PARASITE WARS – THE WORMS STRIKE BACK ….

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……