Chemist Patricia Allen examines a fungal isolate before applying it to a live
tick, to see if it can penetrate the tick's outer cuticle.
Tackling Ticks That Spread Lyme
If just thinking about ticks makes you squeamish, then you'll want to stay
away from Dolores Hill's lab in Beltsville, Maryland. There, in tiny glass
vials and Styrofoam containers, Hill keeps several hundred blacklegged deer
ticks, scientifically known as Ixodes scapularis.
A parasitologist for USDA's Agricultural
Research Service, Hill collects the blood-sucking ticks as part of a 5-year
project to biologically control the pests with microscopic roundworms, called
Hill's goal is to formulate the worms into a product that could be sprayed
in residential areas, like backyards, where homeowners can be bitten by ticks
infected with Borrelia burgdorferi, the bacterium that causes Lyme
disease. In 1996, the Centers for Disease Control in Atlanta, Georgia, received
more than 16,000 reports of the tickborne disease. Ninety percent came from the
northeastern United States, where I. scapularis is most prevalent.
Humans aren't the tick's first choice of hosts. But suburban sprawl into
wooded habitat has placed people in closer contact with white-tailed deer, the
adult ticks' natural host and chief transportation source.
"If deer are feeding on azalea bushes at the back of your yard, that's
where a female tick may drop off and lay her eggs. So that's also where you'd
want to spray the nematodes," explains Hill, who is with the ARS Parasite
Biology and Epidemiology Laboratory.
Her top tick-fighting recruits include nine strains of the nematode species
Steinernema and four strains of Heterorhabditis. Both are well
known to science, she says. While harmless to higher vertebrates (humans and
livestock, for instance), the nematodes are renowned for attacking many insects
"But few people have really looked closely at whether these nematodes
will infect and kill ticks," says Hill. Ticks aren't insects, but
arachnids--like mites, scorpions, and spiders.
Her studies show the worms may be most effective against adult female ticks,
which lay as many 2,000 eggs. "If you find an engorged adult deer tick on
you," says Hill, "it's most likely a female taking a blood meal so
she can reproduce."
But tiny tick offspring are as much of a menace as grown adults.
This is especially true of the nymphs, a juvenile stage about the size of a
pepper flake. They are more likely than adult ticks to transmit Lyme disease to
humans, says Hill. Their tiny size allows nymphs to feed longer without being
detected, giving them enough time to pass the Lyme bacterium into their host's
One tick-fighting strategy is to spray an infested area with an appropriate
insecticide or acaricide. "But many people don't want to have those
chemicals sprayed in their backyards," notes Hill.
Adult deer tick, Ixodes scapularis.
Many of the tick-killing chemicals are toxic, and their use may have adverse
environmental effects. They may also require repeat applications as new ticks
are brought in by deer and other animals searching for food.
The nematodes could offer the advantage of sticking around longer, taking up
residence in soil, leaf litter, or other places ticks seek shelter. "This
may offer a new way of controlling these tick populations without affecting the
environment," says Hill.
Her approach targets adult female ticks, to curb the size of the next
generation of offspring. "We've studied nymphs, larvae, flat (unfed) adult
ticks, and engorged (fed) females," says Hill. "So far, the only
stage that is susceptible to the nematodes is the engorged adult female."
Getting the Wherewithal for Testing
For her research, Hill must first rear large numbers of the nematodes inside
the bodies of greater wax moth larvae. She keeps these host insects in a sealed
plastic container with oatmeal and other high-fiber cereals for food.
To extract the worms, researchers place the infected larvae in two petri
dishes--one inside the other. After feeding inside their hosts, the worms crawl
out of the larvae into the first petri dish and then into the second, which is
filled with water. Hill then drains the water into another container and stores
it at room temperature.
To test their ability to infect ticks, Hill adds either 500, 1,000, or 1,500
of the test nematodes to a petri dish containing five ticks. She uses the
procedure with each tick growth stage.
Her tests show that adult females that have fed on blood are most vulnerable
to nematode attack because feeding expands natural body openings. And the
nematodes aren't picky about where they enter: It can be the tick's breathing
holes (spiracles), mouth, anus, or genital pore.
In an isolated test plot seeded with engorged adult female ticks,
parasitologist Dolores Hill sprays infective juvenile Steinernema
nematodes as a biological control.
The Steinernema nematodes, more so than the Heterorhabditis,
rely on these openings. The latter species sports a short, sharp tooth it
can use to gnaw through a tick's hard outer cuticle. This may give the toothy
worm a slight advantage over Steinernema--especially in trying to
penetrate the flat bodies of unfed ticks.
Hill has viewed Steinernema's unsuccessful attempts under a
microscope. The unfed ticks, she says, "bat away the nematodes that are
trying to get into them."
But engorged female ticks aren't so lucky. Once infected, death comes
quickly. "Less than 50 nematodes will cause the death of fed females in 24
hours," notes Hill.
Interestingly, it's not the worms that kill the ticks. Rather, it's a
species of symbiotic bacteria of the genus Xenorhabdus that the
nematodes carry in their gut. Once inside a tick, the worms release the
bacteria, which in turn liquefy the tick's fat bodies and other tissues.
The nematodes then feed on this nutritious soup and mate, giving rise to
tens of thousands of offspring, called infective juveniles. Once the nutrition
source is exhausted, these juveniles leave the tick's body in search of a new
host to infect.
Though successful in the lab, the true test of the nematodes' tick-killing
prowess will come with outdoor studies. Hill is now planning several small
experiments to that end at ARS' Beltsville (Maryland) Agricultural Research
Center. Such tests will help in compiling data on where, when, and how best to
apply the nematodes--and at what concentrations.
She also hopes to identify cold-hardy strains that can withstand fluctuating
temperatures that characterize much of the East's fall, winter, and spring
months. "Female ticks are abundant in the late fall and early
spring," she says, "so you want to have nematodes capable of
withstanding temperature extremes at those times of the year."
Plotting a Fungal Demise
Entomologist John Carroll sweeps grass and brush areas for adult ticks to use
in laboratory studies.
Hill's colleague, ARS chemist Patricia C. Allen, is pursuing another form of
biological tick control. Her approach calls on naturally occurring fungi that
infect adult ticks--as well as nymphs, larvae, and eggs. Allen has already
isolated several fungi from ticks that have died of natural causes in the
Beltsville area and nearby Patuxent Wildlife Center.
Allen tests each new fungal isolate for production of a protease enzyme.
This protein-degrading enzyme is essential to a fungus' penetration of a tick's
outer protective cuticle.
To test the infectivity of a promising candidate fungus, the researcher dips
healthy, live ticks into a preparation containing 100,000 to 1,000,000,000
fungal spores. She may also place the ticks in small plastic tubes with
spore-treated paper wedges at the bottom. She maintains the tubes at room
temperature with 100 percent humidity and observes the ticks daily for signs of
Allen and ARS colleague Gary Samuels have tentatively identified one of the
most promising fungi as Gliocladium roseum. In one experiment, the
microbe killed 70 percent of unfed ticks, compared to 100 percent killed by a
well-known fungus--Metarhizium anisopliae--that Allen uses in her
studies as a kind of benchmark to help gauge the infectivity of new isolates.
Her research lays the groundwork for building an arsenal of tick-fighting
microbes that could perhaps be deployed under varying environmental conditions.
"The ultimate aim of this research, however," says Allen, "is
to develop a practical method of applying pathogenic fungi to the Lyme disease
One possibility is to formulate fungal spores into a preparation that could
be sprayed onto leaf litter, shrubs, or other vegetation where ticks wait for a
But first, says Allen, any new fungus with biocontrol potential would have
to undergo toxicity tests to ensure human, animal, and environmental safety.
Fungi grown on petri dishes were taken from ticks assumed to have died in
nature from the fungi. The isolate on the right is Metarhizium
anisopliae, the bottom one is Gliocladium roseum, and the
other two are unknown pathogens.
On another front, deer could bring ticks to their doom in a new, ARS-led
experiment to fight Lyme disease. ARS and collaborating university scientists
began the 5-year, USDA Northeast Regional Tick Control Project last fall at
test sites in Connecticut, Rhode Island, New York, and New Jersey. Maryland's
participation in the project begins this spring, says entomologist John F.
Carroll of ARS' Parasite Biology and Epidemiology Laboratory in Beltsville.
Carroll will coordinate the installation and use of 25 experimental
deer-feeding stations on a 2-square-mile test site along Maryland's
Baltimore-Washington corridor. The site will include residential areas, says
Carroll, "because they typify situations where people are at risk of Lyme
The stations, called four-posters, treat deer with amitraz, a chemical that
kills ticks but doesn't harm the animals. Each station has a bin of corn and
four upright rollers coated with amitraz. To get to the corn, deer must brush
their head, neck, and ears against one of the rollers at the station's four
ARS entomologist Mat Pound, engineer Allen Miller, and biological lab
technician Craig LeMeilleur developed, tested, and patented the four-poster
technology. They are at ARS' Knipling-Bushland U.S. Livestock Insects Research
Laboratory in Kerrville, Texas.
In studies there, use of the four-posters controlled up to 97 percent of
lone star ticks, which plague cattle and humans. [See "Ticked Off!"
Agricultural Research, April 1994, p. 4.]
Carroll and other scientists participating in the Northeast project hope for
similar success against I. scapularis. The project's goal: to reduce up
to 90 percent of the tick population, especially nymphs, at each state's test
site by August 2000.
Despite the use of insecticides, repellents, protective clothing, and other
measures, blacklegged deer ticks continue to pose a serious public health
concern--particularly in midwestern and northeastern states.
Hiking, hunting, and other outdoor recreational activities during spring and
summer months also place humans at greater risk of contracting Lyme disease
from infected nymphs. "The nymphs are out there at a time when people are
engaged in activities that heighten their risk of exposure to ticks," says
If the Northeast project is successful, the scientists will transfer the
four-poster technology to public health officials, pest control experts, and
others in Lyme-plagued communities.--By
Agricultural Research Service Information Staff, 6303 Ivy Lane, Greenbelt,
Maryland 20770, phone (301) 344-2173. ARS.
Dolores E. Hill,
Patricia C. Allen, and
John F. Carroll are at the
Biology and Epidemiology Laboratory, Bldg. 1040, 10300 Baltimore Ave.,
Beltsville, MD 20705-2350; phone (301) 504-8300, fax (301) 504-5306.
"Tackling Ticks That Spread Lyme Disease" was published in
the March 1998 issue of Agricultural Research magazine. Click here
to see this issue's table of contents.