Deworming

 Common Endoparasites of the horse.

Strongyles:  

The most significant family of worms, strongyles are responsible for the most severe parasite related diseases, even death in adult horses. Strongyles are cylindrical nematodes with red or white bodies and can range in size from ½ inch to 2 inches long. They are transmitted directly between hosts without involving different species of animals, i.e. a direct life cycle where the horse will shed worm eggs in the feces, these develop in the grass to larvae and other horses can become infected by ingesting these larvae. Development of the larvae to an infective stage can take days in warm weather to weeks in cooler weather. This family is divided into large and small strongyles.

Large Strongyles:

These larvae are ingested, but they leave the intestine and migrate through the horses’ body via the arteries or liver but they eventually return to the gut to mature and lay eggs, which will complete the cycle to larvae in the pasture.  Clinical signs of large strongyle infection include the well known rapid weight loss, lethargy, and loss of appetite, but because of their migratory pattern, damage is also throughout the liver, peritoneum and blood vessels.

Small Strongyles:

After ingestion these larvae immediately invade the lining of the large Intestine, here they become encysted and they follow 1 of 2 developmental patterns. They either mature to the stage L4 larvae within this cyst or they can remain encysted at this stage for up to 2 years. This arrested development can occur if there already are a large number of adult small strongyles in the gut lumen. A mass emergence of these small strongyles occurs when these adults die or are killed.

Disease
Due to the life cycle of these parasites they can cause disease both when they encyst and when they emerge. At invasion there is a local inflammatory reaction of the large intestine, this can cause diarrhea, weight loss and suppressed appetite. The larvae can remain encysted for an extended period of time without causing any clinical signs. However when the larvae emerge there is a very intense local reaction, with hemorrhage, edema this can manifest as diarrhea weight loss and hypoproteinemia. Occasionally due to environmental factors these encysted larvae can emerge simultaneously in very large numbers. This is called larval cyathostomiasis and is associated with very severe clinical signs including severe colic and may even be fatal.

Control
Individual horses vary dramatically in their susceptibility to strongyle infections, some horse will have very low fecal egg counts while others will have very high counts, these horses are responsible for most of the pasture contamination. This variation means it is very important to do fecal egg counts on all horses in a pasture. All broad spectrum de-wormers are effective against the adult egg laying stages, but as we can understand now from the lifecycle most of the damage is caused by the encysted small strongyles and the migrating large strongyles. Ivermectin and moxidectin are effective against migrating large strongyles. Only Moxidectin and 5 day course of Fenbendazole  (5 day Power Pack)  are effective against encysted larvae.  

Ascarids: Roundworms

Parascaris equorum infection is important in young horses up to about 15 months. It is unique in the fact that the adult horse develops complete immunity to it. The female ascarid is a prolific egg layer and can lay hundreds of thousands of eggs a day. These eggs are extremely tough and very resistant to dessication or freezing; they are also very sticky and will adhere to any surface.
These infective eggs are ingested the adults are usually found in the small intestine however like the large strongyles they have to migrate through the gut wall to the liver via the blood or lymphatic vessels  developing through the stages as it goes. They then travel to the lungs, where they enter the alveoli and migrate up the airways to the trachea. When they reach the throat they are swallowed and develop into adults in the small intestine.

Disease
The clinical signs of these worms reflect the migratory path they take and severity depends on total parasite burden. During the migratory phase typically signs of respiratory disease are noticed that do not respond to antibiotic therapy. It is rare for an endoparasite to cause a physical impaction in the small intestine, however this is the case with ascarids especially just after de-worming the dead worms can cause an obstruction in the gut and the foal would develop severe colic signs.

Control
Fecal exams are the gold standard diagnosis of ascarid infection, despite only showing the presence of egg laying adults. Ascarids are also relatively difficult to kill they require a much higher dose of benzimidazole wormers than other parasites, i.e. double dose of panacur compared to what is necessary in adult horses. Ivermectin and Pyrantel (Strongid) are effective at normal doses. Moxidectin is effective but not suitable for foals under 6 months old.

TAPEWORMS

Tapeworms are only in the last few years becoming recognized as one of the main causes of disease in horses, however recent studies suggest that more than 50% of horses have tapeworm infections.
Tapeworms have an indirect lifecycle; therefore they depend on a mite called the orbatid mite for infection. Tapeworms are ingested when the horse ingests the mite when grazing on pasture, eating hay or in bedding. The orbatid mite picks up tapeworm eggs from the feces while ingesting organic material, the eggs develop to an infective stage inside the mite over a period of 2-4 months. When the horse swallows the mite the tapeworms mature in 6-10 weeks. The mature adults attach to the gut lining near the junction of the ceacum and the ilieum with hooks.

Disease
Tapeworms initially cause a problem as there is inflammation where they attach, if they are present in large numbers this can actually block food and cause ileoceacal intussusceptions, impaction colic or gas colic; they also absorb nutrients through their skin depleting the horses supply.
Tapeworms are difficult to diagnose clinically as the eggs are difficult to detect on fecal egg counts. Blood tests to detect antibodies to the tapeworm are available but these will only indicate the horse has been infected, not always the current status of infection.

Control
We have to assume that all horses have tapeworms as it is impossible to eliminate the mite that carries them. The mite is also beneficial for pastures so it is also detrimental to healthy pastures to decrease mite numbers. With this in mind we recommend de-worming twice yearly with a product containing praziquantal: first in spring and again after the first frost in fall.

The Bot

Bot flies are common everywhere horses are kept, there are 2 major species in the US.
Gasterophilius intestinalis lay their eggs on the hairs of the forelimbs and shoulders. 5-6 days after they are laid these larvae are ready to hatch out instantly the horse scratches himself with his muzzle or teeth. The larvae then enter the horse’s mouth and bury into the epithelium of the tongue.
G. intestinalis larvae spend about 1 month in the mouth, living in the tongue and doubling in size. They then enter pockets between the upper cheek teeth where they develop into 2nd stage larvae, they briefly attach to the root of the tongue before heading to the stomach where they moult into full grown bots. These larvae attach to the non-glandular part of the stomach.
 Gasterophilius nasalis: This honeybee like fly lays its eggs on the hairs of the inter-mandibular space under the jaw. These eggs hatch in 5-6 days and the larvae crawl down to the chin until they pass between the lips. Parasitologists believe these larvae develop between the cheek teeth and the third stage larvae are usually found in the first few inches of duodenum.
In late spring the larvae release their hold and pass out in the manure to pupate to adult flies in about 3-9 weeks.

Disease
As you can imagine the oral lesions caused by these bots can be extensive and can have an impact on your horses’ oral and dental health. Currently there is very little evidence associating bot infections with clinical disease; however it may just be too subtle to detect.
Ivermectin is highly effective in removing bots from the equine stomach.

LUNGWORMS

Lung worms are particularly important if your horses share pasture with donkeys. Dictylocaulus arnfieldi is a primary parasite of donkeys that can be also be found in horses. As its name suggests this long (about 3inches) white worm lives in the lungs. The egg contains first stage larvae which hatch before they are even passed in the manure, these larvae become infective in 5 days. When ingested they migrate to the lungs via the lymphatic system in another 5 days. Eggs are laid 4 weeks after initial infection and the larvae are coughed up the trachea, swallowed back into the intestinal tract. It is rare for these worms to successfully reproduce in horses.
The adult worms live in the larger air passages of the lungs; they can cause bronchitis, coughing and decrease the ability of that part of the lung to exchange oxygen. A minor worm burden may not cause obvious clinical signs however larger infections can be difficult to differentiate from other respiratory problems, and we have to rely on a history of being housed with donkeys.
Ivermectin wormers are effective against lungworms.

Pinworms

Oxyuris equi, these large parasites are well known for cementing their eggs en masse to the skin around the anus, each sticky mass can contain up to 60,000 eggs. They develop into infective stage in 4-5 days and the sticky fluid dries up and the eggs fall off, effectively contaminating the environment of the stall. Their life cycle is complete if they are wiped off the perineum with a cloth or sponge and the same cloth is used to wipe the bit. The entire life cycle takes up to 5 months to complete.

Worming Regime
In our equine wellness program we use 3 different types of wormers to combat all the above mentioned endoparasites. They are rotated every 2 months.
Equimax is given in January and July: Contains praziquantal especially to treat tapeworms.
Strongid is used in March and September.
Anthelcide or Quest is used in May and November.
Your fecal should be done at least 1 month after the previous deworming, preferably just before the next deworming.