A critical reflection on current control of Toxocara canis in household dogs

Abstract

Toxocara canis is a roundworm that is common worldwide and also in Dutch household dogs. Adult stages of T. canis can be present in the small intestines of dogs where they produce large numbers of eggs that are shed in the environment (patent infection). Because very young dogs and humans can develop symptoms after infection with infective eggs, the current advice is to deworm all dogs four times a year. However, the vast majority of dogs older than six months will not develop a patent infection although they probably will be re-infected on a regular base. In those dogs infection will not lead to adult worms in the intestines, but the larvae will migrate to the somatic tissues and become arrested in their development, waiting to be reactivated. Reactivation of these larvae is known to occur during pregnancy in dogs. This thesis focused on a more evidence-based control policy of roundworm infections in household dogs older than six months, their contribution to the overall environmental contamination with Toxocara eggs compared with that of cats and foxes, and the owner’s attitude towards deworming, including simulating the effect of different scenario’s using different compliance of owners concerning deworming and cleaning up their dog’s faeces. A total of 570 dog owners, owning 938 dogs, participated in this study. They provided faecal samples of their dogs on a monthly basis and answered accompanying monthly questionnaires. Owners did not deworm their dogs without knowledge and consent of the project team. Coprophagy among dogs proved to be a disturbing factor for interpreting results of coproscopical examination for parasitic infections. All analyses, therefore, took coprophagy into account as a variable. The relative contribution of dogs, cats and foxes to the environmental contamination with Toxocara was estimated using a stochastic model. Household dogs accounted for 39% of the overall egg output. However, the relative contribution of dogs, compared to household cats, stray cats and foxes, can differ significantly with the degrees of urbanisation, due to a difference in preferred habitat for these final hosts. Household cats and stray cats, when considered as one group, would transcend the relative contribution of household dogs. Simulation of different deworming compliances of owners indicated that a higher compliance than is to be expected on a voluntary basis is needed to have a meaningful effect. Risk factors were assessed in several analyses, one of which included a prospective study on recurrent Toxocara infection. A major finding was that a minority of ‘wormy animals’ was found to be responsible for the majority of the positive samples containing Toxocara eggs. Therefore, targeted treatment of these ‘wormy dogs’ might be more efficient in reducing the household dogs’ contribution to the environmental contamination with roundworm eggs than blind deworming of all household dogs.