Fingerprinting Parasitic Worms
According to recent data reported in a Global Biodiversity Assessment by the
United Nations, the world is home to about 13 million species of organisms.
Roughly 400,000 of these are nematodes, about 10 percent of which are
parasites known to attack humans, other animals, and plants. Many others are
still unknown or insufficiently described with regard to features and behavior
to be recognizable.
"Nematodes are so numerous that if every part of the world were to
disappear, except for them, you could still see the outline of the Earth,"
says J. Ralph Lichtenfels. A world expert on parasites, he is a zoologist with
USDA's Agricultural Research Service and curator of the U.S. National Parasite
Collectionthe largest in the world.
He and his small staff in the Biosystematics and National Parasite
Collection Unit at Beltsville, Maryland research and classify all manner of
parasitesfrom tapeworms to round worms.
"Correctly identifying a strange or mysterious parasite is critical to
tracing the originof a specific disease and taking steps to combat
it," he says.
Recently, Lichtenfels discovered that medium and large stomach
roundwormstwo of the most important pathogens causing diseases in cattle
and sheep in the United States and the worldhave distinctive patterns of
ridges on their outer cuticular surface. About 40 of these ridges run along the
length of the worms. He has proven that these ridges can he used as
"fingerprints" to identify them.
The pattern of ridges, called the synlophe can be used by parasitologists
and veterinarians to identify the highly pathogenic species of medium and large
stomach worms afflicting ruminants such as cattle, goats, and sheep. Magnified
400 times under a microscope, the synlophes of both living and preserved
specimens become visible.
Identifying harmful species is important, says Lichtenfels, because "in
the southeastern United States, these parasites are called the bankrupt worm.
The severe losses of young animals, cost of treatment, and loss of use of
contaminated pasture cost about $400 million each year."
Ranchers have to leave pastures empty when parasite infestations are
heavyas in spring, when the weather warms up. Since roundworm parasites
live in the stomachs of ruminant animals, their eggs get deposited on pastures
during grazing. Animals pick up the worms by grazing grass infected with the
larvae that hatch.
Worldwide, three species of large stomach worms, Haemonchus, are
parasites of sheep, cattle, and goats.
These half-inch-long worms are nicknamed the "barber pole worm"
because of the red and while stripes caused by intertwining of the blood in the
intestines with white, egg-filled ovaries along their spaghetti-like bodies.
H. contortus, is mainly a pest of domestic sheep but can infect many
other domestic and wild ruminants. H. placei infects mainly domestic
cattle but has been found in domestic sheep, white-tail deer, and pronghorn
antelope. And H. similis infects domestic cattle in a few southern
stales and white-tail deer in Florida.
Using the pattern and numbers of ridges in the synlophe, Lichtenfels has
developed an illustrated key that makes it possible, for the first time, to
identify individual male and female worms of these three species of
Haemonchus. Even more importantly, Lichtenfels' new identification
method ends a century-long controversy over whether the large stomach worms of
cattle and of sheep are different species; they are.
The new method also distinguishes among species of the medium stomach worm,
or Ostertagiinae, that are also parasites of domestic and wild
ruminants. "In many parts of the world, they are the most pathogenic
nematode parasites in cattle, sheep, and goals," says Lichtenfels.
So he developed a key for identifying all 15 species of the subfamily
Ostertagiinae in North American ruminants. Nine attack domestic ruminants,
and six infect wild native North American ruminants. One species, O.
leptospicularis, is a parasite of deer and a severe pathogen of cattle in
some parts of the world. Recently, it has been discovered in Oregon and
Lichtenfels explains why he has included in his descriptive keys the six
worm species that infect native wildlife but are not known to be hazardous to
"Native wild ruminants like caribou, moose, and other deer and some
wild sheep and goals often share pasture with domestic cattle, sheep, and goals
in many regions of North America," he says. "Because the medium
stomach worms can attack a broad range of hosts, we must regard the species
known to be present in wild ruminants as potential parasites of domestic cattle
Recognizing the nematodes in cattle and sheep as separate species will
improve control measures for them, says Lichtenfels. It will also speed
development of other diagnostic tools that can be used to identify eggs passed
by infected hosts. And having accurate, easy-to-use diagnostic tools will
prevent new parasite pests from being imported into the United States along
with exotic hosts.
Veterinarians will welcome the new information. By more accurately
identifying the nematode causing an infection, they will be better able to
target treatments and prescribe drugs, And new strategies can be devised so
that drugs can be used more judiciously in treating heavily infected animals.
Their overuse can cause these roundworm parasites to develop immunity.
By Hank Becker, ARS.
Parasitology Laboratory, Beltsville, MD
"Fingerprinting Parasitic Worms" was published in the
issue of Agricultural Research magazine.