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Sheep - don’t get caught out by Worms this Winter

Lleyn at Kirkton

Think again if you consider worms to be just a summer problem.  In both 2014 and 2015 SAC’s veterinary laboratories made double the number of diagnoses of parasitic gastroenteritis (worms) in the October - December period compared to June - August. 

Sheep become infected by eating worm larvae along with the grass.  These larvae happily survive through the winter and are not killed by frosts or snow.  The mild temperatures will have allowed worm eggs passed in dung to continue to hatch and the larvae to develop.  Fields stocked with lambs since the spring could have high numbers of worm larvae on the pasture.  Fields grazed by cattle during this time will be lower risk for sheep.

The type of worms that infect sheep can change with the season.  The best known example of this is nematodirus which usually causes problems in the spring.  During autumn and winter the black scour worm (Trichostrongylus sp.) is often the most common worm living in the gut.  Does this matter?  Yes - if you are relying on long acting wormers to control worms at this time of year.  Most people know that wormers containing moxidectin are long acting and prevent re-infection with worms for a period of time.  What you might not realise is that the length of persistence varies depending on worm type and the product used. 

The table below summarises the differences.  More information can be found on the product data sheets.

Product  Brown Stomach Worm
(Teladorsagia sp.)
 Black Scour Worm
(Trichostrongylus sp.)
 Meat Withdrawal
Moxidectin 0.1% Drench  5 weeks  0  14 days
Moxidectin 1% Injection  5 weeks  2 weeks  70 days
Moxidectin 2% LA 20mg/kg  14 weeks  6 weeks  104 days

If you have ill thriven or scouring hoggs don’t assume it can’t be worms because you have treated them with a long acting wormer.  Also remember that worms can become resistant to moxidectin as identified on 21% of Welsh farms screened in autumn/winter 2014. (See and search WAARD for more details.)  Talk to your vet/advisor and investigate any problems.  At this time of year worm infection and cobalt deficiency often go hand in hand so it can be worth checking for both.  This is a good time of year to monitor groups of hoggs by collecting ten dung samples for a pooled worm egg count in order to decide whether treatment is required or not. 

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English and Welsh clients please contact: SAC Consulting Veterinary Services, Allan Watt Building, Bush Estate, Penicuik, EH26 0QE
Tel: 01315353130
Scottish clients please contact your local disease surveillance centre

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