Roundworm Resistance – Backyard Poultry

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Common intestinal parasite affecting poultry still being widely researched. Learn more about roundworm resistance.

Story by Sue Norris

Roundworms are found wherever mammals and birds are found. As the most common intestinal parasite in birds and mammals worldwide, even healthy animals — and humans — can suffer from roundworms. The chicken roundworm, Ascaridia galli, is a type of parasitic nematode worm. These worms can grow to 4.5 inches (115 millimeters) long or longer. A severe overload of worms can lead to intestinal blockage, which can kill the bird.

Roundworm Life Cycle

The life cycle of these parasites is known as a direct life cycle, meaning it can be transmitted directly between host animals without an intermediate host from another species.

Chickens ingest roundworm eggs from a contaminated source, and the hatched larvae head for the intestinal lining where they attach themselves, extracting nutrients from the host. After they are fully grown, the worms will start to shed eggs to be expelled in poultry poop.

While primarily found in the intestinal tract, roundworms can migrate to the esophagus, proventriculus, or oviduct. If the infection’s particularly severe, worms can occasionally be found inside a hen’s newly laid eggs. The full life cycle can be completed in as little as 35 days in a chicken.

Roundworm in chicken feces.

Immunity and Infection

Chicks will develop some immunity to these parasites by three months, but if the worm load is high, they’ll start to exhibit symptoms. Older birds may not handle an infestation as well as younger birds.

Symptoms of infestation are failure to thrive, decreased appetence, weight loss, prominent keel bone, pale comb and wattles, tatty-looking feathers, decreased activity, and depression. A fecal count, usually analyzed by your veterinarian, can confirm worms and the severity of infection.

Generally, roundworms are easily treated with worming medication, but this may change in the future. Veterinarians have known for some years that these parasites are developing resistance to the current treatments available. In fact, roundworm resistance was seen in sheep as far back as the 1980s, reported by Dr. Glen Fahnestock, a practicing rural veterinarian in Yates County, New York.


Roundworm resistance presents unique challenges for controlling intestinal parasites not only in chickens but also in turkeys. The cecal worm, Heterakis gallinarum, is of particular concern. Their resistance is bad news, according to Dr. R. Kaplan, who discovered the connection while working at the University of Georgia, because the worms spread the protozoa Histomonas meleagridis that causes blackhead disease.

Blackhead damages the liver and intestines of affected turkeys. There’s no effective treatment for the disease and infected flocks may suffer between a 70% to 100% mortality rate.

Resistance and Research

What does increasing resistance mean for us and our flocks? To start, now may be a good time to re-evaluate your worming protocols.
Some folks worm their flock regularly. Others have fecal float tests performed periodically and treated accordingly. Yet others never worm or test as long as their flock is asymptomatic.

Dr. Fahnestock believes, along with many of his colleagues, that the overuse of worming medication is leading to higher resistance and that poultry should be assessed prior to anthelmintic treatment (worming). Some flock members may carry a moderate load of worms and be in fine health while others may carry the same load and present as unthrifty birds.

Checking your birds by “eyeball” evaluation is an important diagnostic tool. How does your bird look and feel? How does its poop look? If the poop looks suspicious, get a fecal test done.

There are a few treatments available, but fenbendazole (Flubenvet) was the only one currently FDA-approved that did not require an egg-withdrawal period. Other drugs, such as albendazole, ivermectin, levamisole, Hygromycin-B can be effective but require a period of egg withdrawal. All come with various warnings and should only be used under veterinary direction. Severe infestations may require more than one drug for treatment.

There are herbal products out there that put an emphasis on gut health. Products such as Verm-X claim to enhance gut health and say that a healthy gut is much better equipped to deal with parasites. Dr. Fahnestock actively promotes the use of pre- and pro-biotics for the improvement of gut health in all species, not just poultry.

Several companies are investigating the use of the more traditional and nutritious worm treatments that have been used by small-scale poultry farmers for years. Known as ethno-veterinary practices, you can read articles on the topic at

Pumpkin seeds and wormwood are two plants currently under the microscope in the world of phytogenics. Scientists find that, though pumpkin seeds themselves aren’t strong enough to deworm, concentrated extracts from these two plants are quite effective.

The Olmix Group is investing in seaweed extracts. So far, the extracts are showing great promise, not only as nutritional supplements, but also for their effect on helminths (worms).

While the work continues to uncover the hidden powers of these and other plants, as small-scale poultry keepers, we can, of course, continue to use plants that are available to us. According to Dr. Fahnestock, some “old-time” practices may continue to have value in keeping worm loads at an optimal level.


While some folks may think the practice old-fashioned and a step backward to cages, keeping the birds off the ground — that is, allowing the poop to fall through a wire mesh grid and not allowing the birds to walk on the ground — can effectively limit the number of worm eggs that birds are exposed to. This may be a reasonable solution if you keep birds in a small area.

Concrete or other similar flooring material that can easily be hosed off will help to diminish the chances of re-infestation.

If the flock is unable to free-range, the flock owner can still provide good forage to the flock by feeding greens, such as kale, chard, and other vegetables on a regular basis, in addition to the regular feed. Having mobile tractors that move regularly can keep the ground from becoming heavily contaminated.

Dr. Fahnestock recommends using this rotational system in conjunction with mowing the pasture frequently and keeping the grass short, allowing the sun to kill worm eggs. Some people still use lime on the ground to sanitize the area, but no significant data points to whether this really helps to keep the worm load low.

Given that the resistance to current treatments is likely to increase over time, you might consider limiting the use of medications if your flock is not showing any signs of infestation. Using probiotics, along with a good nutritional diet of quality feed, seeds, and greens, will help to keep your flock’s guts healthy and hearty.

SUE NORRIS was born and raised in the United Kingdom. After traveling around the world as a registered nurse, she settled in New York State about 25 years ago with her partner. She’s happily retired on 15 rural acres with 40-ish chickens, four rabbits, two dogs, three cats, and assorted wildlife.


Originally published in the October/November 2023 issue of Backyard Poultry magazine and regularly vetted for accuracy.

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