How to avoid wormer resistance in poultry

With only three different active ingredients on the market, failure to follow best practice protocols could result in rapid development of resistant worm strains, warns vet Stuart Young from Mount Vets.

“Worms are a problem that all producers face, and we have to be careful not to get in the same situation as other species where we use a particular type of wormer so much that we develop resistant strains,” he said.

See also: Getting on top of worms

Speaking at the annual general meeting of the West Country Layers Association in Devon, Mr Young said that rotating different wormers was vital. “There are several products that are licensed for use in poultry, and the more products that are developed with a nil egg-withdrawal period the better it will be for the industry.”

Regular sampling

However, rotating wormers was only part of the process, he added. “You need to carry out faecal egg counts and only worm when necessary – a lot of wormer is wasted by routine dosing every eight weeks. So look for clinical signs and carry out post-mortems to get an accurate picture of what’s going on at your farm.”

Producers should select the appropriate wormer to target the strains present on farm, and administer the correct dosage. “Inappropriate dosing has led to massive problems with resistance in the sheep industry, and we need to learn from that.”

To carry out a faecal egg count, Mr Young recommended placing four 12x12in boards per 2,000 birds beneath the roosting area, and pooling the faeces before submitting for testing. “If you just go and collect solid faeces from the field you may miss the eggs – you want a representative sample.”

This should be repeated every four to six weeks. Producers should then choose an appropriate wormer, if necessary, to be administered either in the feed or in water. “If you have gapeworm you’ll need to treat immediately, so will need an in-water wormer, as to order medicated feed and wait for delivery could take a couple of weeks.”

Worm control

According to Kate Pitman from MSD Animal Health, poultry worms were classified into two groups; roundworms and tapeworms. And the best forms of control, alongside anthelmintic use, were basic hygiene measures.

“Keep feeders clean and off the ground, don’t stock too heavily and place stones outside your popholes to keep the birds’ feet clean,” she said.

Keeping grass on the range short would expose eggs to sunlight and kill them off, with hot and dry weather particularly beneficial. “If you do manage to find worms, there will likely be a heavy infestation.”

The three active ingredients currently licensed for poultry use are flubendazole, piperazine and fenbendazole – the latter of which featured in MSD’s new Panacur AquaSol. “MSD wanted to design something that you could administer to birds in water without sedimentation and blocking up of nipple lines,” said Miss Pitman.

“We used a wet-milling technique to create a product that, when in suspension, doesn’t settle out or need to be stirred during administration. Treatment is required for five consecutive days; a zero withdrawal period applies to eggs and a six day withdrawal period is necessary with regard to meat and offal.”

Common worms


  • Ascaridia galli. Grow up to 12cm long. Direct lifecycle: eggs are excreted in faeces, become infective after 10-12 days and are eaten by the birds. Can last up to a year in litter. Larvae embed in duodenal mucosa causing haemorrhage and anaemia. Birds can also suffer from gut obstruction if large numbers of worms are present.
  • Several different strains, which have both direct and indirect lifecycles. The most damaging strains are C annulata and C contorta, which damage the crop and oesophagus in young birds. Also cause production losses, pale yolks and enteritis.
  • Heterakis, or cecal worms. A small worm, 15mm long, with a direct lifecycle. Eggs are excreted in faeces and become infective in hours – but can remain in litter or soil for a year or longer. After consumption, the larvae reach the cecum within 24 hours. Can act as a vector for Histomanas meleagridis (blackhead), which is transported from the ceca to liver, causing necrosis and lesions. Blackhead symptoms include depression, morbidity and sulphur yellow diarrhoea; mortality may reach 30%.
  • Syngamus trachea (gapeworm). More common in pheasants, these bright red worms block the trachea, causing coughing, mucus and breathing difficulties. Direct lifecycle also involving worms, slugs and snails, which can last up to three years.


  • Tapeworms are not as common in poultry as roundworms. They are clearly segmented and require an intermediate host such as slugs or snails to complete the lifecycle. The two most common strains are Davainea proglottina and Raillietina cesticillus. Birds under the age of 10 weeks are most susceptible and symptoms include weight loss and breathing difficulties.