Anti-parasiticides are fatal to parasites while not harming the mammal that they are given to, largely because of different body chemistry. Most anti-parasiticides have a wide ‘safety margin’ (i.e., how much of an overdose can be given without toxicity). However, for some there is a much higher risk of toxicity to the animal and there may be a risk present even if the correct dose is given.
There is also a risk of toxicity to other species. Humans are exposed by inhalation or skin contact and can suffer accumulation of chemicals (ensure you read the safety data sheet (SDS) and wear appropriate PPE); and dogs are endangered by their creative appetites (i.e., eating chemicals directly, or dung of drenched livestock). Some dog breeds are especially sensitive – e.g., Border collies. Other species in the environment can be at serious risk too (e.g., aquatic life and dung beetles).
This general advice is applicable to all drenches:
Doses: To reduce the risk of resistance developing, it is standard good practice to dose a mob based on the heaviest animal (to avoid underdosing). However, if there is a very large spread of bodyweights that crosses several dose ranges, it may be best to divide the mob into heavier and lighter groups when using more-toxic drugs (i.e. those with a narrower safety margin). An excessive dose can increase the risk of toxicity as well as possibly breach residue limits.
Animal health: Stressed or ill (including malnourished) animals are at greater risk of toxicity. Dehydrated animals also may be more sensitive, so take precautions of offering food and water in hot and/ or dry periods and work in the cooler periods of the day.
Drench shaking: if the drench requires shaking, ensure that this is done as per label directions. For these products, ingredients settle at the bottom during storage. Failure to shake will mean that some animals will not get enough of the active ingredient (drench will not work) while some will receive a very concentrated (toxic) dose.
Mixing drenches: some drugs act in different ways and combining them off-label can increase the risk of toxicity from what would be a safe dose when given alone. It is generally not recommended to mix or dose with other drugs unless the label says that you can.
Mineral-enriched drenches: do NOT use unless you know that mineral deficiency is present. Excess minerals (e.g. selenium, copper) may be toxic.
Pour-ons: Licking pour-on product from each other can contribute to toxicosis in a group – products are absorbed more rapidly from the gut than the skin (e.g., closantel, levamisole).
Goats – as goat producers know, not many products are registered for goats. They often require higher doses than sheep by bodyweight. Check with your vet regarding a suitable product and dose, as too high a dose may be toxic. Safe effective doses for goats may require an off-label veterinary prescription. More information on how to use drenches in goats.
If you suspect toxicity: phone your vet! If a group of animals suffer possible harmful effects (e.g. neurological signs or sudden death shortly after drenching) it can be important to find out why to avoid it happening again in the future, and to rule out anything else, e.g. infectious disease. Any adverse event should be reported to the manufacturer or direct to the Australian Pesticides and Veterinary Medicines Authority.
Drenches which require particular care
The following are the drugs most commonly associated with toxic effects, although toxicity is possible with any anti-parasiticide:
Naphthalophos (registered for sheep as a sole ingredient, and as combination drenches) is a highly toxic ingredient with a much narrower ‘safety margin’ than other drenches. Naphthalophos causes severe fatal pneumonia if inhaled (direct chemical damage to the lungs). An overdose as low as 2x may cause toxic symptoms. Labels for naphthalophos products usually advise the following (check the label and the SDS for information on the exact product that you intend to use) – if not followed, your animals are at risk of death:
- Do not mix with other drenches
- Do not dose at the same time as other drenches
- Avoid risk of dosing into the lungs (e.g., check length of nozzle appropriate for the animals’ mouths, do not dose in a cradle, do not dose stressed/ hot/ panting sheep)
- Do not treat thirsty, exhausted, or malnourished sheep
- Do not use a power drenching gun
- Ensure accurate dose by bodyweight
- Thorough shaking before use required
Abamectin is available in various products, as a pour-on, drench and injectable. It should not be used in lambs under 6 weeks/15kg and calves under 16 weeks/100kg, as they are especially sensitive to toxic effects. The age and weight limits vary between formulations, so check product labels. Toxicity has even been reported in young calves at foot whose dams were treated. However, toxicity can occur at any age, with older cattle showing toxic signs starting from a 5x overdose. Clinical signs are neurological: wobbly gait, weakness, drooling, pupil dilation, flaccid lips and tongue, coma, and death.
Levamisole is available in various products, as a pour-on, drench and injectable for cattle and sheep. Acute toxicity can be seen from doses at 3x the appropriate dose rate. An immediate overdose may cause – salivation, trembling, wobbly gait, urination, defecation, collapse, and death due to breathing difficulty. Longer-term exposure may cause depletion of white blood cells, i.e., reduce ability of your animals to fight infections.
Closantel (anthelmintic and flukicide) is available for sheep. Toxicity has been reported in sheep, goats, and cattle, at doses from 2.5x the recommended dose. Effects of toxicity include loss of appetite, wobbly gait, weakness, blindness, and death. Vision loss has been reported in humans exposed to it.
Albendazole can cause bone marrow depression, intestinal damage, diarrhoea and death at repeat doses in goats. In the case reported by Harm et al, the problems (anaemia, diarrhoea) seen by animal’s owner were wrongly assumed to be due to surviving worms when in fact it was drench toxicity. This shows the value of testing before treating!
Residue violation
Mis-using drenches in production animals can lead to excess chemical residues in animal products intended for human consumption. This endangers human health, product reputation, and Australia’s export markets.
In general –
- stick to the label guidance (this is APVMA approved, and the withhold periods depend on correct dosing),
- don’t repeat a dose sooner than the label permits (i.e. within the ‘re-treatment interval’ – if it hasn’t worked, you need to think about why, e.g. consult your vet or Drench Test,
- avoid inadvertently ‘doubling-up’ on an ingredient by checking the chemical name of what’s actually in a mixture & combination drenches carefully. This is true regardless of how the drenches are given (pour-on, injection or oral) and what purpose they are for (worms, ticks etc) – some ingredients may be included in many types of drench (e.g. macrocylic lactones, the mectin drugs). Overuse of one active may lead to residue violation (as well as increase the risk of resistance developing). It is best to use different actives for internal and external parasites
Explanation of drenches which contain more than one active ingredient.
More information on why combination drenches are effective.
Explanation of the broad drench chemical ‘families’ and their uses against worms, lice, flies and ticks.
Dairy: Many fewer products are registered for use in dairy animals than meat animals, due to residues making their way into milk for a long time. Some simply have a long milk WHP and may be used in the dry period, others must never be used in animals who will at some point have a career in producing milk for human consumption (including heifers/ ewe lambs/ young nanny goats).
Further advice directed towards management of parasites in dairy cattle and dairy goats & milking sheep.
Goats: often require higher doses to kill parasites than cattle or sheep, which can lead to residues lasting longer in the animal’s body. Read the label carefully. For any product not registered for goats, or which you might want to use at a dose rate different to the label, you legally need to obtain a veterinary prescription.
More information on drench use in goats.
References
Beynon, SA (2012) “Potential environmental consequences of administration of anthelmintics to sheep” Veterinary Parasitology 189 (1) 113-124
Borges, DGL; Borges, FA; Vieira, MC; Pupin, RC; de Araújo, MA; de Castro Guizelini, C; de Lemos, RAA (2021) “Poisoning of calves reared with cows treated with abamectin” Toxicon (203) 115-116
Harm, TA; Radke, SL; Burns, LE; Schrunk, DE (2022) “Enteropathy and bone marrow hypoplasia associated with presumptive albendazole toxicosis in a juvenile Boer goat” J Vet Diagn Invest. 34(6) 1015-1019
Müller, KR; Dwyer, C (2016) “Suspected levamisole intoxication in calves” NZ Vet J. 64(4):257-60.
Otter, A., Bell, S., Loewenstein, N., Hemingway, J., Floyd, T., & Wessels, M. (2018). Closantel toxicity in ewes through drinking cattle pour-on. The Veterinary Record, 183(17), 540
Pitman, B (2013) “Acute post drenching mortality in lambs” www.flockandherd.net.au/sheep/reader/drench-mortality-acute.html Flock & Herd accessed April 2023
Tabatabaei, SA; Soleimani, M; Mansouri, MR; Mirshahi, A; Inanlou, B; Abrishami, M; Pakrah, AR; Masarat, H (2016) “Closantel; a veterinary drug with potential severe morbidity in humans” BMC Ophthalmol 29;16(1):207
Watt, B; Bunker, E (2016) “A case of closantel toxicity in crossbred lambs” Flock & Herd www.flockandherd.net.au/sheep/reader/closantel-toxicity.html accessed April 2023
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