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Parasites and Free-Range Poultry: Cause, Effect and Treatment

By Dr Rod Reece, Veterinarian

Posted Flock & Herd July 2019


General Sun Tzu, in the Art of War, notes that to attain victory we must know our enemy, and the enemy’s intention; in this case, internal parasites of free-range poultry. In an older text (Proverbs 3), King Solomon exhorts us to know about things (i.e. information) but more so, to understand or comprehend; but beyond that, we are to pursue wisdom so we know what to do and how to apply the knowledge and understanding effectively. The purpose, privilege and responsibility of poultry farmers is to produce safe and nutritious food stuff, at reasonable cost, and fit for the consumer.

Intensification of poultry production, and indoor year-round housing, became possible after the elucidation and supplementation of diets with vitamin D, then vitamin D3, along with extended lighting periods, genetic selection etc. In the 1930s, it was observed that laying hens housed in cages tended to produce more and better eggs, with less labour cost, than comparable birds housed in non-caged systems (Arndt 1931). Caging of layers is a contentious issue and has considerable animal welfare implications; however, it was realised that one advantage of caging of layers was the elimination of internal parasites (North & Bell 1990).

With the encouragement of non-caged systems, especially free-range poultry production, there has been a concomitant rise in the prevalence (number of cases of disease, in this case parasitism, as percentage of the flock or group at a particular time point, i.e. today), incidence (number of new cases in a flock, as %, over a specified period of time, i.e. between 20 and 30 weeks of age; this is a cumulative tally), and significance of internal parasites, but at the same time, a reduction in suitable chemotherapeutics for treatment and control. The focus of this presentation is on internal parasites of free-range layers in Australia.


Internal parasites that are of concern to us can be classified into Nematodes (round worms) and Cestodes (flatworms or tapeworms). Trematodes (flukes) and Acanthanocephalids (thorny-headed worms) are not a significant problem in chickens in Australia (but they can be for other free-living, wild and semi-commercialised avian species).

All Cestodes have a complex life cycle requiring an intermediate host, whereas most Nematodes have a direct life cycle, although some will take advantage of a suitable transport (paratenic) host if available. 

Most internal parasites are host species specific: i.e. the common round worm of chickens, Ascaridia galli, looks like (morphologically resembles) the roundworm of pigs (Ascaris suis), but is a completely different species and will not affect pigs, nor will the pig parasite affect chickens: similarly, the different round worms of pigeons and other species in parrots etc., will not infect chickens.

Nematodes have a mouth-part and a digestive tract, they will eat their way into tissue to some extent at some stage in their life cycle, and will obtain nutrients by eating ‘bits’ of their host (this causes damage to the host itself); some adults may ‘swim’ free in the lumen itself and obtain nutrients by engulfing ingesta the host has eaten (thus diverting nutrients away from host’s use). Specific details relating to some common parasites will follow.

Adult Cestodes have suckers at the anterior or ‘head’ end (scolex), that allow them to attach to the intestinal inner wall (mucosa) causing some focal damage, whilst the body of the parasite waves free (protrudes) out into the intestinal lumen. Each segment is self-contained with both male and female components, and nutrients are absorbed, from the passing ingesta, through the cuticle as required (thus depriving the host of nutrients eaten). When the segment (proglottid) is full of fertile eggs, it is released into the intestinal lumen as a packet. This is passed in the droppings. The developing hexacanth embryos must be eaten by the intermediate host (ant, housefly, beetle etc.), and develop therein to cysticercoids, which are the infectious stages for the next chicken host. The pre-patent period of the developing cestode within the chicken is about 3 weeks.

The nematodes have distinct genders, and the adult females lay eggs. The eggs are released more-or-less continuously and passed in the faeces (droppings). In the environment, the embryo in the egg hatches into a larva; the larva may or may not be released from the egg in the environment, and may or may not moult in the environment (for some nematode species, e.g. spiruroids, the eggs sporulate before being laid). The larvated egg or free larva is ingested by a new host and proceeds through a number of moults whilst it burrows into the gastro-intestinal wall mucosa or other host tissues. After a certain stage (L5?), the adult emerges and matures.


Ascaridia galli, the common roundworm of poultry.

The adults sit free within the small intestine lumen and remain there by ‘serpentine’ swim-like motility. The adult females initially lay up to 100,000 eggs per day (EPD). The females are 8-11 cm long and white; the males are somewhat shorter (4-8 cm). The adult female may lay eggs for several months; the eggs are laid more-or-less continuously, although some studies have shown a peak in faecal egg count in the afternoon (that observation may be related to movement of the chicken and/or eating and thus movement of ingesta and parasite eggs etc., along the gastro-intestinal tract, rather than an inherent property of the parasite).

The eggs are voided in the droppings and thus discharged into the environment, they embryonate in the first few days, then undergo moult until larva stage 2 (L2), all within the egg still. This is the infective stage reached after about 2 weeks in the environment. The larvated egg will survive moderate frost, and under ideal conditions in litter will remain infectious for a year or more. The role of earthworms, ants, flies etc. for transmission of infective larva is minimal.

Upon ingestion by a new host chicken (and possibly turkey), the larva is released from the egg in the gizzard. Larvae undergo a series of moults in the proximal (upper) small intestine, with the final larval stage burying the proximal part of its body within the mucosa, i.e. it will cause some damage. Hungerford (1969) considered that 10 adult ascarids was a significant burden. Heavy burdens will cause intestinal obstruction. The parasites utilise nutrients in the feed that the host has ingested. 

On extremely rare occasions, adult ascarids may be found in the abdominal cavity, or within a hen’s egg. This comes about because an adult ascarid has been displaced, probably partially paralysed following treatment, and within the cloaca it recommences movement and then ascends the oviduct: if it encounters the next hen’s egg being formed and descending the oviduct, the ascarid may become entrapped within the egg white and be found by a customer. Aesthetically, this is upsetting, but of negligible health concern to people, as this ascarid is chicken specific. Not even the eggs within an adult female A. galli are at a stage ready to immediately infect another chicken.

Table eggs are routinely candled to detect cracks so that such eggs are excluded from sale; and an intra-egg ascarid may be observed as feint, serpentine-like shadow, but they are subtle and readily overlooked or missed because of the angle of viewing, so this is not a reliable method of eliminating this rare problem.

Heterakis gallinae, the caecal worm.

The adult female sits free in the caecal lumen but lays close to the mucosal surface. Females are up to 1.5cm, males are shorter. Adult females may lay about 500 EPD (maximal is ~800). Eggs are voided in the droppings and they embryonate in the environment; over the following 2 weeks, they undergo moult to L2 which is the infective stage. They can remain viable for years at this stage. Alternatively, the larvated egg can be eaten by an earthworm wherein the larva is released and remains in the earthworm for several years. Upon ingestion by a new host, and this can be a chicken, turkey, pheasant, quail, partridge etc., the L2 larva is released, moves down the small intestine and enters the caecum. The larvae undergo a series of moults and are embedded deep within the caecal crypts rather than specifically burrowing into the mucosa.

Adult Heterakis emerge onto the surface and by 4 weeks post-ingestion, the adult females are laying eggs. Hungerford (1969) considered that 50 adult Heterakis was a significant burden. Heterakis gallinarum eggs and larvae can transmit the protozoan Histomonas meleagridis, the cause of ‘blackhead’ (so-called because of the cyanosis of the head of dying turkeys). This is a serious disease and can cause significant mortalities (there is no current registered chemotherapeutic control or treatment for histomoniasis in Australian poultry). The eggs of Heterakis gallinarum are indistinguishable from those of Ascaridia galli. A faecal egg count must take into account that an adult female A. galli can produce 100,000 EPD, whereas an adult female H. gallinarum produces about 500 EPD.

Capillaria, fine hair worms.

Capillaria annulata is a parasite of the crop and/or oesophagus; C. obsignata of the small intestine. These are small, fine, thread-like nematodes and are not host-species specific: the same or similar species infect chickens, turkeys, quail, guinea fowl, and pigeons can be infected with C. obsignata. C. annulata can utilise earthworms as an intermediate host. The life cycle is relatively direct and quick: from eggs laid to infective larva in the environment is a little more than a week; from ingestion of infective larva to production of fertile eggs is under 4 weeks. These belong to SuperFamily Trichuroidea and the eggs are operculated. The parasites burrow into the mucosa and create significant tissue damage leading to localised inflammation and congestion. Infection can cause morbidity, and sometimes death. The adult parasites are extremely difficult to observe in affected tissues.

Syngamus trachea (tracheal worms, gapeworms), occurs in Australia, but does not appear to be a significant problem in free-range poultry thus far. Adult parasites are found deeply attached within the trachea and full of blood. Following ingestion of infective larva, they have a complex migratory pattern within the host. They have a wide variety of final avian hosts – chickens, pheasants, turkeys, emus, kookaburras, magpies, etc., and the larva can utilise intermediate hosts such as earthworms, slugs and snails, or spread by direct faeco-oral ingestion of infective larva. Severely affected birds have open-mouthed breathing due to respiratory distress, hence ‘gape worm’. The adult females are about 2cm long, and permanently joined with a smaller male (6mm). Eggs are laid into the tracheal lumen, coughed up, swallowed and passed out in the droppings. The eggs are typical strongylid-type. 

Oxyspirua mansoni (poultry eye worm) affects a wide variety of birds, including poultry. Sometimes it is referred to as the tropical eye worm. It is a spiruroid nematode; the adult female is under the nictitating membrane of the eye, and lays embryonated eggs which are swallowed, and then hatch in transit through the gut. The larvae require a cockroach as an intermediate host. They have been reported in Australia, but the significance in free-range poultry, thus far, is uncertain.

CESTODES (Tapeworms)

Davainea proglottina is a very small tapeworm affecting the duodenum of chickens. It requires an intermediate host of snails and slugs. The tapeworm is comprised of less than 10 proglottid segments, and is <5mm long, and deeply embedded in the duodenal mucosa. Observing these grossly, with the unaided eye, is almost impossible. The adult D. proglottina in the chicken duodenum can persist for years. Heavy infestation may result in weight loss, emaciation and death, and in some cases there may be paralytic-like signs and weakness, hence the suspicion in some earlier reports of a neurotoxin (not thus far confirmed).

Choanotaenia infundibulum is a large (20 cm long), vivid white, tapeworm inhabiting the proximal small intestine. The intermediate hosts are house flies and beetles. Pathogenicity is related to intestinal obstruction by multiple parasites within the lumen.

Raillietina cesticillus is another large tapeworm (15 cm) that utilises beetles as intermediate host; whereas R. tetragona (25 cm) and R. echinobothridia (>30 cm) use ants. Infection with the latter results in large intestinal wall nodules up to 6 mm in diameter, associated with the attachment of the parasites.


Faecal egg count

Faecal egg count is available commercially through private and government veterinary laboratories in Australia, and it is possible for owners, managers or staff to learn the procedures and to recognise parasite eggs so that testing can be done on-farm. (Most courses are targeted for sheep farmers examining faeces for strongylid eggs. Courses could be tailored for poultry farmers. Some courses are available online).

Faecal egg count using the modified McMaster method is a well-recognised procedure for diagnosing and interpreting parasite burdens, particularly in sheep. Basically, a fixed weight of faeces is mixed with a fixed volume of diluent, and the mixture is examined in a chamber of known volume on a slide under a microscope. The number of parasite eggs is counted, and then a calculation is performed to give the number of eggs per gram of faeces. Different diluents are used for different parasites of concern but saturated salt appears to be best for most nematode and coccidian ova.

However, extrapolation to chickens is not straightforward. In sheep, we are mainly concerned with strongyles such as Haemonchus not Ascaridia or Heterakis, and the diet and digestive tract of sheep is significantly different from that of chickens. It must be remembered that reverse peristalsis is common in chicken intestines with minor reverse waves every 15 minutes or so, and a major expulsatory wave every hour producing an intestinal dropping with ingesta fragments etc. surrounded by a curtain of precipitated urate crystals derived from the kidneys; several times per day the caecae are emptied out producing a relatively homogenous semi-fluid dropping. 

Detection of free larva, as may be present in litter samples, or for parasites that produce free-larva (eyeworm), requires the Baermann technique.

The presence or absence of nematode eggs is certainly useful, and some indication of the abundance of nematode eggs is important. Faecal egg counting does not give a reliable indication of cestode burdens as the tapeworm segments are released as separate packets.

A variation of this procedure is to collect droppings or litter, and to examine that for parasite eggs. When Ascaridia and Heterakis eggs embryonate, the larvae remain within the egg until eaten by the next chicken, so they are countable.

Ascaridia eggs and Heterakis eggs are indistinguishable, but differ in significance. Egg production by an adult female ascarid can reach 100,000 EPD, whereas adult Heterakis rarely exceeds 800 EPD. 


Birds can be killed and their intestinal tract examined for adult worms. Hungerford (1969) observed that 10 adult Ascaridia or 50 Heterakis was a significant burden in an adult hen. Adult Heterakis are small and can be easily overlooked. Adult Capillaria are very small and fine, and buried in the mucosa, so are rarely observed on naked-eye examination, but faecal examination should indicate if there are adult female worms producing eggs (Capillariad eggs are operculated). Large adult tapeworms can be seen with the naked eye, but species such as Davainea proglottina are tiny and buried deep between duodenal villi. Individual proglottids (packs of eggs released by adult) may be observed in the droppings. 


No specific serological tests are readily available for poultry parasites. Studies of serological ELISA in chickens infected with A. galli have shown promise, including modification to detect IgY antibody in egg yolk.

Parasite identification

ALWAYS contact the laboratory to which the specimens are to be despatched for clarification, prior to preservation or despatch. Parasites do not transport well in water. Do not chill live parasites in a refrigerator. If using preservative, do not transfer parasites into ‘cold’ fluid. 70% methylated spirits may be a satisfactory temporary preservative, but check with the laboratory first. Note that methylated spirits itself may create difficulties with handling and delivery. Adequate extraction of the entire parasite is important. One ‘good’ nematode preservative fluid (alcohol-acetic acid-formalin) is composed of 100ml of 50% ethyl alcohol, 6.5ml of commercial grade buffered 40% formalin, and 2.5ml glacial acetic acid. Another preservative (alcohol-glycerol-formalin; I routinely used this) is 700ml 95% ethanol, 50ml glycerol, 220ml 40% formalin, 220ml water. (Use with caution and read the Material Safety Data Sheet (MSDS)). Some websites etc. claim better parasite detail in preservatives by addition of other substances.


A wide variety of ‘natural’ products are promoted for control of internal parasites in poultry; including diatomaceous earth (a soft siliceous sedimentary rock with embedded fossilised diatoms), garlic, cinnamon, peppermint, grated carrots, onions, elder leaves, wormwood (sagewort), hyssop, pumpkin seeds, apple cider vinegar, cucumber seeds etc. The relative efficacy of these (i.e. their ability to reduce or clear parasite infection), and consequences as far as taint in the chickens’ eggs from onions, garlic and the like is concerned, are not generally detailed. The only claim noted is often: ‘may assist in parasite control’. It is also possible to purchase herbal gut conditioners and similar products, for organic poultry etc.: the benefits of these may be uncertain.


Chemotherapeutics for use in poultry must be approved by the Australian Pesticides and Veterinary Medicine Authority (APVMA.gov.au). Refer to their website under ‘Search PubCRIS’. The ‘with-holding period’ is the minimum time between last administration of a particular chemotherapeutic product, and the earliest at which the chickens’ product, i.e. eggs, can be collected for sale. Eggs produced during that period are not to be used for human consumption.

There are relatively stringent standards for veterinary involvement in treating poultry flocks, including, the veterinarian must have current registration in the state concerned, and dispense APVMA registered products. The farmer or owner has to be a bona fide client of the veterinarian. Further, the veterinarian must have some responsibility for, and current knowledge of, the health of the flock concerned and to have established a therapeutic need; and information is conveyed to the person responsible for the flock, regarding withdrawal periods, supportive treatment etc. It is possible for a veterinarian to prescribe products ‘off-label’, but this is only undertaken with great caution and consideration.

Some products are NEVER to be used in poultry that at any stage of their life may produce eggs for human consumption. That should include backyard poultry. Included amongst these are praziquantel for treatment of cestodes in birds, dimetridazole for treatment of histomoniasis, and the coccidiostats ethopabate, nicarbazin and clopidol.

Only the following products can be used in laying hens, with NIL with-holding period for the eggs produced* (NB check with supplier and APVMA for up-to-date information):

PIPERAZINE# is for ascarids only. Treatment paralyses the ascarid and the host expels them. It is particularly effective for adult worms but only mildly effective against larval stage (eliminates about 2/3rds);

LEVAMISOLE# paralyses adult and larval Ascaridia, Heterakis and Capillaria;

MALDISON# for external parasites;

CHLORTETRACYCLINE, or LINCO-SPECTIN, or NEOMYCIN in-feed, as antibiotics. 

# These products can be obtained and used without veterinary intervention. Note: some products do not have a designated with-holding period for particular classes of poultry because they are not registered for use in that class, EVER. 

In many articles and books from overseas, there is reference to chemotherapeutic products that are not registered for use in poultry in Australia, e.g. fendendazole. There are some products registered in Australia, for use in non-poultry birds that should never be used in poultry, such as, praziquantel. Check before acquiring and using, as the integrity of the eggs produced may be at risk.


Prevention of infection by these internal parasites is by stopping access of chickens to droppings and/or intermediate hosts. In free-ranging poultry, this is impossible. As infection is common in most, if not all, non-caged poultry, acquiring truly parasite-free free-range chickens is almost impossible.


Regular chemotherapeutic, or other, treatments may reduce build-up of parasite burdens. Various programmes can be tailored to specific needs; however, regular treatment will encourage anthelminthic resistance to some extent. Reducing intermediate hosts will assist with control of parasites that have intermediate hosts i.e. cestodes, Heterakis; but there are a variety of intermediate hosts to be targeted. The life cycle of the parasite of concern must be known and understood for effective control to be implemented. Sanitation of the environment, reducing wet damp areas, general cleanliness and tidiness, etc. may assist in reducing parasite burdens.


External parasites

There is insufficient time to deal with external parasites of poultry: but those that need to be considered are lice (6-legged; they complete their life cycle on the bird, and lay eggs amongst the feathers), mites (adults have 8 legs; the nymphs move off the host and moult in the environment; the eggs are not on the birds; there are separate species of mites that infect the subcutis, anther within the skin of the shanks, and still another in the lungs themselves), ticks (Argas persicus, the fowl tick; these can transmit Borellia anserina, poultry tick fever – not Lyme disease, and both have a restricted occurrence, within NSW), stick-fast fleas; as well as flies, beetles, ants, and the like.

Protozoan diseases

There is insufficient time in this short presentation to deal with internal protozoan diseases, especially coccidiosis and blackhead – their spread, effect, immunity, vaccination for coccidia, their treatment and control, and significance. There are also other protozoan diseases that are potentially significant in free-range poultry in Australia, namely trichomoniasis, spironucleosis and maybe haematoprotozoans.


  1. Anonymous (2008). Poultry AgSkills. NSW DPI, Tocal College
  2. Anonymous. Poultry Hub.Poultry CRC, University of New England
  3. Arndt,M (1931). Battery Brooding, 2nd ed.
  4. Bolla,G, Robinson,S & Arzey,G (2003) Poultry keeping on a small scale. NSW Agriculture, AgNote
  5. Hungerford,TG (1969). Diseases of Poultry. 4th ed
  6. North,MO & Bell,DE (1990). Commercial Chicken Production Manual; 4th ed.
  7. Swayne, DE (2013). Diseases of Poultry, 13th ed. [14thedition is in preparation]
  8. Various Agfacts and notes and short leaflets by state and territory governments in Australia


Image of vial of poultry nematode larval ascarid
Image of vial of poultry nematode adult ascarid
Image of vial of poultry nematode heterakis
Image of vial of poultry nematode capillaria
Image of vial of poultry cestode davainiidae
Image of vial of poultry cestode choanotaemia
Image of vial of poultry body lice eggs
Image of vial of poultry body lice
Image of vial of poultry northern fowl mite
Image of vial of poultry red mite
Image of vial of poultry fowl tick nymphs
Image of vial of poultry fowl tick


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