Article By Jenn Webster
When Dr. Ela Misuno, DVM, MVSc first came to Canada from Denmark to pursue her veterinary residency program, she was surprised to learn of the differences our country presented in terms of equine deworming strategies. By comparison, Denmark had been employing routine fecal egg examinations since the 1990s and dewormers were only sold to horse owners by veterinarians – after they delivered a fecal sample for testing. Only horses that were determined to be moderate and high shedders in respect of the level of parasitic eggs found in one gram of manure, were then given a dewormer.
“When I first came to Canada, it seemed as though no one was talking about fecal egg exams and pasture management,” says Dr. Misuno, now a technical veterinarian for Vetoquinol. “And learning about parasites in vet school was not an exciting subject. I felt it was a highly important topic for horses in North America, so chose parasitology research project for my master’s studies.”
With internal worms developing increased resistance to deworming drugs, the war against equine parasites has changed. Rotational deworming is a thing of the past. Here Dr. Misuno guides us through new parasite considerations such as geographic location, herd management, manure control and targeted deworming for better practices to suit our needs as horse owners today.
All horses carry some amount of a parasite burden. The big questions are, are they carrying numbers high enough to cause disease? And are any of those burdens large strongyles, tapeworms or small strongyle encysted larvae? No amount of deworming will eliminate parasites completely however, the point of a parasite control program is to prevent horses from amassing such high parasite burdens that cause those animals to experience diarrhea, colic, weight loss or even death.
The parasitic cycle is such that to develop parasites, a horse will ingest larvae from their surroundings. Next the larvae develop and migrate through the body. They become egg laying adults in the gut and eggs are passed through the horse’s manure. The eggs hatch and larvae live in the horse’s environment – and the cycle starts all over again. The parasitic cycle is very dependent on weather conditions and the environment.
“A freeze / thaw cycle will kill larvae because they are sensitive,” states Dr. Misuno. “Except for one specific worm – parascaris (roundworms). In Canada the cycle is generally halted in the winter because the cold will stop larval development. It all depends on temperature and humidity. Larvae like moderate temperatures and high humidity, hence, they can develop quickly in the spring early summer and fall.
Eggs are much more hardy than larvae. Eggs can start to develop slowly in a cool, Canadian spring. Any temperatures above 30-degrees Celsius can kill both eggs and larvae however, the ambient temperature must also be dry – no humidity. That’s why the Canadian prairie provinces get a winter break from parasites, but British Columbia can have a problem all year long. Not all provinces are the same. Parasite burdens depend on susceptible horses and favorable environments.
Dr. Misuno states that every equine property needs to be assessed on an individual basis. The best way to create a tailored parasite control program is to first identify “herds” of horses in each property. A herd is a group of horses who are in close enough contact to transmit parasites to each other. This would include horses who are housed together on one pasture or pen. Each herd would then have a parasite control program based on the concentration of horses per acre, feeding practices, age and fecal shedding levels. Horses kept in individual stalls should be treated individually.
“Larvae develop on grass where there’s organic material and moisture. That’s why their development is a bit halted on dirt paddocks. Paddocks aren’t perfect but at least they have less parasitic transmission. In a pasture, the concentration of horses to land is crucial. That’s why there are certain things an owner can do for management practices to help stop parasitic transmission.”
These include cleaning up the areas in pastures where horses eat regularly. In the wild, horses eat grass and walk away. In a pasture situation, they walk around in a circle and come back the eating area.
“If you can only do one thing like clean around those high-traffic areas in your pastures, you would be making a great difference in parasite control,” Dr. Misuno says.
“Notice the trends of your pasture to help you make a difference. And why are we talking about this in the first place? Because of the accelerating problem of resistance to current deworming drugs. We have to start thinking about what else we can do to manage parasites. The simple fact is, if you provide your horses with an environment that has very few parasites in it, you help decrease the infection level in your animals..”
Additionally, not all horses on the same property are the same. Based on research we have to date, it seems that adult horses tend to follow the 80/20 rule in regards to their egg shedding levels. If you follow a fecal egg exam on horses over the years, you will see that only 20-30% of horses will be considered “high shedders.” Why does this happen? Because the immune system of every horse is different.
“We believe that horses of three years of age and throughout their adult life, are consistent in their shedding levels. Young horses need time to prime their immune systems against parasites. An old (geriatric) horse’s immune system changes as they get older – so older horses may change their shedding levels.”
FECAL EGG COUNTS – A HOW-TO
There is actually a proper way to submit a fecal sample for testing. Two to three fecal balls are necessary. Also, “A sample must be fresh (‘steaming’) but that still means it can be kept in the fridge for two to three days to be considered ‘fresh.’” says Dr. Misuno.
This allows horse owners, or boarding facilities time to collect samples from numerous horses for a simultaneous submission – since it’s often difficult to collect samples from several horses on the same day.
Ziploc bags are the best way to store samples and each bag must be clearly labelled on the outside as to which horse it belongs, the age of horse and the time of last treatment with dewormer. Samples should never be frozen or left at room temperature. When samples are submitted to a veterinarian, horse owners should also make the vet aware of any current symptoms occurring in a particular horse. These include things like diarrhea, colic or weight loss.
Ideally, another fecal sample should be submitted to your veterinarian two weeks after deworming your horse. It is called a fecal egg count reduction test and helps you choose the most effective drug for your herd of horses and assure that no resistance is developing to it. Parasites of foals may be sensitive to different dewormers than parasites of adult horses. It is recommended to perform fecal egg reduction test on around 30% of moderate to high shedders, and repeat it at least once every three years.
“If we can kill all the adult parasites, there will be no new egg production,” explains Dr. Misuno. “In a moderate to high-shedding horse, a rechecked fecal example two weeks after deworming means there should be zero eggs – we killed 100% of all adult forms.
TAKE HOME MESSAGE
The best way to develop a parasite control program for your needs is to contact your local veterinarian and have them devise a plan for you. Fecal egg samples are crucial for success as is appropriate pasture management. Do not spread horse manure on your pastures. Cross-species grazing is a smart technique to keep parasite levels down – it’s better to rotate one year with cattle, if possible. Also remember that if your system is to typically deworm only in the spring and fall, you’re not protecting any high shedders on your property.
With only four drugs to rely on and drug resistance becoming a very real problem, Dr. Misuno points out the time for action is now.
“Parasites are a problem that affect 100% of horses. Not addressing this problem is no longer an option.”