Here you’ll find details on the different anthelmintic classes available in New Zealand – what worms they target, how they act, and what we know about drench resistance for each class.
What’s in a name?
- Drench is the common name for anthelmintic.
- These kill certain species of internal parasites (worms) inside your animals.
- Dozens of drench BRANDS are available
- However they all belong to:
- Five 'broad-spectrum' drench families (cover gut worms, lungworm)
OR
- Three 'narrow- spectrum' drench families (cover one or two other parasites, e.g. tapeworm, liver fluke)
- Each family has a different mode of action (MOA).
- Get to know your drench families, so you can make better decisions when choosing treatments for your stock.
Family matters
Above: Timeline of approximate year of release of different drench actives in New Zealand. Courtesy of Elanco Animal Health.
Key
- White = Benzinimidazoles
- Yellow = Levamisole
- Tan = Macrocyclic Lactones
- Green = Derquantel
- Orange = Monepantel
Broad-spectrum drenches
Names to look for: Albendazole, oxfendazole, fenbendazole and triclabendazole.
Mode of action: starvation of the worm.
Target parasites: most BZs have broad spectrum activity against most important worms in sheep and cattle. The exception is triclabendazole, which only kills liver fluke. Some BZs are also active against tapeworm.
Given as: oral (most common); extended-release capsule (historical); pour-on (historical).
Widely used? Yes.
Resistance status: as of November 2022, Gribbles Veterinary data (PDF, 474 KB) show most common worm species are now resistant to BZ drenches on NZ sheep farms: From 17% of farms for Cooperia to 64% of farms for Nematodirus. Resistance was first diagnosed in sheep in 1979. There is one reported case of liver fluke resistant to Triclabendazole in NZ.
In a survey of 62 beef properties in the North Island in 2004, 47 farms (76%) had worms that were resistant to BZ drenches. BZ’s may be useful in combinations but would not generally be recommended on their own in cattle.
Often called ‘clear’ drenches, but again, not all are clear. Used in NZ since 1969.
Names to look for: Levamisole
Mode of action: paralysis of the worm.
Target parasites: broad-spectrum. Acts against most worms except inhibited L4 larvae of Ostertagia in cattle.
Given as: oral, injection, pour-on.
Widely used? Yes.
Resistance status: as of November 2022, Gribbles Veterinary data (474 KB) show three key worm species are now resistant to levamisole drenches on many NZ sheep farms – Teladorsagia (48%); Trichostrongylus (35%) and Nematodirus (20%). Resistance was first reported in sheep worms the 1980s.
In a survey of 62 beef properties in the North Island in 2004, 4 farms had worms (all Ostertagia) that were resistant to levamisole. However, there are now multiple reports of both Ostertagia and Cooperia that are resistant to levamisole, including in combinations. It is not wise to assume that levamisole will adequately control Cooperia in intensive cattle systems nowadays. Do your own testing.
Also known as ML or mectin drenches. A broad-spectrum family with many different drugs in it, used since 1981.
Names to look for: Ivermectin, abamectin, moxidectin, eprinomectin, doramectin.
Mode of action: paralysis of the worm.
Target parasites: acts against all key worms, including lungworm and inhibited larval stages of worms in both sheep and cattle.
Given as: oral, injection, pour-on, extended-release capsule (historical).
Widely used? Yes.
Resistance status: varies on sheep farms by ML type and parasite species. As of November 2022, GGribbles Veterinary data (474 KB) show many common worm species are now resistant to ML drenches. Teladorsagia is most resistant overall, ranging from 83% of farms for ivermectin to 44% for abamectin and 40% for moxidectin. Trichostrongylus was resistant to moxidectin on 38% of farms and abamectin on 23% of farms. Nematodirus was similar. Cooperia resists both abamectin (20%) and moxidectin (16%). There have been field reports of resistance in Haemonchus in sheep, but no laboratory data. ML resistance was first reported in NZ sheep worms in 2000.
In a survey of 61 beef properties in the North Island in 2004, 56 farms had worms (Cooperia) that were resistant to ivermectin. Ivermectin-resistant Ostertagia were first reportedby AgResearch in 2016. There are now multiple reports of Ostertagia that are resistant to ML drenches, including in combinations. It is not wise to assume that levamisole will adequately control Cooperia in intensive cattle systems nowadays. Do your own testing.
Also called AADs. One of our newest, novel drench families. Broad spectrum; first registered in NZ in 2009.
Name to look for: Monepantel. (Zolvix Plus (R))
Mode of action: paralysis.
Target parasites: all key roundworm species in NZ sheep and cattle.
Given as: oral
Widely used? Becoming so.
Resistance status: Gribbles Veterinary data (474 KB) suggests resistance in sheep is just beginning to appear for both Teladorsagia and Trichostrongylus (7% each). NB – monepantel was tested only in combination with abamectin.
One of our newest, novel drench families. Broad spectrum; first registered for NZ sheep in 2010.
Names to look for: Derquantel (Startect (R)).
Mode of action: paralysis.
Target parasites: all key worm species in NZ sheep.
Given as: oral.
Widely used? Becoming so.
Resistance status: Gribbles Veterinary data (474 KB) from November 2022 suggests resistance in sheep is just beginning to appear for both Teladorsagia and Trichostrongylus (6% and 4% respectively). NB – derquantel was tested only in combination with abamectin.
Are there going to be any new drenches? Link to Future Farm coping with drench resistance – Parasite management with Dr Dave Leathwick 6.55-7.57
Narrow-spectrum drenches
Narrow spectrum drench family, first registered in NZ in 1991. Used only in sheep.
Names to look for: Closantel.
Mode of action: disrupts worm metabolism. It binds strongly to blood proteins, so is only effective against blood feeding parasites.
There are 2 different forms of Closantel: Closantel Salt and Closantel Base. Closantel Salt can prevent re-establishment of Barber’s Pole worm in sheep for up to 4 weeks. Closantel base is short acting, treated sheep can become re-infected with Barbers Pole worm larvae after a few days.
Target parasites: Liver fluke, Haemonchus (Barber’s Pole worm)
Given as: oral.
Widely used: in areas where Haemonchus (Barber’s Pole worm) is prevalent.
Resistance status: unknown.
Small, narrow spectrum family first registered 1993. Used only in cattle.
Names to look for: Clorsulon
Mode of action: disrupts metabolism.
Target parasites: Liver fluke; most effective against adult fluke and late-stage immature flukes (6-8 weeks old). No action against tapeworms or roundworms.
Given as: injection.
Widely used? Reasonably common as a liver fluke treatment/insurance.
Resistance status: unknown.
Names to look for: Praziquantel
Mode of action: Uncertain, thought to cause skin damage and paralysis.
Target parasites: Tapeworm.
Given as: oral
Widely used? Yes, for early drenches of lambs.
Resistance status: cases of resistance were reported in the early 2000’s as part of another study. No formal survey has been done since, but veterinarians report seeing tapeworm eggs in faeces of lambs treated with praziquantel.