Young piglets are often born with the
whole body trembling and shaking to a variable extent. The
consequences of this trembling will depend on its severity. The
condition is known as Conditional Tremor or Myogenesis
congenita.
It is widely assumed that, because
pigs are born affected, it must be an inherited (genetic) condition
and a common reaction is to blame and cull the boar. This is only
rarely the cause.
THE DISEASE
Five specific types of Congenital
Tremor are recognised. The first two are extremely rare and are
inherited conditions:
| A sex linked inherited
defect occurring in pure Landrace pigs affecting only a small
proportion of males (Type A III).
| An inherited recessive
genetic defect in pure Saddleback pigs tending to affect only 25%
of a litter (Type A IV). It is unclear whether this genetic defect
is present in the Hampshire breed which was originally part of the
Saddleback group (the others being the Essex and Wessex breeds)
The most common causes are infectious agents
which attack the na•ve gilt or sow, usually in the first half of
pregnancy and can be one of two viruses:
| Classic Swine Fever (Hog Cholera). Over the last 15 years in
some outbreaks in Germany, associated with mild strains, Congenital
Tremor was the only sign seen with Classical Swine Fever infection.
In the recent UK outbreaks in 2000, Congenital Tremor was not seen.
(Type A I.)
| Congenital Tremor Virus. Despite extensive research work over
many years, the specific causal agent remains unclear. There is
some suggestion that it may be Porcine Circovirus type II or even
type I but this is doubtful. This type of Congenital Tremor (Type A
II) is by far and away the most common form occurring in the UK
herd and can present as:
1. A sporadic disease affecting odd litters – usually most of
the pigs within a litter are affected.
2. An outbreak over 6-8 weeks affecting a proportion of all
farrowings.
3. An ongoing problem affecting only gilt litters.
In addition to these four types of Congenital Tremor, specific
toxins to which the pregnant sow is exposed can cause a further
type (Type A V). Organophosphates have been specifically implicated
in some outbreaks.
 |
|---|
Pigs affected with CT do not tremble
when asleep |
CLINICAL SIGNS
Piglets are usually born shaking,
trembling and nodding whilst active stimulation (eg handling,
noise) will often exaggerate the shaking; conversely it will tend
to stop when the piglets fall asleep. It is common for trembling
pigs to have a simultaneous splay leg of the hind legs (fig 2). In
rare cases the trembling may not become apparent until 24-48 hours
of age. There is no fever.
The effect on the pig will depend upon
the degree of trembling and its impact on the ability of the pig to
suck on a teat. Mild cases may be barely inhibited, whilst severely
affected pigs can only suck if physically held onto the teat. Where
a high level of stockmanship can be applied, mortality levels can
be kept within manageable levels Ð 15-20%. Particular attention is
needed to ensure colostrum intake in the first 6-12 hours of life
but thereafter assistance may be required to prevent starvation
over the prolonged period on the sow. In severe outbreaks – most
commonly encountered in gilt herds or where a high level of gilt
intake has occurred (eg sudden herd expansion) – mortality levels
can be much higher – reaching 30-40%.
 |
|---|
Trembling pigs are often complicated
with splay leg |
In the individual pig, providing care can be given, the trembling
will gradually reduce over the sucking period to the point of
ceasing by the time the pigs are weaned. In rare cases, it can take
up to 8 weeks of age for complete resolution.
Pathologically, 2 distinct syndromes
are recognised:
| Lack of development of the cerebellum – the part of the brain
that controls co-ordination – which does not resolve. This is
typical of CSF (Type A I) for which there would be compulsory
slaughter of the whole herd.
| Lack of myelin development. Myelin is effectively the
insulation material around the nerve fibres, its absence leading to
uncontrolled nerve stimulation. It is these cases that can resolve
by delayed development of the myelin sheaths.
 |
|---|
FYarded gilts must be fully
acclimatised before service to ensure immunity to CT virus (and
other herd pathogens) |
(Tremor in young pigs can also be a feature of other diseases such
as Aujeszky's disease – tending to occur after the first week of
life – and as a sporadic sign in PMWS in weaner pigs. In such cases
it does not resolve.)
TREATMENT
The reason that pigs are born
trembling is that there is damage to the nervous control system and
this takes time to heal. There is no specific treatment. However,
assisted suckling and provision of an environment where chilling
and overlaying can be avoided will allow more pigs to recover with
time, although weaning weights may be depressed by 1kg or more.
PREVENTION
If purebred Landrace or Saddleback
pigs are involved and the pattern of disease suggests that the
cause may be one of the two inherited conditions, then clearly the
breeding programme must be closely examined and culling of
"carrier" animals or even families may be appropriate.
If type A V is suspected, then the key
is to stop poisoning the sows! Identify the cause of the toxin and
remove it.
Presence of organophosphates in diets
may produce other signs in other classes of stock such as posterior
paresis (complete loss of use of back legs) in sows. (Demyelination
is also a feature of organophosphate toxicity in man).
No vaccine is available as the
causative organism of Type A II is not known.
Classic Swine Fever is a notifiable
disease with a compulsory slaughter policy. Suspicion of this
disease must trigger notification to the DVM of DEFRA and an
investigation will be undertaken.
Control of the most common form of
Congenital Tremor (Type A II) hinges on ensuring that gilts are
exposed to the causative agent long before service.
The fact that Congenital Tremor type A
II is uncommon and sporadic in nature suggests that gilts produced
on most multiplication farms – or indeed in-house – are exposed to
Congenital Tremor virus before breeding, but there are occasional
herds where the virus is absent – hence unexposed gilts are
produced – and in three site production systems it is possible to
"lose" the virus from the finishing stage – even where it is known
to be present in the breeding herd Ð with the same effect.
Whilst three-site production – which
is designed to produce pigs of the highest possible health – is a
valuable tool in commercial pig keeping, it may not be applicable
for seedstock population if it produces animals that are Ôtoo
cleanÕ for the recipient herds.
It is difficult to say what a reliable
source of virus is for na•ve gilts but experience suggests that
weaners of 8-12 weeks of age can provide satisfactory exposure for
gilts over a 4-week period, stopping at least two weeks prior to
service. Cull sows are also widely used to acclimatise incoming
gilts to a range of potential pathogens.
In the rare herds which are free of
the virus – and this will only be known if a parent herd is
believed to be free, or retrospectively following a breakdown – the
aim must be to keep the virus out. As the cause is not known and,
therefore, no test is available, this is very difficult,
particularly as there is circumstantial evidence to suggest that
the virus will pass through the semen.
In practice, most herds carry the
virus and see little or no disease. Any change in the source of
gilts must be treated with caution to avoid upsetting the
equilibrium that exists with this and may other infectious agents,
particularly if high numbers of gilts are introduced.
COSTS
At the mildest level, the disease has
minimal cost, other than the time to care for affected piglets. In
a severe outbreak in a new herd of 400 gilts, total mortality for
the first 5 months was 35% - estimated that 25% of all deaths were
associated with Congenital Tremor. The loss of 1000 pigs cost more
than £35,000 to that farm.
Copyright © NADIS
2009www.nadis.org.uk
Comments
made in this paper are based on the practical clinical experiences
of the author working in the field and opinions expressed are
derived from those experiences; others may have different
experiences.
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