Download What are worms and why are they harmful? Parasites such as gut

What are worms and why are they harmful?
Parasites such as gut worms, lung worms and fluke selfishly use their host to feed, grow and
reproduce often to the detriment of the host. Some parasites are relatively harmless (tapeworm),
others cause reduction in growth rates. There are some parasites that can cause severe health issues
and even death due to the damage they cause to vital tissues (eg Fluke, Lungworm, Nematodirus).
Can stock become immune to worms?
Yes. The extent of the immunity depends upon the age and health of the animal along with the
species of worm. Lungworm stimulates an excellent immunity in cattle but exposure needs to be
repetitive. Sheep and cattle develop reasonably good immunity to most gut worms including
Nematodirus. Liver fluke stimulates moderate immunity in cattle, however, sheep develop no
immunity.
What products are available to treat worms?
Gut worms and lungworms can be treated with a multitude of drugs but many of them have similar
methods of action. Due to their characteristics the classes of available wormer are commonly known
as white (Benzimidazoles), yellow (Macrocyclic Lactones) and clear (Avermectins). In recent years
there has been the development of a new class now termed orange (Monepantel).
Treatment of fluke requires separate drugs, which drug is chosen depends upon the time of year and
the stage of the fluke’s life cycle, veterinary advise should be sought.
What is wormer resistance and is it a problem?
A small proportion of worms may be able to survive after the animal is wormed. These surviving
worms then produce eggs passing on their resistance to further generations. Over time the
population of resistant worms grows.
The development of wormer resistance is a huge problem and there are farms in the UK which have
wormer resistance to all 3 main classes of wormer. Without the development of new wormers sheep
farming on these farms would become impossible.
When should ewes and tups be wormed?
Adult sheep do not need to be routinely wormed for gut worms since they develop good immunity.
Ewes, at most, should only be wormed once a year, the most appropriate time is at lambing as this
will help to prevent contamination of the pasture with worm eggs. The only sheep that should be
wormed prior to tupping are tups and any ewes which are in poor body condition. To reduce the risk
of wormer resistance never worm more than 90% of a group at any one time
How often should youngstock be wormed?
In the past it was advised to worm growing stock every 3-4 weeks and move to fresh pasture. This
cost farmers money and time; it also increased the rate at which a resistant worm population
developed and prevented the development of natural immunity in the animal.
Current advice is to worm stock only when they need to be wormed. This will depend upon
monitoring for signs of worm infection; these include slower growth rates and mucky tails or
coughing in the case of lungworm. Regular worm egg counts are invaluable to help confirm the
diagnosis of worms and to assess the level of infection. These are easily done at your vets and they
can advise you on what treatment to use.
Why is Nematodirus a concern?
This is a particular species of worm which can cause rapid death in young lambs around spring as
they start to graze. Fortunately Nematodirus stimulates good immunity in sheep and is also very
susceptible to wormers. Due to the lifecycle of nematodirus the risk of infection can, in some
instances, be predicted and ‘worm forecasts’ made. Keep in touch with your vet about these
forecasts. Submit regular worm samples for egg counts from lambs and immediately investigate any
outbreaks of scour or sudden deaths in grazing lambs.
Has my farm got wormer resistance?
This can be determined by taking a worm egg count from a small group of animals, treating the
animals with a wormer, then resampling the group after 7-14 days. If there is no wormer resistance
then there will be no worm eggs at the second sample!
How do I prevent resistance?
The more a wormer is used the more we select for resistance, to prevent this, wormer use needs to
be minimised and it must be used accurately. A farm also needs to prevent ‘importing’ resistant
worms from other farms.
Only worm stock when there is evidence that worms are affecting performance. Always confirm the
diagnosis as there are diseases, such as coccidiosis, that can simulate the symptoms of worms. Avoid
worming more than 90% of a group. Prevent moving stock to fresh pasture after treatment as this
would lead to contamination of the pasture with resistant worms.
Good worming technique involves dosing for the heaviest animal in a group and regularly calibrating
the dosing gun to ensure the correct dose is being delivered.
Take precautions when purchasing stock; they should be isolated for 24-48 hours and given a
‘quarantine treatment’ of wormer to kill all internal worms including resistant worms. This involves a
‘yellow’ drench and an injection of ‘clear’ wormer. Alternatively a dose of a new class of wormer
(‘orange’) can be given.
What is the value of a worm egg count?
A worm egg count will help confirm that worms, not coccidiosis, is the cause of scour; in some cases
it will identify the species of worm involved. It will also reveal the extent of worm infection and
hence a decision can be made whether treatment would be justified. Animals which have mucky
tails should be targeted for worm egg counts.
Should I dose and move?
Moving stock to fresh pasture after treating with a wormer is a great way to select for resistant
worms as any worm eggs shed on to the fresh pasture will be from resistant worms. This practice
should be avoided and only used if a small proportion of stock to be moved have been treated, or it
has been a few days since the treatment was administered.
Should I rotate the class of wormer I use?
Regularly rotating the class of wormer used does not affect the rate at which resistant worms
develop. How often you change the class of wormer you use depends upon a farms individual
circumstances, the species of worm that is being treated and any history of resistance on the farm.
You should seek advice from your vet when choosing which wormer to use.
Is fluke a big problem?
Fluke has become a huge problem in the last 7 years and it has spread to areas which have
previously never had fluke problems. Much of this is due to the wet summers we have encountered
and the movement of infected stock to uninfected farms.
Cattle build up some resistance to fluke but sheep develop no immunity. Sheep in fluke areas
require regular treatment, the timing dependent upon weather conditions and grazing history. In
general sheep should be targeted from late-summer through to winter. Cattle should be fluked after
housing to eliminate any infection over the winter months. Speak to your vet to get advice about
which is the best flukicide to use and when to use it.
Is Lungworm still a problem in cattle?
Lungworm is still a problem and there are cases each year where animals have died, surprisingly
there are also cases seen in adult cattle. The key to lungworm control is maximising an animal’s
immunity to it and this requires repetitive expose to it in small doses.
Lungworm vaccine is a great tool to give initial protection to an animal but it is important to be
aware about the adverse effects that wormers can have on the vaccine. A dose of wormer at an
inappropriate time will nullify the vaccine; the overuse of wormer will also prevent the repetitive
exposure that is essential to stimulate strong immunity in cattle.
If you would like to discuss any of the issues raised in the above article visit the XLVets stand at
North sheep on the 8th June.