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Overview of selected parasitic diseases in dogs

  • Ecommercy
  • 2026-09-22
  • 16:17
  • 0 comments

Parasites are among the most common health threats affecting dogs. Both external and internal parasites can pose a risk, and parasitic infections may lead to a wide range of serious health problems. This article discusses selected parasitic diseases occurring in dogs: toxoplasmosis, cardiopulmonary and subcutaneous dirofilariasis, hepatozoonosis, thelaziosis and angiostrongylosis.

Toxoplasmosis

In most cases, Toxoplasma gondii infection in dogs is asymptomatic or subclinical. Clinical disease occurs mainly in immunocompromised animals or those with concurrent diseases. Cases of latent toxoplasmosis becoming reactivated when the immune system is weakened have been described in the literature.

Neurological signs predominate in dogs and may include seizures, ataxia, paresis, cranial nerve paralysis and hypersensitivity to noise. Clinical signs of toxoplasmosis may also depend on the location of the infection. Cysts can exert pressure on nerves and muscle tissue, resulting in signs such as muscle tremors, lameness, severe pain and a stiff gait.

Cutaneous toxoplasmosis is also possible and may cause nodules and, in some cases, ulcerative skin lesions. As in humans, dogs may develop ocular toxoplasmosis, which can result in partial or complete loss of vision.

T. gondii infection may occur through ingestion or licking of oocysts shed by cats, for example through contaminated water, faecal material present on plants or food containing infective stages of the protozoan. Infection may also occur through consumption of raw or undercooked meat containing tissue cysts.

Venereal transmission has also been confirmed in dogs, with the protozoan detected in the semen of seropositive animals. Transmission through blood transfusion is also possible.

Interestingly, it has been hypothesised that the parasite could potentially be transmitted by blood-feeding arthropods, including ticks. However, tick-borne transmission of toxoplasmosis has not yet been confirmed.

Diagnosis can be challenging because of the morphology of the protozoan and the characteristics of its life cycle. Both direct methods, involving detection of the parasite or its genetic material, and indirect methods, involving antibody detection, are used. Standard diagnostic tests include ELISA for the detection of IgG and IgM antibodies and PCR.

Dirofilariasis

Dirofilariasis is a parasitic disease caused by nematodes of the genus Dirofilaria. It is the major mosquito-borne parasitic disease affecting dogs. Two forms are distinguished: cardiopulmonary dirofilariasis caused by Dirofilaria immitis and subcutaneous dirofilariasis caused by Dirofilaria repens.

D. immitis is primarily found in the pulmonary arteries. In severe infections, parasites may also be present in the right ventricle of the heart. Heartworm disease is rarely diagnosed in Poland. The parasite favours warmer climates and is therefore more commonly encountered in southern Europe.

Infected animals may remain free of clinical signs for many months or even years. Signs develop gradually and may include coughing, dyspnoea and exercise intolerance.

Over time, exercise-induced syncope may occur. Prolonged presence of parasites in the pulmonary vessels can lead to pulmonary hypertension and secondary fibrosis of the lung parenchyma.

Kidney damage may also occur because chronic infection can cause substantial activation of the humoral immune response. This leads to the formation of immune complexes, which are primarily deposited in the kidneys. Their accumulation may contribute to the development of membranous glomerulonephritis. Dead nematodes may also cause embolism of blood vessels.

Diagnosis involves serological testing and examination of blood collected from dogs suspected of infection. Microfilariae may be detected in blood smears or using the modified Knott test.

D. repens causes subcutaneous dirofilariasis. The disease is characterised by the formation of usually painless nodules located within the skin, subcutaneous tissue, muscle tissue and even the eye. Adult parasites can be found within these nodules.

The nematode can migrate through tissues and may therefore also be found in body cavities, the pericardial sac or the knee joint. Compared with D. immitis, D. repens is diagnosed considerably more frequently in Poland. Infection is often asymptomatic, and parasites may be detected incidentally.

Thelaziosis

Thelaziosis is a parasitic disease caused by Thelazia callipaeda. Originating from East Asia, this parasite has considerable potential for rapid geographical spread. Nematodes of the genus Thelazia cause ocular disease in companion animals.

Until relatively recently, thelaziosis was regarded as an exotic disease imported from Asia. This small nematode is found in the nasolacrimal ducts, conjunctival sac, beneath the third eyelid and on the surface of the cornea.

In Europe, the parasite is transmitted by fruit flies of the species Phortica variegata. Thelaziosis in dogs occurs seasonally, reflecting the presence and activity of its vector. Adult parasites may remain in the conjunctival sac of an infected animal for up to one year.

Infection may be either asymptomatic or symptomatic. The pathogenic effects of the parasite result from mechanical irritation of the conjunctiva and cornea as well as the toxic effects of nematode metabolites.

Clinical signs may include excessive tearing, swelling and increased temperature of the eyelids, photophobia, acute conjunctivitis and conjunctival hyperaemia, as well as serous or mucoid discharge that may eventually become purulent and cause the eyelids to stick together.

Prolonged untreated infection may lead to corneal opacity and ulceration. Eventually, uveitis, corneal perforation, cataracts, glaucoma and even loss of vision may develop.

The disease may affect only one eye, and the severity of clinical signs does not correlate with the number of nematodes present.

Thelaziosis is a zoonotic disease. The parasite primarily affects dogs, although cats can also become infected.

Diagnosis is usually based on identifying the parasite within the conjunctival sac. If adult parasites are not visible, diagnosis may be more difficult. In such cases, flushing the conjunctival sac and nasolacrimal ducts and examining the collected material can be effective.

Hepatozoonosis

Hepatozoonosis is caused by the parasite Hepatozoon canis. The disease occurs mainly in countries of the Mediterranean region, although its geographical range has expanded considerably in recent years. The first case of canine hepatozoonosis in Poland was confirmed in 2021.

Unlike most other tick-borne diseases, infection occurs when a dog ingests an infected tick, for example while grooming its coat. Wild or hunting dogs may also become infected after consuming prey or carrion carrying infected ticks or containing parasite cysts within their tissues.

Clinical signs in affected dogs may include fever, stiff gait, lethargy and muscle weakness. Mucopurulent discharge from the conjunctival sacs may also occur, and some animals may develop muscle atrophy.

Diagnosis can be challenging and usually requires several diagnostic tests. Microscopic examination of stained blood smears may reveal gamonts within leukocytes, most commonly neutrophils and monocytes. PCR is considered the most sensitive diagnostic method.

Angiostrongylosis

Angiostrongylosis is caused by Angiostrongylus vasorum, commonly known as the “French heartworm”. It is a nematode belonging to the family Metastrongyloidae.

It is an emerging disease affecting dogs in Poland and represents a diagnostic challenge for vets. The parasite is considered cosmopolitan and is widespread in countries including Spain, France, Belgium, Denmark, Italy, Germany, Hungary, Sweden and the United Kingdom. It has also been confirmed in Australia.

In Poland, A. vasorum was first detected in 2007 during post-mortem examinations of wolves. The first clinical case in a dog was diagnosed in Lublin in 2014.

The main definitive hosts are foxes and dogs. Intermediate hosts include aquatic and terrestrial snails and slugs, which are widespread in Poland, particularly species belonging to the genera Helix and Limax.

Dogs may become infected by ingesting an entire snail or slug, or through contact with their faeces or mucus. Paratenic hosts, including rodents, birds and frogs, may also play an important role in transmission. Mild winters favour the survival of the parasite.

Early diagnosis can be difficult because there is no single characteristic clinical presentation and a wide variety of signs may occur. Angiostrongylosis may have a moderate, acute or chronic course. Infected dogs shed larvae in their faeces, and infection may persist for between two and five years.

During the initial stage of infection, migrating larvae cause lung damage. Affected dogs may develop coughing, dyspnoea and difficulty breathing, as well as exercise intolerance. Neurological signs may occur in some cases.

Adult nematodes are primarily located in the pulmonary vessels and the right ventricle of the heart. Obstruction of the pulmonary artery may result in sudden death.

Parasites may also occur in atypical locations, including the brain, urinary bladder, anterior chamber of the eye or kidney. In such cases, the clinical signs will depend on the organs involved.

Ante-mortem diagnosis is based on faecal examination using the Baermann technique. Larvae may also be detected in respiratory mucus collected during bronchoscopy. ELISA serological tests can be used to detect circulating parasite antigens or antibodies. PCR testing is also available.

References

  1. Adaszek Ł., Szczepaniak K., Szadkowski M., Pisarek M., Mazurek Ł. (2023). Sytuacja epizootyczna telazjozy psów w Europie. Magazyn Weterynaryjny; 9.
  2. Hildebrand W. (2024). Toksoplazmoza psów – nie tylko koci problem. Weterynaria w Praktyce.
  3. Kandefer-Gola M., Żebrowski K. (2022). Inwazja Dirofilaria immitis – opis przypadku. Magazyn Weterynaryjny; 1.
  4. Karaś-Tęcza J., Łukaszewska J., Dawidowicz J., Słobodzian I. (2016). Zarażenia Dirofilaria repens u psów okiem dermatologa i hematologa. Magazyn Weterynaryjny.
  5. Mierzejewska E.J., Dwużnik D., Bajer A. (2017). Angiostrongylus vasorum – nowe wyzwanie diagnostyczne. Weterynaria w Praktyce; 3.

Author: Adrianna Iwan

Microbiologist, Veterinary Technician, Animal Behaviourist and postgraduate graduate in Animal Rights. Veterinary Medicine student. Member of the Polish Council for Companion Animal Parasites (ESCCAP Poland). Creator of the educational profile Mikrobiolog z przypadku.