thought to have contributed to the extinction of at least 10 species of native birds and may threaten another 22 species.
Each of the mosquitoes Atkinson is examining has been exposed to one of seven amakihi. The birds have lived in an aviary at the park and are known to have been exposed to malaria, even though blood tests show no presence of the parasite. When Atkinson examines the mosquitoes that fed on four of the birds, he finds the telltale oocysts. Mosquitoes exposed to the three others have no sign of the parasite.
To learn that that mosquitoes can pick up malaria from such birds is an important step forward in understanding the enemy that Atkinson and others are fighting.
Life History of a Disease
Malaria in birds, like the human variant, requires mosquitoes as a vector. The disease is caused by a protozoan parasite. After oocysts develop in the gut of a mosquito for one to two weeks, they rupture, releasing infectious cells into the insect’s circulatory system. Ultimately, the cells, called sporozoites, invade the salivary glands of the mosquito. When the mosquito “bites” a bird, these sporozoites enter the bird’s bloodstream. In the spleen, they develop and reproduce as cryptozoites. From there, other organs are invaded and parasites eventually enter and begin to destroy red blood cells, leaving the host weak. In this weakened state, birds are more vulnerable to predators and may have difficulty finding food and reproducing. Anemia and anorexia resulting from malarial infections are often fatal.
If the bird survives the initial acute phase of the disease, it may recover completely, but, like the birds in Atkinson’s aviary, may still serve as a reservoir for the disease. Some species of native birds seem to have developed genetic resistance, while others – `i`iwi, for example, — are notoriously susceptible, with nine of 10 birds infected with just one mosquito bite dying as a result. Even if a bird survives, it remains chronically infected, Atkinson said, which could put it at a disadvantage: “We don’t know if chronically infected birds are affected in terms of their ability to reproduce or to find and complete for food resources, et cetera.”
The cycle of disease is completed when a mosquito draws blood from an infected bird. In the mosquito’s midgut, the malarial cells exit the blood and begin the process of reproduction.
Without mosquitoes – specifically, without Culex quinquefasciatus, or the southern house mosquito – there is no avian malaria. As Dennis LaPointe, an entomologist and colleague of Atkinson’s at Volcano, has written, this variety of mosquito was the first to arrive in the islands, borne in water casks aboard the ship Wellington, which pulled into port in Lahaina in 1826. By the end of the 19th century, two more species – the yellow fever mosquito Aedes aegypti Linneaus and the forest mosquito Aedes albopictus Skuse – had become established. In the 1960s, Aedes veans nocturnes was found on O`ahu, probably brought to the island by aircraft. And in 1981, the most recent immigrant, Wyeomyia mitchelli, was found in Manoa Valley. Wyeomyia is known from the Caribbean and is thought to have arrived in an illegal importation of bromeliads from Florida. Two non-biting species of mosquito that prey on other species have also become established on O`ahu and Kaua`i; they were introduced for biological control of Aedes albopictus.1
Culex quinquefasciatus is the most abundant mosquito in Hawai`i. Its larvae have been found in small ponds, roadside ditches, stock troughs, cisterns, cesspools, hapu`u cavities, treeholes, barrels, cemetery, vases, and refuse containers. When Atkinson wants a supply of larvae to raise for his experiments, for example, he can always count on finding them in abundance at the Pana`ewa Zoo south of Hilo – where researchers have found the highest abundance of mosquitoes.
The mosquito carries not only avian malaria, but other diseases, including the virus causing avian pox, dog heartworm, and human filariasis (a worm that invades the blood and lymph systems). It is a potential vector for several other viruses, including some forms of encephalitis and Rift Valley fever.
Building Insect Highways
Not until 1939 was avian malaria noted in Hawai`i. Five years later, disease began to be suspected as a cause of extinctions and declines in native bird populations. In the 1960s, scientists began to suspect a link between the absence of native forest birds and the presence of mosquitoes in what was called a coastal “mosquito belt” up to about the 2000-foot (600 meter) elevation.
Since then, this model has been substantially refined. The range of malaria-carrying mosquitoes is not fixed, but can vary with the season and the availability of habitat – particularly pools, pig wallows, and tree fern cavities where mosquitoes can breed.
In his research, LaPointe identified another factor that can help in the spread of avian malaria: disturbed habitat. “Mosquitoes disperse further in fragmented landscapes than in intact forests,” he says. This could account for differences in the abundance of Culex quinquefasciatus on Mauna Loa and Mauna Kea.
“The Waiakea Forest Reserve on Mauna Loa is fragmented, bisected along its slope by a paved roadway and disturbed throughout by a network of forestry roads and former logging operations. Mosquitoes associated with the residential and agricultural lowlands could rapidly disperse into the upper elevation forests of Mauna L9oa aided by prevailing winds and corridors in this forest landscape,” LaPointe wrote in his dissertation. “In contrast, the lower elevations of Hakalau Forest [National Wildlife Refuge] on Mauna Kea have not been penetrated by roads and remain relatively undisturbedÉ. On Mauna Kea, cattle ranching has opened up most of the upper elevation forestland and it is through this open landscape that mosquitoes probably dispersed into Hakalau Forest NWR.”
Perhaps the greatest limit to the spread of avian malaria is temperature. Average temperatures decrease with elevation. LaPointe writes that areas above 900 meters (about 3000 feet) can support the growth of mosquito larvae for just a few months out of the year at the same time that the lower temperatures extend the parasite’s development time. At even higher elevations, the window of opportunity for larval growth – and parasite development — is further narrowed. While mosquitoes may be present at high elevations, the low temperatures prevent development of the malaria parasite.
This “thermal constraint,” as LaPointe calls it, affords a certain degree of protection to native birds at high elevations. But that could be in jeopardy if climate change brings warmer average temperatures higher up the mountains.
“Depending on the accuracy of general circulation models, climatic change could present the greatest threat,” he says. “The predicted 2¡C warming in global mean air temperature would greatly alter the altitudinal range of avian malaria in Hawai`i just as it would the geographical range of human malaria transmission. Relictual bird populations currently located on the upper edge of their forest habitat would be particularly vulnerable to the altitudinal advance of disease.”
Large-scale Studies
Although avian malaria has been studied in Hawai`i for half a century, researchers still have much to learn. But they’re hopeful that a large grant recently awarded by the National Science Foundation may help fill some of those gaps.
The $4.1 million, five-year grant will bring together scientists in three separate fields: emerging diseases, conservation biology, and invasive species. Institutions participating in the project, led by the Pacific Cooperative Studies Unit at the University of Hawai`i at Manoa, include the U.S. Geological Survey, the Smithsonian Institute, and Princeton University.
Project leader David Duffy is hoping that by applying chaos, or complexity, theory, the mechanisms involved in the spread of avian malaria may be better understood. In the past, studies of the disease have focused on small parts of the problem. “Traditional scientific approaches have not been able to explain or predict” the development of avian malaria, Duffy said. “The theory and application of biological complexity may help.”
One of the factors that Duffy and his teams will be considering is that of climate change. “If the current warming trend continues, there may not be any malaria-free areas except at the highest, tree-less elevations hostile to most Hawaiian native land birds, so that more such species will soon become extinct,” Duffy says.
Atkinson, who is one of 10 principal investigators on the grant, is looking forward to the expanded work. “Instead of doing discrete projects, we’ll be doing a lot of projects all at once over a big area and try to integrate them. For example, we’ll look at land use change and mosquito abundance – how changes in land use may translate into the movement of mosquitoes in the forest, and potential disease outbreaks.”
“We’ll incorporate sociological aspects as well,” he said. “We’ll be looking too at the genetics of the host, the vector, and the parasite, how they’re co-evolving.” For example, even as birds develop some genetic resistance to the parasite, the parasite itself may evolve so as to be less virulent. In the interaction of parasites and hosts, after all, the parasite succeeds only if it does not kill the host.
The research will focus on avian malaria, but could have implications for human malaria and other diseases as well.
Ultimately, says Atkinson, “we’ll be looking at how the sum of all the parts might lead to something not easily apparent. It could be just one key thing in the system that you can change, and so shift the disease cycle in one direction or another. Or it could be a combination.”
- The source of this and other information used in this article is the dissertation of Dennis A. LaPointe, “Avian Malaria in Hawai`i: The Distribution, Ecology and Vector Potential of Forest-Dwelling Mosquitoes.” LaPointe, a colleague of Atkinson’s received a Ph.D. in entomology from the University of Hawai`i in August of this year.
— Patricia Tummons
Volume 11, Number 7 January 2001
Leave a Reply