Through the continued use of dewormers, parasites develop resistance to the dewomers.
The dewormer that you are using may not be effective on the worms in your animal.
Many of our horses are not infected with parasites & therefore do not need to have dewormer given.
By running fecal parasitology tests, we can figure out if your animal has a worm burden and if it does, whether that parasite may be dewormer- resistant.
Cameron Veterinary Clinic has a lab set up to run quantitative fecal egg counts (EPG or Eggs Per Gram tests) and Egg Count Reduction tests on your animals. These sensitive tests, run by centrifugation rather than flotation, will allow us to determine which animals need to be dewormed. They will help us determine which dewormers to use and which dewormers the parasites infecting your animals have built up resistance to.
To perform a fecal egg count test, we take a measured amount of fresh manure and after centrifuging it in a concentrated sugar solution at specific RPM for a set period of time, look at the sample under the microscope in a calibrated counting chamber and identify the parasites present to a sensitivity of 8 eggs per gram (EPG). A significant worm burden is considered anything greater than 500 EPG, an insignificant worm burden under 200 EPG.
To perform a fecal egg count reduction test, a second fecal sample is analyzed using the method described above approximately two weeks after deworming. Comparing the egg count in the second sample to that in the first allows us to determine whether the dewormer was effective at reducing the parasite burden in that animal. If not, the test can be repeated after deworming with a different class of dewormer.
Are intestinal parasites a problem?
New research, and a fresh look at old research, is showing that intestinal parasitism continues to be a problem for livestock species. Additionally we are finding that our continual use of dewormers has helped to select populations of parasites which are resistant to those deworming agents. I continue to be surprised by the horses and alpacas that I examine who do have parasite infections despite regular administration of dewormers. Worm infestations are a common cause of diarrhea, and can cause impactions (blockage of the intestine) and colic.
It helps to understand how these organisms get into your animals. Very generally, intestinal parasites get into their host (your animals) by being ingested. They then develop and live in the intestines and begin to lay eggs, which are passed out of the animals in their stool, ending up on the ground where they are capable of reinfecting the same animal or infecting a herdmate or neighbor when ingested as the animals browse around looking for something to eat.
There are many different species of intestinal parasites. Their common names, “roundworms”, “hookworms”, “whipworms”, “tapeworms”, are the same for all animal species, but for the most part they only infect one species of animal. In other words, the roundworms that infect horses (Strongylus spp.) are different from the roundworms of camelids and different from the roundworms of dogs (Toxocara spp).
Should I deworm my animals?
It is no longer a matter of deworming or not. Regular deworming does not work as well as we ththought, due to the increasing incidence of parasites that are resistant to the dewormers. For this reason it is important to regularly test your animals for parasites. As mentioned above, the majority of horses in a herd may not be carrying a worm burden that requires attention. For these animals, after checking a fecal sample, we may recommend skiping the dewormer altogether.
For those animals that do have a significant parasite load, after deworming a fecal sample should be rechecked to see if the dewormer was effective at killing the parastites carried by the animal. If parasite eggs are still present in the same numbers as before, the dewormer wasn't effective and it never will be for that animal, so it can be removed from the deworming "rotation."
In our local environment of dry lots, paddocks and stalls, infectious larvae can get spread from one area to another by wind and by birds and rodents moving manure about. Manure carrying infective larvae can get baled up in hay, carrying eggs from the pasture into the feeder.
Even animals housed in a dry environment, who never leave their paddock, can become infected with parasites. Research in horses tells us that the distribution of parasite infestation is not evenly distributed throughout the population. In a herd of horses, most will have low to moderate parasite burdens and a few will have very high worm burdens, and continue to contaminate their environment.
When you travel with your animals, they are at increased risk of exposure to parasites left behind by other animals. That little pile of dried up fecal balls in the stall that you put your horse in for a few minutes at that show? The breeder from somewhere far away who stepped in to your alpaca’s pen with manure stuck to the bottom of his shoe? The lush green high-mountain meadow the horses are on that was grazed last week by another group of packers? These are all areas of potential exposure to GI parasites.
Deworming your animals is important, but it is only part of the solution to keeping parasites under control. Regular parasite testing and removing manure from their environment are just as important. Parasites are very adept at surviving in the environment and can infect an animal years after their eggs have been laid. The most common and pathologic parasite of horses, Strongyles, will develop into infective larval stages in 8-20 days after eggs are shed in the manure (Geogis' Parasiology for Veterinarians, D. Bowman, 1995: p177)so removing manure on a regular (weekly) basis should prevent these infective stages from developing in your horse's environment.
How often should I deworm?
The latest research shows us that many parasites have developed resistance to one or more of the dewormers that we commonly use. Ideally, we will regularly run fecal tests on your animal’s manure to know what the “worm burden” is, that is what type and how many worms (in eggs per gram or EPG) are present in the animal’s GI tract, and come up with a deworming schedule based on the test findings. Without doing so, you may be unnecessarily deworming animals who don't need it or pouring ineffective dewormers into an animal with resistant parasites.
Travel and exposure to other animals or the manure that they have left behind is another area for attention. I recommend parasite testing on new additions to your barn before you introduce them as well as separating them from the rest of the herd for a few weeks after arrival and regularly removing their manure from the pen.
How do I know which dewormer to use?
A population of worms can become resistant to a particular class of dewormer. A population of parasites does not acquire resistance to a deworming agent due to repeated use of the drug. Rather repeated use of the drug kills off the susceptible population, leaving behind those that, through genetic variation, are able to survive despite the drug. Those resistant worms continue to reproduce, creating a large population of worms for which the deworming drug is ineffective.
We categorize dewormers by the class of drug: bendazoles, pyrantel, avermectins being the “big three,” approved for use in horses. Historically we recommended switching from one class (not brand) of dewormer to another with each deworming.However, because of parasite resistance to dewormers, after running a fecal egg count reduction test, we may eliminate one or more of these from use forever for a given animal.
With the new- found information that parasite populations have developed resistance to many of the common deworming agents, I recommend running fecal egg counts and egg count reduction tests in order to determine which dewormer to use.
The cost of fecal egg count testing may easily be offset by decreasing the amount you spend annually on dewormers. For an animal with a resistant parasite infestation, the cost may be a bit more initially, but the savings will be seen in not purchasing an ineffective dewormer in the future and more importantly in providing the most effective health care for your animal.
Aren’t dewormers poisonous?
Not if used appropriately. At the proper doses, they are not toxic to the animal they are labeled for, even though they are to the parasite. Many of the deworming drugs are not even absorbed into the blood, but just pass through the GI tract, killing worms on the way through.
Most dewormers have been tested and found to be safe at 4 to 10 times the effective dosage in certain species. They have been tested and found to be safe in breeding and pregnant animals at all stages of gestation. Although you may have heard that deworming during certain periods of gestation may cause birth defects or abortion, there is no evidence in safety studies to back this up. The best place to look for reliable information on this is the Food and Drug Administration (FDA)’s website.
There are no deworming agents that, to my knowledge, have label claims for use in camelids. (A "label claim" for a drug means that the drug has been tested and proven to be effective against that parasite in that species.) Thus we have to rely on the experience of other veterinarians who have used a particular drug and knowledge of how a drug treats similar parasites in other species of animal. This is referred to as "extra-label use" of a drug, and is something that the FDA allows veterinarians to do in order to avoid the time and cost-prohibitive work of testing every drug against every bug in every species of animal that we treat.
Overdosages or use in sick or severly infested animals may cause illness, though. A horse with a very large parasite burden may experience serious colic &/or impaction due to the die-off of all those worms shortly after deworming. Collie breeds of dog (Australian shepherd, border collies, etc.) are very sensitive to avermectins. While they can be safely given ivermectin at low doses for heartworm preventative, I have seen these breeds poisoned from getting the remainder of a tube of horse paste. Other species that are sick with inflammation of the central nervous system, such as an alpaca with meningeal worm infection, may be similarly sensitive to avermectins.
It is important to test your animals for parasites regularly. Appropriate deworming is important and different for each group of animals, depending on their home environment, how they are fed, where they travel, and what they are exposed to. It is just as important to clean their environment and pay attention to potential parasite exposures when you travel. Your deworming protocol is one of the topics we should discuss during semi-annual wellness exams. Give me a call to schedule an appointment or talk about your parasite control plan.