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Larval Cyathostominosis in Horses: Please, Don’t Panic.

Generally speaking, worms do not cause serious sickness or death in horses. This is because we now have access to excellent deworming treatments, which have significantly reduced the incidents of worm-based colics in horses.

That being said, small strongyles (cyathostomes), are still capable of causing serious illness through their ability to encyst in the intestinal wall before emerging at a later time. It is this emergence of the larvae that is the cause of larval cyathostominosis and it can actually occur in three different ways: acute, chronic (leading to colitis/peritonitis) and subclinical. It is very important to note here, that encysted larvae cannot be diagnosed. They do not lay eggs (so no detection of a FEC), and when encysted they cause very little damage so there is no changes in bloods that can currently be detected.

Acute larval cyathostominosis is a serious illness that occurs when there is a mass emergence of thousands of larvae all at one time. It presents suddenly, with symptoms of profuse diarrhoea (sometimes with worms present in the manure, as the horse attempts to ‘shed’ out as many worms as possible) and colic related to the pain caused by the emergence. Horses will become dehydrated very quickly, and will likely be lethargic. Treatment should be through your vet, with fluid replacement and anti-inflammatory medication to aid in pain relief and to limit the damage caused by the larvae. An anthelmintic with larvicidal properties should also be administered – all under the guidance of your vet.

Chronic larval cyathostominosis occurs when the larvae slowly emerge over a period of time. This will lead to a constant irritation of the lining of the intestine and may go entirely unnoticed by horse owners. The main issue of chronic larval emergence is that the low level, but constant, irritation and inflammation of the gut will lead to a secondary opportunistic infection by common gut bacteria. Infection of the gut lining by bad bacteria will lead to the classic colitis/peritonitis symptoms: fever, inappetence, pain related colic, lethargy. If caught early, this can be treated via anti-inflammatory medication and a round of antibiotics. If left too long more intense treatment (e.g. IV-antibiotics) will be required. It is very important to note here, that the colitis/peritonitis is being caused by bacteria – not by the worms. Additionally, identifying the causative agents (i.e. what bacteria) of these cases of colitis/peritonitis is impossible as they are simply caused by an infection with bacteria already present and common in the intestines. (Just a note here so that you don’t panic. In a healthy gut, bacteria such as E. coli and Salmonella happily live their life playing an important role in food fermentation etc, and it is only when there is a disruption to the normal balance that they have an opportunity to overgrow and cause a problem. This is much the same as how if you keep scratching at a mosquito bite, it will break the skin and give your normal skin bacteria access to somewhere very nice for them to grow.)

This chronic larval cythastominosis does not occur overnight. Horse owners may be able to detect signs of it up to a month out from it developing into colitis. Look for horses that are not thriving, or if a horse starts to become girthy or irritable when worked, it may be a case of chronic larval emergence. If you are worried, have a chat to your vet or FEC service about the chances of it being larval cythastominosis and what you can do to treat/prevent it.

Subclinical larval cyathostominosis is the most common – where, ever so slowly, worms are emerging from the gut lining. This is actually perfectly normal as part of the relationship between the horse’s immune system and the lifecycle of the worm. If a horse is in excellent health, a subclinical (or asymptomatic) case of larval cyathostominosis will not cause any issues and will likely never be detected by the horse owner.

How can you avoid acute or chronic cases of larval cyathostominosis?

Keep your paddocks clean. This is the number 1, most important, most useful, way to prevent this from happening. When horses ingest larvae off the pasture, the larvae have two choices. One: mature into an adult worm or two: encyst in the intestinal wall. This choice is actually often made by the already existing population of adult worms in the gut – an overload of adult worms will trigger an immune response and so it is in the best interest of the larvae to wait their turn by encysting. The more larvae present on pasture, the more larvae will be ingested and forced into cysts within the horse, leading to the chance of a mass emergence later on.

Keeping your paddocks clean doesn’t necessarily mean poo-pick up very day (which is not possible for many people). What you should be mindful of is to not overstock, to cross-graze, to rest your paddocks for appropriate amounts of time, to worm your high shedding horses when needed and to keep young horses (or those at higher risk), on the ‘cleanest’ pastures you have.

It is much better to prevent larval cyathostominosis occurring, than thinking you can detect it and treat it. If you have clean paddocks and you horses have good nutrition then you have already significantly minimised the chances of it happening.

Most importantly, do not feel tempted to treat for encysted larvae simply because the potential of larval cyathostominosis worries you. Encysted larvae can only be removed with either a treatment of moxidectin, or a 5-day course of fenbendazole. 5-day courses of fenbendazole are actually extremely harsh on the lining of the gut and should only be given under guidance of a vet. In contrast, a single dose of moxidectin will have a similar result, however, I will stick to my guns here, and say only use moxidectin when you really need to (i.e. not for routine treatments) due to the ever increasing drug resistance developing.

If you want to avoid larval cyathostominosis occurring in your horses here are the three most important things to do:

1. Keep your pastures clean.

2. Monitor your horses for gut discomfort.

3. Get advice from your vet/FEC service if you are worried before you make any treatment decisions. The “just in case” attitude is driving us very quickly down the path of drug resistance.

And lastly – don’t panic. Preventing larval cyathostominosis is far more a long-term management issue than a quick-fix treatment. If you are mindful of paddock conditions, EPGs of your horses, their risk of developing LC (young horses, immuno-compromised horses) and every deworming treatment you give, it is very likely you will never have a problem.


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