There are a limited number of worm parasites that affect the pig
in the UK. The most prevalent of these is Ascaris suum, most
commonly associated with milk spot liver. Whilst severe
infestations are generally only associated with the poorest levels
of hygiene, modest levels of worms are present in many herds and
can have a significant effect on growth and feed efficiency.
Over the last 11years producers have had access to abattoir
reports (generated under the British Pig Health Scheme)
clarifying questions regarding milk spot liver condemnation
and the need for control of the disease on farm.
Fig1 Extensive milk spot in a single
liver. (Photo courtesy of British Pig Health
The different type of lesions that migrating
Ascariasis suum larvae. (Photo courtesy of British Pig Health
Scheme) Lymphonodular lesions in the centre (The 2 linear
marks are unrelated hepatic capsular scars)
Fig 2b Dispersed lymphoid infiltration
The lesions are found within the tissue of the liver - not just
on the surface - and can take the form of discrete lymphonodular
accumulations (Fig 2a) or more commonly dispersed lymphoid
accumulations - hence the term milk spot. Very early lesions may
contain a small haemorrhagic centre where the larva has penetrated
the capsule. Migrating larvae do not induce the formation of scar
tissue (fibrosis) which explains why they resolve in a matter of
weeks - generally fibrous tissue once formed is permanent.
Fig 3 Intestinal section completely blocked with Ascaris
suum worms - an extremely rare occurrence in growing pigs
Like all worm parasites, there is a complex life cycle with
Ascaris suum. The adult worm - which can be up to 40cm long -
lives in the small intestine of the pigs; in sows there may only be
a few of these worms present. Each adult produces huge
quantities of eggs intermittently and so examination of faeces for
worm eggs can be an unreliable method of diagnosis.
The specific feature of the eggs of most significance is that
they are covered with a sticky protective coat, which means that
the egg will survive many years outside the body of the host.
Its sticky nature makes it difficult to wash away by cleaning and
allows very easy spread between units on animal and mechanical
carriers. Birds are probably quite significant in the spread
of the eggs. The protective coat also renders the egg very
resistant to drying and disinfection. The only reliable
methods of destroying the eggs are with fire (flame gun) or caustic
soda. Oocide (Antec) may also be effective, although the
nature of this product is such that it is difficult to use in the
types of buildings where the eggs build up e.g. dry sow housing and
In the environment, the eggs undergo a maturation phase, which
occurs more rapidly in higher temperatures but will take at least 2
weeks. This phase leads to the hatching of a larva, which
will then be ingested by the pig. There is reason to believe
that the sucking pig receiving milk is resistant to these larvae
but the eggs can easily be picked up in the farrowing area, stuck
to the body and then mature and infect after weaning.
Once the larva has been swallowed, it will begin one of its most
destructive phases. The larva penetrates the wall of the
intestine and migrate around the body specifically first to the
liver and then to the lungs, all the while continually
maturing. Eventually the larvae will be coughed up,
re-swallowed and re-enter the gut to mature to adult worms.
The whole life cycle will take a minimum of 8 weeks.
Due to the effects of temperature on larval development outside
the body there is a marked seasonality in the incidence of milk
spot with levels typically rising in late summer and autumn. This
was particularly seen in 2010 with nearly a 3-fold increase in the
proportion of pigs affected Fig 4. However, the overall incidence
of lesions at least in mainstream commercial herds assessed under
BPHS has steadily declined. Fig 5.
Fig 4 Courtesy of BPHS
Fig 5 - courtesy of BPHS
Clinical signs seen with Ascaris infestation will depend on the
level of contamination and the site of the larvae or adult.
In the mild cases most commonly seen, the only evidence of the
worms is at slaughter where white specks are seen on the liver,
giving the term "milk spot". The liver is condemned as a
Migration through the lungs can present as coughing in growing
pigs - impossible to differentiate from enzootic pneumonia and the
lungs may contain petechial haemorrhages throughout the tissue at
slaughter although this is often grossly obscured by other
pathological lesions and slaughter process artifacts.
With very heavy infestation in growing pigs, the young mature
worms can block the intestine leading to vomiting, constipation,
jaundice, weight loss and death. This is extremely rare.
Milk spot lesions are themselves transient and will resolve
after 40 days. Therefore, if there is evidence of liver
damage at slaughter, the problem must be occurring in the finishing
area. Where liver damage is severe, weight loss, jaundice and
death can occur, although more typically there is a reduction in
growth of up to 10% and a degeneration in food conversion
efficiency of up to 13% in individuals.
Figure 6 Maturing young adult worms from the gut of a
clean pig at slaughter
Within the slaughter house it is also not uncommon to see
maturing worms present in the intestines within the gut room. Pigs
affected in this way must have been initially infested earlier in
life to allow complete migration and the full life cycle. There may
well be no or limited milk spot lesions in the liver as these have
Slaughter house monitoring
BPHS reports and to a lesser extent CCIR feedback provide a
reliable monitoring system for the levels of parasitism in
slaughter pigs. However, lesions need to be differentiated from
hepatic scaring which is the result of fibrous tissue build up in
the capsule of the liver and is of unknown cause.
Abattoir data collected as part of the
Batch Pig Health scheme indicates that up to 90% of herds show no
evidence of milk spot livers at slaughter and less than 5% of herds
have significant levels of livers affected.
Fig 7 - courtesy of BPHS
In the slaughterhouse batches of pigs fall broadly into four
- The vast majority of batches of slaughter pigs show no evidence
of milk spot suggesting the husbandry system is sufficient to
maintain full control of the parasite or any challenge occurs early
in life and lesions have resolved
- Batches containing livers of which many are affected with a
small number of lesions suggesting low levels of late
challenge either from a low contaminated environment or in a
situation where active control of a known problem situation is
- High levels of contamination in most livers representing a high
challenge situation. This is most likely to occur in poor hygiene
situations especially where there is continuous occupation and is
seen as a particular problem in the organic and free range sectors
or where pigs are reared in yards with non-cleanable floor
- The majority of livers are milk spot free but 1 or 2 are
heavily infested. Assuming that slapmarks are readable and are
accurately read, confirming the anomaly is genuine, the most likely
explanation can be that the affected pigs have been grown in a
separate environment from the main group. It is highly likely that
they will have come through a hospital area which is never cleaned
or emptied and contains compromised animals that are most likely to
be affected with parasites .
It should also be noted that there are other related non-pig
worm parasites which in specific circumstances can spill over into
pigs. The most likely scenario is where cats contaminate growing
pig environments and the cat roundworm -Toxocara cati - sheds eggs
that can produce larvae to be picked up by the pigs. These will
migrate to the liver inducing milk spot like lesions but
generally do not complete the life cycle.
It should also be noted that A suum being related to the dog and
cat roundworms (Toxocara species) have the potential to act as a
zoonosis and infect man. Migrating larvae in theory can cause
similar problems to these pet parasites (blindness etc) although it
is dubious as to whether this has ever been definitively
Where Ascaris has been demonstrated as a significant problem,
(for instance more than 25% livers condemned at slaughter) a
rigorous cleaning programme is needed to reduce the levels of
environmental contamination. Use of a detergent in the
cleaning will help to break down the sticky coat of the egg,
probably allowing eggs to be washed away. Conventional disinfection
is unlikely to have must effect. Final treatment of a washed
area with a flame gun is effective, allowing for the obvious health
and safety concerns.
Worming of adult sows is advisable to stop further production of
worm eggs, although alone is inadequate to control an established
problem where environmental contamination is most significant.
Where treatment of growing pigs proves necessary, it is
important that the product chosen is effective against the larvae
as well as the adults. Such products include the Avermectins
(Ivomec, Dectomax) and the benzimidazole group including
fenben,(Panacur:Intervet) and Flubendazole (Flubenol:
Janssen). Obviously, care must be taken over withdrawal
periods - particularly the injectable products. The Avermectins are
generally only justifiable on cost grounds if there is a need to
control sarcoptic mange as well as worm burdens.
Abattoir data collected as part of the Batch Pig Health scheme
indicates that approximately 70% of herds show no significant
evidence of milk spot livers at slaughter and only 5-8% of herds
have an incidence of more then 25% livers affected.