Important internal parasites of the horse.
Small strongyles - a group of parasites broadly known as cyathostomes or small redworm, which are now recognised as a major causal agent of poor performance and loss of condition in the horse. They can also cause subclinical disease which may go unrecognised and untreated. The cyathostomes are highly prolific with it being possible for a horse to carry many thousands of worms with the mature females (inhabiting the large intestine) producing eggs which are released into the gut and expelled onto the pasture in the faeces. They can complete their life cycle in only 6-8 weeks, enabling a massive growth in population and subsequent spread through a group of horses.
Eggs shed onto the pasture develop into infective larvae (also known as third-stage or L3 ) which crawl up stems of grass and are ingested by the horse as it grazes. They pass along the digestive tract and, once in the large intestine, burrow into the walls and complete their development through to emerge as mature adults capable of starting the cycle again. HOWEVER, there is a further problem associated with these worms. Some larvae burrow into the intestinal wall and undergo a period of arrested development whereby they encyst and can remain in this state for over 2 years, even in horses receiving routine worming chemicals. The larvae eventually emerge, sometimes in very large numbers, leaving a damaged and ulcerated gut wall.
A further problem associated with small strongyles is the development of resistance to certain worming chemicals. This is aided by the short life cycle which, when subjected to doses of worming chemicals, creates a 'selection pressure' so that worms with some degree of resistance to the chemical are more likely to survive than the others, so they go on to produce more worms with this same trait, and so on until a population of resistant worms has developed. Naturally, this presents certain problems:
the owner, being unaware of resistance, continues to worm using the chemical to which resistance has developed, believes that his/her animal is free of worms. Numbers of resistant worms build up through successive cycles of reinfection leading to a slow decline in fitness of the horse.
the choice of chemicals for use in the worming strategy becomes limited so that it becomes difficult or impossible to target certain developmental stages of some parasites.
the horse carrying resistant worms sheds eggs onto shared grazing where they are picked up by other horses and ponies who then become carriers of resistant worms. The owners of these horses may be worming diligently and believe that their horses are worm free.......
the horse travels to shows or camps both in and out of the district and sheds eggs. Horses grazing these paddocks later pick up the larvae.......... by this means, resistance spreads and many owners will not even be aware that they have a problem building up.
To monitor for resistance many owners like to include an egg count reduction test as part of their worming routine
Large strongyles - large redworms, bloodworm. As with the small strongyles, eggs passed out onto the pasture hatch and develop into infectious larvae which are ingested during grazing. The larvae burrow into the gut wall and make their way into smaller arteries, then to the mesenteric artery which supplies the gut. They remain in this migratory phase for around 6 months and can cause constriction of the arteries, reducing the supply of blood to the gut and leading to tissue death. Blood clots may also form which dislodge and travel to other organs where they can have devastating results. The larvae develop into adults which attach to the intestinal wall and suck blood, causing inflammation and ulceration.
Equine Tapeworm - Anoplocephala perfoliata. This parasite can grow to about 8cm in length and attaches to the gut using four suckers. It is usually found at the ileo-caecal junction where the large and small intestine join. There is normally no sign of infection although horses may show some condition loss and if untreated the intestinal wall can be grossly affected leading to more serious problems.
In order for the life cycle of the tapeworm to be completed, it is essential for the eggs to enter a secondary host, in this case a small oribatid mite found in the pasture. Eggs are passed out via the horse's faeces and eaten by the mites inside which they hatch and mature over 2-4 months. The mites are then eaten by horses as they graze, and the tapeworm continues its development to maturity within the horse over a period of months.
Roundworm - Ascarid, Parascaris equorum. Signs of infection with these parasites are most often seen in young horses as immunity has usually built up by the time the horse reaches 2 years of age. The worms are very long and mature adult females produce vast numbers of thick-walled, tough, sticky eggs which can adhere to anything they come into contact with and can remain viable for several years. The life cycle takes approximately 10 weeks and begins when the foal ingests the infective eggs which then hatch and the larvae burrow through the intestinal wall and travel to the liver. One week later, the larvae are carried in the blood to the lungs, are coughed up and reswallowed, ending up in the small intestine where they finish their development to adults. Although there is often no clinical sign of infestation, large numbers of roundworms may slow down movement of food through the gut, potentially leading to problems.
Bots - Gasterophilus intestinalis. Although these are not actual "worms", but are in fact the larvae of a fly, they are still important parasites of the horse. The adult female bot fly deposits her small, very sticky cream/yellow eggs on the individual hairs of the legs, throat and abdomen of the horse during late summer, often causing extreme agitation to the horse despite the act being painless. The larvae hatch, are licked off by the horse, and, after burrowing within the oral cavity and passing through 2 maturation stages, eventually travel to the stomach where they attach to the stomach lining. They are responsible for the formation of pits in the stomach wall which may perforate and can, in some cases, lead to peritonitis or rupture of the stomach if the infestation level is high. They remain here for about 9-10 months, growing to around 2cm in length, before detaching and being passed out in faeces after which they burrow into loose soil, pupate and emerge as adults 3 weeks later. Bots do not reach adulthood and do not shed eggs whilst in the horse, so they can not be detected using a faecal worm-egg count.