What is toxocariasis?
Human toxocariasis is caused by infestation of the larvae of two widespread roundworms, Toxocara canis and Toxocara cati. Dog and cat intestines are the reservoir for T. canis and T. cati respectively. T. canis is more commonly responsible for human disease. Human toxocariasis is widespread and is usually symptom-free. Almost 14% of the United States population is estimated to be infected, with even higher rates in rural areas and tropical countries.
Most eggs enter the environment via the faeces of puppies and lactating dogs. Humans usually become infected by ingesting eggs from the environment. The disease commonly affects young children with a history of eating soil, exposure to dogs, and those living in poor socioeconomic conditions.
What are the clinical features of toxocariasis?
The ingested toxocara eggs develop into larvae, which penetrate the gastrointestinal mucosa. The larvae are then carried in the bloodstream to the liver, lungs, and other organs, where they can remain for long periods. A specific type of white blood cells, called eosinophils, accompany the larvae and infiltrate the organs and tissues in response to the infection. Raised levels of eosinophils can usually be detected in the blood.
Mild infections with Toxocara larvae often produce no symptoms. When symptoms are present, a variety of clinical syndromes have been described.
|Clinical syndrome||Clinical features|
|Visceral larva migrans (VLM)||The liver and lungs are most commonly involved producing abdominal pain, decreased appetite, enlarged liver, coughing, wheezing, restlessness, fever, and asthma. Skin disease occurs in around one in four cases and includes rashes, urticaria, and hypodermic nodules (small lumps below the epidermis).|
|Ocular larva migrans (OLM)||Migration of T. canis larvae to the eye causes loss of vision.|
|Common toxocariasis||Usually affects adult patients living in rural areas of the Midi-Pyrénées region of France. Presents with chronic lack of energy and weakness associated with digestive disorders. Blood tests show raised levels of eosinophils and total IgE. Skin manifestations such as itch, eczema/dermatitis, and urticaria are sometimes present.|
|Covert toxocariasis||Non-specific symptoms such as cough, abdominal pain, headache, sleep disorder, vomiting, enlarged liver, and occasionally a rash. Blood tests show raised eosinophils in around half of all cases.|
VLM and OLM are usually more severe, whereas common and covert toxocariasis are less severe and are spontaneously cured in most cases.
What are the skin manifestations of toxocariasis?
Skin manifestations are relatively common in patients with toxocariasis, particularly in VLM and common toxocariasis. Occasionally skin manifestations may be the sole feature of the disease. Given the high prevalence of toxocariasis worldwide, the diagnosis should be considered in any case of undiagnosed suspicious skin lesions that have been recurring for several months or years, such as:
- Chronic urticaria — the most common skin manifestation
- Chronic pruritus (itch) or prurigo
- Miscellaneous eczema including including atopic dermatitis and dyshidrosis
- Rare skin manifestations include panniculitis, vasculitis, Henoch-Schölein purpura, Reactive arthropathy, Wells syndrome, and eosinophilic folliculitis.
How is toxocariasis diagnosed?
Although the definitive diagnosis is based on detection of Toxocara larvae in tissue samples, obtaining biopsy material containing larvae can be difficult and is usually not necessary. The diagnosis is usually based on clinical features and results of blood tests:
- Markedly raised white cell count with raised eosinophils.
- Raised total IgE.
- Raised specific IgE or IgG antibodies against Toxocara can be detected with an enzyme linked immunosorbent assay (ELISA). IgG antibodies are the most sensitive, but remain elevated for at least four years after infection; specific IgE antibody levels decrease significantly during the first year after treatment.
- Eosinophil levels are usually normal in patients with chronic skin manifestations of toxocariasis. Furthermore patients with chronic urticaria, pruritus, or prurigo may have lower antibody levels than those with standard VLM. In these patients the Western blot serology method may be more suitable to diagnose toxocariasis.
The ELISA test can be modified to confirm OLM using fluid samples from inside the eye.
What is the treatment for toxocariasis?
- Common or covert toxocariasis usually resolves spontaneously.
- Symptomatic VLM or OLM requires treatment with an anthelmintic agent such as albendazole, diethylcarbamazine, or mebendazole (the latter is the only agent registered in New Zealand). If there is eye or neurological disease, corticosteroids may also be used.
- Appropriate anthelmintic treatment can often cure skin manifestations of toxocariasis; particularly prurigo and, to a lesser extent, chronic urticaria.
How can toxocariasis be prevented?
- Dogs and cats must be treated for roundworm.
- Children should be taught to not eat soil.
- Discourage dogs from public playgrounds and areas that children play.