New Perspectives on an Ancient Disease

DNA studies provide insights on the evolution of avian malaria and reveal that the disease is widespread among the birds in our own backyard

By Irby Lovette

Malaria is a disease that most people in North America associate with tropical climates and distant lands. That may be an accurate view of human malaria, but the story is entirely different for birds. Many—perhaps even most—birds in North America have contracted avian malaria at some point in their lives, and most of these infected individuals carry the disease for life. They may not show any symptoms, but many of the birds in your backyard or at your bird feeder probably have chronic malaria infections. For birds, North America is a malaria hot zone.

Researchers at the Cornell Lab of Ornithology and elsewhere are just beginning to document the complex interactions between avian malaria parasites and the birds they infect—including how parasites and hosts evolve in response to one another. By employing new and powerful genetic methods to find and identify the parasites that cause malaria, we have found that a surprising number of parasite species occur in an unexpectedly wide diversity of bird species. The severity of malarial disease differs dramatically among locations, from Hawaii, where some bird species have gone extinct following epidemics, to North America, where infections are common but usually less deadly.

Searching for a purple needle in a red haystack

Malaria is a general term for infection with parasite organisms in the genera Plasmodium and Haemoproteus. The parasites that cause malarial disease in humans and other mammals are classified as Plasmodium, but birds are infected by both groups. Each group of parasites infects a particular class of host, meaning that humans can’t catch malaria from birds, or vice versa.

All malaria parasites share a similar life cycle that involves a primary host, such as a bird or mammal, and an organism that transmits the parasite, such as a midge or mosquito. When an infected insect bites a bird, its saliva transmits the parasite to the bird. Once in the bird’s bloodstream, the parasites travel to the bird’s internal organs, where they start to multiply. Some then move into the red blood cells, where they continue to reproduce. When a few of these infected red blood cells are taken up by another midge or mosquito, the chain of transmission continues.

Until recently, the preferred method of screening for malaria required an extraordinary level of patience, expertise, and time—and it missed many infections. After capturing a bird, a parasitologist would draw a drop of blood and smear it in a very thin layer across a glass microscope slide. As the blood dried, the red blood cells would adhere to the glass, fixing them in place. The next step was to wash the slide with a bright purple stain that adhered only to the malaria parasites, but not to the bird blood cells. Under a microscope, the bright purple parasites would stand out from the background of dusky red blood cells, making them easy to identify and count.

Or at least that is how it works in theory. In reality, the purple blobs that represent parasites are easy to miss because they are very small and are only found inside the red blood cells. Making the screening process even more laborious, the parasites often show up in only one blood cell out of many thousands, meaning that a diagnostician had to spend many minutes at the microscope looking for a tiny purple needle in a large red haystack. If the bird’s immune system was doing a good job of holding the parasites in check, or if the parasites were present in the organs but not reproducing in the blood, it was very easy to classify an infected bird as parasite-free.

Revelations from DNA

In our research, we use new genetic methods of searching for malaria parasites. Instead of examining the blood under a microscope, we bring it back to the lab and purify the DNA it contains. When uninfected birds are sampled, that purified DNA will be entirely from the bird, but when infected birds are screened, the DNA will be a mix of bird DNA and parasite DNA, because the parasites are present in the blood sample along with cells from the bird.

We then use a set of laboratory tricks that allow us to target, copy, and record the presence of the parasite DNA, leaving the bird DNA behind. Hundreds of samples can be screened quickly using this method. An additional advantage is that it takes a tiny amount of parasite DNA to set the process going, meaning that we can often detect low-grade infections that would be missed using the blood smear technique.

These techniques have already revealed that infection by malaria parasites is very common in many species of North American birds. For example, traditional blood smear techniques have found an average infection rate of about 25 percent in European birds, whereas the new molecular techniques suggest that a much higher proportion of individuals carries these parasites.

Another major advantage of this genetic technique is that we can use it to study the diversity of the parasites themselves, because each species of parasite has a slightly different sequence in its DNA. By looking for these differences at the genetic level, we can distinguish various parasite species that look similar to the eye.

This approach is revolutionizing our idea of how many parasite species are out there in the birds, and whether parasites are capable of infecting a wide range of bird species. The traditional view was that most malaria parasites are highly specialized, targeting a particular group of animals; for example, parasites that researchers found in one avian family would generally be unable to infect a different avian family.

By using the genetic screening technique, Megan Szymanski, a Cornell undergraduate, showed that diverse parasites infect many kinds of birds on a scale that previous surveys had missed. Working in the Lab of Ornithology’s molecular genetics laboratory, Megan screened DNA from warblers, sparrows, finches, and swallows that nest around Ithaca, New York, in the summer. She identified a surprising diversity of parasites in birds from this single geographic region—8 parasite species from 14 bird species. Even more surprisingly, most of these parasite species were not restricted to a small group of birds, but infected many of the bird species that Megan surveyed from four taxonomic families (Journal of Parasitology, in press, 2005).

On a broader geographic scale, Lab of Ornithology graduate student Mari Kimura is exploring the diversity of parasites infecting House Finches at sites all across North America. Working in conjunction with the Lab’s multifaceted studies of House Finch diseases, Mari has documented clear differences in the parasite faunas at different sites. The causes of these differences—the presence of different insects that transmit the parasites, for example—are a topic of ongoing study.

Evolutionary battles between parasites and birds

No discussion of avian malaria would be complete without mentioning the most famous—and most tragic—interaction between birds and malaria parasites. Until the mid-1800s, the Hawaiian archipelago was free of mosquitoes capable of transmitting avian malaria. Birds carrying malaria almost certainly visited the islands on a regular basis, but the parasites were unable to spread without the insects to transfer them from bird to bird. That changed when a whaling ship dumped its musty water casks into a Hawaiian stream, thereby introducing larvae of a mosquito that could spread the parasites to the native bird fauna.

The ensuing epidemic caused massive mortality among the native birds, which had no resistance because they had evolved for millions of years in the absence of malaria. A number of Hawaiian bird species subsequently went extinct, many probably as a direct result of the avian malaria epidemic. Today, very few native Hawaiian birds are found at the lower elevations where the mosquito thrives. Instead, the birds you see at low elevations are exotic species introduced from locations where malaria has long been present.

This Hawaiian malaria story is well known to many birders and most ornithologists, but an important recent discovery adds a new twist while shedding light on why most North American birds infected with malaria show few if any symptoms of disease (Proceedings of the National Academy of Sciences USA, February 1, 2005). In surveying forests in the lower slopes of Mauna Loa volcano on the island of Hawaii, a group of researchers found a population of a native bird called the Amakihi that appeared to have escaped the worst effects of avian malaria. Studies of blood samples and trapped mosquitoes showed that the malaria parasite and its vector were both present in abundance and that many of the birds were infected. Unlike Amakihi elsewhere, however, these birds appeared not to suffer high mortality from malaria infection.

The most likely explanation for this newfound ability to survive in the malaria-plagued lowlands is that this population of Amakihi has recently evolved partial resistance to the parasites. Amakihi are one of the most abundant of the remaining native Hawaiian birds, and this species may have harbored more genetic diversity than rarer species, enhancing the likelihood that the raw material for genetic resistance would arise in the population. Unfortunately, rarer Hawaiian species are less likely to be able to evolve resistance.

In North America, however, most bird species have evolved under constant pressure from malaria parasites. Selection pressure on the birds in favor of resistance, and on the parasites for the ability to escape the birds’ immune system, has been intense over many millennia. This has favored the type of interaction we now see when we look closely at the birds in our backyards: most birds are infected by malaria, but most of these infections cause little harm to the birds.

Irby Lovette is director of the Lab’s Evolutionary Biology Program.

 

For permission to reprint all or part of this article, please contact Miyoko Chu, editor, Cornell Lab of Ornithology, 159 Sapsucker Woods Rd., Ithaca, NY, 14850. Phone: (607) 254-2451. email: mcc37@cornell.edu