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Endoparasites: Nematodes

Nematodes are considered to be the most numerous multi-cell organisms on the planet. The phylum Nemata contains over 20.000 classified species. Structurally they are simple organisms. Adult forms of the organism are comprised of around 1.000 cells, most of them as a part of the digestive system. Nematodes possess digestive, excretory, reproductive and nervous system and lack circulatory and respiratory system. The ‘tube within a tube’ phenomena refer to the long alimentary canal extending from the mouth of the parasite to the anus (anterior end). They can range from 0.3 mm to over 8 meters in size.

We will take you back to those parasitology classes by describing the most relevant Nematode species in the animal kingdom. For the purpose of simplifying the article and making it easy-to-use we divided them in two categories, intestinal and non-intestinal and subsequently in genuses.

Intestinal Nematodes

Ascarids (Roundworms)

Toxascaris leonina is a parasite whose infestations usually results in asymptomatic illness. They can be found everywhere in Europe and final hosts are dogs, cats and foxes. The infestation occurs by ingestion of embryonated eggs present in the soil or paratenic hosts larvae.   For cats there is a pre-patent period of 13 weeks, 8 weeks in dogs, and patent period of 4-6 month both for dogs and cats. Toxascaris leonina eggs can be detected using flotation method using 3-5 g of feces. There is a risk of zoonosis, especially in children.

Toxocara canis can infest dogs and foxes as final hosts causing asymptomatic to moderate illness manifested with cachexia, intussusceptions or intestinal blockage. In pups there is a pot-bellied appearance. Hosts get infested by ingesting embryonated eggs from fur or soil or ingesting larvae present in milk or paratenic hosts. Vertical transmissions also occur. The pre-patent period varies 3-5 weeks and depends on the time of infestation (pre or post-natal) and the route of infestation, while the patent period is 4-6 months. Again, children are at greatest zoonotic risk and diagnosis is determined by performing flotation tests using 3-5 g of fixed or fresh feces.

Toxocara cati can be found in cats as final hosts and the infestation is due to ingestion of embryonated eggs present in soil and larvae from milk and paratenic hosts. The infestations can result in asymptomatic cases or can cause intestinal symptoms due to intestinal blockage and intussusceptions. Young kittens suffer from cachexia, occasional pneumonia and are presented with pot-like bellies. Pre-patent period is 6 weeks and patent period is 4-6 months. Even though they can be transmitted to humans, the risk is lower than Toxocara canis. Identical flotation method is used for detection of eggs.


Ancylostoma caninum, also known as dog hookworm can cause acute or chronic illness with diarrhea (sometimes bloody), anemia and weight loss in dogs and foxes. The ingestion of third-stage larvae (L3) from the environment or milk/paratenic hosts larvae are one way of transmission, but percutaneous infestation has also been reported. Detection of eggs is performed by using standard flotation method.

Ancylostoma tubaeforme infects cats resulting in chronic or acute signs of diarrhea, bloody diarrhea, anemia and weight-loss. The parasite can be predominantly found in Europe and isn’t a threat to human’s health.

Uncinaria stenocephala has cats, dogs and foxes as final hosts. Ingestion of embryonated eggs from soil and larvae from paratenic hosts results in a disease with acute or chronic gastro-intestinal clinical signs (diarrhea, weight-loss). The pre-patent period is 3-4 weeks and the patent period varies depending on the immunological status of the final host.


The last representative of intestinal nematodes is Trichuris vulpis. Mild infestations are usually asymptomatic, but heavy cases can cause illness manifested with diarrhea, weight-loss and anemia. Dogs that ingest embryonated eggs present in the environment are the final hosts. The pre-patent period lasts for about 8 weeks followed by long 18 months of patent period.

Non-intestinal Nematodes


Dogs and foxes as final hosts for Angiostrongylus vasorum get infected by ingesting infective larvae present within molluscs or paratenic hosts. The patent period of up to 5 years follows a pre-patent period of 40-49 days. Initially, the disease is asymptomatic after which generalized respiratory signs are apparent with cough, tachypnoea and dyspnoea in most cases. Other pathologies include coagulopathy and neurological signs. Sometimes sudden death occurs without initial respiratory signs. Serological test for detecting Angiostrongylus vasorum is commercially available. Other diagnostic methods used are the Baermann method for detection of live larvae using at least 4 g of feces and microscopic larvae detection from bronchial lavage.

Dirofilaria immitis occurs rarely in cats and the transmission of the infective 3rd stage larvae is via intermediate host (mosquito). The pre-patent period lasts approximately 8 months and most of the times the infections are asymptomatic. Once the worms reach the heart, acute respiratory and circulatory symptoms appear with coughing, tachycardia and tachypnoea. The definitive diagnosis of heartworm can only be done by combination of serological tests, hematological test with echocardiography and thoracic radiography. Even though reported, human infections rarely occur.

In dogs, Dirofilaria immitis has a pre-patent period of 120-180 days and patent period of several years. Mild infections result in asymptomatic disease, but severe infections may pose a serious threat for the dog’s life and can prove to be fatal. 5-7 months after infection the initial clinical signs appear: coughing, dyspnoea and lack of stamina. The disease can develop into chronic form manifested through ‘Caval syndrome’, tachycardia, tachypnoea and frequent coughing. Detection after concentration of microfilaria using Difil or Knott’s Test 180 days after infections shows great diagnostic results. Serological antigen detection 5 months after infection can potentially have 100% sensitivity if at least 1 female worm is present.
Read more about Heartworm resistance here.


Aelurostrongylus abstrasus is generally prevalent in stray cats (final hosts) that get infected ingesting an intermediate host. Most of the cats show no signs of illness, and the clinically apparent cases experience exercise intolerance and respiratory symptoms. The patent period is several years and the pre-patent period lasts 7-9 weeks. Bronchial lavage larvae microscopic detection and Baermann method are used as diagnostic tools.

Capilaria spp. can cause asymptomatic to fatal diseases in dogs, cats and foxes depending on the specific parasite species and quantity in the organism. The infection occurs by ingestion of infective larvae from the environment followed by 4 weeks of pre-patent and 10-11 months of patent period. Capilaria hepatica causes hepatic lesions and usually has fatal outcome and Capilaria philippinensis damages the small intestine resulting in fatal enteropathy. The infections are generally discovered during routine autopsies. C. hepatica, C. philippinensis and C. aerophila can infect people.

Final hosts for Crenosoma vulpis (fox lungworm) are dogs and foxes that get infected by ingesting larvae present in molluscs and paratenic hosts. The parasite generally affects the respiratory system with corresponsive clinical signs. Microscopic evaluation of bronchial lavage and Baermann methods are used for detecting larvae.

Filaroides hirthi infects dogs by unknown route of transmission and potentially inflicts respiratory symptoms (coughing, exercise intolerance). The pre-patent period lasts 10-18 weeks and there is a lack of information regarding the patent period. Diagnostic methods for detection of larvae are Baermann method and bronchial lavage microscopic evaluation.

Oslerus osleri is directly transmitted orally from bitches to pups in dogs and foxes. The usual respiratory signs are apparent and besides bronchial lavage and feces evaluation, endoscopy and radiography are useful for diagnosing.

Subcutaneous worms, oesophageal worms and threadworms

Dirofilaria repens infects cats, dogs and other carnivores and is transmitted via mosquitoes (cutaneous infection). The pre-patent period is up to 34 weeks followed by several years of patent period. Even though mostly asymptomatic, cutaneous lesions sometimes occur. The diagnostic methods are the same as for the previous Dirofilaria species using 2-4 ml EDTA blood sample. There is a zoonotic risk and infections result in subcutaneous nodules in the conjunctiva.

Spirocera lupi (oesophageal worm) infections can cause alimentary disruptions in dogs, cats, foxes, wild dogs and wild cats. Many cases are without clinical signs and the ones clinically apparent exhibit vomiting (with worms in the content) and difficulty swallowing. Endoscopic and radiographic images show granulomatous lesions inside the oesophagus.

Infections with Strongyloides stercoralis in dogs, cats and humans as final hosts, result in bloody diarrhea, severe dehydration and sometimes death. The route of transmission is by ingestion of embryonated eggs from fur and soil, ingestion of larvae present in milk and paratenic hosts and also vertically. In people the parasite causes several forms of the disease: chronic intestinal syndrome, subcutaneous lesions, mild transient form and neurological illness. The pre-patent period is short (9 days) and the patent period ranges from 3 to 15 months.


Thelazia callipaeda is transmitted onto dogs and cats via arthropod vector. The patent period lasting few months to couple of years follows a pre-patent period of about 3 weeks. The infections result in epiphora and blepharospasms. Material used for detection of adult forms of the parasite and larval stages is tear film taken from the surface of the conjunctiva.

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