Once upon a time all we had to do is buy a dewormer at our local tack
shop, feed it to our horse, repeat that 4x times a year and our parasite
problem was solved – or so we thought.
The sequel to the story goes to the parasite: The Ontario
Veterinary College has seen an increase of parasite related colics and colic
surgeries in the last few years and the live threatening disease
“Cyanosthomiasis” comes into any equine vets mind when presented with a young
horse with fulminant severe diarrhea.
What is going on?
Long story short - decades of routine use of “anthelmintic” drugs have
left us with drug resistant parasites, similar to the antibiotic resistance
problem we have with bacterial infections.
The good news is:
We might have lost a battle, but not the war – yet.
During my career as an equine veterinarian deworming protocols and
recommendations have changed several times. From annual rotations with
different dewormers in the same year to rotating on a yearly basis – and back.
In the good old times things were easy, if a client asked me what to
use, I would ask what did you use last time and depending on the season and my
mood I would suggest a product. Nowadays the conversation goes like this:
Client: What dewormer should I use this year?
Me: I have no idea…
Client: what do you mean?
Me: I don’t know if your horse has worms, or not; I don’t know if the
previous dewormings you did where effective, or not; I don’t know if you have a
shedder in your herd that keeps contaminating your pastures, or not…
I am not good at role play so I have to
stop it here!
These are the Facts:
- 20% of all horses carry 80% of the worm-burden
- Foals have roundworms
- The young horse (under 2) has the highest risk
- Old horses – might look shiny, but can still be high shedders
- Pasture rotation and hygiene is VERY important.
The most important offenders are:
1) Parascaris equorum – the roundworm: This is mainly a foal
parasite, and the growing horse develops normally a strong immunity once older
than a year.
The eggs have a thick protective shell and can survive for many years. The
foals of the previous year “infect” the new ones. Transmission via the
milk of the mare can also occur.
Foals should be dewormed with an appropriate dewormer starting at the 6
to 8 weeks of age, and then every 2 months. Ivermectin or Moxidectin might not
be the perfect drug for foals. Benzimidzole class dewormers seem to be the best
choice in the moment.
2) Strongylus vulgaris, the large strongyle or ‘bloodworm” was for centuries
the number one parasite of the horse. Their migration in the blood supply of
the intestine could cause dramatic “embolic-thrombotic colic. They can be
effectively controlled by most available dewormers and are the big losers in
our drug war, BUT this parasite took over:
3) Cyanosthomins - Small Strongyles : The larval
stage of this parasite encysts in the intestinal wall during its live
cycle and the synchronized hatching of the larvae ,or the rapid death due
to deworming can lead to life threatening diarrhea caused by the damage to
the intestinal lining.
4) Tapeworms are different. The horse tape-worm
needs an intermediate host like a grass mite in order to complete its life
cycle, the whole process takes about 6 months. They have been identified as one
of the risk factors for colic. Most of the time their eggs do not show up on
fecal testing. Thanks to their long life cycle one or two treatments per year
are enough. However only a drug called “Praziquantel “is effect at normal
dosages, the only other drug that is been used to treat and prevent is
“Pyrantel” in a double dose regimen.
5) Pinworms can be bothersome parasites, but actually
cause very little damage. Adults crawl out of the horse's rectum to lay their
eggs, which then can cause intense itching. This can certainly be a pain in
your horses’ butt.
We have learned – 20% of horses shedding 80% of the parasite eggs. All
we need to do is to identify these animals. Fecal egg counts (FEC’s) are our
most valuable tool nowadays. The better we know the status of the herd, the
more strategic can we treat the horses that need to be treated. We also will
have to accept the fact that we cannot get them all, and that is OK!
There are pitfalls to the testing protocol. While it gives important
information about the situation in the herd, the individual horse might not
shed eggs when we pick our sample and sometimes the undetectable larval stadium
is more dangerous than the adult worm.
Therefore: If I have a horse that looks like it suffers from
internal parasites, I treat first and ask questions later, same with foals,
they need to be dewormed regularly and the testing is only in order to monitor
how successful we are – or not…
Also, every horse should have at minimum an appropriate dewormer in fall
to keep botflies, strongyles and tapes under control.
TALK TO YOUR VETERINARIAN ABOUT IT……
