by Joe Eaton
(Full article from RATS Tales July 2025)
Among other unusual attributes, hedgehogs are resistant to viper venom. (However, they don’t harvest fruit by impaling it on their spines, a myth promulgated by the Roman naturalist Pliny the Elder and given credence by Charles Darwin!)
According to a recent Belgian study, though, these resilient little pincushions are susceptible to anticoagulant rodenticides (ARs) to which they’re exposed by eating slugs that have fed at poison-bait boxes. This is the latest of several studies documenting ARs in European hedgehogs, a possible factor in their alarming decline.
Hedgehogs are venerable as well as vulnerable creatures, with a fossil record extending back eight million years after the demise of the non-avian dinosaurs. Seventeen species occur in Europe, Asia, and Africa; hedgehogs once inhabited North America as well, with fossils found in British Columbia, Oregon, California, Florida, and Oaxaca. The 52-million-year-old British Columbia specimen was only two inches long. Hedgehogs are related to moles and shrews and more obscure mammals like gymnures, desmans, and solenodons. They’re omnivores, with a diet that includes land mollusks, insects, carrion, eggs, and pet food.
The European hedgehog is classified as “near threatened”; for instance, the current English population is 97 percent lower than in the 1950s. Habitat loss (including the destruction of hedgerows), urban sprawl, agricultural intensification, traffic, disease, and predation by badgers have been implicated, and there’s growing concern about ARs as a factor. (Ironically, in New Zealand–where they were introduced by homesick colonials–hedgehogs have become a pest endangering native wildlife).
The first report of AR exposure in European hedgehogs came from Britain in 2010. Claire Dowding and co-authors detected the second-generation AR (SGAR) difenacoum in 47.5 percent of 120 hedgehogs admitted to rehabilitation facilities in Scotland, Wales, and southern England. Two-thirds had residues of both first-generation ARs (FGARs) and SGARs. Frequency of exposure was higher in males than females, and levels were comparable to those previously reported for British polecats and owls. Incomplete location data prevented a comparison of rural and urban exposures.
A subsequent study of mammals and birds in two regions of Spain found that half the sampled hedgehogs had residues of the SGAR brodifacoum and that hedgehogs had even higher AR levels than raptors. Last year, Sophie Rasmussen (widely known as “Dr. Hedgehog”) and other Danish scientists documented ARs in 84 percent of 115 hedgehog carcasses; the SGAR bromadiolone had the highest prevalence (79 percent) with an average concentration of 118 nanograms per gram. Hedgehogs killed by cars had higher bromadiolone levels than those that died of natural causes, but this could reflect the high number of roadkills in the sample. Habitat, age, sex, and disease were not associated with AR exposure.
Other researchers investigated the importance of slug consumption as an exposure pathway. In 1976 D. W. Yalden published the first analysis of hedgehog stomach contents, with slugs accounting for four percent by weight in samples from an English country estate. Hussein Alomar and other French scientists reported in 2018 on a study in which slugs were allowed to consume brodifacoum in a field. Their AR intake was used to determine a toxicity/exposure ratio for slug-eating hedgehogs based on Yalden’s data and the known toxicity of brodifacoum to rats. (No lethal dose has been determined for hedgehogs.) Alomar and his co-authors calculated that hedgehogs would receive a lower dose of brodifacoum than shrews or starlings, which also eat slugs.
In the most recently published study, appearing this year in Science of the Total Environment, Valérie Miserez of Ghent University and her collaborators looked at AR exposure in hedgehogs and slugs in Belgium and experimented to determine how hedgehogs were affected by ARs. Of 128 hedgehogs brought to two Belgian wildlife rescue centers in 2023 and 2024, all had been exposed to at least one AR. Prevalence rankings: 100 percent for the SGAR difenacoum, 99 percent for brodifacoum, 98 percent for bromadiolone, 72 percent for the FGAR coumatetralyl, and 30 percent for the FGAR chlorophacinone. Brodifacoum had the highest average (437 nanograms per gram) and maximum (6743 nanograms per gram) concentrations. Highest concentrations were found in areas with the densest human populations. Exposure wasn’t associated with hedgehog diphtheric disease, a bacterial disease widespread in Belgium. None of the hedgehogs had signs of internal bleeding.
Slugs were collected from four different habitats (residential, park, forest, and agricultural) around Antwerp. Thirteen of 120 had traces of ARs, averaging 0.27 nanograms per gram. Chlorophacinone was the most prevalent compound. The urban samples ranked highest in AR exposure; one specimen, from a residential area, had residues of five SGARs.
In the laboratory, the researchers used captive-bred African pygmy hedgehogs as stand-ins for European hedgehogs, giving them an oral dose of brodifacoum based on the French’s groups calculations on slug consumption and New Zealand data on beetle consumption. “The aim was to simulate a realistic but subclinical brodifacoum exposure scenario,” Miserez explained in an e-mail. At the end of six weeks, two of the nine experimental subjects had bleeding inside their kidneys. “The findings of this study indicate that short-term exposure to low doses of ARs may have sublethal effects on animals without manifesting clinical symptoms,” Miserez and co-authors concluded.
“We do not know whether there is a difference in slug uptake [by hedgehogs] between rural and urban areas,” Miserez noted. “However, our study did show that slugs in urban areas are more exposed to rodenticides, which was related to the concentrations found in hedgehogs from urban environments. Since hedgehogs are omnivores, it cannot be ruled out that they occasionally feed on the carcass of a poisoned rodent or directly from bait.” But the Belgian data strengthens the case for a slug-based exposure pathway.
