Parasitism – are you under control?
There are 2 main types of parasite affecting our horses. These are ecto-parasites (that live on the outside of the host) and
endo-parasites (living and feeding on the inside). A typical example of an ecto-parasite could be a louse infestation in the
horse’s coat. The typical endoparasite problem for horses is a redworm burden in the intestine.
Parasites are living off their hosts, and often at the detriment of the host animal. Some parasites may have a mutually
beneficial arrangement with their hosts whereby they do not cause damage to the host, but impart some benefit to them.
This is called a symbiotic relationship, and it is quite rare in our domestic equines. There are some schools of thought that
mild worm burden may help horses with building up an immune system, which could be seen as a symbiotic relationship.
Overall, most equine parasites cause either a nuisance or damage to our horses.
Endo-parasites
Diagnosis of Worms
Sometimes we can reach a suspicion of worm infestation, through careful examination of the history and symptoms. Worm
damage can have devastating consequences for the function of the intestine, sometimes years after the horse was exposed
to large numbers of worms.
Most worms will only be detected in the droppings when they die. Even then, they can be so small that the naked eye would
not necessarily notice them. The only time when worms shed eggs is when they are mature and the only reliable way to
diagnose a presence of worms in the droppings is to check for worm eggs under a microscope. Faeces samples submitted for
worm egg counts should be taken just before worming is due.
Faecal egg counts can be negative in spite of the horse having worms inside them. There is a period of 4-12 weeks after
ingesting new worm eggs or larvae where the worms are immature and not laying eggs. Redworms is a particular type of
worm, which may arrest its development over the winter period (hibernating larvae). This could result in negative worm egg
counts, but horses that may still be at risk of parasitic disease.
In spite of these shortcomings of egg counts to monitor worm burdens, it is by far the best way to manage worm control in
this day and age. Worm egg counts are much more effective to monitor whether a whole herd is under control. Samples of all
horses in a herd should be monitored, and that will keep individual horses safe, because we can deduce that they are not
over-exposed to worms.
By monitoring egg counts and treating horses with moderate to high burdens all horses in the herd are protected and
resistance of the parasite to treatments is avoided
Prevention or cure?
Daily muck picking of the fields can help prevent larvae spreading. This is a cumbersome task and a few people manage to
keep this chore going. The droppings should not be dispersed over the paddocks unless very hot and dry weather can be
guaranteed. Grazing other species, such as sheep, in the same fields will have some diluting effect on the amount of worms
but will not prevent infection. Similarly, extensive grazing is beneficial.
Strategic worming programs should be implemented in most horse populations to avoid parasitic disease, which can at its
worst lead to death. The worming programs will serve three main purposes, namely preventing disease in individual horses,
avoiding contamination of the pastures and also avoidance of the build up of the resistance against wormers.
Although some exposure to parasites can be beneficial by helping horses build up immunity against them, some horses do
not build up good protection and become ill. Many parasites will cause scarring of the intestinal tract which can result in poor
digestion and weight loss, colic and even death.
To monitor whether worm control programs are effective, dung samples of at least 20% of the horses in each herd should be
analysed by the practice laboratory.
Ecto-Parasites
Leg Mites (Chorioptic Mange)
This mite commonly infects the legs of feathered horses and causes itchiness, skin irritation and crusty lesions. The
discomfort often manifests itself as frequent stamping of the feet, or rubbing of the legs, either against one another or
against objects. The mites can also lead to mud-fever like lesions. The lesions can spread to the groin and abdomen. Some
horses have mites without showing too many signs, which is why it is sometimes necessary to treat all in-contact horses and
disinfect their environment. Leg mange can be confirmed by analysing skin scrapes or sticky smears under a microscope. This
mite does not spread to humans. Modern sprays and more recently, injections, have made this condition easier to treat and
many cobs or draught horses can be spared suffering thanks to these anti-parasitic treatments. If you suspect that your horse
could suffer from leg mites, you must contact your vet urgently to avoid further suffering immediately.
Louse Infestation
Lice are parasites that fall into two major groups, ‘biting’ lice and ‘sucking’ lice. Lice are most prevalent in the winter months
when horses are congregated together in warm stables and wear rugs. The lice are spread by direct contact via grooming and
tack. The ‘biting’ lice prefer the back and the sides of the neck, while the ‘sucking’ lice can be mainly found on the neck, tail
and limbs.
All lice cause itchiness, which produces irritation, rubbing and biting and very often a ‘moth-eaten’ appearance of the coat.
Some severely affected horses can lose condition and have anaemia. Nits can often be found in the coat and occasionally
some of the lice can actually be seen crawling around.
We spend a lot of time, money and effort trying to control parasite burdens in our horse’s world. An insight in how to keep
our horses free from parasite disease can help with managing them. In order to keep them disease free, we may have to
adopt a zero tolerance policy to eradicate the parasites in our animal’s environments, or it may be more useful to keep the
numbers of parasites low (and below the level where they harm our horses), or to treat them and eradicate parasites when
we see early signs of their harmful effects. An example of this last tactic would be treating for leg mites when the first signs
of itching are seen.