Equine parasites come in many sizes and shapes. Some commonly grow to 20cm in length and can even reach 50cm. In sufficient numbers, they can cause a potentially fatal intestinal blockage.
Many are very tiny indeed and can be present in their millions.
Some develop into highly efficient egg-laying machines. Infected horses can be passing out millions of eggs a day in their faeces, supporting a parasitic life cycle that will see other horses infected.
Most parasites take up residence in a horse's gut. It's not difficult to see why. There's a plentiful supply of nutrients and access is pretty easy, via the horse's mouth.
Exiting presents few problems, either. Their eggs simply pass out in the faeces, or, in some cases, the mature parasite simply lets go of the gut wall and journeys out through the intestine and into the pasture.
We refer to most of these gut-based parasites as worms. It's a good general term, but scientists have more precise scientifically accurate terms for them. For example, the different roundworms that infect a range of animals, including horses, are more properly called nematodes.
What exactly is a parasite? In its simplest terms, it is an animal that lives in another host animal. It draws sustenance from the host and will nearly always cause some harm in doing so. If enough parasites are present, their collective "harm" can cause disease. Scientists who study these worms are called parasitologists.
You'll notice our simple definition of a parasite uses the term "animal".
There are many organisms - some are actually too simple to even be called an organism - that fit neatly within the definition of a parasite, such as prions, viruses, bacteria, and some fungi.
Parasitologists are concerned with those organisms that fit into the definition of animals - albeit simple ones. These include the likes of nematodes, flatworms and protozoa.
They also include a bunch of arthropods - lice, fleas, ticks and the like - which have very different life cycles from those we generally refer to as worms.
About 150 different internal parasites are known to affect horses.
Obviously, organisms of interest to parasitologists are far more complex than the likes of bacteria or viruses, but there are other important differences.
A virus, for example, is a tiny bundle of genetic material which can multiply only within a host cell. Bacteria are a step up the evolutionary chain, being single-celled organisms but without a nucleus housed within a membrane.
Viruses and bacteria infect their subject and, if conditions are to their liking, multiply rapidly within that host, resulting in illness or disease once their numbers reach a high enough level.
Internal parasites, on the other hand, generally do not multiply within their host.
They may, once established, produce millions of eggs for returning to the pasture but, with only a few exceptions, the arrival of one parasite egg inside your horse's gut will result in one case of infection.
One swallowed roundworm egg, for example, will result in one roundworm within your horse's gut. If they ingest more parasites, numbers will naturally rise.
This is an important consideration. While viruses or bacteria will multiply rapidly within their hosts, the level of parasites found in any particular horse will be related directly to its exposure to the eggs of infective larvae in the paddock.
Like viruses or bacteria, once numbers reach a certain level, they will start to affect the health of the host animal, with symptoms ranging from coughing, ill-thrift and malnutrition to colic.
The aim of your parasite control programme is to keep these parasite numbers at a level where they cause no health issues.
 Infective larvae lie in wait on grass in the hope of being ingested by grazing horses.
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You will not eliminate all parasites from your horse's gut and maintain that glorious state of affairs. Some can be so plentiful (small strongyles) that horses may re-infect themselves from virtually their first mouthful in a paddock.
That said, horse owners can employ several important strategies to minimise horses' exposure in its environment.
Horse owners will be familiar with the brands of drenches available to them.
These products will contain one, or a combination of different worming agents, to tackle your horse's worm burden. They will be agents such as ivermectin or fenbendazole.
The various worming agents - and it has to be said that mankind does not have a vast array at its disposal - are not necessarily effective against all worm varieties, or indeed at all stages of each worm's lifecycle.
While proprietary worming agents will clearly identify which parasites they target, horse owners are well-advised to be familiar with the different worming chemicals available to them. Why? Because worm resistance is a greater problem in some drenches than others, and part of your strategy will be to change drenches from time to time to boost the effectiveness of your worming programme.
This means you'll be reading the fine-print on the drench containers so you understand what you're buying.
Regular faecal eggs counts will also allow you to monitor whether a particular drench is proving effective against the key worm varieties.
However, your drenching programme will be only one part of your worm control programme.
If you can minimise a horse's exposure to eggs in a paddock environment, the horse is likely to maintain a lower worm burden for longer, thus reducing your need for drenching.
However, none of this should be a matter of guesswork. That is why faecal egg counts should be a part of your worming strategy, as we'll discuss later.
Unfortunately, in the world of parasites, there are exceptions to the general rule. There exist several protozoal parasites which can cause your horse grief and these behave in a different way to the major non-amplifying parasite.
These tiny single-celled organisms are able to multiply within their host and, once numbers reach a critical level, cause disease. But even protozoas must eventually leave their host to complete their life cycle.
And, yes, there is a common parasite that can hang out on one host and reproduce generation after generation without the need to return to the environment. It is the humble louse. More on them later.
The take-home message for control is pretty simple. Drenches will play an important part in parasite control, but your management of your horses and their environment is just as important.
Not all worming agents are created equal and how you use them will have a major bearing on their effectiveness.
» Next: How tough will the battle be?
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