Deworm to Prevent Rather Than Treat Worms
By Kelli Gunderson
Livestock Editor, Agri-View
According to Dave Thomas, UW-Extension sheep specialist, sheep
producers could stand to benefit from deworming their flocks before they show
signs of worms.
“This is a little different from what we’ve traditionally done,” Thomas told
sheep producers at the Southeast Wisconsin Shepherd’s Clinic held recently in
East Troy. Traditionally, he said, shepherds turn their sheep out on pasture in the
spring, and, come June or July, often begin to notice lambs with runny stool or
ewes with swelling under the jaw – both telltale signs of nematode infection.
Considering nematodes (also known as roundworms) can cause much economic
loss and even death in sheep flocks if left uncontrolled, shepherds generally take
action against the parasite right away. Once they see signs of worm infestation,
they typically treat their flocks with an oral drench, and then re-drench the sheep
every three weeks thereafter, often through the remainder of the grazing season.
But Thomas says there’s a better way. “If we can avoid nematode larvae
buildup on pasture, we don’t have to drench our sheep all summer and fall,” he
said.
Thomas explained that the nematode life cycle lasts 21 days (see Figure 1).
The larvae generally start out in relatively low numbers in spring pastures,
overwintering from the previous fall. The larvae go through several larval stages
on pasture and eventually crawl up blades of grass, where sheep ingest them as
they graze. Once inside a sheep’s digestive tract, the larvae develop into adults
and attach to the lining of a sheep’s stomach and intestines, where they suck
blood. These adult nematodes undergo sexual reproduction in the sheep, laying
eggs that are passed into the environment in the sheep’s manure. The eggs then
hatch into larvae, beginning the cycle over again.
Of utmost importance in understanding how to control nematodes is
understanding that these worms don’t increase in number while living in the sheep,
Thomas said.
“The only way to have a heavily parasitized sheep is to have a heavily
infested pasture,” Thomas said. When grazing begins in the spring, he explained,
adult ewes and rams often ingest nematode larvae but don’t shed as many
nematode eggs as might be expected. “Adult sheep are fairly well immune to
these worms after they’ve been exposed to them one season,” Thomas said. The
problem tends to come with new lambs. “Lambs don’t have that natural immunity,
so they’re throwing eggs back out on pasture and we end up with high levels of
nematode infestation on the pasture by summer,” he said.
Because of the tendency for pastures to become highly infested with
nematodes, thereby allowing sheep to become infected, Thomas suggested control
measures that won’t just control nematodes in sheep, but on pasture too.
“The first thing we need to do is try to reduce the number of infective
larvae on pastures,” Thomas said. “We can do this through strategic early
deworming.” Rather than waiting for sheep to show symptoms of nematode
infestation, shepherds should deworm their sheep with an oral drench two weeks
after first putting the sheep on pasture, Thomas said. By drenching early,
shepherds will kill the nematodes while they’re still inside the sheep, preventing
them from re-infecting the pasture. In this system, Thomas suggests shepherds redrench the flock three weeks after the first drenching and again three weeks after
the second drenching. After that, “You should be able to go the rest of the season
without drenching,” Thomas said. He did, however, recommend a mid-summer
and fall drenching for parasite-prone lambs that might still be picking up small
amounts of larvae from the pasture.
By strategically deworming early, “You’ll save a lot of drench and a lot of
time,” Thomas said. Shepherds will also benefit from better-performing sheep and
lambs, and from decreased parasite levels on the pasture the following year.
For producers who have more than one pasture, and for those who raise
other species of livestock, Thomas suggested deworming the sheep flock and then
moving them to another pasture.
“You could graze your sheep on an infected pasture (one that was grazed
by sheep the previous year) until late June or so, deworm the sheep and then move
them to a ‘safe’ pasture that wasn’t grazed by sheep the previous year,” Thomas
said. Considering nematodes are infective on pastures for several months, “safe”
pastures might be hard to come by for some producers. For that same reason,
pasture rotation isn’t an effective control for nematodes, Thomas said. Options for
the safe pasture would be an empty hay field, for instance, or a pasture that was
previously grazed by cattle, since nematodes that affect sheep differ from those
that affect cattle. Even on a safe pasture, re-drenching would likely be warranted
in lambs, Thomas noted.
Whichever treatment method a shepherd prefers, “There’s no way we can
get by without using chemical dewormers,” Thomas said. However, he stressed
that shepherds should use dewormers strategically both to maximize their results
and reduce worms’ resistance. He said studies in Virginia have shown that sheep
on 90 percent of farms surveyed showed resistance to Ivomec (ivermectin) and
Valbazen (albendazole) treatments, and another 40 to 50 percent showed
resistance to Tramisol (levamisole). Considering these three products are the only
deworming treatments approved for sheep in the United States besides
albendazoles (which aren’t effective against nematodes but are very useful against
liver flukes and tapeworms), Thomas said it’s important that shepherds use them
tactically.
“Use one drench product each year and then switch to another product the
next year,” Thomas said. Or, use one drench product for as long as possible until
it stops working, and then switch, he said. But, whatever you do, “Don’t switch
products every time you drench because you might develop a population of
worms that’s resistant to every drench,” he said.
Thomas also warned shepherds not to under-drench. Although he
acknowledged that drenches are expensive, he said that, if anything, shepherds
should overdose rather than underdose. “Drench to the heaviest lamb in the group,
not to the average lamb in the group,” he said. “If you under-drench, you tend to
build resistance up faster.”
So far, the one deworming treatment that’s been shown to have the least
amount of resistance from nematodes is Cydectin (moxidectin), Thomas said.
Although Cydectin has seen much use in the cattle industry as a pour-on treatment,
it’s not yet approved in the United States for sheep, and it doesn’t work as a pouron for sheep either. Sheep respond to oral treatment of Cydectin, Thomas said, but
such treatment needs to be approved by a veterinarian. Other deworming
treatments needing a veterinarian’s approval for use in sheep are Panacur and
Safeguard (fenbendazoles). Worm resistance to these and other dewormers is of
growing concern, Thomas said, as he knows of no new deworming products being
researched or produced.