Update on Toxocara species and toxocarosis

01 March 2014
6 mins read
Volume 5 · Issue 2


Toxocara species eggs are produced by adult worms in the intestine of the fox, dog and cat definitive hosts. In the environment, third stage larvae develop inside the eggs and once developed, the larvae are infective. The relative contribution of the three species of definitive hosts depends on the population size, the prevalence and intensity of infection in those hosts and what steps are taken to prevent infection or to pick up faeces, particularly in the case of dogs. Infection in paratenic hosts results in larval migration which can cause behavioural changes when migration occurs in the central nervous system. Infection in the lungs of the definitive host must be distinguished from other causes of lung disease. Of the syndromes associated with infection in humans, the varied signs associated with covert toxocarosis are becoming better understood and are now believed to include asthma, reduced lung function, reduced cognitive function, chronic coughing. Diagnosis in humans remains challenging with elimination of other possible causes an important constituent.

Toxocara canis and Toxocara cati are both ubiquitous, every day parasites that we take for granted and there may be a tendency to believe that we know it all. However, recent work, for example, on the pathology caused by T. cati in the lungs of cats and the body of data that exists on the effects of this parasite on humans, have produced sufficient new data to merit a review of what is known about this enigmatic parasite. This review does not go systematically through all aspects of the infection and these details can be found on, for example, the www.esccapuk.org.uk website.

Female Toxocara spp. are prolific egg layers, with each worm able to lay up to 20 000 eggs. Eggs are passed into the environment within faeces and take a period of time to develop within the egg, depending on environmental conditions. Traditionally it was believed that the second stage larva was the infectious stage that remained protected by the egg shell until ingested by a host. Recent work has demonstrated that there are two moults of the larva within the egg, hence the infectious larva within the egg is actually third stage (Figure 1) (Schnieder et al, 2011).

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