Capillaria spp (also called hairworms) is a genus of parasitic roundworms (=nematodes)Adult Capillaria putorii = Aonchotheca putorii. Picture from that infects dogs and cats but also livestock, poultry and numerous other wild mammals, birds and fish. Some species are also human parasites. About 300 species of this genus have been described, with a complex and still "unstable" systematic.

Capillaria worms are found worldwide, but occurrence and abundance depend on the worm species, the final host and the region. As a general rule, the incidence of Capillaria worms in dogs and cats is rather low (up to ~10%) compared with other roundworms that affect pets. Incidence in wild animals is often much higher: in endemic regions 100% of wild animals may be infected with some Capillaria species.

The systematics of this genus has been revised and several species are now included in other genuses.

The most relevant species for dogs and cats are the following ones (modern names in orange):

  • Capillaria aerophila = Eucoleus aerophilus,affects occasionally dogs and cats. Main final hosts are wild carnivores (foxes, wolves, coyotes, hedgehogs, lynx, raccoon, etc.). Found worldwide.
  • Capillaria boehmi = Eucoleus boehmi, the nasal worm, affects occasionally dogs. Main final hosts are wild mammals.
  • Capillaria feliscati = Pearsonema feliscati, the cat bladder worm, affects occasionally cats, seldom dogs. Main final hosts are wild carnivores (foxes, wolves, coyotes, hedgehogs, etc.). Found worldwide.
  • Capillaria hepatica = Calodium hepaticum, affects occasionally dogs and cats. Main final hosts are rodents (e.g. rats, mice, rabbits) and monkeys. Found worldwide.
  • Capillaria plica = Pearsonema plica, the dog bladder worm, affects occasionally dogs, seldom cats. Main final hosts are wild carnivores (foxes, wolves, coyotes, hedgehogs, etc.). Found worldwide in temperate and tropical regions, excepting Australia.
  • Capillaria putorii = Aonchotheca putorii, the cat stomach worm, affects occasionally cats. Main final hosts are wild mammals such as badgers, otters, wild boars, etc. Found worldwide.
  • Capillaria phillipinensis (= Paracapillaria phillipinensis) and Capillaria hepatica (= Calodium hepaticum) are also human parasites. A few cases of human infections have also been reported for Capillaria aerophila (= Eucoleus aerophilus).

Several Capillaria species affect chicken and other domestic and wild birds, mainly in the gastrointestinal tract and can be rather harmful. The major species relevant for poultry are Capillaria annulatus (= Eucoleus annulatus), Capillaria contorta (= Eucoleus contortus), Capillaria caudinflata, Capillaria obsignata, Capillaria annatis and Capillaria retusa. Click here to visit the specid article on poultry Capillaria.

Capillaria bovis is found in the small intestine of cattle, sheep, goats and other ruminants; and Capillaria longipes in the small intestine of sheep and goats. Little is known about their pathogenicity (capacity to cause a disease) but it seems that they are rather harmless for ruminants.

The disease caused by Capillaria worms is called capillariasis (seldom capillariosis).

Are dogs or cats infected with Capillaria worms contagious for humans?

  • Not directly. Most Capillaria species of dogs and pets are not parasitic for humans. Capillaria hepatica and Capillaria aerophila have been reported in humans, but are very rare and the infective stages are not passed directly from pets to humans, but need to spend some time in the environment. Capillaria philippinensis, the most common and harmful human parasite among the Capillaria species is not parasitic of dogs or cats.

You can find additional information in this site on the general biology of parasitic worms and/or roundworms.

Final location of Capillaria worms in dogs and cats

Predilection sites of Capillaria worms are species-specific.

  • Capillaria aerophila: respiratory tract 
  • Capillaria boehmi: nasal mucosa and sinuses
  • Capillaria feliscati: urinary bladder
  • Capillaria hepatica: liver
  • Capillaria plica: urinary bladder
  • Capillaria putorii: stomach and small intestine

Anatomy of Capillaria worms

Adult Capillaria worms are medium-sized worms (10 to 80 mm long, depending on the species) and all very thin, usually less than 1 mm, hence their name "hairworms".  Males are shorter than females. As other roundworms, the body of Capillaria worms is covered with a cuticle, which is flexible but rather tough. The worms have a tubular digestive system with two openings. They also have a nervous system but no excretory organs and no circulatory system, i.e. neither a heart nor blood vessels. In most species males have only one chitinous spicule for attaching to the female during copulation.

The eggs are barrel-shaped (20-30 x 50-65 micrometers), with a thick membrane and typical plugs on both poles, rather similar to those of Trichuris whipworms.

Life cycle and biology of Capillaria worms of dogs and cats

Egg of Capillaria philippinensis. Picture from

The life cycle of many Capillaria species is not completely elucidated.

Some species (e.g. Capillaria hepatica) can follow a direct life cycle, i.e. without an obligate intermediate host. The eggs in the environment develop to infective L1-larvae in 3 to 5 weeks. Final hosts ingest such larvae with contaminated food or water. The larvae penetrate into the intestinal wall and reach the portal vein system to be carried to the predilection sites, where they complete development to adult worms of both sexes and reproduce.

Other species (e.g. Capillaria aerophila and Capillaria plica) have an indirect life cycle with earthworms as intermediate hosts. In this case the earthworms eat eggs shed by infected final hosts into the environment. Out of these eggs infective L1-larvae develop in the body cavity of the earthworms. Final hosts become infected after ingesting such earthworms or other transport (= paratenic) hosts (e.g. birds, rodents, etc.) that eat the earthworms.

Other species (e.g. Capillaria putorii, Capillaria feliscati and maybe Capillaria aerophila) are able to follow both the direct and the indirect path.

Depending on their predilection sites, the eggs are shed directly through the feces (e.g. Capillaria hepatica and Capillaria putorii), indirectly through the feces after coughing and subsequent swallowing (Capillaria boehmi and Capillaria aerophila), or through the urine (e.g. Capillaria feliscati and Capillaria plica). In the case of Capillaria hepatica, the eggs laid in the liver do not develop further and are not directly shed with the feces.

For further development the eggs have to spend some time in the environment: either the host dies and after decomposition the eggs continue development in the environment; or another host eats the infected host and the ingested eggs are shed with the feces of the predator. After being shed they continue development to infective larvae in the environment.

Harm caused by Capillaria infections in dogs and cats, symptoms and diagnosis

Capillaria infections of dogs and cats are rather unusual and often benign and without clinical signs. Massive infections cause clinical signs depending on the affected organs. Secondary infections of affected organs with bacteria are also possible.

  • Capillaria aerophila = Eucoleus aerophilus can cause inflammation of the airways (bronchitis, rhinitis, etc.) manifested through with wheezy cough, sneezing, etc. Bacterial infections can lead to bronchopneumonia.
  • Capillaria boehmi = Eucoleus boehmi can cause rhinitis, sneezing and nasal discharge, sometimes with blood.
  • Capillaria feliscati = Pearsonema feliscati and Capillaria plica = Pearsonema plica, can irritate the mucosa of the urinary system (bladder and tract), sometimes with painful urination and incontinence.
  • Capillaria hepatica = Calodium hepaticum can lead to liver enlargement and fibrosis due mainly to the migrating larvae and the accumulation of eggs. Jaundice, vomits, depression and thirst have been reported.
  • Capillaria putorii = Aonchotheca putorii can cause several gastrointestinal symptoms such as vomits, diarrhea and gastric ulcer.

Diagnosis is not always easy. Eggs require adequate microscopic examination of the feces, the urine or expectorations, but false negatives are anytime possible, because egg shedding is intermittent. Capillaria hepatica eggs are not shed and therefore diagnosis can only be confirmed in liver biopsy or necropsy samples.

Prevention and control of Capillaria infections in dogs and cats

In endemic regions dogs and cats should be prevented from eating wild hosts (rodents, birds, etc.) that could be infected, but this may be rather difficult to achieve in rural regions

Some anthelmintic active ingredients (e.g. febantel, fenbendazoleivermectinmebendazole, milbemycin oxime) are known to be effective against Capillaria infections. However, since most commercial dewormers are not approved for use against this worm, the veterinary doctor has to determine a special treatment regime.

There are so far no true vaccines against Capillaria worms. To learn more about vaccines against parasites of livestock and pets click here.

Biological control of Capillaria worms< (i.e. using its natural enemies) is so far not feasible.

You may be interested in an article in this site on medicinal plants against external and internal parasites.

Resistance of Capillaria worms to anthelmintics

So far there are no reports on resistance of Capillaria worms to anthelmintics.

This means that if an anthelmintic fails to achieve the expected efficacy, chance is very high that it was not due to resistance but to incorrect use, or the product was unsuited for the control of these worms. Incorrect use is the most frequent cause of failure of antiparasitic drugs.

Ask your veterinary doctor! If available, follow more specific national or regional recommendations for Capillaria control.