Use of anthelmintics in equine practice

Purpose

To provide guidance on the responsible use of anthelmintics in horse populations to support effective parasite control and minimise resistance.

Policy

  1. Veterinarians must recommend the judicious use of anthelmintics in horses to both reduce the risk of disease and limit the development of anthelmintic resistance.
  2. Routine interval-based or rotational deworming is outdated; treatments should be targeted using diagnostics and seasonal risk factors. Faecal egg counts (FEC) and Faecal egg count reduction tests (FECRT) should be used to support informed and responsible decisions about anthelmintic use.
  3. Anthelmintics are not a standalone treatment for parasite control and ongoing environmental management is essential.
  4. Veterinarians should be familiar with published guidelines but be aware that further research around appropriate anthelmintic use and anthelmintic resistance is required.

Background

The primary targets of nematode control in equine practice are the large and small strongyles and ascarids (Parascaris spp). Other common equine parasites (bots [Gasterophilus spp], pin worm [Oxyuris equi], tapeworm [Anoplocephala perfoliata], Habronema, Draschia spp, and Onchocerca spp) are also managed in parasite control strategies.

Anthelmintic resistance to all three major drug classes for nematodes – macrocyclic lactones (MLs), benzimidazoles (BZs), and tetrahydropyrimidines (THPs) – has been reported in Australia in both cyathostomins and Parascaris spp.

Resistance of other parasites to the major worming classes, including cestode resistance to praziquantel, has been reported in other countries (Nielsen, 2022 and Nielsen, 2023), but is, at this time, largely unknown and anecdotal in Australia.

Control strategies involving diagnostic tests and environmental management are essential to minimise further development of resistance in strongyles and Parascaris spp.

Cyathostomins (small strongyles) and Parascaris spp. are primarily seen in horses of different age groups and require different control strategies. Cyathostomins are the major target of control strategies in mature horses (from 12 months of age), whereas Parascaris spp. mainly affect foals and weanlings and can cause coughing, ill-thrift, colic and occasionally fatal intestinal obstruction (Beasley, 2025).

Cyathostomins generally cause subclinical infections, although rare synchronous larval emergence events (larval cyathostominosis) can lead to diarrhoea, weight loss, and death. They are ubiquitous in both horses and the environment and are present in large numbers. This has made complete elimination unrealistic. Management of cyathostomins should therefore be focussed on simultaneously minimising both disease and the development of resistance, which requires the utilisation of susceptible worm populations (refugia) to dilute resistance alleles. The propensity of individual horses to shed varying levels of worm eggs allows the targeting of individual horses (categorised as low, medium, or high shedders of strongyle eggs) in refugia-focussed selective worm control strategies.

Guidelines

Cyathostomins

Current recommendations by the Australian Equine Parasite Advisory Panel (AEPAP) for parasite control aim to reduce the risk of parasitic disease to acceptable levels, while minimising the further development and spread of resistance (Beasley, 2025). These guidelines focus on managing strongyle egg shedding into the environment, particularly when conditions favour egg and larval survival, and on reducing the frequency of anthelmintic treatments. Faecal egg count reduction testing (FECRT) using FECs pre-treatment and 14 days post-treatment, with or without further periodic FECs to determine egg reappearance periods (ERP), should be used to identify effective anthelmintics and support the maintenance of strongyle refugia on individual properties. Quarantine protocols are also described to reduce the risk of transmission of resistant parasites between farms.

The Australian climate favours year-round parasite survival unlike those in Europe or America. In general, horses aged between 1-3 years are likely to require 3-4 treatments for strongyles per year based on hygiene, stocking density, and the environment. These treatments should be guided by FEC results and ideally timed to coincide with peak transmission periods (Spring and Autumn) (Beasley, 2025). Mature horses (5 – 15 y) generally develop natural immunity and may only require one or two treatments a year based on FEC. Young horses (<2yo) require a different approach as their immunity is still developing – and treatment 3-4 times per year is recommended. Macrocyclic lactones are recommended for use against strongyles, either alone or in combination with praziquantel when tapeworm treatment is also indicated.

Parascaris spp.

The prepatent period of Parascaris is 10-12 weeks and treatment of foals with any anthelmintic prior to 2 months of age should not be considered unless Strongyloides westeri (threadworm) is of concern. There are no defined refugia-based resistance management strategies for Parascaris spp at this time, and interval-based treatment is likely to be required in young horses infected with Parascaris spp to limit the clinical impact.

The AEPAP recommends treating foals at two and five months of age using a combination of THP and BZ class anthelmintics. Subsequent treatment decisions should be guided by the presence or absence of Parascaris eggs in faecal samples. Anthelmintic resistance in Parascaris spp. to macrocyclic lactones is common and these drugs should be avoided unless their use is supported by FECRT.

Environmental management

Anthelmintic use does not replace good pasture management. Removing faeces two to three times a week during warm months reduces egg contamination on pasture. Harrowing or spreading of faeces is not recommended unless ambient temperatures exceed 40C. Freezing / frosts have not been shown to affect Parascaris egg survival.

While pasture spelling may help reduce cyathostomin egg loads, Parascaris spp eggs can persist in soil for years. Most equine parasites are host-specific and so co-grazing will help clean up infected pastures, however, cattle tend to graze the top of the pasture, potentially leaving higher worm burdens in the lower pasture.

Other approaches including dung beetles, use of nematophagous fungi (eg: BioWorma®) and plant extracts have been tested for the control of equine gastrointestinal nematodes in Australia and may form part of an integrated approach to reduce pasture contamination.

Donkeys/mules

Donkeys often co-habit with horses and other livestock, and whilst a member of the Equus family, are genetically distinct. The pharmacology of anthelmintics in donkeys is largely unknown and requires off-label prescription. Anthelmintic resistance to the major anthelmintic classes has been reported in donkeys and treatment at this stage is recommended to be the same as for horses until further studies are conducted (Buono et al., 2023).

Recommendations

  1. Anthelmintic use should be both strategic and targeted, based on age and relevant parasites.
  2. Anthelmintic use for adult horses should be used in conjunction with FEC/FECRT testing.
  3. Veterinarians must take responsibility to educate themselves regarding appropriate anthelmintic use in horses. A limited list of references is supplied, with the recently published guidelines from the AEPAP (Beasley et al, 2025), being a most valuable resource.
  4. Owners may consider quarantining new horses when first introduced to a property to limit the transmission of resistant parasites in conjunction with advice from their veterinarian on appropriate FEC testing and treatment protocols for new introductions.
  5. Product manufacturers and veterinarians should ensure appropriate up-to-date education and training of all parties involved in supplying anthelmintics to horse owners, and that horse owners are offered every opportunity to be informed on appropriate deworming of their horses.
  6. Veterinarians must document any adverse reactions observed to anthelmintics, including declining efficacy, and these should be reported to the product manufacturer or directly to the Australian Pesticides and Veterinary Medicines Authority.
  7. Responsibility and accountability ultimately are individual matters and education (of suppliers, owners, and veterinarians) is key to addressing anthelmintic resistance.

Related AVA policies and documents

AVA Guidelines for prescribing, authorising and dispensing veterinary medicines (2013): https://www.ava.com.au/siteassets/resources/prescribing-guidelines/guidelines-for-prescribing-authorising-and-dispensing-veterinary-medicines-october-2023.pdf

Guidelines-for-veterinary-personal-biosecurity-2017-final.pdf: https://www.ava.com.au/globalassets/authors/guidelines-for-veterinary-personal-biosecurity-2017-final.pdf

Infection prevention and control in veterinary workplaces (2025): https://www.ava.com.au/policy-advocacy/policies/surgical-medical-and-other-veterinary-procedures-general/infection-prevention-and-control-in-veterinary-workplaces/

Responsible use of veterinary medicines on farms (2010): https://www.ava.com.au/policy-advocacy/policies/use-of-veterinary-medicines/responsible-use-of-veterinary-medicines-on-farms/

Veterinary use of compounded pharmaceuticals (2010): https://www.ava.com.au/policy-advocacy/policies/use-of-veterinary-medicines/veterinary-use-of-compounded-pharmaceuticals/

References

Abbas, G., Ghafar, A., McConnell, E., Beasley, A., Bauquier, J., Wilkes, E. J., … & Jabbar, A. (2024). A national survey of anthelmintic resistance in ascarid and strongylid nematodes in Australian Thoroughbred horses. International Journal for Parasitology: Drugs and Drug Resistance,24, 100517.

Beasley, A., Abbas, G., Hughes, K., El-Hage, C., Jacobson, C., Bauquier, J., Wilkes, E., Carrigan, P., Cudmore, L., Hurley, K, Beveridge, I., Nielsen, M., and Jabbar, A (2025) ‘Australian guidelines for equine internal parasite management’ Australian Veterinary Journal

Buono, F., Veneziano, V., and Molento, M.B. 2023 ‘Horse and Donkey Parasitology: differences and analogies for a correct diagnostic and management of major helminth infections’ Parasitology 150, 1119-1138

Nielsen, M. K. (2022). Anthelmintic resistance in equine nematodes: Current status and emerging trends. International Journal for Parasitology: Drugs and Drug Resistance, 20, 76-88.

Nielsen, M. K. (2023) Apparent treatment failure of praziquantel and pyrantel pamoate against anoplocephalid tapeworms. International Journal for Parasitology: Drugs and Drug Resistance 22 (2023) 96–101