Roundworms, also called nematodes, are a group of worms from which more than 28'000 species have been described. They are considered as an own systematic group called Nemalthelminthes.

More than 16'000 roundworm species are parasitic of plants and animals, including cattle, sheep, goats, pig, poultry, horses, dogs, cats as well as many other wild and domestic animals, humans and also plants. Roundworms of veterinary importance are all obligate parasites, i.e., they cannot complete development without parasitising their hosts.

Gastrintestinal roundworms (Trichostrongylus axei)

Roundworms have a more o less long tubular form, and a digestive system with two openings, mouth and rectal opening, in contrast with flukes that have only one opening, and tapeworms that have no digestive tube at all. The mouths of many species have species-specific structures for attaching to the host and/or feeding on its tissues, e.g. teeth, cutting plates, hooks, etc.

Most roundworms have a slender form, with thinner head and tail. Most species are rather small (< 1 cm) but several parasitic roundworms can be over 50 cm long. Their body is covered with a cuticle, which is flexible but rather tough and has various types of species-specific structures such as ridges, rings, bristles, etc. They have no external signs of segmentation. Besides a tubular digestive system they have a nervous system but no excretory organs and no circulatory system, i.e. neither a heart nor blood vessels. Most species are bisexual, i.e. there are male and female individuals, males being often smaller than females, whereas a few species are hermaphroditic.

In female roundworms the uterus ending often shows species-specific morphology. Males have usually one or more chitinous spicules for attaching to the female during copulation, which also show a species-specific morphology. Males of many roundworm species have a so-called copulatory bursa, which is a funnel-like extension of the cuticle used for grasping the female during copulation. The bursa has species-specific forms and structures (rays, lobes, etc.) that are used for determining the species.

Infections with roundworms are called nematodosis.


Within the parasitic worms (= helminths) roundworms include the largest number of species parasitic of livestock, horses, dogs and cats, which can be very harmful, sometimes even fatal, e.g. lungworms (Dictyocaulus spp), stomach worms (e.g. Haemonchus spp, Ostertagia spp), etc. In fact, most veterinary medicines against parasitic worms target roundworms, particularly gastrointestinal worms, whereas flukes and tapeworms are often a minor issue, or at least not as harmful as roundworms.

In this site you can find additional general information on parasites and on parasitic worms.

Life cycle of roundworms

Copulatory bursa of Haemonchus contortus

Most parasitic roundworms have direct life cycles, i.e. the free-living stages do not need an intermediate host for development but infect directly their final host, where they migrate to their predilection sites and complete development to adults.

Inside the final host pregnant females produce thousands of eggs that are usually excreted with the feces of the host and contaminate pastures, rivers, lakes, etc.

Under favorable climatic conditions young L1 larvae hatch out of the eggs in a few hours. Under adverse conditions egg hatching is delayed or the eggs may even die. 

Hatched L1 larvae live in the vegetation or in the aquatic environment and feed on bacteria and other microorganisms. Since their skin is rigid, they have to molt for growing. L1 larvae molt to L2 larvae, and later L2 larvae molt to L3 larvae. In most species L1 and L2 larvae are not infective, i.e. if ingested by a suitable final host they will not complete development but are digested. The infective stage is usually the L3 larva. L3 larvae of several gastrointestinal nematodes often swim upward in the water the covers the plants to reach their highest parts where they are more likely to be ingested by grazing animals. 

Once inside the final host L3 larvae migrate to their predilection site (usually one or a few related organs) where they complete their development to adults and reproduce. Depending on the species migration is short and simple (e.g. down the digestive tube to the stomach or the intestine), or rather long and complicated (e.g. to the kidneys through the liver and the lungs).

Under certain conditions (drought, cold weather, etc.) L4-larvae of certain species can interrupt their development inside the host for several months and become inactive, a phenomenon called hypobiosis. Such larvae are called arrested, inhibited, hypobiotic or dormant larvae. The mechanisms that initiate or finish hypobiosis are not completely elucidated.

Some roundworm species (e.g. Onchocerca spp, the eyeworm Thelazia spp, etc.) have indirect life cycles that require a passage through specific intermediate hosts (e.g. insects, snails, etc.).

Mixed infections

Female genital opening of Haemonchus contortus

Livestock infections with gastrointestinal roundworms are mostly mixed infections, especially in ruminants (cattle, sheep, goats) and horses. This means that several worm species infect simultaneously all the animals in a herd or a property. 

The typical species involved in mixed infections in cattle, sheep and goats are those of the genera Bunostomum, Cooperia, Haemonchus, Mecistocirrus, Nematodirus, Oesophagostomum, Ostertagia (=Teladorsagia) and Trichostrongylus. The specific mix within a herd and the predominance of one or more species strongly depend on climatic and ecologic conditions, season of the year, livestock breeds, pasture management, etc. The same happens in horses with the Small & the Large Strongyles and other equine gastrointestinal roundworms.

Such mixed infections complicate preventative measures. Prevention is often based on the life cycle of the worms, their behavior and survival off the host, etc. When several species are involved, what is best against one species may favor other species.

Mixed infections make chemical control with anthelmintics more difficult. For example, some worm species may be controlled at a given dose of an anthelmintic, but other species may need a higher dose. Or some species may be already tolerant or even resistant to several anthelmintics, others not, etc. This is an increasing problem in sheep and goats in many countries where several gastrointestinal roundworms have developed resistance to almost all available anthelmintics.

Roundworm species of veterinary importance

In the following those roundworm species most relevant for cattle, sheep, pig, poultry, horses, dogs and cats are listed. For each species the predilection site(s) and most affected hosts are mentioned. Some species are quite host-specific; other species can infect a wide range of hosts. Many species are cosmopolitan, whereas other occurs only in certain regions. Incidence and prevalence within similar climatic regions are highly dependent on particular ecologic conditions, farm management practices, season, etc.

Depending on the predilection site of the adult worms, parasitic roundworms are often subdivided into:

  • Gastrointestinal worms: infect several organs of the digestive system; often abbreviated with the acronym GIN (Gastro Intestinal Nematodes)
  • Lungworms: infect the lungs and other organs of the respiratory system
  • Other roundworms that infect the skin, the eyes, the muscles, internal organs, etc.

However, many worms cause damage not only at their predilection sites, but also in other organs where they migrate through during development.

For viewing specific articles on each species click the corresponding link in the list below or use the site map.

Gastrointestinal roundworms


Worms in other organs