When horses, sheep, or cats are treated for intestinal worms, the poison is spread through their faeces.
But it's not just the worms that are being wiped out.
These substances are harmful to a range of other small creatures, says soil scientist Erik Joner.
Insects that come to break down the faeces ingest deadly doses. The substances can also be spread with fertiliser. Medication residues can affect soil life.
Insects and other critters in the soil are very useful.
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"If we didn't have soil life breaking down waste, we'd be wading knee-deep in insect shells and dead plant matter," says Joner.
He is head of research at the Norwegian Institute of Bioeconomy Research (Nibio)'s Division of Environment and Natural Resources.
The dung remains
"When a cow pat falls to the ground, insects, worms, and roundworms will help themselves and ensure it disappears quickly," says Joner.
The numbers came from the Veterinary Medicinal Product Register. Among the drugs they examined, most were used for sheep and horses.
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Much is given preventively
More than half of the antiparasitic medications were given as a preventive measure – meaning the animals had not been confirmed to actually have the parasites they were treated for.
"A lot of treatment is administered based on poor assessment of need, whether it's for pets, horses, or livestock. It runs a bit on autopilot," says Joner.
A 2021 survey among sheep farmers showed that only 11 per cent took faecal samples beforehand to determine whether treatment was necessary.
At the same time, Joner stresses that untreated parasites can cause serious suffering in animals that genuinely need care.
Most of the antiparasitic drugs in the Nibio study were used for sheep and horses.(Photo: Joan Vadell / Shutterstock / NTB)
Several antiparasitic medications are given as a single oral dose. These substances are expelled from the body after a few days. It is during this short period that their faeces can harm insects.
Another type of treatment is called a bolus. It is given to heifers going out to pasture for the first time. In that case, a dose is released every three weeks throughout the summer season.
Dung beetles and springtails
There have been studies in Scandinavia on how small creatures are affected by antiparasitic drugs, says Joner, referring to several studies (see reference list below).
"They show quite strong effects, especially on dung-dwelling organisms like dung beetles. Dung flies lay their eggs in this toxic pool," says Joner.
In Norway, about half of the dung beetle species are considered threatened.
"Those that live in cultivated land are declining the most. That could indicate that antiparasitic drugs are having an impact," he says.
Other organisms affected include mites, springtails, roundworms, and single-celled organisms.
"There are effects on nearly the entire food chain in the soil," he says.
Unclear what the consequences are
How serious is the spread of antiparasitic drugs?
Are we just seeing a few insect deaths over a short period, or does the use have broader, long-term impacts?
"We really don’t know. There’s not much solid information on this even across Europe," says Joner.
Still, Joner believes antiparasitic drugs could be one of the reasons why some international studies report a decline in insect populations.
"Anything that systematically harms soil life, like antiparasitic drugs, will eventually come back to affect us," he says.
Even the small creatures we consider annoying are part of the ecosystem and can play important roles in food production and plant growth.
Ask the vet
Questions that Joner and other researchers are investigating include how quickly the toxins break down in different climates and whether they disappear when dung is composted.
"Is there anything animal owners can do to limit the spread?"
"Just being aware of the issue helps – and asking the vet whether treatment is truly necessary," he says.
You can also ask what it would cost to first check if the animal actually has parasites, Joner suggests.
Brings attention to an important issue
Marit Jørgensen Bakke is an associate professor at NMBU's Faculty of Veterinary Medicine and researches pharmacology, drug resistance, and parasites in animals.
She says Erik Joner is bringing attention to an important issue.
Bakke says it is possible that the substances can harm soil organisms and be washed into water sources, affecting aquatic life as well.
She explains that new guidelines are being developed that will include environmental considerations.
"We must prioritise the health of the animals. Beyond that, the medications should be used sensibly, and use should be limited where possible," she says.
For example, animals can be kept off pasture for a few days after the treatment, or the dung can be collected afterwards.
Tick medication in water
What about the treatments used on dogs and cats? Do they spread into the environment?
Bakke points to a study from England where flea and tick treatment chemicals were found in 19 out of 20 rivers. In several of them, concentrations were above what is considered a safe limit.
"These are the same substances that have been banned in agriculture because they are so persistent and harmful to the smallest organisms," says Bakke.
Not all dogs may need tick treatment, says Marit Jørgensen Bakke.(Photo: Chutima Chaochaiya / Shutterstock / NTB)
Tick treatments are often applied to the neck and spread through the skin's fat layer.
If a treated dog sleeps in your bed, the chemicals can transfer to the sheets. When the bedding is washed or the dog is bathed, the tick medication ends up in wastewater.
"Sheep and cattle are also treated with products that are poured onto the skin," says Bakke.
These substances may then rub off on the grass and end up in nature.
Bakke suggests we should reconsider whether all dogs and cats really need to be treated for ticks.
"How much tick activity is there in different parts of Norway? Do pets need protection year-round? A lot of the use probably stems from owners feeling uneasy about parasites," she says.
"Avoid unnecessary treatment"
Inger Sofie Hamnes is a veterinarian and senior researcher at the Norwegian Veterinary Institute. She works on parasitic diagnostics in animals.
Hamnes finds it reasonable to believe that parasiticides affect organisms living on and under dung, and that the chemicals can bind to particles and spread into water sources.
"The guiding principle is that we should avoid unnecessary treatment and preferably treat based on test results, using the narrowest-spectrum drug possible," she says.
According to Hamnes, there is still room for improvement here.
However, sometimes preventive treatment is necessary.
"Sometimes you know from experience that if you skip treatment, lambs will fall ill and die within a few weeks. That doesn't mean you should stick to the same routine forever," she says.
Depends on the animal's lifestyle
There are some exceptions to the recommendation of testing first. Kittens and puppies, for example, are often infected with a type of roundworm through their mother's milk and should be treated when young.
Adult cats and dogs should be treated based on actual needs.
"There are many dogs and cats that don't have a great need for treatment," says Hamnes.
She explains that it depends on the animal's lifestyle. A dog that roams outdoors a lot is more likely to be exposed to parasites, unlike an indoor cat.
"If you look at the amount of faeces containing antiparasitic substances left in nature, dogs may not actually be the worst offenders. Many owners pick up after their pets and dispose of the waste properly," she says.
