Internal parasites remain a significant health threat to cats, and some are also relevant to human health. Understanding how to recognise clinical signs, how infections occur and how parasitic diseases can be prevented is therefore important. This article discusses four important parasitic diseases in cats: giardiasis, toxoplasmosis, lungworm infections and opisthorchiasis, together with key information on their clinical course, diagnosis and zoonotic potential.
Selected parasitic diseases in cats
- Introduction
- Giardiasis
- Toxoplasmosis
- Lungworm infections – an underestimated threat
- Opisthorchiasis
Introduction
Internal parasites in cats remain a common problem encountered in veterinary practice. Protecting cats from parasites is important not only for their own health and well-being. Many parasites can also pose a risk to humans, meaning that looking after our animals also helps protect our own health.
It is important to remember that endoparasites are not limited to those living in the gastrointestinal tract. Lungworms affecting the respiratory system are being diagnosed increasingly frequently in Poland.
This article discusses selected parasitic diseases found in cats: giardiasis, toxoplasmosis, lungworm infections and opisthorchiasis (liver fluke infection).
Giardiasis
Giardiasis is caused by the protozoan Giardia intestinalis (syn. Giardia duodenalis, G. lamblia). It is one of the most common gastrointestinal parasites.
The parasite colonises the intestine, particularly the proximal small intestine, and occasionally the biliary tract. It can cause acute or chronic diarrhoea in both animals and humans.
The protozoan has been divided into eight genogroups, known as assemblages A to H. Cats are primarily infected with assemblage F of G. intestinalis.
G. intestinalis cysts are resistant to environmental conditions. They may remain viable in soil for three to six months, provided the temperature does not exceed 20°C. In water at 6°C, they may remain viable for 30 days.
Infection occurs through ingestion of infective cysts present in water, soil or contaminated food, particularly raw vegetables.
Once Giardia enters the gastrointestinal tract, it rapidly colonises the small intestine. It may damage the intestinal villi, resulting in impaired absorption and weight loss.
Cysts are shed by both clinically affected animals and those showing no clinical signs. Giardiasis may therefore be symptomatic or asymptomatic. In adult animals, infection is usually asymptomatic, while clinical infections are most commonly reported in young cats.
The most common signs include:
• recurrent watery diarrhoea,
• lethargy,
• weight loss,
• abdominal bloating,
• abdominal pain,
• fatty stools.
The mechanism by which diarrhoea develops is not fully understood but is thought to be associated with impaired intestinal absorption.
Diagnosis is based primarily on microscopic techniques, including direct faecal smears and centrifugal faecal flotation, as well as rapid immunochromatographic tests or SNAP tests.
Infections are often self-limiting but may recur and become chronic. G. intestinalis can evade recognition and elimination by the host immune system through variable surface proteins (VSPs).
Recent research also suggests that trophozoites may produce metabolites that create favourable conditions for the growth of certain microorganisms. This may alter the composition of the intestinal microbiota and contribute to dysbiosis.
Toxoplasmosis
Domestic cats and other felids, as the definitive hosts of Toxoplasma gondii, play a major role in the dissemination of oocysts.
The parasite occurs in three infective stages: sporozoites, tachyzoites and bradyzoites. Sporozoites are found within sporulated oocysts, which are exceptionally resistant to environmental conditions.
In temperate climates, they can survive for very long periods, ranging from several months to several years. Oocysts may survive for up to 54 months at 4°C, 106 days at −10°C and one minute at 60°C.
Importantly, oocysts are not immediately infective after being shed. They usually undergo sporulation within 2–5 days and become infective only after this process. Oocyst shedding generally lasts between one and three weeks and is most intense between days two and five.
Intermediate hosts include all mammalian species as well as birds, reptiles and fish. Infection occurs primarily through ingestion of tissues containing the protozoan. In addition to oral transmission, intrauterine transmission and infection through blood transfusion are also possible.
In recent years, an interesting hypothesis has suggested that the parasite could potentially be transmitted by ticks, supported by the detection of T. gondii DNA in Dermacentor reticulatus. However, tick-borne transmission of toxoplasmosis has not yet been confirmed.
Clinical toxoplasmosis occurs considerably more frequently in cats than in dogs. Clinical signs are generally observed in young cats, while infection in adults is usually asymptomatic.
Congenital toxoplasmosis is the most severe form in cats. It may cause cholangiohepatitis, pneumonia and encephalitis. Affected cats may develop ascites, dyspnoea and lethargy, while infection in kittens may be fatal.
Some cats may develop the intestinal form of toxoplasmosis, accompanied by signs typical of coccidiosis. Generalised toxoplasmosis is characterised by pneumonia, which may progress to acute respiratory distress syndrome and septic shock.
In clinical cases, signs most commonly involve the nervous system, liver, lungs and eyes. T. gondii can cause necrotic lesions within affected organs. Acute toxoplasmosis is rarely diagnosed in cats.
It is still commonly claimed that cats pose the greatest toxoplasmosis risk to humans, particularly pregnant women. In reality, a healthy, well-cared-for domestic cat does not pose a significant risk to humans under normal hygienic conditions.
Cats most commonly become infected by consuming infected birds or rodents containing tissue stages of the protozoan, including tachyzoites and bradyzoites. Raw or undercooked meat may also represent a potential source of infection.
Cats begin shedding oocysts approximately 3–10 days after ingesting tissue cysts. Shedding may continue for up to 20 days and is most intense between days two and five. Oocysts require at least 24 hours to become infective and usually become infective after 2–5 days.
Indoor cats therefore pose little risk provided litter trays are cleaned regularly.
Humans may become infected by ingesting oocysts present on unwashed fruit or vegetables or by consuming tissue cysts in raw or undercooked meat from infected sheep, goats and pigs. Drinking unfiltered water should be avoided, and fruit and vegetables grown in gardens should always be washed thoroughly.
Diagnosis and interpretation of results require a comprehensive approach. Standard diagnostic methods include ELISA for the detection of IgG and IgM antibodies, the indirect fluorescent antibody test (IFAT) and PCR.
IgM antibodies usually appear 2–4 weeks after infection. Their presence does not necessarily mean that the cat is shedding oocysts. Monitoring changes in antibody titres over time is therefore important, and diagnosis should not be based on a single measurement.
IgG antibodies detectable by ELISA appear approximately three weeks after infection, with titres increasing over the following 6–8 weeks. A single positive result does not necessarily indicate active or recent infection. A fourfold increase in titre over a period of 2–4 weeks is indicative of active or acute infection.
Lungworm infections – an underestimated threat
The number of lungworm infections diagnosed in domestic cats has increased across Europe in recent years.
The most common infection is caused by Aelurostrongylus abstrusus. Infections with Capillaria aerophila and Troglostrongylus brevior are also being identified increasingly frequently.
Aelurostrongylosis is one of the most clinically significant parasitic diseases affecting cats. Adult A. abstrusus nematodes live in the lungs of infected cats, primarily in the alveoli and bronchioles.
Infection occurs following ingestion of infective L3 larvae present in intermediate hosts such as slugs and snails, or in paratenic hosts including rodents, birds and amphibians.
The larvae migrate through the lymphatic vessels to the lungs, where they mature. Females lay eggs in the lungs, from which larvae hatch. These larvae are coughed up and swallowed, enter the gastrointestinal tract and are subsequently shed in the faeces.
The life cycle of Troglostrongylus brevior is similar to that of A. abstrusus. Important differences include the possibility of intrauterine or lactogenic transmission to kittens and the parasite’s location within the lungs, as T. brevior inhabits the small and large bronchi.
The life cycle of Capillaria aerophila (syn. Eucoleus aerophilus) is not yet fully understood. Foxes act as an important reservoir, and earthworms are thought to serve as intermediate hosts. Paratenic hosts are similar to those of A. abstrusus.
The parasite is located within the connective tissue surrounding the trachea and bronchi. In most infected cats, C. aerophila causes chronic bronchitis. Importantly, this nematode also has zoonotic potential.
Cats with unrestricted outdoor access are at the greatest risk of infection with these parasites.
Clinical signs may include:
• coughing,
• sneezing,
• difficulty breathing.
Early non-specific signs may include fever, lethargy and weight loss.
The standard diagnostic method for detecting these nematodes is faecal examination using the Baermann technique. However, a negative result may occur if the sample is collected during the prepatent period, when larvae are not yet present in the faeces.
For C. aerophila, faecal flotation is sufficient for detecting infection. The Baermann technique is useful for identifying or excluding concurrent lungworm infections.
Outdoor cats presenting with acute or chronic respiratory signs should therefore be tested for lungworm infection.
Opisthorchiasis
The cat liver fluke, Opisthorchis felineus, together with two related Asian species, O. viverrini and Clonorchis sinensis, forms a group of three important liver flukes affecting dogs and cats.
Within the European Union, O. felineus infections have been reported in Greece, Germany, Romania, Spain, Lithuania, Italy and Poland.
The first intermediate hosts of the cat liver fluke are snails of the genus Bithynia. Various species of cyprinid fish act as second intermediate hosts.
Humans and animals become infected by consuming raw infected freshwater fish, primarily from the carp family, containing metacercariae.
Importantly, metacercariae are resistant to drying, high salt concentrations and low temperatures. They are destroyed only by high temperatures.
Within the gastrointestinal tract of the definitive host, metacercariae emerge from their cysts and the immature flukes migrate into the bile ducts of the liver.
Interestingly, adult liver flukes may survive in humans for more than 20 years. Eggs deposited at the bottom of freshwater reservoirs may remain viable for up to 104 days when they enter the water in early spring.
Clinical signs depend on the location and number of adult flukes and on the animal’s immune response.
Approximately 10% of O. felineus infections develop into acute disease. Clinical signs may include fever, diarrhoea, vomiting, pain in the liver region, loss of appetite, ascites, jaundice and oedema.
These signs may persist for 2–4 weeks and vary considerably in severity.
Liver flukes can mechanically damage the liver or contribute to cirrhosis as a result of toxic metabolites produced by the parasite. Patients with chronic infection are at risk of developing bile duct cancer.
Low-intensity infections are asymptomatic in most cats.
Diagnosis is based on parasitological examination of faeces for parasite eggs. Adult flukes may also be identified during post-mortem examination.
Animals at greatest risk include those fed raw freshwater fish and those with unrestricted access to freshwater environments.
There are currently no medicinal products specifically authorised for eliminating liver fluke infections in dogs and cats. Spot-on products containing praziquantel are, however, authorised for use in cats.
References
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- Jańczak D. (2023). Pierwotniaki o potencjale zoonotycznym. Cz. I. Magazyn Weterynaryjny; 2.
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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.