Barberpole Worms—Parasites of Another
The barberpole worm is among the world's most loathsome and widespread
stomach parasites of sheep, goats, and cattle. They feed on host animals' blood
with a voracious appetite.
For farmers, the headache begins when approved drugs fail to protect their
animals from this parasitic nematode that is better known by its scientific
name, Haemonchus contortus. There seems to be evidence that the pest is
showing signs of resisting the effects of such pharmaceutical controls. But new
research under way in Beltsville, Maryland, could undermine the parasite's
costly mischief by means of biochemical sabotage.
Agricultural Research Service
zoologist Raymond H. Fetterer is leading the effort, along with chemist Marcia
L. Rhoads, parasitologist Dolores E. Hill, and technician Ruth Barfield. Their
chief focus is to identify and exploit natural substances that the barberpole
worm secretes while using its host as both room and board.
By taking this route, they hope to set the stage for developing new drugs
or, preferably, natural control agents "relatively specific to the
nematode and nontoxic to the host," says Fetterer, who is at ARS' Parasite
Biology and Epidemiology Laboratory in Beltsville.
The team's work comes at a time of growing concern over evidence that many
nematode parasites may be developing drug resistance.
"In many parts of the world, including the United States, drug
resistance is becoming a problem, particularly with Haemonchus in
sheep," says Fetterer.
Going for the Gut
Scrutinizing the barberpole worm's biochemistry could yield much-needed
alternatives, including new substances for creating animal vaccines. "We
have a long way to go before that point," cautions Fetterer, "but we
have some good candidates."
These include a potent cocktail of digestive proteins and enzymes the
scientists recently discovered after cutting into the parasite's long, thin
intestine. Their focus now: confirm a suspicion that the cocktail helps the
barberpole worm digest its host's red blood cells. They're also trying to
purify the cocktail's active ingredients so the size, molecular structure, and
function can be described.
Fetterer refers to a key group of proteins as hemolytic factors (HFs). He
believes the proteins are localized within membranes of the parasite's gut.
There, like "molecular drill bits," they punch holes into the walls
of ingested blood cells, causing hemoglobin to seep out.
Millions of years of evolution have equipped the barberpole worm with a
single, toothlike structure for scraping its host's stomach tissues to cause
"It feeds by sucking up the blood, tissues, and anything that happens
along," says Fetterer.
By secreting the HF proteins, he proposes, the parasite can then extract
hemoglobin and other important nutrients from the blood cells.
Another player in the digestive process is the cysteine protease enzyme.
"It helps break down the hemoglobin and other ingested proteins into
smaller fragments," further aiding absorption, says Fetterer.
His lab is now conducting an informal study with scientists who are with
Enzyme Systems Products in Livermore, California, to see if substances called
inhibitors will obstruct cysteine protease activity in the barberpole worm. If
so, the approach could deprive the parasite of a key nutrient-gathering tool.
In addition to the barberpole worm, "some insects also have these types
of enzymes," notes Fetterer. One such insect is the stable fly, from which
researchers took their cue in searching for the barberpole worm's digestive
cocktail. "If you just think about getting nutrients from a cell,"
says Fetterer, "then what you end up having to do is break it open."
The scientists began their search for that cell-breaking factor by grinding
a barberpole worm's intestine and mixing a small amount of extract material
with red blood cells from sheep. Normally, sheep red blood cells appear as
smooth-surfaced disks with a slight dimple, or indentation, in the center. But
exposing the cells to the parasite's hemolytic proteins quickly distorted their
Images captured with a scanning electron microscope's high-powered eye
revealed that the surface of the cells began to buckle after 15 minutes. By 30
minutes, jagged projections emerged, making the cellular surface appear harsh
and alien. The cells also began losing hemoglobin, which escaped through holes
that had opened up. By 90 minutes, the cells began to break apart, deflating as
the last of the hemoglobin escaped.
"These observations support a hypothesis that the hemolytic factors
serve as pore-forming agents," says Fetterer. "They insert themselves
into red blood cell membranes, altering their structure and thus causing them
"One of our goals," Fetterer continues, "is to further purify
and characterize this hemolytic factor and see if we can develop an antibody to
An antibody also raises the prospect for developing a vaccine. Injected into
a lamb, for example, a vaccine could help prime the young animal's immune
system for nematode attack.
"There are no such vaccines currently available for controlling these
parasites," says Fetterer.
Until then, anthelmintics--drugs that destroy or expel parasitic worms--will
remain the staple defense. While effective, such chemotherapy isn't a magic
bullet. That's because even a treated animal can reacquire the parasite by
grazing on infected pasture, necessitating yet another round of treatment. To
avoid this costly cycle, farmers must be diligent about where and when they put
their animals out to graze.
Sheep are especially vulnerable to barberpole worm assaults. For one, sheep
aren't averse to grazing pasture where manure is present that may contain
parasite eggs, according to Virginia Cooperative Extension scientists in
Sheep also graze plants close to their roots, where parasite larvae are most
concentrated. To make matters worse, "the larvae are actually specialized
at crawling up grass blades where they can be ingested," adds Fetterer.
Once ingested, parasite larvae quickly find their way into the host's
abomasum, or true stomach. There, they mature, mate, and produce eggs in about
In susceptible or young animals, parasite numbers can build so high that the
animal loses more blood than its body can replace. Severe blood loss can kill
an animal, says Fetterer. Milder or chronic infections can cause lethargy and
loss of appetite and can interfere with weight gain. This affects a producer's
profit when animals show a drop in their milk, meat, or--in the case of
Setting the Stage for Parasite Sabotage
Fetterer's hope is that what is learned about the barberpole worm's key
weaknesses will also apply to some other nematode parasites, like the brown
stomach worm, Ostertagia ostertagi. In the United States, this
parasite--more than the barberpole worm--is a menace to beef cattle, especially
heifers and breeding stock.
"Ideally, we'd like to find something common to the
Trichostrongylidae," says Fetterer. This nematode family includes
Haemonchus, Ostertagia, and about a half dozen other species
known to parasitize ruminant animals.
Next to antibiotics, administering anthelmintic drugs is often a farmer's
second highest production cost, Fetterer says. So, "if you could reduce
the reliance on chemotherapy, you could reduce costs." That's especially
important in light of the barberpole worm's increasing drug resistance.
Additionally, organically grown produce, including milk and meat products,
is becoming more popular with American consumers--all of which opens the door
to novel methods for protecting livestock and farmers' profits.
"The long-term rationale for doing this research is to understand how
nematodes survive and adapt to their hosts," says Fetterer. "Such
knowledge can lead to the development of new tools for producers to use so they
can choose what's best for their particular management styles."By Jan Suszkiw, ARS.
Raymond H. Fetterer, Marcia
L. Roads, and Dolores E. Hill are at the USDA-ARS
Parasite Biology and
Epidemiology Laboratory, Rm. 103, Bldg. 1040, 10300 Baltimore Blvd.,
Beltsville, MD 20705-2350; phone (301) 504-8300, fax (301) 504-5306.
"Barberpole WormsParasites of Another Stripe" was
published in the January 1999 issue
of Agricultural Research magazine.