Curing Guam's Snake Epidemic
by Tim Stoddard
Five steps inside the door of the National Zoos Bird House, behind a thin veil of netting, two unlikely roommates bed down next to each other. Horatio, a diminutive, dark-plumaged Guam rail (Rallus owstoni), excavates a small pit next to a large plastic box in which Sid, a nine-foot-long brown tree snake (Boiga irregularis) basks angelically. Its an ironic image of prey coexisting with an alien predator, like the lion lying down with the lamb. Sidnamed after Sid Vicious, the late guitarist of the British band The Sex Pistolsflicks his tongue like a true punk-rocker, inches away from the rail. Naïve Horatio pecks at the plastic inquisitively, oblivious to any potential danger.
Since they were accidentally introduced to Guam in the late 1940s, brown tree snakes (BTS) have decimated the islands endemic birds and reptiles, causing the local extinction of five lizard and nine bird species, including the Guam rail. The snakes arent simply a nuisance, theyre an ecological and economic nightmare. Every time the sun sets over the western Pacific, more than a million brown tree snakes emerge from their daytime hideouts on Guam, prowling secluded forests as well as residential neighborhoods.
Horatio and Sids peculiar housing arrangement serves to educate visitors about the impact of the brown tree snake on Guams birds. What zoogoers dont see, however, are the dozens of brown tree snakes living in a room in the Zoos pathology department. The typical BTS on Guam is shorter and skinnier than the well-fed Sid, with coloration that varies from yellowish green to darker browns and grays. Donald Nichols, a veterinary pathologist for the National Zoo, has collected these snakes from Guam on seven occasionsmost recently in November, 1998. Nichols transports the snakes back to Washington to learn how they react to certain snake-specific viruses. It now appears that Nichols and a collaborating biologist, Elaine Lamirande, may have found an elegant solution to Guams BTS epidemic: Make the entire brown tree snake population on Guam sick.
When Guam was occupied by the Japanese during World War II, a U.S. bombing campaign razed most of the islands infrastructure. In the post-war reconstruction, Guam imported large shipments of lumber from New Guinea. It is believed that brown tree snakes, which are native to New Guinea, Northern Australia, and the Solomon Islands, were stowaways on these shipments.
The brown tree snakes silent invasion of Guam didnt draw much attention until the mid-1970s, when the U.S. Fish & Wildlife Service (USFWS) realized that several endemic bird species were declining at an alarming rate. Biologists initially guessed that a mysterious disease was ravaging Guams birds. But in the early 1980s, Julie Savidge, then a staff biologist at the Guam Division of Aquatic and Wildlife Resources (DAWR), discovered that the snakes were the culprits. Further surveys revealed the severity of the BTS problem: Guams forests were seething with as many as 14,000 individuals per square mile in some areas.
Like countless other introduced pests, the BTS is far less abundant in its native range, where it has to compete with other predators for limited resources. Guam, however, offered all the right conditions for a BTS population explosion. Small introduced geckos and skinks are ubiquitous on Guam, supplying young brown tree snakes with a steady food supply that sustains them until they can move on to larger prey. The islands endemic birds evolved in a predator-free safe haven that didnt select for the evolution of any of the defensive behaviors that snake-wary birds exhibit. While diurnal Guamanian birds sleep, nocturnal brown tree snakes raid their nests, taking eggs, chicks, and adults like candy from a baby. At the Zoo, when Horatio sleeps inches away from Sid, he isnt brazenly flirting with deathhis neural circuitry simply doesnt recognize the large snake as a threat.
The BTS poses far less a threat to Guams human population. Unlike rattlesnakes, the BTS does not have large, hollow front fangs that simultaneously pierce the skin and deliver a venomous cocktail. Instead, the snakes latch onto their prey and chew until their rear-fangs have opened a small wound. Weak venom drips down from modified salivary glands at the back of the mouth and flows into the open wound. The venom subdues the prey while the snake slowly squeezes the victim to death or eats it alive.
Nichols, who catches his study snakes by hand, has been bitten on several occasions, and claims that the bite is no worse than a bee sting. With readily accessible medical care on Guam, none of the 200 plus cases of BTS bites there have caused more than discomfort in adults. Nevertheless, in several instances infant victims have allegedly gone into respiratory arrest before medical treatment relieved their symptoms. Guamanians tell horror stories of waking in the middle of the night to find a BTS wrapped around a screaming baby. The snakes are only following their instincts, of course, probably mistaking the crib for a nest and the infants small hands and feet for squirming chicks.
Public health and ecological issues aside, brown tree snakes are a considerable economic burden on Guam. Following their tree-climbing instincts, the snakes spiral up power poles and across electric lines, causing short circuits that trip off power outages on average once every four days. Living with these outages is not cheap, what with the cost of repairing damaged power facilities, replacing downed computer systems, and restocking perishable foods.
Along with their penchant for powerlines, BTS naturally seek out small hiding spaces, which often include cargo containers at airports and harbors. As a result of Guams high-volume of outbound ship and air traffic, BTS are frequently transported to far-flung locations, including the Federated States of Micronesia, Okinawa, Diego Garcia (a remote island in the Indian Ocean), and, in one case, Corpus Christi, Texas. The snakes have already colonized the island of Saipan in the Northern Marianas, and, with daily flights from Guam to Honolulu, theres an imminent threat of them arriving in Hawaii, a state already reeling from the demise of native species. From 1981 to 1994, six BTS were found dead or dying near runways at Honolulu International, sometimes hiding in the wheel wells of jumbo jets or in sealed cargo containers. An additional seven were captured alive in a military warehouse several miles from the airport.
Female brown tree snakes may be able to store sperm for several months to delay fertilization until environmental conditions are ideal, so, in theory, a single female snake could arrive in Hawaii and establish a new colony. Even with U.S. Department of Agriculture (USDA) officials inspecting all aircraft from Guam with sharp-nosed, reptile-sniffing beagles, its likely that a few brown tree snakes will settle in Hawaii.
In an effort to quarantine the BTS, the USDA has set out an extensive trapping system around the perimeters of military and commercial airports on Guam. The traps haul in about 6,000 snakes each year, keeping most of the snakes off of aircraft. But the traps have virtually no effect on the overall BTS population, which has reached two million individuals by some estimates. Thirty years ago, a trapping program might have taken care of the burgeoning BTS population. But today, like a raging infection, there are simply too many snakes for this kind of cure.
Don Nichols learned of the brown tree snake problem in the early 1980s, while completing a post-veterinary school pathology residency at the Zoo. At that time, the wild populations of Guams endemic rails and Micronesian kingfishers (Halcyon cinnamomina) were so close to extinction that the National Zoos Scott Derrickson coordinated a rescue effort with DAWR to place the remaining birds in breeding programs at various U.S. zoos. This is how Horatio ended up in Washington. A few years later, while Nichols was freelancing as a pathology consultant, receiving slides of tissue from animals that had died at other zoos, he noticed that snakes in some collections were suffering from paramyxovirus outbreaks. This family of viruses comprises a wide variety of pathogens, including the distantly related viruses responsible for mumps, measels, and canine distemper. When infected with a paramyxovirus, snakes exhibit flu-like symptoms, usually involving mucus discharges from the nose and mouth, laryngitis, and pneumonia, that often progress to death. Nichols is quick to emphasize, however, that these snake viruses are only distant relatives of human pathogens. Temperatures above 95 F inactivate these viruses, making it impossible for them to survive at the body temperature of mammals or birds.
The epiphany came to Nichols one day while separate thoughts about brown tree snakes and sick zoo snakes collided in his mind. "Wouldnt it be nice," he thought to himself, "if there were a paramyxovirus outbreak on Guam?" Nichols, who normally searches for ways to prevent animals from getting sick, suddenly saw a useful application for the problematic pathogens.
With a laboratory full of brown tree snakes, but with no idea which strain of paramyxovirus would actually affect the BTS, Nichols and Lamirande spent several years inoculating their snakes with different viruses and observing their responses. Nichols also has been collaborating with Jim Winton, a fish virologist in Seattle, who was able to build a family tree of the viruses based on a genetic analysis. Using this information, the Zoo researchers have now isolated two promising virus strains: one named KSV (for king snake virus, after the type of snake in which the strain originated); the other EEGV (for English Elaphe guttata virus, after the corn snake acquired from England in which they isolated the strain).
In laboratory experiments, KSV consistently kills about half of the brown tree snakes it infects, and EEGV two-thirds. At first glance, these figures might not seem very impressive. Wouldnt an effective biological control need to kill all of the snakes it comes in contact with? According to statisticians, no. The trick is finding a virus that takes a moderate amount of time to kill its host. Too virulent a virus will wipe out all of the infected snakes before they have a chance to spread the disease across the island. Computer models predict that a 40 to 60 percent mortality rate would have the most effective results.
A human can catch the flu without ever actually making physical contact with someone who is sick. While snakes dont have diaphramsmaking it impossible for them to sneeze on each othera healthy snake likewise can contract a paramyxovirus via secondary contact, such as slithering through a sick snakes secretions. Nichols envisions the following scenario: A contingent of infected snakes is released around Guam and slowly transmits the virus until the brown tree snake population has disappeared for good.
Even with these solutions on the horizon, conservationists cant wait for Guam to become snake-free before initiating recovery programs for the islands endangered wildlife. The recovery of Guam rails such as Horatio offers a glimmer of light on an otherwise dark drama. When the population of Guam rails plummeted from 80,000 individuals in the late 1960s to 50 birds in 1985, DAWR, with the help of Scott Derrickson, began capturing rails for breeding. Biologists were anxious to get the zoo-bred rails back into the wild as soon as possible. To side-step the snakes on Guam, they began an introduction program on the tiny island of Rota, Guams snake-free neighbor. Located just 36 miles north of Guam, Rota offers a dramatic contrast, with large tracts of native forest, minimal human development, and abundant birdlife. There are no feral goats or pigs on the island, and, best of all, there are no endemic rails to compete with introduced Guam rails for food or nesting sites.
Between December 1989 and February 1991, a team of scientists from DAWR, the Zoo, and the University of Tennessee released 55 zoo-bred rails on Rotas Sabana, a vegetation-covered coral mesa about 1,200 feet above sea level. Sadly, feral cats, cars, and farm vehicles wiped out this first wave of rails, and the project stalled until 1995, when introductions resumed and biologists documented the first successful reproduction of the birds. Since that time, 215 Guam rails have been introduced to Rota in annual releases, and, with a long-term program to remove predatory feral cats, survival rates continue to improve.
In another ambitious project, conservationists are bringing rails back to the snake hot-zone on Guam. In 1997, DAWR acquired an abandoned portion of Andersen Air Force Base with old runways bordering almost 60 acres of open grassland and limestone forest. The air force erected a fence and a snake barrier around the area and began a rigorous trapping program that removed the brown tree snakes from the area. In November 1998, DAWR biologists released 16 rails in the enclosure, and have since observed nine nesting birds exhibiting territorial behavior.
Spurred on by this hard-won success, DAWR is now exploring similar projects in other localized areas on Guam. Even if these birds are restricted to a tiny safe-zone within a sea of snakes, they might someday be part of the first wave of rails to repatriate Guam in the wake of the brown tree snakes eradication.
Tim Stoddard, a former intern for ZooGoer, wrote about kiwis in our November/December 1999 issue.
ZooGoer 29(1) 2000. Copyright 2000 Friends of the National Zoo. All rights reserved.
