How to Treat Worms in Pigs: Dewormers and Schedule

Treating worms in pigs relies on a small number of proven deworming drugs given at key points in the production cycle, combined with good hygiene and periodic fecal testing. The three main drug classes used in swine are benzimidazoles (like fenbendazole), macrocyclic lactones (like ivermectin and doramectin), and imidazothiazoles (like levamisole). When you use them, how often, and in what combination with management practices matters as much as which product you pick, because worm burdens that go unchecked can cut daily weight gain by roughly a third and worsen feed conversion considerably.

The Worms You Are Actually Dealing With

Pigs worldwide harbor a fairly predictable cast of internal parasites. On most farms, the dominant species are large roundworms (Ascaris suum), nodular worms (Oesophagostomum spp.), and whipworms (Trichuris suis). A survey of pig farms in Poland found parasite eggs or oocysts on over 80% of the farms examined, with Oesophagostomum detected on nearly 69% of farms and Ascaris on about 29%.¹ In tropical and subtropical climates, parasite diversity and prevalence tend to be even higher. A study of pig farms in Thailand detected gastrointestinal parasites in about 85% of fecal samples, with strongyle-type worms in over 82% and Ascaris in nearly 69%.²

A few less common but still important parasites round out the picture. Threadworms (Strongyloides spp.) mainly affect young piglets. Lungworms (Metastrongylus spp.) are a concern for pigs with outdoor access, since the larvae live inside earthworms that pigs root up and eat. In one documented outbreak, about 11% of affected pigs died over a three-week period, with treatment only partially effective once disease was advanced.³ Kidney worms (Stephanurus dentatus) appear mainly in warmer regions and can persist for years in the urinary tract and surrounding tissue. The key point for your deworming plan is that different worms hit different age groups hardest. Coccidia and threadworms peak in suckling piglets, Ascaris and whipworms peak in growers and finishers, and nodular worms are most common in sows.¹

What Worms Actually Do to Your Pigs

Light infections often fly under the radar. Pigs can carry moderate worm burdens without obvious clinical signs, which is part of why routine deworming matters: by the time you notice a problem, the damage to growth and feed efficiency has been accumulating for weeks. A meta-analysis pooling data from multiple studies found that parasitized pigs gained about 31% less weight per day and ate about 5% less feed compared to parasite-free pigs, while also converting that feed into body weight about 17% less efficiently.⁴ Those numbers represent serious economic losses over the course of a finishing period.

Heavy Ascaris infections cause the most visible damage. As larvae migrate through the liver, they trigger white spots (sometimes called “milk spots”) from multifocal inflammation and fibrosis. When larvae pass through the lungs, they can cause pneumonia, pleurisy, and asthma-like respiratory signs.⁵ Liver condemnations at slaughter are one of the most common ways producers discover they have a roundworm problem, since the white spots persist long after the larvae have moved on. Whipworms burrow into the lining of the large intestine and cause bloody diarrhea in heavy infections. Nodular worms create small abscesses in the intestinal wall that reduce nutrient absorption. Even when pigs look clinically healthy, the hidden drag on growth is real and measurable.⁶

The Three Main Dewormer Classes

Nearly all pig deworming programs rely on drugs from three chemical families. Each works differently and has its own strengths, so understanding the basics helps you pick the right tool.

Benzimidazoles

Fenbendazole (sold under brand names like Safe-Guard and Panacur) is the most widely used benzimidazole in swine. It is typically mixed into feed, which makes it practical for treating groups. In a controlled trial, fenbendazole given at a total dose of 9 mg per kilogram of body weight was 100% effective against Ascaris suum regardless of whether the dose was spread over three, six, or twelve days. Against whipworms, the shorter three-day course performed best at 100% removal, while longer courses were slightly less effective.⁷ Fenbendazole’s broad spectrum, feed-mixing convenience, and wide safety margin make it a workhorse for many operations. It works by disrupting the worm’s ability to absorb nutrients at a cellular level, killing them relatively slowly over a few days rather than all at once.

Macrocyclic Lactones

Ivermectin (Ivomec and generics) and doramectin (Dectomax) are injectable products that work by paralyzing worms. They cover a broad range of internal parasites and also kill external parasites like mange mites and lice, which makes them a popular two-for-one choice. Ivermectin given by injection at 0.3 mg/kg is the standard swine dose for most gastrointestinal worms. For kidney worms specifically, a higher dose of 0.5 mg/kg proved effective in a trial where treated sows shed all their kidney worms within four days and remained parasite-free through necropsy two months later.⁸ The drawback of injectables is that each pig needs individual handling, which adds labor on larger operations.

Levamisole and Pyrantel

Levamisole is available in injectable, pour-on, and water-soluble forms. It paralyzes worms by overstimulating their nervous system and is effective against roundworms and nodular worms, though it has a narrower safety margin than fenbendazole or ivermectin. Pyrantel tartrate is an in-feed dewormer often used continuously at low doses in starter and grower rations to prevent Ascaris establishment. One important safety note: combining levamisole and pyrantel is dangerous. Research showed that pyrantel, while safe on its own at deworming doses, lowered the lethal dose of levamisole from about 40 mg/kg to about 28 mg/kg when both drugs were given together.⁹ Never use these two drugs at the same time or in close succession.

Building a Deworming Schedule

There is no single universal schedule, because the right timing depends on your housing system, climate, herd size, and which parasites are present. That said, most veterinary guidelines share common principles you can adapt.

For breeding herds, the most critical treatment window is sows before farrowing. Treating sows about one to two weeks before they enter the farrowing crate reduces the parasite load in the environment where newborn piglets will live. Many producers use ivermectin or doramectin for this pre-farrowing dose because it also knocks out mange mites, preventing sow-to-piglet transmission of sarcoptic mange. Some operations add a second sow treatment at weaning or when sows return to breeding pens.

For growing pigs, a common approach on indoor operations is to treat once when pigs enter the nursery or grower barn (around 8 to 10 weeks of age) and again when they move to finishing, roughly 16 to 20 weeks. Outdoor and pasture-raised pigs generally need more frequent treatment because their exposure to infective eggs and larvae in soil is continuous. Treating every 6 to 8 weeks through the growing period is a common recommendation for pastured pigs. Pyrantel tartrate fed continuously at low levels is another strategy some producers use in nursery and grower diets to suppress roundworm development between strategic treatments.

Gilts and boars entering the herd should be treated and quarantined before introduction, ideally with a fecal test to confirm they are not bringing resistant parasites with them. This quarantine deworming is one of the most overlooked steps and one of the easiest ways new parasite problems enter a clean herd.

Why Fecal Testing Matters More Than Most Producers Think

Treating on a fixed schedule without ever checking whether the treatment is working is a gamble. Fecal egg counts are the standard diagnostic tool: you collect fresh fecal samples, process them in a flotation solution, and count the eggs under a microscope. The test is cheap and gives you two critical pieces of information. First, which parasites are present and roughly how heavy the infection is. Second, and perhaps more important, whether your dewormer is still working. A fecal egg count reduction test, where you compare counts before and after treatment, is the simplest way to catch early signs of drug resistance.

Fecal testing has real limitations, though. Egg counts can underestimate the true number of infected pigs. Serological testing in one study detected Ascaris exposure in over 50% of fatteners, while fecal egg counts in the same group came back positive in only about 16%.¹⁰ False positives are also a known issue. Pigs can pass Ascaris eggs in their feces simply from having swallowed eggs that transit through the gut without establishing an actual infection. In indoor experiments, the proportion of false-positive fecal samples ranged from 4% to 36%.¹¹ On pasture, where pigs constantly ingest contaminated soil, false positives are less common because most egg-positive pigs truly are infected. The practical takeaway is that fecal egg counts are a useful but imperfect tool. A negative result does not guarantee your pigs are worm-free, and a positive result in indoor pigs should be interpreted with some caution. Testing multiple animals and testing repeatedly gives you a more reliable picture than any single sample.

The Growing Problem of Dewormer Resistance

Pig producers have historically worried less about drug resistance than sheep or goat farmers, where resistance to all major dewormer classes is widespread and sometimes devastating. That era of complacency may be ending. Resistance to pyrantel, levamisole, and benzimidazole drugs in Oesophagostomum (nodular worm) species in pigs has been documented in various countries.¹² More concerning, ivermectin resistance in the same worm genus was confirmed for the first time in UK pigs, meaning all three major drug classes now have documented resistance in at least one pig parasite species.¹²

Swedish data paints a similar picture. On farms tested after deworming, three out of nine herds positive for Oesophagostomum showed fecal egg count reductions below 90%, the threshold below which treatment is considered to be failing. All three of those herds had been treated with injectable ivermectin. In contrast, five farms using fenbendazole achieved reductions between 95% and 100%.¹³ Ascaris suum treatments remained effective across the board regardless of drug used, which is one piece of good news.¹³

Practical steps to slow resistance development include rotating between drug classes rather than relying on a single product year after year, treating based on diagnostic results rather than on a purely calendar-based schedule, avoiding underdosing (which gives partially resistant worms a survival advantage), and quarantine-treating all incoming stock. If you suspect resistance, work with your veterinarian to run a proper fecal egg count reduction test before and after treatment. Catching reduced efficacy early gives you time to switch strategies before the problem becomes entrenched in your herd.

Environmental Management and Pasture Hygiene

Dewormers kill the worms inside the pig, but they do nothing about the millions of eggs already sitting in the environment. Ascaris eggs are remarkably tough. They can survive in soil for years, even through freezing winters, and yearly pasture rotations are not enough to break the cycle for parasites with such long-lived eggs.¹⁴ This is why even well-dewormed pigs on contaminated ground can quickly become reinfected.

Indoor systems with slatted floors have a natural advantage. Fecal material falls away from the pigs, and the concrete or metal surface can be cleaned and disinfected between groups. Intensive production systems using slatted floors, all-in/all-out management, and thorough pressure-washing between batches tend to have much lower parasite burdens than operations using bedding, straw, or outdoor lots.¹⁵ For outdoor and organic systems where eliminating soil contact is not an option, shorter pasture rotations, avoiding overstocking, composting manure at high temperatures before spreading it, and keeping wallowing areas well-drained can all reduce transmission. The Polish farm data confirmed that smaller farms with paddock access and litter floors had higher parasite prevalence than larger, more intensively managed operations.¹

Natural and Alternative Dewormers

Interest in herbal and non-chemical dewormers is strong among organic and small-scale producers, partly because organic certification programs restrict or prohibit conventional anthelmintics. Plants with historical use as pig dewormers include garlic, wormwood (Artemisia species), pumpkin seeds, black walnut, and peppermint. An ethnobotanical survey in British Columbia documented the use of these and other plants by organic farmers, holistic veterinarians, and herbalists, finding that some had moderate to high traditional validity as anthelmintics, though rigorous clinical evidence for most remains thin.¹⁶

One of the few field trials on an alternative product tested SwineX, a proprietary herbal blend, against levamisole in pasture-raised pigs. Pigs treated with SwineX had statistically similar final weights, daily gains, and hanging weights compared to pigs treated with levamisole.¹⁷ That sounds encouraging, but a single trial measuring growth performance rather than actual worm counts is a long way from proof that the product reliably kills worms. The pigs may have performed similarly because parasite loads were low in both groups, or because the production environment mattered more than the dewormer. If you choose to use herbal products, treat them as supplements to good management rather than as replacements for proven drugs, and monitor fecal egg counts to verify they are working.

Farm Size, Housing, and Infection Patterns

The type of operation you run shapes your parasite risk profile dramatically. Large commercial indoor farms with slatted floors, strict all-in/all-out flow, and regular power-washing between groups often have low worm burdens and may need only strategic sow treatments and occasional monitoring. Small and medium farms, especially those using deep bedding, outdoor runs, or continuous-flow housing where different age groups share space, face much higher parasite pressure. The Polish survey found that simultaneous infection with multiple parasite species was more common than single-species infection, and that farms not using all-in/all-out management had higher odds of carrying parasites across the board.¹

Pasture-based and heritage-breed operations have the highest worm challenge because pigs are rooting in soil that accumulates eggs over time. These systems benefit most from frequent fecal monitoring, shorter treatment intervals, and creative pasture management such as rotating pigs with ruminants (which do not share the same worms) or leaving paddocks vacant for extended periods. Even so, research on Ascaris eggs suggests that a single year’s rest may not be enough to make a pasture safe again.¹⁴

Can You Breed Pigs That Resist Worms?

One of the more intriguing long-term strategies is genetic selection. Research has found that the tendency to resist roundworm and whipworm infections is surprisingly heritable in pigs. In experimental infections, between 29% and 73% of the variation in fecal egg counts could be attributed to genetic factors, depending on the parasite species and time after infection.¹⁸ There were also strong genetic correlations between resistance to Ascaris and resistance to Trichuris, meaning that selecting for resistance to one would likely improve resistance to the other as well.¹⁸

This has led researchers to suggest that breeding for parasite-resistant pigs could become a practical reality.¹⁹ In practice, no major breeding company currently markets parasite resistance as a selection trait, in part because indoor commercial herds have low enough worm pressure that it has not been an economic priority. But as drug resistance grows, outdoor and organic production expands, and consumer demand for reduced antiparasitic use increases, genetic resistance could become a more attractive complement to chemical control. For individual producers, the takeaway is simpler: if you consistently notice that certain sow lines carry heavier worm burdens than others despite identical management, the difference may be genetic, and favoring the cleaner lines in your breeding decisions is a free form of parasite control.

Lungworms and the Earthworm Connection

Lungworms deserve special mention because their transmission route is unlike any other pig parasite. Metastrongylus larvae cannot infect pigs directly from the environment. Instead, they develop inside earthworms, and pigs pick them up by eating those earthworms while rooting. This means lungworms are almost exclusively a problem for outdoor pigs. Indoor pigs on concrete or slatted floors virtually never encounter them.

Lungworm infections cause coughing, labored breathing, and reduced growth. In a documented outbreak, roughly a quarter of affected pigs needed antibiotic treatment for secondary bacterial pneumonia, and about a quarter of those treated pigs still died.³ Early treatment mattered: pigs treated at the first signs of respiratory disease fared much better than those treated later. Standard dewormers like fenbendazole, ivermectin, and levamisole are all effective against adult lungworms, but the damage from larvae migrating through lung tissue and the secondary bacterial infections that follow are harder to undo. If your pigs are outdoors and you start seeing widespread coughing, act fast rather than waiting for a fecal test, since lungworm eggs are shed intermittently and can be tricky to detect.

Kidney Worms in Warm Climates

Stephanurus dentatus, the swine kidney worm, is a parasite of tropical and subtropical regions. Larvae penetrate through the skin or are ingested, then migrate extensively through the liver, lungs, and other organs before settling around the kidneys and ureters, where adult worms can live for years. The damage to kidneys and liver leads to organ condemnations at slaughter, and heavy infections cause unthriftiness and poor growth in young stock.

Ivermectin at 0.5 mg/kg by injection proved highly effective in a trial on naturally infected sows. Treated sows passed worms in their urine within the first four days and were completely free of kidney worms at necropsy 61 days later. Control sows, by comparison, harbored an average of 65 kidney worms each.⁸ Prevention in endemic areas relies on keeping young pigs off heavily contaminated pastures and treating all breeding stock regularly to reduce environmental contamination. Because the worm’s life cycle can involve earthworms as transport hosts, the same outdoor-access risk factors that promote lungworm transmission apply here as well.

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