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Helminthiasis in Poultry

(Nematode and Cestode Infestations)


Rüdiger Hauck

, DVM, PhD, DECPVS, Department of Pathobiology, College of Veterinary Medicine, Auburn University

Reviewed/Revised Jan 2024
Topic Resources

Helminthiasis is infestation by parasitic worms. In rare cases, affected birds develop clinical signs such as apathy or diarrhea. The influence on zootechnical parameters is usually negligible. Infestations are diagnosed on the basis of the presence of worms in affected organs, mostly the intestines, or by detection of eggs in the feces. Control relies on improvement of management and sanitation.

Causative Species of Helminthiasis in Poultry

Nematode and cestode helminths are common GI parasites of commercial poultry. Approximately 100 worm species have been recognized in wild and domestic birds in the US. Nematodes (roundworms) are the most important by number of species and economic impact.

Parasitism by acanthocephalans (spiny-headed worms) and trematodes (flukes) Fluke Infections in Poultry Fluke infections are rare in commercial poultry, mainly because housing indoors prevents access to snails and other intermediate hosts. Most birds with light infections show no external signs... read more is less frequent in poultry. Annelid parasites of poultry are of comparatively minor importance, although leech infestation has been reported in waterfowl species.

Of helminth species found in commercial poultry, Ascaridia galli is by far the most common (see A galli and A galli photographs).

Field studies show that poultry maintained under free-range conditions may be heavily parasitized; therefore, control measures such as preventing infestations or chemotherapy can improve weight gain and egg production. Prevalence of helminthiasis is greater in poultry raised under nonconfinement conditions than in those housed in cage production systems (1 References Helminthiasis is infestation by parasitic worms. In rare cases, affected birds develop clinical signs such as apathy or diarrhea. The influence on zootechnical parameters is usually negligible... read more References ).

Generally, nematodes have separate sexes that can be morphologically differentiated; eg, males of Tetrameres spp are elongated and slender, whereas gravid females are globe-shaped. The size and shape of nematode species vary widely; ascarids are sturdy and long (up to 116 mm [4.5 in]); capillarids are more delicate, slender, and long (60 mm [2.3 in]); and other nematodes, like the cecal worm Heterakis gallinarum, are much shorter (2–12 mm [0.08–0.48 in]; see H gallinarum).

Cestodes (tapeworms) also vary in size. Raillietina spp may be longer than 30 cm (12 in), whereas Davainea proglottina often is shorter than 4 mm (0.16 in). The proglottids of individual tapeworms are hermaphroditic. Tapeworms have been recovered in the thousands from individual chickens and turkeys.

See table for information on common nematode and cestode parasites of poultry.


Transmission of Helminthiasis in Poultry

Modern confinement rearing of poultry has substantially decreased the frequency and variety of endoparasite infestations such as helminthiasis, which are common in ranged birds and in backyard flocks. However, severe parasitism still may occur in floor-reared layers, breeders, turkeys, or pen-reared game birds where management problems may exist. Contributing factors include the use of poorly managed built-up litter, which fosters the propagation of intermediate hosts and the accumulation of infective eggs, and resistance of the parasites to therapeutic drugs.

Range infestations of nematodes such as H gallinarum and Syngamus trachea may increase because of seasonal or climatic abundance of specific invertebrate intermediate hosts, eg, large numbers of earthworms brought to the surface by spring rains. Some species have been associated with large numbers of darkling beetles, which may act as mechanical vectors of infective eggs.

Nematodes either have a species-specific direct life cycle with bird-to-bird transmission by ingestion of infective eggs or larvae or have an indirect cycle that requires an intermediate host (eg, insects, snails, or slugs). Eggs of many nematode species are resistant to low temperatures and disinfectants but may be more susceptible to heat and desiccation. Eggs of A galli and H gallinarum can survive up to two years in soil.

The life cycle of A galli is simple and direct:

  • Eggs in the droppings become infective in 10–12 days under optimal conditions.

  • The infective eggs are ingested and hatch in the proventriculus, and the larvae live free in the lumen of the duodenum for the first 9 days.

  • The larvae penetrate the mucosa, causing hemorrhages; return to the lumen by 17–18 days; and reach maturity at 28–30 days.

Levels of infestation are often underestimated because early larval stages are barely visible and can remain for long periods within intestinal tissues, whereas adult stages in the lumen are generally fewer in number because most larvae will die before they reach maturity. Maturation of larval stages can also be hampered by adult worm numbers, thereby increasing the time larval stages remain in intestinal tissues and continue to cause damage.

The life cycle of H gallinarum is similar to that of A galli, but the development to mature worms after infestation takes only ~2 weeks. The greatest production of eggs for each hatched egg ingested occurs in the ring-necked pheasant, followed by the guinea fowl and chicken.

The larvae are closely associated with the cecal tissue; however, a true tissue phase rarely occurs. Most of the adult worms are found at the blind end of the ceca (see H gallinarum cecal pouches ). Earthworms may ingest the eggs of the cecal worm. Heterakis larvae will translocate into the coelomic cavity of the earthworms and remain alive for years. The earthworms then serve as a source of infestation when ingested by poultry. Darkling beetles may also serve as a mechanical vector.

The life cycle of Capillaria may be direct (C obsignata), require an intermediate host such as earthworms (C annulata and C caudinflata), or be a combination of the two (C contorta). Larval development in the egg takes 8–15 days, depending on temperature. Worms reach maturity 20–26 days after ingestion by the final host (see Capillaria and photomicrographs).

The gapeworm Syngamus trachea inhabits the trachea and lungs of many domestic and various wild birds (see S trachea ). Transmission may occur directly by ingestion of infective eggs or larvae; however, severe field infestation is associated with ingestion of transport hosts such as earthworms, snails, slugs, and arthropods (eg, flies).

Many gapeworm larvae may encyst and survive within a single invertebrate for years. Although gapeworms are not a problem in confinement-reared poultry, they cause serious economic losses in game-farm pens and in range-reared chickens, pheasants, turkeys, and peacocks. Cyathostoma bronchialis is the gapeworm that affects geese and ducks.

Eggs of Oxyspirura mansoni, Manson eyeworm, are deposited in the eye of chickens; reach the pharynx via the nasolacrimal duct; and are swallowed, passed in the feces, and ingested by the Surinam cockroach (Pycnoscelus surinamensis). Larvae reach the infective stage in the cockroach. When infested intermediate hosts are eaten by chickens, liberated larvae migrate up the esophagus to the mouth and then through the nasolacrimal duct to the eye, where the cycle is completed. Other insect species may also serve as the intermediate host.

Cestodes require an intermediate host (eg, insects, crustaceans, earthworms, or snails). Floor layers, breeders, and broilers are infested with Raillietina cesticillus by ingestion of the intermediate host, small beetles that breed in contaminated litter. Cage layers in unscreened houses may become infested with Choanotaenia infundibulum by eating its intermediate host, the housefly. Darkling beetles in proximity may also serve as intermediate hosts.

More than 3,000 microscopic Davainea proglottina tapeworms have been recovered from a single bird. Several species of slugs and snails serve as intermediate hosts, and > 1,500 infective parasites have been recovered from a single slug.

Pathogenesis and Clinical Findings of Helminthiasis in Poultry

Ascaridia, Heterakis, and Capillaria spp are widely distributed helminths. They can cause nonspecific clinical signs such as general unthriftiness, inactivity, depressed appetite, and decreased performance; in severe cases, death may result. Larger numbers may block the intestinal tract.

Ascarids may migrate up the oviduct via the cloaca to become enshelled later within the egg, which is an aesthetic rather than a public health problem, avoidable by careful egg-candling before the release of eggs to market (see A galli and A galli ).

A dissimilis (turkey roundworm) may also migrate out of the intestine, through the portal system, and into the liver, causing hepatic granulomas.

H gallinarum is an even milder pathogen. In large numbers it may cause thickening, inflammation, or nodule formation in the cecal walls. H gallinarum parasitism has been associated with cecal and hepatic granulomas. H isolonche, highly pathogenic in pheasants, may cause death in 50% of affected pheasants. H gallinarum carries Histomonas meleagridis, the protozoan that causes histomoniasis Histomoniasis .

C contorta in the mucosae of the crop and esophagus and C obsignata in the wall of the small intestine cause marked thickening and inflammation of the organs (see C contorta ). Birds harboring large numbers of these threadlike worms become weak and emaciated and may die.

Young birds are the most severely affected by gapeworms. Sudden death and verminous pneumonia characterize early outbreaks. Gasping, choking, shaking of the head, inanition, emaciation, and suffocation may follow. Necropsy reveals adult gapeworms obstructing the lumina of the trachea, bronchi, and lungs (see S trachea ). Respiratory inflammation may be present.

The blood-red female gapeworm is usually found in copulation with a much smaller, paler male with its head embedded deep in the host tissue. The joined pair have a Y-shaped or forked appearance.

Oxyspirura mansoni is a slender nematode, 12–18 mm long, found beneath the nictitating membrane of chickens and other fowl in tropical and subtropical regions. The parasite causes various amounts of inflammation, lacrimation, corneal opacity, and disturbed vision.

Other nematodes include the following:

  • Amidostomum anseris attacks the gizzard lining of ducks and geese and causes dark discoloration, necrosis, and sloughing at the parasitic loci.

  • Dispharynx nasuta causes ulceration, thickening, and maceration of the proventriculus; heavily infested birds may die.

  • Tetrameres americana, a bright red worm discernible through the proventricular wall, causes diarrhea, emaciation, and, with heavy infestation, death.

  • Trichostrongylus tenuis causes inflamed ceca, weight loss, anemia, and death, especially in young birds.

  • Ornithostrongylus quadriradiatus, a blood-sucking parasite, causes pigeons to regurgitate bile-stained fluid mixed with food. Greenish mucoid diarrhea from hemorrhagic intestines, emaciation, and death follow.

Most pathogenic tapeworms are found in the small intestine; the scolex, usually buried in the mucosa, generally causes mild lesions. Davainea proglottina may cause weight loss. Raillietina tetragona causes weight loss and decreased egg production; R echinobothrida produces granulomas at its attachment sites (“nodular disease”).

Diagnosis of Helminthiasis in Poultry

  • Identification of individual parasites by morphology or molecular biological tests

  • Detection of eggs in feces by flotation or sedimentation

A reliable diagnosis of helminthiasis can be made by accurate identification of the individually recovered parasites by their morphology or increasingly by molecular biological methods. Only specific recognition of the parasite allows meaningful recommendations for flock treatment and management (see photograph).

To determine the species by morphology, worms detected during necropsy should be carefully removed, put into a saline solution, and examined under a microscope. However, identification of the often fragile worms can be difficult for an inexperienced individual and is complicated by intraspecies variation.

Detection of worm eggs by fecal flotation or sedimentation allows for the reliable confirmation of the presence of worms. However, the species cannot usually be identified this way. Furthermore, eggs are shed intermittently and in varying numbers; the absence of eggs in one sample does not necessarily mean that worms are absent.

ELISA systems to detect antibodies against A galli have been described. However, they are not commercially available, and the detected antibodies are not species-specific.

Species identification by molecular methods can be done by PCR assay using universal primers that amplify the partial cytochrome c oxidase subunit 2 (COX2) gene, a fragment of the rDNA gene comprising the internal transcribed spacers, or the partial nicotinamide adenine dinucleotide dehydrogenase subunit 1 gene, followed by sequencing of the PCR products.

Increasingly, PCR assays and loop-mediated isothermal amplification (LAMP) assays for detecting specific worms, including A galli, several Heterakis spp, and Raillietina spp, have been described. However, the specificity of some assays has not been validated against other genera, but only against other species in the same genus. Generally, there are few published reference sequences, which are necessary to validate the tests for routine diagnosis.

Treatment and Control of Helminthiasis in Poultry

  • Few drugs available

  • Treatment only for birds with severe infestation that show clinical signs

The number of medications approved for treatment of helminthiasis in poultry is decreasing. There are also reports of resistance developing.

To decrease the potential spread of resistance, treatment should be limited to birds with severe infestation that show clinical signs of disease. Such targeted treatment also seems to more effectively decrease worm burden and cumulative environmental parasite egg numbers than untargeted routine treatment. Worm loads have been reported to rebound quickly after deworming, however.

Improvement of management and sanitation in confined operations will generally lower the parasite levels in birds. In range birds, the only option is to move to new pasture, although the benefit that may result will be of short duration.

Application of approved insecticides to soil and litter when premises are unoccupied may interrupt the life cycle of the parasite by destroying its intermediate host. When the premises are restocked, groups of birds of different species or ages should be separated to avoid transmission of parasites. Migration of darkling beetles or other insects may contaminate new or widely separated housing.

Approved compounds are limited in the US. Because of frequently changing regulations, the status of any medication should be checked before its administration. Approved animal drugs for use in the US are listed online at Animal Drugs @ FDA (aka the Green Book), and approved medicated feeds are listed in theFDA Center for Veterinary Medicine's Blue Bird labels.

Only approved drugs may be used in birds producing eggs or meat for the commercial market. Label directions and recommended doses should be followed precisely, with scrupulous adherence to withdrawal times. Veterinarians seeking assistance in assigning withdrawal times for drugs used in food-producing birds should consult the Food Animal Residue Avoidance Database (FARAD).

Several compounds are reported to be effective against nematode infestations but are not approved for use in poultry or other avian species in the US.

Fenbendazole is a benzimidazole anthelmintic and is approved in the US for use in chickens against A galli and H gallinarum (1 mg/kg, PO in the drinking water, every 24 hours for 5 consecutive days). Fenbendazole, a type C medicated article, is also approved in the US against A dissimilis and H gallinarum for use in growing turkeys when administered at a rate of 16 g fenbendazole per tonne of feed for 6 days. Extra-label use may not be permitted in some jurisdictions.

Fenbendazole has also been shown to be effective against Ascaris spp when administered once at 10–50 mg/kg; if needed, the treatment can be repeated after 10 days (extra-label drug use in the US). At 10–50 mg/kg, fenbendazole administered daily over 5 days is effective against Capillaria (extra-label drug use in the US).

Fenbendazole is also effective against other nematodes when administered at 10–50 mg/kg every 24 hours for 3–5 days or as a single dose of 20–100 mg/kg, or when added to the drinking water at 125 mg/L for 5 days or to the feed at 100 mg/kg (all extra-label dosages in the US). At 20 mg/kg for 3–4 days, it effectively removes gapeworms in pheasants (extra-label drug use in the US).

Treatment in the face of heavy roundworm infestations in poultry should be performed cautiously:

  • Visceral migration of larvae has been reported and treatment of such severe infestation may kill migrating parasites, which could trigger a shocklike syndrome or reaction culminating in sudden death.

  • Toxicosis has been reported in pigeons that received fenbendazole at the rate of 30 mg/kg for 5 days.

  • Fenbendazole should not be administered during molt, because it may interfere with feather regrowth.

Flubendazole (1.43 mg/kg) is used widely in Europe against Ascaridia spp and H gallinarum.

Pyrantel tartrate was more effective then pyrantel pamoate against the adult stage of A galli, and it was somewhat effective against Capillaria spp when administered at 15–25 mg/kg (extra-label drug use in US).

Poultry producers wanting to treat for tapeworms should be aware that expulsion of the parasite will be a short-term remedy if the scolex is not removed or if the intermediate host is not eliminated as a source of reinfestation.

Butynorate in combination with piperazine and phenothiazine as a feed additive or by individual tablets has shown some efficacy. Other promising experimental drugs include chlorophene, niclosamide, and praziquantel, none of which are approved in the US.

The use of diatomaceous earth supplemented at 2% in feed and fed continuously lowers numbers of Heterakis and Capillaria in chickens. The efficacy of several essential oils and plant extracts has been measured, with inconsistent results.

Key Points

  • Helminthiasis is an infestation with parasitic worms and usually does not cause clinical signs.

  • Reliable speciation is done based on the morphology of the adult worms. Detection of eggs in the feces proves infestation but does not always help to differentiate species.

  • Control relies on improvement of management and sanitation. Very few compounds are approved for use in chickens and turkeys and should only be used in severe cases.


For More Information

Approved drugs for use in the US are listed online at the following sites:

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