| What is the main
parasite that can affect my horse? |
| The most common parasite to affect
horses today is the small redworm (small strongyle/cyathostomin),
which can cause diarrhoea, rapid and severe weight loss,
as well as life-threatening colic. |
| I understand that
encysted small redworms are the biggest threat to my horse
– what treatments are available to control this
parasitic stage? |
Traditionally, the only licensed treatment
to control encysted small redworm was a 5-day course of
a fenbendazole–based wormer. However, in 2003 a
moxidectin-based wormer (EQUEST) was granted a licence
for the control of all stages of encysted small redworm
with a unique single standard dose.
It is important to note that with the fact that there
is widespread small redworm resistance to benzimidazole-based
wormers (i.e. fenbendazole), use of fenbendazole is not
advisable where resistance is present (Coles G et al,
1999).
Furthermore recent research has found that a 5-day course of fenbendazole may cause damage to the horse's gut wall, mirroring the situation caused by the actual mass emergence of encysted larvae (Steinbach T et al, 2006). Whereas a single dose of moxidectin was found in the same research not to cause severe damage to the horse's gut wall. |
| When should I worm
my horse? |
| Horses should be wormed routinely
throughout the year for the control of roundworms, with
strategic treatments given at specific times for the control
of tapeworms, bots and most importantly encysted small
redworm. Please refer to the When to worm with what page
for further information. |
When do I need to
worm my horse for tapeworms and
do I need to double dose? |
| Treatment for tapeworm is recommended every 6 months (Proudman C, 1994). As exposure to tapeworm is greatest during periods of prolonged grazing, treatment should be undertaken in the autumn following summer turnout, and thereafter 6 months later in the spring. Tapeworm can be controlled by a single dose of a praziquantel-based wormer or (e.g. EQUEST PRAMOX) or by a double dose of a pyrantel-based wormer. |
| My horse has a heavy
worm burden? How do I start its treatment? |
| If you are concerned that your horse
has a very high worm burden it is advisable to contact
your veterinary surgeon to establish a treatment specific
to your horse’s needs. One possible strategy would
be to use a wormer that controls adult worms only, such
as a pyrantel-based wormer. This treatment can then be
followed 7-10 days later with a moxidectin-based wormer
to treat for the remaining larval parasites. A tapeworm
treatment should also be given if necessary. |
| What is the difference
between strategic and routine worming? |
| A strategic worming programme involves
the use of Faecal Worm Egg Counts (FWEC) throughout the
year with extended dosing interval periods between treatments.
Horses on this programme are treated only when FWEC results
are above a specified level i.e. 200 eggs per gram (epg).
Care must be taken with this strategy, as FWEC do not
give an accurate measure of the worm burden; not accounting
for larval stages including encysted small redworm larvae
present in the horse. Routine worming is the use of equine
anthelmintics throughout the year in accordance with dosing
intervals recommended by the manufacturer to treat for
roundworms, tapeworms and bots. |
| Why do Faecal Worm
Egg Counts (FWEC) not diagnose tapeworm? |
| Tapeworms release their eggs sporadically
(irregular intervals) and so FWEC could show a false negative
i.e. the result would indicate no eggs are present but
in reality the horse does have a tapeworm burden. A tapeworm
antigen blood test will give some indication of any tapeworm
burden. |
| What happens if
I overdose my horse? |
| Although a slight overdose of an anthelmintic
is unlikely to have harmful effects, as with all drugs,
it is important to dose accurately and in accordance with
the manufacturer’s recommendations or those given
by your vet, especially in low bodyweight foals or pony
foals. |
| What about underdosing? |
| Underdosing will not do the horse
any harm, but you would in fact be administering a sub
therapeutic level of the drug and thereby exposing the
worms to the drug but perhaps not at a sufficient dosage
to kill them. Worms that survive treatment may pass on
their “immunity” to subsequent generations
and so causing the potential for resistance to develop
to that drug. |
| How important is
it to weigh horses? Can’t I just guess his weight
or give him a whole syringe? |
| Establishing a horse’s weight
by guessing and not using a weigh tape or weigh bridge,
means you risk giving your horse an insufficient dose
of wormer. By not giving your horse enough treatment for
its body weight you risk giving a sub-lethal dose of the
drug which will not be enough to kill all resident worms.
These will not only be left to continue their life cycle
and breed in your horse, but are also given the opportunity
to build up immunity and resistance by being exposed to,
but not killed by the drug. A small overdose however,
is unlikely to do your horse any harm. |
| If my horse will
not take the syringe easily, how can I give him the wormer? |
In-feed wormers are available. However,
if the active ingredient or product that you plan to use
is only available in a syringe, then the correct dose
of product can be administered and mixed into the horse’s
feed. It is very important that you ensure the horse eats
all of the feed and therefore gets the full dose of wormer.
If wormer is added to the horse’s feed, dogs and
cats must be kept away and the feed container cleaned
thoroughly afterwards. |
| If I don’t
use a whole syringe, can I use the remaining wormer next
time? |
| Always refer to the packaging of the
product to see how long the product can be used once open.
Remember the product must be stored according to the package
instructions and not accessible to children, or other
animals. Empty syringes must be disposed of safely. |
| Do I need to rotate
wormers each grazing season to combat resistance? |
The idea to rotate wormers was actually
derived from the sheep industry, a strategy that has not
yet been widely shown in practice to make a significant
difference in the length of time for resistance to occur.
In fact, computer modelling has suggested that either
alternative of ‘no drug rotation’ or ‘more
frequent drug rotation’ make no difference to the
onset of resistance (Barnes E et al, 1995).
There are a number of factors which may be involved in
the development of resistance, with it having been suggested
that the most important factors in delaying the onset
of resistance is the anthelmintic’s initial kill
of resident resistant worms, and secondly reducing the
frequency of use of anthelmintic (Dobson R et al, 1996).
This means that the ideal anthelmintic is a highly potent
one to which there is no resistance, and which can be
given as infrequently as possible.
With widespread resistance to benzimidazole-based wormers,
and reported resistance to pyrantel and ivermectin-based
wormers, there may be little room for choice as to what
to rotate onto, as such it is recommended that you consult
your vet prior to using one of these.
It has also been shown that of the macrocyclic lactones
(moxidectin and ivermectin), moxidectin selects more slowly
for resistance (Ranjan S et al) and should therefore be
your first choice when using a macrocyclic lactone wormer.
An alternative strategy to rotation is to use a highly
effective wormer with no resistance in horses, such as
a moxidectin-based wormer. By using moxidectin, you can
worm your horse as infrequently as possible (13 week dosing
interval), while still protecting his health.
In addition, pasture management and faecal worm egg counts
(FWEC) can also play a role in delaying the onset of resistance.
Removing dung from the pasture removes most sources of
re-infection, including resistant worms (Wolstenholme
A et al, 2004) and so breaking their lifecycle. It is
possible to extend dosing intervals by using FWEC: only
treating animals when they are producing eggs above a
specific level. It is important to note, however, that
FWEC do not give an indication of larval burden as eggs
are only produced by adult worms, therefore FWEC will
not give an indication of encysted small redworm larvae
harbouring inside your horse. |
| How frequently do
I need to pick up the dung from the pasture? |
| Dung should be removed regularly,
at least twice weekly during the grazing season, and once
a week between November and March. Although harsh frosts
may kill infective larvae on the pasture, with the milder
and wetter climate now experienced in the UK, high levels
of infective larvae can occur at any time of the year
(Love S, 2003). Thus, there is an increased likelihood
of grazing animals acquiring parasite burdens throughout
the year. Therefore if the autumn and winter remain mild
and wet, the twice-weekly removal of dung will be necessary
all year round. |
| Should I worm my
pregnant mare? |
| It is very important to worm pregnant
mares, not only for their own health, but also that of
the foal when it is born. Always check that the product
you want to use is licensed to treat pregnant and lactating
mares and try to worm her before foaling and throughout
lactation according to manufacturer’s instructions.
For more information, see Pregnant Mares, Foals and New
Horses. |
| From what age should
I start worming my foal? |
| As foals have a low tolerance to worms they can quickly acquire massive worm burdens; therefore worming of foals should start at four to six weeks of age, and repeated every four weeks until 6 months of age. Thereafter worming can continue as per wormers recommended dosing interval. Always check 'the age from use' of any wormer prior to use. For more information, see Pregnant Mares, Foals and New Horses. |
