Europe’s Pigs Are Silently Losing Ground to Drug-Resistant Gut Worms

Gastrointestinal nematodes remain one of the most underappreciated threats to European pig production, quietly eroding growth rates, feed efficiency, carcass quality and reproductive performance across a sector that produces roughly 22 million tonnes of pork a year. A comprehensive new review published in Parasites & Vectors has synthesised decades of evidence on the three dominant worm species afflicting European herds, the large roundworm Ascaris suum, the nodular worms Oesophagostomum spp. and the whipworm Trichuris suis, and concluded that the current control paradigm, built overwhelmingly on prophylactic anthelmintic treatment, is neither sufficiently monitored nor sustainably equipped for the production systems now emerging across the continent. The authors, led by Luise Hohensee and Friederike Ebner of the Technical University of Munich, argue that without standardised surveillance and diagnostic-guided treatment, the industry risks both mounting losses and accelerating drug resistance.

The technical case for concern begins with the biology of the parasites themselves. A. suum undergoes a hepatotracheal migration, with larvae passing through the intestinal mucosa, the liver and the lungs before maturing in the jejunum, and this journey leaves characteristic white granulomas, so-called milk spots, on the liver that trigger condemnation at slaughter. Oesophagostomum spp. larvae embed in the submucosa of the caecum and colon, forming nodules before adults emerge into the lumen, while T. suis develops entirely within the large intestine, its whip-shaped anterior end threading into the mucosa of the caecum and colon. Each species follows a direct faecal-oral life cycle built around remarkably persistent eggs: A. suum and T. suis eggs can remain infective in the environment for at least four years, a durability that shapes virtually every management recommendation discussed in the review.

The productivity losses attributable to these infections are far from trivial, even when infections remain clinically silent. Depending on the parasite species and infection intensity, reductions in average daily weight gain range from one to thirty-one percent, corresponding to a daily loss of sixty to 128 grams of growth and roughly a four percent extension of the finishing period. Feed conversion ratios, the efficiency with which feed is converted into body mass, deteriorate by 14.9 to thirty-three percent. Carcass quality suffers in parallel, with infected pigs showing reduced carcass weight, backfat thickness reductions of 5.5 to 12.5 percent and lower lean meat percentage, changes that downgrade carcasses within the SEUROP classification system used across Europe to grade meat based on lean content. Historical estimates of the economic burden are striking: a European study calculated a 60.1 million euro increase in feeding costs attributable to worm infections in 2010 alone, and the same year an estimated 17.5 million euros were lost through liver condemnations.

Beyond productivity, the review highlights a growing body of evidence that gastrointestinal nematodes actively modulate the pig immune system in ways that worsen other diseases. Ascaris suum infection has been linked to increased lung pathology and reduced seroconversion following vaccination against Mycoplasma hyopneumoniae, one of the most economically important respiratory pathogens in swine. Migrating larvae impair pulmonary bacterial clearance, and experimentally infected pigs developed more severe disease after challenge with Bordetella bronchiseptica and Escherichia coli. Coinfection studies have associated A. suum with greater pulmonary consolidation in combination with M. hyopneumoniae, elevated mortality in piglets coinfected with Actinomyces pyogenes and Pasteurella multocida, higher Salmonella enterica burden, and more frequent detection of porcine circovirus type 2. The authors caution, however, that much of this mechanistic evidence derives from small experimental studies conducted in the 1990s, and its quantitative relevance to modern genetics, vaccination programmes and housing conditions remains untested.

There is also a zoonotic dimension that extends the significance of porcine worm control beyond the barn. Ascaris and Trichuris species infect humans, and molecular studies have documented hybridisation between human and pig Ascaris populations, indicating a bidirectional exchange between the two host species. People living near pigs show a 4.65-fold higher risk of ascariasis, and swine veterinarians are 1.5 times more likely to be seropositive for A. suum than their non-swine colleagues. Nematode eggs have been detected in wastewater and sludge used as fertiliser, with concentrations reaching up to 867 eggs per gram, and on fresh fruits and vegetables, raising food-safety concerns. The therapeutic use of Trichuris suis ova in experimental treatments for Crohn’s disease has even produced documented iatrogenic infections, a curious footnote that nonetheless illustrates how readily these parasites cross host boundaries.

Prevalence data across Europe, the review finds, are frustratingly inconsistent. Reported farm-level prevalence for A. suum spans from one to one hundred percent, for Oesophagostomum spp. from two to twenty-seven percent and for T. suis from 0.3 to seventy-five percent. The discrepancy reflects genuine differences between production systems, outdoor, organic and small-scale farms consistently report higher infection levels, but also profound methodological heterogeneity. Faecal egg counting methods such as the McMaster and FLOTAC techniques dominate surveillance for Oesophagostomum and T. suis, while A. suum assessment increasingly employs serological assays including the Ascaris haemoglobin ELISA and L3-larva ELISA, alongside slaughterhouse milk-spot inspection. Serological studies report farm-level seroprevalences of 91 to one hundred percent for A. suum, whereas faecal-based surveys frequently record individual-level prevalence below eight percent, a gap that likely reflects differences in test sensitivity, infection biology and sampling design rather than true epidemiological variation.

Slaughterhouse data provide one of the few large-scale, ongoing indicators. Recent German statistics from 2024 recorded 3,621,397 livers discarded from fattening pigs due to hepatitis parasitaria, the formal designation of milk-spot lesions, out of 43,511,330 pigs subjected to official meat inspection, implying at least an 8.3 percent liver condemnation rate, down slightly from approximately 9.8 percent in 2023. But even this figure demands careful interpretation: German inspection practice historically classified only livers with more than five milk spots as affected, and European regulations, while mandating liver inspection, do not define standardised criteria for recording or quantifying parasitic lesions, undermining cross-country comparability. Denmark reported milk-spot prevalence of up to 96 percent in a survey of organic farms, while Austria and Italy reported rates near 19 and 30 percent respectively, figures that almost certainly reflect both husbandry differences and inspection standards.

On the control side, the review delivers a sober assessment. Under EU Regulation 37/2010, only four chemical classes are authorised for deworming food-producing pigs: benzimidazoles, macrocyclic lactones, piperazine and imidazothiazoles. Prophylactic, blanket deworming without prior diagnostics remains the dominant practice in Europe, a strategy the authors identify as a key driver of anthelmintic resistance. Reduced efficacy and outright resistance have already been documented for Oesophagostomum spp. against both flubendazole and ivermectin, although comparable resistance has not yet been reported for A. suum or T. suis. Intriguingly, treatment efficacy appears to depend on dietary fibre content, with high-fibre rations potentiating drug activity. Management measures offer complementary leverage: all-in/all-out production, easy-to-clean housing surfaces, adequate drying periods between batches and pre-farrowing treatment of sows all reduce transmission, while slatted floors lower liver condemnation risk compared to littered systems. The welfare-driven move away from fully slatted flooring, however, threatens to reverse these gains by returning pigs to bedding systems where eggs persist.

The review’s forward agenda is unambiguous. The authors call for standardised, EU-wide surveillance integrating abattoir monitoring with serological testing, a shift from prophylactic to diagnostic-guided anthelmintic treatment modelled on equine parasite control programmes, routine tracking of emerging resistance, and long-overdue field-based intervention studies evaluating integrated control under commercial conditions. They also identify promising but immature alternatives, including dietary interventions with fermentable carbohydrates such as inulin and chicory-derived fibres, which reduce worm burdens in the large intestine through short-chain fatty acid production, probiotics that moderately decrease milk-spot risk, and selective breeding, since Danish hybrid studies revealed strong heritability of resistance to both A. suum and T. suis. Vaccine development remains a research priority but, given the substantial biological barriers and the absence of a market-ready product, is positioned as a future component of integrated control rather than an imminent replacement for diagnostics, management and targeted drug use. As organic production expands under the European Green Deal and welfare regulations reshape housing, the conditions favouring nematode transmission are set to intensify, making the review’s message increasingly urgent: Europe’s pig sector must modernise its approach to parasite control before the worms, and their resistance genes, outpace it.

Subject of Research: Control strategies and surveillance gaps for gastrointestinal nematode infections in European pig production

Article Title: Current strategies for controlling gastrointestinal nematodes of pigs in Europe

Article References: Current strategies for controlling gastrointestinal nematodes of pigs in Europe. (n.d.). https://doi.org/10.1186/s13071-026-07715-4

Image Credits: AI Generated

DOI: 10.1186/s13071-026-07715-4

Keywords: Ascaris suum, Oesophagostomum, Trichuris suis, gastrointestinal nematodes, pigs, anthelmintic resistance, Europe, pig production, zoonosis, surveillance, animal welfare, organic farming