Intestinal parasites are becoming resistant
Dr. Jeske Noordergraaf

Horses are a lot of work to take care for but horse owners obviously think that it is time well spent and enjoyed.  Along with many other horse owners, I prefer cleaning the barn to cleaning my house.  Are my horses more appreciative than my family?  Maybe not, but the barn just smells good.  I don't mind scooping manure at all but seeing worms in the manure is unpleasant especially if the worms are roundworms or ascarids and you were planning on spaghetti for dinner. 

Intestinal parasites are harmful to horses.  They can cause colic, weight loss, unthriftiness, and make the horse rub its tail.  Regular deworming is very important but how do you know if your program works?  Fecal egg counts determine what type of eggs your horse is shedding and how many of each is present.  At a recent seminar that I attended the presenting veterinarian discussed resistance.  The drugs we have are no longer as effective since intestinal worms are adapting and unfortunately there are no new drugs ready to be released.  The recommendation is thus to do more testing to determine if there is a problem along with pasture management and knowledge of the parasite lifecycle. Horses do not need to be 100% parasite free as they do develop some resistance. 

There are eight common types of intestinal worms in horses: roundworms, small strongyles, large strongyles, tapeworms, pinworms, bots, lungworm and threadworm.  Roundworms tend to be a problem in the young and usually by 18 months of age the horse has developed resistance and it is then unusual to see  these passed after deworming.  The main clinical sign seen in a horse infected with roundworms is unthriftiness with rare reports of death due to impaction or rupture of the small intestine.  The large adult parasites (up to 40 cm long) reside in the small intestine where they produce eggs that are passed in the feces.   It only takes 10 – 14 days for these eggs to develop into the infective larval stage and foals become infected by ingesting them.  The larvae then migrate through the liver and lungs on their way back to the small intestine where the cycle repeats itself.  The entire cycle takes 3 months to complete.  To stop this life cycle, foals need to be dewormed to kill the adults parasites before they start passing eggs which would then be available for other foals of the same age or succeeding generations.  It is extremely difficult to remove the eggs that are already present in the pasture so ideally, young stock should be moved to pastures where only adults have been.  The eggs are very resistant and can survive in the environment for years.  Most dewormers are effective at killing these roundworms. 

In adult horses, large strongyles are an important problem.  Heavy infections are common in all horses but most common in those 1 to 3 years of age.  Clinical signs include loss of condition and anemia.  The large strongyle larvae spend extended periods migrating through blood vessels and other organs but cause the most serious problems when they are in the cranial mesenteric artery and cut off the blood supply to the intestine.  This is a common cause of colic especially in young horses.  The adult parasites are plug feeders which means that they take big bites out of the intestinal lining as they attach.  Spreading out the manure in a pasture is not recommended as the interior of even scattered manure is moist enough for development to the third stage and then this stage can survive even Minnesota winters. 

Small strongyles cause the most common parasite problem of horses.  The adults reside in the cecum and colon where they feed on plugs of mucosa, leaving small erosions and hemorrhage at each feeding site.  Adult small strongyles also cause the clinical signs of ill-thrift and anemia.  This is the most common parasite we see on farm calls as they can be on our rectal sleeve after doing a rectal exam.  This is when the expression that a picture is worth a thousand words holds true.  These parasites can encyst in the wall of the small intestine for many months before continuing their development into the lumen.  In the wall of the intestine, they cause inflammation and decreased gi motility leading to colic.  The only dewormer effective against the encysted stage is the double dose of fenbendazole for 5 consecutive days.   

Bot flies are visible in the late summer and fall.  The females lay eggs on the horses legs and mane.  The life cycle of this parasite takes about 1 year.  As we all know, these eggs are very sticky. When the horse itches the area with the eggs, the bot eggs end up in the horses mouth where they live for a few weeks burrowing into the mucous membranes of the lip and tongue causing a transient irritation.  The larvae then migrate to the stomach and grow there for 10 months until being passed in the manure where they pupate in the ground.  Stomach bots cause mechanical blockage, colic, or rupture of the stomach wall and resultant peritonitis or infection of the abdomen.  We often wait till after the first frost to deworm with a dewormer that will kill the adults in the stomach but remember that eggs left on the legs are still infective. 

We all blame pinworms for a horse rubbing its tail.  The female worm lays eggs around the anus which cause the itchiness.  The egg masses can be seen as yellow/grayish streaks when the tail is lifted.  The infective larva develops within the eggshell 4 to 5 days after the pregnant female has cemented it to the skin of the anus.  Massses of cement gradually dry, crack, and detach from the skin in flakes containing thousands of infective eggs.  These flakes adhere to mangers, water buckets, and walls, thus contaminating the environment of the stable. 

Threadworms are generally just a problem in foals aged 2 weeks to 4 months and seldom persist in animals older than 6 months.  Foals become infected either by nursing as the larvae are in the mares milk or by penetration of the skin by infective larvae.  The signs seen include diarrhea and enteritis and a fecal exam will show a large number of parasite eggs.  The larvae seem to go dormant in the mammary gland of mares and only cause problems for the foals.  Mares can be infected without these eggs showing up on a fecal exam. 

Tapeworms are the most recent target of dewormers.  The horse becomes infected by ingesting forage mites which contain a developmental stage of the mite.  The adults live in the small intestine and cecum near the valve between them and can cause colic as a result of a blockage.  The new praziquantel dewormers are effective for these parasites.   

The last parasite that I am going to mention is the lungworm.  There are also several other types of parasites but these are less common.  Lungworms are frequently found in fecal sample from donkeys but infection is rarely associated with signs of respiratory disease in this host.  In horses, lungworms can be the cause of chronic coughing.  Infection occurs by ingestion of the infective third-stage larvae while grazing.  These larvae then travel from the intestine via the lymphatics and blood to the lungs where they develop to the adult stage in the bronchial tree.  To reduce the risk of infection in horses, contact with untreated donkeys should be avoided. 

Our recommendation over the years has been to deworm horses every 6 to 8 weeks.  There are no new dewormers ready to be released and thus we must work with what we have.  To keep the parasite load down in the pasture, collect feces frequently.  Fecal collection before parasite larvae have migrated out from the fecal pile is the single most important measure that can be taken to prevent problems.  The next recommendation is to actually test the feces for parasite eggs.  There may be a few horses that have the majority of the parasites and these horses will need to be treated more often.  Management factors such as good stable hygiene and paddock rotation and reserving the least contaminated pastures for horses and foals is helpful.  At the end of the grazing cycle, treatment with a compound effective against small strongyle larvae in the intestinal mucosa should prevent outbreaks during the late winter or the following spring.  One other recommendation is to deworm any new horse that comes to your facility and keep the horse isolated for 72 hours to prevent introduction of anthelmintic resistance.