Anthelmintics are important drugs used to protect our horses from internal parasites. When used according to veterinary recommendations, correctly, and in the right quantity, they treat intestinal worms and shield our horses from poor health, disease and colic.
Unfortunately, their widespread and uncontrolled use has resulted in unwanted secondary effects, causing harm to both horses and the environment. Their misuse has produced resistance amongst parasites, which means it has reduced their effectiveness against them, and leaving horses more exposed to the harm of worms (Rendle, 2022).
However, horses are not the only ones to pay the price for the misuse of such drugs. According to the UK Parliament, residues of veterinary medicines such as anthelmintics and ectoparasiticides, are a contributing factor to the decline in insect populations and diversity in the UK, leading some species to extinction (UK Parliament Post, 2020).
Following worming, pharmaceutically-active products are expelled through horses’ faeces into the environment, where other, off-target species are likely to be affected. For instance, after oral administration of ivermectin and moxidectin, the peak excretion in horses’ faeces occurs after 2 to 3 days for both drugs. For ivermectin, approximately 90% is excreted within four days of worming, whereas it takes twice as long for moxidectin (Perez et al., 2001).
Amongst the insects which are indirectly harmed, the case of one in particular has been investigated for its ecological value: the dung beetle. Dung beetles live and feed upon animal dung and, in doing so, play a significant role in helping modern grazing systems. As they break down dung, they provide a number of other free services, as shown in the image below:
Dung Beetles Direct (2015)
In a paper by Beynon et al. (2015), four functions carried out by dung beetles in cattle-grazed pasture have been modelled with the DEFRA ecosystem service framework to help account for their economic value to society. It has been estimated that the total monetary value of such services in the UK could amount to over £300 million a year, in a so-called ‘conventional’ system. Although such services might be more relevant to the cattle industry, the fields where our horses live can benefit from the same functions.
Unfortunately, the ecotoxic effect of anthelmintic residues is a contributing factor to the decline of dung beetle populations and the services they provide. In addition, some damage can be caused even when drug concentrations in dung is sub-lethal, by impairing dung beetles’ reproductive abilities (Weaving et al., 2019).
To address and improve the use of anthelmintics, the equine industry has recently launched two initiatives: Project WORMS and the pan-industry CANTER group, initiated by the VMD. Together, these are acting to unify recommendations and implement best practices to find solutions against resistance and the unwanted impacts on off-target species.
Reducing our reliance on anthelmintics would be one simple way of mitigating their side effects. By monitoring the parasitic burden in our horses' droppings, using faecal egg counts and serological testing, we could administer treatments on a necessity basis, whilst monitoring their health and sparing other species.
In addition, timely poo-picking and pasture management after worming would also help to reduce harmful effects of anthelmintics by limiting the amount that goes back into the environment.
Beynon, S.A., Wainwright, W.A., Christie, M., 2015. The application of an ecosystem services framework to estimate the economic value of dung beetles to the U.K. cattle industry. Ecological Entomology 40, 124–135. https://doi.org/10.1111/een.12240
Dung Beetle Direct, 2015. Factsheet 1 Dung Beetles: what do they do for us?. available at https://www.thebugfarm.co.uk/wp-content/uploads/2018/07/dung-beetles-direct-fact-sheet-1.pdf (accessed 11.22.22).
Pérez, R., Cabezas, I., Sutra, J.F., Galtier, P., Alvinerie, M., 2001. Faecal Excretion Profile of Moxidectin and Ivermectin after Oral Administration in Horses. The Veterinary Journal 161, 85–92. https://doi.org/10.1053/tvjl.2000.0521
Rendle D., 2022, “Best practice for worming and anthelmintic resistance”, Vet Times 52.09 available at https://cpd.vettimes.co.uk/cpd-plus/equine/cpd-preventive-health/best-practice-for-worming-and-anthelmintic-resistance (accessed 11.22.22).
UK Parliemant, Post, 2020. UK insect decline and extinctions. UK Parliement Post note, Number 619, March 2020 available at https://post.parliament.uk/research-briefings/post-pn-0619/ (accessed 22.11.22)
Weaving, H., Sands, B., Wall, R., 2019. Reproductive sublethal effects of macrocyclic lactones and synthetic pyrethroids on the dung beetle Onthophagus similis. Bulletin of Entomological Research 110, 195.