There is no
doubt about it, worming horses effectively can be complicated.
Horses play host to a number of different types of internal
parasite and, unfortunately, there is no one wormer which
will treat them all.
An effective,
sensible worming program is one which controls all the potentially
harmful parasites without overuse of drugs. To be able to
create such a program, we need to understand what worms we
are trying to control and which drugs to use for each.
The major
internal parasites of horses can be divided into three broad
groups – roundworms (nematodes), tapeworms (cestodes)
and fly larvae (insects). Each group creates its own
challenges for control.
Roundworms
This group
contains most of the common parasites affecting horses in
the UK.
| Large Redworm (Strongyle) – historically the most feared group of worms, their
larvae can block the arteries to the gut causing a potentially
life-threatening colic. Thankfully, as most modern wormers
control large strongyles, disease is now relatively uncommon. |
|
| Small Redworm (Cyathostomes) – this group
has taken over as the commonest parasite. Part of the
lifecycle involves the larvae becoming ‘encysted’ (forming
a protective membrane around themselves) in the gut wall.
They may remain in this state for months or even years
before suddenly all emerging together and causing severe
illness with diarrhoea, weight loss or colic. If this
‘larval cyathostominosis’ is not prevented, the success
rate for treatment is only about 50%. Unfortunately, the
encysted larvae are well protected and very few drugs
are able to remove this most important stage of the worm. |
|
| Large Roundworm (Ascarid) – a very
large worm (up to 40cm long) usually only found in animals
under a year of age. Large numbers of ascarids in the
small intestine of a foal may cause a partial, or even
complete, blockage. Migrating ascarid larvae may travel
to the liver and lungs and may cause respiratory disease
in foals. |
|
| Threadworm –
these worms cause dullness, ill-thrift and reduced weight
gain in foals and are interesting in that they can be
passed to the foal through the mare’s milk. This is
prevented by worming the pregnant mare. |
| Pinworm (Oxyuris)
– more of a nuisance worm than a serious health threat.
Pinworms, also known as seatworms, glue their eggs onto
the skin under the tail. This glue causes irritation
and horses rub their hindquarters – often to the extent
that they cause hair loss on the tail. |
|
| Lungworm – live
in the airways and are common in donkeys. They are usually
only a problem in horses which graze with donkeys.
|
| Tapeworm
There are, in fact, 3 species
of tapeworm which infect horses in the UK. They have
been associated with several serious types of colic
if present in large numbers. Contrary to popular opinion,
equine tapeworms seldom reach over 5 cm in length.
Tapeworms
are picked up when grazing horses inadvertently eat
forage mites which contain the intermediate stage of
the worm. Studies have shown that up to 69% of horses
in the UK are infected with tapeworms. |
|
| Fly Larvae
Stomach bots are the larvae of
the bot fly. This is the fly which lays the incredibly
sticky small yellow eggs on the legs of horses in the
summer. These eggs then enter the horse’s mouth and
develop into reddish-brown ‘maggots’ in the stomach.
They spend the winter inside the horse before being
passed in the dung the following spring. |
|
A
well-planned worming program should aim to control all the
parasites already mentioned. At first glance, the array of
wormers to choose from appears mind-boggling.
However,
by looking at the active ingredient it can be seen that many
of the brands contain the same drug and perform the same function.
Below is a summary of the active ingredients available in
the commonly used wormers.
Fenbendazole
A 5 day course
is effective against encysted small redworms. Resistance to
this class of wormer is very common in small redworms in the
UK. Routine use - repeat every 8 weeks. No activity against
tapeworms or bots.
Pyrantel
Frequently
used as a double dose for tapeworms. As a routine wormer it
has the limitation that it does not kill larval worms of most
species. Additionally there is some resistance to pyrantel
in the UK in both large and small redworms. Routine use -
repeat every 4-8 weeks. No activity against bots or encysted
small redworms.
Ivermectin
Historically,
a good general purpose wormer which treats most roundworms
and bots, including those which are resistant to fenbendazole
and pyrantel.
Routine use
– repeat every 8-10 weeks. No activity against tapeworms or
encysted small redworms.
Moxidectin (Equest)
The longest
lasting wormer available – repeat every 13 weeks. No reported
resistance in the UK. Kills approximately
80% of developing encysted small
redworms with a
single dose. Also, treats most roundworms and bots, including
those which are resistant to fenbendazole and pyrantel. No
activity against tapeworms.
Praziquantel (Equitape)
Praziquantel
is a specific tapeworm treatment and is effective against
all three species of tapeworms found in the UK with a standard
dose. Routine use – once or twice per year for tapeworms.
A worming
program may be created using a combination of any of these
drugs, but there are some general principles that should be
adhered to:
1.
Treatment
for encysted small redworms should be included at least once
per year, ideally in the autumn/winter.
2.
Treatment
for tapeworms should be included in the autumn and spring.
3.
Bot
treatments should be timed for the autumn/winter, after the
adult flies have died.
4.
Do
not underdose. Use a weigh tape.
5.
All
horses on a single yard should be treated at the same time,
using the same type of wormer.
Well thought
out use of wormers along with good pasture management, such
as muck-picking and resting fields, allows for cost-effective
worm control. Your veterinary surgeon will be able to advise
you on a yearly program tailored for your needs.
Finally, an
example of a worming programme. The example below effectively
treats all the parasites mentioned above and provides year-round
control of roundworm egg output. It also eliminates the need
for complications such as double-dosing for tapeworm or five
day courses for encysted small redworm.
