Goals Clarify the concerns regarding parasite
management Explain the history behind current deworming
practices Discuss emerging issues in equine parasite
control Review ways in which parasite control can be
optimized
Why do we care about parasites anyway? Parasites can affect multiple organ systems The intestine is the obvious primary site affected Can impair feed digestion and absorption Can alter normal intestinal function Can physically damage intestine Can impair blood supply to the intestine Can cause substantial blood loss and anemia
The liver can be affected by migrating larvae The lungs are also a site for migration of some larval
stages
Why worry? For almost 30 years we have been somewhat
complacent regarding parasite management As long as we did our two-month rotations everything
was ‘okay’ Unfortunately this is not always true and this has led
to a number of problems New types of parasites are becoming a problem Parasites that we have considered as ‘controlled’ are
emerging as problems again Some that we never regarded as a serious problem are
turning out to be associated with disease
How did we get here? Three primary reasons Research in the 1960’s showed that deworming resulted
Historical Perspective Forty years ago the primary parasite problem was with
large strongyles
in reduced shedding of strongyle eggs for ~8 weeks
These were known as ‘blood worms’
This led to the current recipe for treating with
Strongylus vulgaris was the worst
anthelmintics every eight weeks The development of easily administered paste wormers
simplified anthelmintic treatment Emphasis moved to routine treatment over monitoring
Because we have Ivermectin! It was touted to be impossible for parasites to become
resistant to this drug
Large Strongyles
Invaded the blood vessels supplying the
large intestine resulting in clots which interrupted blood flow This resulted in severe and often fatal
colic Even with surgery many horses died
What happened to large strongyles?
The good news was that they are not very complex
parasites
These parasites have become almost a historical
Simple fecal-oral life cycle Eggs are shed in manure Hatch in manure and larvae
attach to blades of grass where they are eaten by the horse Larvae migrate from intestine into bloodstream and then
back into the intestine where adults suck blood from intestine wall and shed eggs Susceptible to a variety of anthelmintic drugs There is not any stage in the life cycle where the
organisms are ‘protected’ from anthelmintics
footnote for most North American horses thanks to routine deworming
Why did treatment work so well? The simple life cycle of these parasites and
susceptibility to most drugs meant that a single treatment would stop pasture egg contamination for 6 months
The larvae could only survive for ~1 year in the
environment
After only 18 months of routine treatment of the herd
the organism is effectively eliminated
Other Parasites Did not seem to be a major problem back
then… Tapeworms Seen occasionally at post-mortem, but did not
seem to be causing much damage Ascarids Only present in younger animals Small strongyles Rarely present in large numbers, did not seem to
Anthelmintic Therapy Historically this consisted of potentially toxic drugs that
had to be administered by stomach tube Dichlorvos, phenothiazines, piperazine
Realistically this meant that most horses were not
dewormed very frequently
Some highly managed farms would monitor for parasite eggs
in the manure to guide them on when to deworm
Many horses were never treated with an effective
anthelmintic
Due to the poor coverage with anthelmintics colic due to
large strongyles was unfortunately common
cause problems
Paste Dewormers At first only a few drugs were available, but the ease
of administration meant that owners could administer the treatment themselves The frequency with which horses received
dewormers began to increase As more drugs became available the concept of
‘rotational deworming’ became widely accepted
Rotational Deworming By using different drugs alternately we could try to prevent
the parasites from becoming resistant to any single drug Not a bad concept, but because of the frequency of
administration resistance still developed What we were seeing is evolution on an incredibly fast
timeline Kill off the most sensitive organisms and only the resistant
ones survive to reproduce Continue selecting for the MOST resistant organisms until the
population does not include any that are sensitive to the drug anymore
Along Came Ivermectin Suddenly we had a new drug that supposedly had a
mechanism of action that parasites could not become resistant to… The rotational concept was then revised for two
reasons
So what happened next?
First, if nothing can become resistant to ivermectin why
not use it all the time? Second, by using ivermectin as the primary drug we could
stop using the less effective drugs and improve our ‘kill rate’
Equine Tapeworms While the early thought was that they did not cause
many problems for the horse this opinion began to change
Veterinarians began to see more horses with
obstructions of the small intestine at the ileum, where the small intestine empties into the large intestine Large numbers of tapeworms were often present at
this site
In some cases the wall of the intestine was very
thickened, perhaps secondary to the tapeworms
Research in the UK began to show that there might
be a link between tapeworm infestation and colic
Tapeworms So why were they an increasing
problem? Most of the commonly used
anthelmintics did not eliminate these parasites Exposure in the environment is
widespread We did a better job controlling the
other intestinal parasites Tapeworms may alter the normal
motility of the intestine
Tapeworm Antibody Positives
Tapeworm Life Cycle The life cycle is more complex than for the
large strongyles
The parasite releases body
segments known as proglottids These contain numerous eggs Eggs are released into the
environment and then ingested by mites The infected mites are then
ingested when the horse eats grass The tapeworm then matures in the intestine
New Deworming Rotations Ivermectin was used every two months except for late
fall/early winter when Strongid double dose given This is thought to be the best time to eliminate
tapeworms Mites are unable to survive hard freezes in winter
Some programs include tapeworm therapy once every
six months Use of these rotations has decreased the incidence and
severity of tapeworm infestations
Tapeworm Treatment Even as the appreciation of the potential importance of
tapeworms increased there were few treatments available
The primary therapy was pyrantel pamoate (Strongid™) This treatment is only effective when administered at twice
the normal dosage
“Double-dose Strongid” - 13.2 mg/kg orally once
A newer therapy is the drug praziquantel This treatment is highly effective Single dose of 1 mg/kg Marketed in combination with ivermectin Zimectrin Gold™, Equimax™ Quest Plus™ (moxidectin and praziquantel)
So, now what? With the use of these new ivermectin and double dose
Strongid (or praziquantel) regimens parasite problems should be a thing of the past, right? But, along come the small strongyles! No one paid them much attention, and they probably
were not a major problem when they were competing with the large strongyles for resources in the intestine Even more importantly, these parasites have a complex
life cycle Includes a stage where they are ‘protected’ from most
drugs which allowed them to survive despite deworming
Small Strongyles Cyathostomes
Adult worms produce eggs
Eggs shed in manure
Eggs hatch and develop through three larval stages in environment
Third stage larvae (L3) are infective and horse ingests them while grazing
L3 larvae penetrate gut wall and ‘encyst’ The cyst wall protects them from most drugs
L4 larvae ‘excyst’ and enter intestine where they develop to L5 stage and then to adulthood
Small Strongyles The encysted L3 larvae do little
harm
But they tend to all excyst
(‘hatch’) simultaneously, usually in the spring
The cumulative effect is to
damage the intestinal wall and cause substantial inflammation
The inflamed gut wall can lose
www.power-dose.com/strongyles.asp
it’s normal motility pattern
This may predispose to colic
Do they cause a real problem?
Small Strongyle Treatment
The small strongyles are not typically a major
The adult parasites are sensitive to all commonly used
pathogen in healthy horses that are well nourished and maintained at low stocking densities Unfortunately low stocking densities are pretty rare in
urban and suburban areas
Most horses probably suffer from sub-clinical disease Poor feed efficiency, decreased performance
Horses with severe infestations show clinical disease Cyathostomiasis Weight loss, hypoproteinemia, anemia, poor growth, colic
drugs But the adults are not the problem Treatment of L3 and L4 stages requires drugs that can penetrate
cyst wall
Only two currently available drugs accomplish this Moxidectin Quest® – 0.4 mg/kg moxidectin
Fenbendazole is only effective at higher doses and when given daily for five days Panacur® ‘Powerpack’ – 10 mg/kg once daily for 5 days
What now? The tapeworms and the small strongyles
should be effectively addressed with our new plans Yet another parasite was waiting in the
wings
Ascarids An emerging problem in young horses Typically weanling age Horses normally develop natural immunity by
2-3 years of age Clinical signs Infected animals are usually in good condition
and are growing well Primary sign is typically acute colic
Ascarids Parascaris equorum Interesting life cycle Eggs are shed in feces Very hardy (up to 10 years)
Infective larva (L2) develop within
the eggs
Eggs are ingested and hatch Larvae migrate through the liver
to the lungs
Larvae are coughed up and then
swallowed
Mature, reproduce and lay eggs in
intestine
Ascarids Can be seen as an incidental finding Gastroscopy or ultrasound Can cause sub-clinical disease GI or respiratory May cause poor growth, poor hair
coat, pot belly
Can cause severe or even fatal colic Especially following deworming
Why are ascarids a problem now? Ivermectin resistance Long term use of frequently dosed ivermectin has
resulted in resistance This has been reported in the US, Canada and Europe More intensively managed farms are more likely to have
resistant organisms develop Often treat foals monthly with ivermectin for first year
The problem is hard to detect without monitoring
Ascarid Ivermectin Resistance Typically resistant to moxidectin as well Only a few drugs appear to be effective Oxibendazole Panacur PowerPak for the full five days Pyrantel (Strongid) - variable
Monitor response to treatment with fecal egg counts
fecal egg counts on a farm-wide basis
This will ensure that treatment is effective
Typically sub-clinical, as these foals look and act pretty
Will also allow for modification of treatment if needed
healthy unless they develop acute signs
What about Bots? Do not cause serious health
issues for most horses
But control is indicated to
minimize damage to the stomach
Best addressed during the winter Eggs are laid by Gasterophilus
flies
Flies don’t survive in winter Larvae migrate from mouth to
the stomach and develop on the wall
Ivermectin remains the primary
treatment and is given in late fall or early winter
What the heck am I supposed to do now?
Manage to Minimize Parasitism Do not overstock pastures
Designing Treatment Regimens Use anthelmintics most in the horses that
Rotate pastures
actually need to be treated
Pick up manure from paddocks
The horse population on any farm typically includes low,
medium and high ‘shedders’
Do not allow overgrazing Avoid feeding from the ground
In one study 3/16 horses shed 85% of eggs in the herd Pasture ‘picking’ of manure
Compost manure for at least one year Graze pastures with other species of livestock
The medium and high shedders are the ones most likely
to be affected adversely by parasites By treating low shedders less frequently they are not
harmed but serve to maintain a population of parasites that remain sensitive to the drugs we have available
Cattle and sheep
How do we determine risk of shedding? As few as 20-30% of the horses in the herd are
responsible for harboring the majority of parasites Therefore responsible for most of the pasture
contamination Use fecal egg counts to determine who they are Perform fecal egg counts on all members of herd after a
pause in deworming (several months) High counts > 500 eggs per gram Medium = 200-500 eggs per gram Low < 200 eggs per gram
When to Treat? When is the contamination of the pasture and
infection of the horses the greatest? In our region this occurs in the Spring and Fall Cold winter temperatures limit hatching of larvae in
pasture but do not kill small strongyle larvae Hot, dry summers have an even greater negative effect Temperatures over 85 degrees dramatically reduce
transmission rates and will kill small strongyle larvae For this reason we can consider that the Fall, Winter
and Spring are actually the ‘Deworming Season’ for us
When to Treat?
Adult Horses
Due to seasonality of excysting of cyathostome larval
All horses (Low, Medium and High)
stages in areas with a ‘true’ winter you can time the treatments to best effect
Larvacidal therapy in spring and fall
Ideally one larvacidal treatment should be applied in
the early spring, prior to excysting of the
4th
stage larvae
A second larvacidal treatment in the fall reduces the
burden of encysted larvae and aids in controlling the other intestinal parasites Animals with higher risk of shedding or farms with
severe pasture contamination may benefit from one or two treatments in the summer as well
Quest (moxidectin) Panacur PowerPak (fenbendazole) Ivermectin (least effective)
Combine a tapeworm therapy with these in
the fall Praziquantel Double-dose Strongid
Medium Shedders
Heavy Shedders
Same as above for all horses
Same as for moderate shedders
Add an additional adulticide treatment in early
Add one more treatment in the late summer
summer to minimize the spread of cyathostomes
period
Pyrantel pamoate (Strongid)
This will minimize the degree to which they
Oxibendazole (Anthelcide EQ)
Ideally fecal egg counts should be monitored to
contaminate the environment Best to use a different drug than earlier summer
treatment
ensure effective treatment
Ivermectin (effective against adults)
Combining these two drugs may increase
Moxidectin
effectiveness
First Year of Life Targeted towards ascarids and strongyles Do not deworm foals before 60 days Treat at 8 week, rather than 4 week, intervals Rotate among all effective drugs If ivermectin resistance is not present then continue to use
it and/or moxidectin as part of the rotation
Also include: Oxibendazole (Anthelcide EQ) Fenbendazole (Panacur PowerPak) Strongid (pyrantel pamoate)
Monitor Response to Treatment Fecal Egg Count Reduction Test FECR =
(FEC prior to treatment – FEC after treatment)
___________________________________________ X 100
FEC prior to treatment
Use this test to determine if the drug being used is effective FECR of at least 80% is minimum required FECR of 90-100% is ideal (>98% for ivermectin, moxidectin)
Timing of testing is variable in the literature Two weeks post treatment represents the best time to
assess for resistance
Monitoring Response Remember, a negative fecal egg count does NOT
mean the horse is free of intestinal parasites. Larval stages do not produce eggs
Conclusions We cannot continue to do things the way we
have for the last 30 years
That is why it is important to test as many horses in
Evolution is constantly shifting the lines on the
Test before AND after treatment at each deworming
Effective parasite control will require assessing
the herd as possible
for the first 1-2 years
battlefield
Can assess response to treatment and identify ‘high
and monitoring the actual parasite problem on each farm
Cost of testing should be recouped in savings on
And monitoring the effectiveness of treatment
shedders’ over time
deworming products (average savings of 75%)
Additional Resources The Marion duPont Scott Equine Medical Center www.vetmed.vt.edu/emc/
The Horse Magazine/AAEP www.thehorse.com
The AAEP www.aaep.org
Proceedings of the International Equine Parasite Drug
Resistance Workshop, July, 2008.
www.vet.uga.edu/ID/conferences/equine/
Questions?
