Roundworms, and in particular Toxocara cati, are the most common intestinal parasites in cats and occur in cats of all ages throughout the world. Regular routine faecal examinations and treatment, when necessary, for roundworms is recommended throughout a cat’s life. While Toxocara canis (the roundworm species found in dogs) has been implicated in most cases of human infections, Toxocara cati also has potential to transmit to humans and may be an underestimated source of human disease.
Toxocara cati is a parasitic roundworm (ascarid) infecting cats, with multiple routes of infection. Although many similarities exist between dog and cat roundworms caused by Toxocara canis and Toxocara cati respectively, these parasites have a different biology. For instance, Toxocara canis can be transmitted via the placenta, as well as colostrum and milk for weeks after birth in the dog. Conversely, Toxocara cati does not infect animals via the placenta but rather transmission occurs via the colostrum and milk during the first days of nursing only, when infection of the queen takes place during late pregnancy. Cats can also acquire the infection, regardless of their age, when they ingest eggs from the environment (which are passed in the faeces and can remain viable in the environment for several years) or intermediate hosts (e.g. invertebrates, rodents, birds) harbouring encysted larvae.
Infection of cats with the eggs of Toxocara cati occurs when ingested larvae penetrate the stomach wall and migrate through the liver, lungs and trachea until they reach the small intestine where they develop into adults. Most larvae ingested via the milk (or with intermediate hosts) continue their development to adult worms in the intestinal tract. Toxocara cati has an incubation period of 8 weeks after ingestion of eggs from the environment or larvae from intermediate hosts, while the faecal excretion of eggs following transmission via milk starts after 6–7 weeks. There is a tendency to consider ‘intestinal worms’ as a health problem of kittens and that adult cats are resistant. However, cats are exposed to roundworm infections throughout their life – for example, adult cats may ingest environmental eggs through self-grooming. Kittens have higher parasitic burdens, egg output and infection rates than adults, but intestinal infections occur in cats of all ages.
Although infected cats usually harbour Toxocara cati, other roundworms may also infect cats. Toxocara leonine, an ascarid able to infect both cats and dogs, has minor disease causing potential in cats compared with Toxocara cati and is not transmissible to humans. Toxocara malaysiensis has been described in cats from some Asiatic regions. At the moment little is known about its epidemiology and clinical impact in Asia or other regions, and no information is available on a potential zoonotic (meaning that they can be transmitted to, and cause disease in humans) role.
Many people wrongly believe that it is normal for kittens to have intestinal worms and that there is nothing really dangerous for a kitten in having the typical ‘pot-belly’. Obviously, a kitten with thick and, in some cases, up to 10 cm-long worms living in the small intestine is a sick animal, regardless of the presence of evident clinical signs. Severe infections in kittens may result in:
- Variable appetite or anorexia
- Vomiting (especially after feeding)
- Diarrhoea alternated with constipation
- Growth disturbances
The expulsion of masses of, usually live, worms in vomit is also quite common. While adult cats usually show milder clinical signs, severe infections can cause the stomach to perforate leading to the life-threatening condition peritonitis.
Impact on human health
It is well known that roundworm species found in pets cause human infections worldwide. To date, Toxocara canis has been implicated in most cases of human infections but the role of Toxocara cati has probably been underestimated. The infection in people usually relies on the ingestion of eggs present in soil or in contaminated food. The eggs of Toxocara cati are not infective immediately after excretion in the faeces, as 2–6 weeks are necessary for the complete development of an infective larva. Other sources of human infection are raw/undercooked meat from intermediate hosts (e.g. chickens or cattle). In humans, animal Toxocara species do not reach the adult stage, and clinical signs, when present, are due to migrating larvae. The disease severity depends upon the:
- Degree of tissue damage
- Tissue involved
- Number of migrating larvae
- Age of the person
- Immune response of the person
The most important clinical syndromes are the so-called ‘visceral larva migrans’ (VLM), encompassing important organs (mostly the liver) and ‘ocular larva migrans’ (OLM), due to damage to eye. Other syndromes, for example, covert, neural and atopic toxocariasis, are also described.
Visceral Larva Migrans (VLM)
Children and, in particular, toddlers, are more at risk of VLM due to low hygiene and frequent exposure to areas (e.g. sandpits, sandboxes, gardens, playgrounds) potentially contaminated by the eggs of Toxocara species. VLM in children is characterised by:
- General malaise
- Abdominal pain
Infection can affect the liver, lungs, heart, and the central nervous system with a variety of signs relating to each. However, experimental models have shown that Toxocara canis has a greater affinity to the central nervous system compared with Toxocara cati, which prefers organs. Indeed, Toxocara cati may migrate to the brain but it localises more frequently in the cerebellum (lower part of the brain) while Toxocara canis prefers the cerebrum (largest part of the brain).
Ocular Larva Migrans (OLM)
OLM occurs at any age but more frequently in 5–10-year-old children. The most important manifestation is impaired vision up to total loss of sight, followed by complaints of ‘seeing lights’, squinting and glaucoma.
Covert toxocariasis (CT) is considered to be a milder form of the disease than VLM and OLM, and may be linked to chronic exposure. Adults with CT may have:
- Breathing difficulties
- Abdominal pain
- Generalised itching
In children, CT is associated with:
- Behaviour and sleep disorders
- Coughing and wheezing
- Limb pain
Toxocara species infections can exacerbate asthma and cause skin conditions such as itching, eczema, and urticaria. Cases of adult Toxocara cati passed from children are documented, but they originate from the ingestion of worms expelled with vomit or faeces by an infected animal.
Treatment and prevention
Different worming products are available in various formulations for the treatment and control of intestinal worms in cats. There is a plethora of products with a narrow or broad spectrum, which can be selected according to each individual possible scenario and to ensure owner and animal compliance. For instance, spot-on formulations have the advantage of being easy to apply, while some oral formulations are highly palatable.
The use of worming products should be in accordance with parasite biology and epidemiological features in different regions. Geographical spread of parasites, clinical importance, and possible high resistance of infectious stages in the environment regardless of season or climate should be considered. Toxocara cati eggs are resistant to harsh chemicals and a wide range of temperatures. Different guidelines are issued by the CAPC in North America and ESCCAP in Europe (https://capcvet.org; www.esccap.org). As no prenatal transmission occurs in cats, kittens can undergo fortnightly treatments from 3 weeks old until weaning. They should be treated monthly until 6 months old and adult cats can be treated once a month (‘year-round’) or should be subjected to faecal examinations once a month and treated accordingly (ESSCAP guidelines, www.esscap.org). Treatment of a pregnant queen with selected protocols and selected drugs prevents transmission of Toxocara cati via the milk. Lactating queens should be treated with licensed drugs once during lactation concurrently with the first treatment of kittens.
Routine faecal examinations and regular deworming are crucial for an effective control of ascarids throughout the cat’s life. Such a regime limits the risk of infection and contributes to the safeguarding of both animal and human health.