Tapeworms in Horses
Tapeworms belong to a group of parasites
known as cestodes (or flatworms). Three
species of tapeworms affect horses in the UK, the most common of which is Anoplocephala perfoliata (1). Tapeworms
are short and flat in appearance, growing up to 8cm long and 1.5 cm wide. They consist of a small round head which
attaches the worm to the wall of the gut, and a body formed of a chain of egg
containing segments known as proglottids.
Life
cycle
Adult
tapeworms release egg-containing body segments into the host horse’s
intestine. These are passed in the
faeces and disintegrate, releasing large numbers of eggs into the
environment. Eggs are eaten by tiny
forage (oribatid) mites, which can be present in very large numbers on pastures. Over a period of approximately two to four
months, these eggs develop within the mite into an infective intermediate stage
of the tapeworm. As the horses graze,
they will inevitably ingest the mites, releasing the infective stage of the
worm into the intestines. The immature
worm migrates to the ileo-caecal junction (the join between the small and large
intestines). Here it develops into the
adult form which, after one to two months, starts to produce egg filled
segments, continuing the life cycle.
Effects
of tapeworm infestation
For many
years tapeworms were considered to be harmless to the host horse. The arrival of more reliable methods for (a):
identifying horses infected with tapeworm, and (b) determining severity of
parasite burden (see diagnosis below) has changed this perception of tapeworm
infections. In a study of UK horses,
researchers discovered that infection with the most common species of tapeworm,
Anoplocephala perfoliata, increases the risk of two types of colic-spasmodic colic
and ileal impaction colic. It also demonstrated that the heavier the tapeworm
burden, the higher the risk of these types of colic.1 Although the level of risk should be kept in
perspective, effectively minimising
tapeworm burden should decrease the risk of tapeworm associated colic.
Diagnosis:
Diagnosing tapeworm
infection can be tricky in horses. Clinical signs are not a reliable indicator,
as horses can have tapeworm burdens and appear clinically healthy. Unfortunately, counting tapeworm eggs in a
faecal worm egg count (FWEC) is also unreliable, as the eggs are encased in the
tapeworm segment, rather than being distributed evenly throughout the
droppings. Also, egg filled tapeworm
segments are only shed intermittently.
As a result, a negative result on a FWEC does not necessarily mean a
horse is free of tapeworm.
Fortunately
we can use a blood sample to test for levels of tapeworm infection- a method
known as the tapeworm antibody test. A horse that has a tapeworm infection will
produce antibodies to the tapeworm, which can be detected in the blood. Instead of giving the exact number of
tapeworms, the test indicates the intensity of infection - the level of
antibodies indicates whether the burden is low, medium or high.
While the
blood test is useful for assessing whether your horse has been infected with
tapeworms in the past, it is not a reliable indicator of response to treatment.
This is because it takes at least 16 weeks for the antibody levels to return to
normal after the worms have been killed (2).
Controlling tapeworm
As well as
the appropriate use of worming treatments (see below), it is important to
manage pastures carefully to minimize the parasite burden that horses are
exposed to. The aim is to prevent contact with faecally contaminated pasture or
feed. This could be achieved by regular
removal of faeces from pasture, prevention of over-stocking, resting heavily
grazed pasture and rotating pasture with other species-eg sheep or cows. There
are two different approaches to tackling tapeworm. Treatment can be targeted,
following a blood test, or administered within an annual worming program.
With
targeted treatment, a decision is made on (a): whether to treat, and (b): with
what product. This is based on the horse’s worming history alongside combined
FWEC and blood test results. Targeted treatment identifies horses which are at
risk of high levels of worm infection. This helps to ensure that the most
appropriate wormers are only used when necessary, thus reducing the risk of
worms becoming resistant to treatment.
Using this
method, horses are blood-tested for tapeworms once a year. Those with a high
burden are treated after the test and then again six months later. The blood
sample is then repeated the following year. New horses entering the yard should
have a blood test and a FWEC to determine whether worming is required, or be
routinely treated for both tapeworms and roundworms and stabled for 48 hours,
before they are turned out with others.
In some
yards, targeted worming may not be suitable or practical. In these instances,
all grazing horses should be treated for tapeworms at least once yearly, and
every six months in high-risk animals or those shown to have high levels of
infection1.
,
There is
little evidence of infection being linked to seasonality, so timing of tapeworm
treatment is not critical. As risk is higher after prolonged periods of
grazing, horses are often treated at the end of a summer out at grass.
It is
important to be aware that not all wormers kill tapeworms. The parasites can
mainly be treated with one of two drugs — praziquantel or pyrantel. Fortunately both drugs are very effective and
cause death and detachment of the tapeworms from the gut wall. They are available as single products or in
combination with other drugs that target different types of worms.
In summary,
careful pasture management combined with correct testing and treatment will
ensure that tapeworm is effectively controlled. thus reducing risk of
associated disease.
1. Proudman
C. and Matthews J. Control of intestinal parasites in horses. In Practice (2000) 22: 90 – 97
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