¶ … dozens of internal parasites that infect horses, the most dangerous and most capable of causing serious harm are the large strongyles parasite. They are considered to be the ones in charge of causing severe parasite related health problems in horses and they have the ability to kill. They have cylindrical bodies and unlike other parasites like tapeworms, strongyles have different sexes- male and female and can be told apart by the shape of their tails. The female lays eggs almost all the time and even though this makes it easier to detect any signs of infection, it does not stop it from growing. Several horses are affected by this disease every year, but careful research and study has been done and soon this disease will be a thing of the past and horses will be able to live a longer and healthier life. Since an encounter against internal parasites was faced, researchers have been quite interested in finding more about them. Internal parasites are small organisms that live a part of their life cycle within a host animal. These parasites feed on organs, cavities within the body and different tissues. It is from these that they gain nutrients and are able to grow. The damage that parasites cause differs with every parasite but they can cause serious health problems to the animal.

There are a variety of internal parasites that affect horses but only a few of them cause serious medical issues. One primary class or group of parasites that infect horses are nematodes and a kind of nematode is the strongyles. These parasites are the most significant and largest group of worms that affect horses and quite interestingly they are also the most dangerous. Strongyles are of two types: large and small. For the purpose of this research focus will be towards large strongyles. (Gore, Gore, and Giffen 692)

Heavy infections in all horses are common due to large strongyles but it is most prevalent in those ages between one to three years. Clinical signs of infection include a loss of conditions and anemia. Larvae of this kind of strongyles spend a long period of time moving through blood vessels and organs but they cause the most serious problems when they enter the cranial mesenteric artery and cause a cut off of blood supply to the intestines of the animal. These parasites are have big buccal cavities and they take huge bites and chew away the intestinal linings as they attach to them.

The large strongyles of horses are also called blood or palisade worms and fresh specimens of this parasite are notmally red brown in colour because of the blood they ingest from the bodies of their hosts. There are majorly three kinds of large strongyles and the eggs of each of these transform into infective larvae within three days if conditions are favourable and moisture is present. The three types of latge strongyles are: (Gore, Gore, and Giffen 692)

Stongyles Vulgaris

Strongyles Edentatus

Strongyles Equinus

All horses are at a potential risk of being infected by this parasite, especially the young ones. The strongyle infection can be transmitted very easily. A horse that is already infested with this disease will pass out strongyle eggs in its droppings. The eggs may be found in soil or on grass and can be swallowed by another horse. Once the eggs are swallowed they will move towards the intestines and the eggs will start to mature. Depending on the kind of strongyles that has entered the horses body, the extent to which it is affected can be determined.

Each of the three stongyles parasites have different life cycles and affect different areas within the horses body.

Strongyles Vulgaris -- almost 25 mm long. The eggs (L1) of this kind of strongyles transform into the infective larvae (L3) within a span of a few days. The larvae penetrate in to the intestinal mucosa of the horse and they change into stage four larvae in a few days. The fourth stage larvae seep into blood vessels and migrate into the arteries in a span of two weeks. After migrating into the arteries the larvae enter the anterior mesenteric arteries that carry blood full of oxygen to the arteries. The time interval that passes before the infection caused by the larvae can be detected is around six to eight months. L4 larvae turn into the L5 adult worm and then go on to damage the intestines severely.

Strongyles Edentatus -- upto 40 mm long. Edentatus larvae penetrate into the intestines and then travel to the liver. Here they molt into the fourth stage larvae and then after a period of nine weeks go to the peritoneum and back to the colon after forming nodules.

The larvae of this kind dig into the submucosa transform into the stage four larvae. They then swim and wander into the liver for around 6 weeks. They go from the liver to the abdominal organs as adults and then finally return to the colon. The perpatant period for these larvae is almost 9 months.
Strongyles Vulgaris is the most pathogenic among the three types of large strongyles. The reason for this is that it makes extensive migrations within the mesenteric arteries of the horse before it goes back to mature in the colon and cecum. almost 48 hours after the horse gets infected with strongyles vulgaris, mucosal haemorrhages begin in the ileum, colon and cecum. Seven days later, inflammation in the small intestinal arteries occur and a significant infiltration of neutrophils in the submucosa begins. Arteries being to extend along the ileo-cecal colic artery and further infiltration occurs after about three weeks. (Gore, Gore, and Giffen 692)

Strongyles vulgaris causes severe damage to the anterior mesenteric artery and its branches. The result of this is aneurysums, emboli and thrombi. Strongyles edentates larvae are concentrated in the cecal and portal veins. This causes the liver to swell up and appear red-blue. Chronic fibrosis may occur due to prolonged infestation and peritonitis may also occur. Strongyles equines forms submucosal cysts in the intestines, liver and pancreas

Horses that graze in pastures usually become infected with large strongyles when they take in feces when the larvae is at its infective stage. In fact most of transmission occurs only in pastures and very little infestation happens in stables or in dry lots. Once larvae is swallowed the horse is said to be infected with the disease. The eggs of the large strongyles require a suitable temperature to hatch in. The optimal temperature is 25 degrees Celsius, where 68% of the hatched larvae transform into infection causing stage three parasites within the span of five days. During cold winters or even in very hot temperatures the larvae of the strongyles becomes less able to cause infection. However that does not mean they are not living, they just lose the ability to infect greatly at the time.

The most common signs that are shown by the animal suggesting that it is infected by the strongyles parasite are:

Significant weight reduction

Anaemia and dull appearance of the animal coat

Diarrhoea

Colic

Enteritis

Lethargy

Decreased food intake

Pneumonia

If a horse shows some or all of these signs, care must be taken and diagnosis should be done. The longer it takes to control the infection, the lesser are the chances of recovery and the animal may die. ("illustrated veterinary encyclopedia for horsemen" 702)

The large strongyles is found throughout the world but in North America these parasites are of particular importance. Individual horses vary markedly when it comes to being susceptible to the large strongyles infection. Many horses have a very low fecal egg count even without any anthalmintic treatment while others who do receive treatment may have a greater fecal egg count. Areas with a colder climate are more prone to being affected by the disease. In the United States large stronglyes infestation increases during the winter months, while survival and multiplying chances of the eggs decreases when the weather is warmer. The prevalence of Strongyles Vulgaris is 80.5%, Strongyles Equinis is 9.8% and Strongyles Edentatus is 4.5%. Almost every horse is effected with either of the three types of strongyles in its lifetime. The presence of the parasite may or may not be outwardly shown but it is there. Due to the higher prevalence of Strongyles Vulgaris it should be considered in the diagnosis of the disease among horses.

There was a time when several horses used to die because of infestation by large strongyles and no successful form of treatment was available. But due to thorough research, the extent to which this disease prevails has significantly reduced. In order to make treatment successful anthalmintics have to be used. Horses that are affected with large strongyles often are also infested by small strongyles and therefore dewormers that kill both types of strongyles have to be used. Small strongyles are resistant to many medications and thus care has to be taken to use only those which attack both parasites effectively. The most common medicines used for treatment are pyrantel pamoate, oxibendazole and ivermectin. The…