Global prevalence of Toxocara infection in dogs

Abstract

Dogs serve as the most important definitive hosts for Toxocara canis—a causative agent of human toxocariasis and one of the most widespread zoonotic helminth worldwide. The present study was undertaken to assess the global prevalence of T. canis infection in dogs. PubMed, Scopus, Web of Science, EMBASE and SciELO were searched to identify relevant studies. A random-effects model was used to estimate the overall and the subgroup-pooled prevalences across studies, and heterogeneity was assessed via the I² test. The data were categorized according to WHO-region, different types of dogs, risk factors and environmental variables. From a total of 4370 peer-reviewed publications, 229 articles that studied 13,010,004 dogs in 60 countries met the final inclusion criteria. The overall prevalence of Toxocara infection in dogs was 11.1% (95% CI, 10.6–11.7%). The estimated prevalence in the different WHO-regions ranged from 6.4% to 19.2%: Eastern Mediterranean (19.2%, 13.7–25.5%), Africa (18.5%, 13.7–23.9%), South-East Asia (11.9%, 6.8–18.2%), North America (11.1%, 10.6–11.7%), South America (10.9%, 7.6–14.6%), Europe (10.8%, 8.9–12.9%) and Western Pacific (6.4%, 3.3–10.2%). Young (< 1 year of age), stray, rural and male dogs had a significantly (P < 0.001) higher prevalence of infection than older, pet, urban or female dogs. The prevalence was higher in low income countries and regions at a low geographical latitude, close to the equator, characterized as having tropical climates. From this review, it is estimated that ≥ 100 million dogs are infected with Toxocara around the world. This highlights the need for an increased focus on implementing affordable, appropriate control programs to reduce the public health threat of toxocariasis as a zoonosis of global importance.

Introduction

Dogs (Canis familiaris) are ubiquitous and have the widest diversity of roles of any domesticated animal in human societies worldwide. They serve as companion, working or service animals, entertainment or show dogs, working dogs on farms, police or military dogs, shepherds, hunters, guardians, transport, and play an important role in a wide range of comparative animal medicine studies of diseases such as diabetes, narcolepsy and cancer. The dog is widely considered to be ‘man's best friend’ and has important roles in the psychological and physiological well-being of many people, particularly young children, as well as disabled and elderly people (Serpell and Barrett, 2016). The global dog population is estimated at 900 million (https://www.worldatlas.com), ~ 500 million of which are stray or feral animals (https://www.carodog.eu). These numbers probably underestimate the true numbers of dogs, as dogs are not registered in many countries. In 2017, the total number of pet dogs in the European Union was estimated at 85 million (https://www.statista.com). According to the 2017–2018 National Pet Owners Survey in the United States, ~ 60% of households owned a pet, and the number of pet dogs was estimated at 89 million (https://www.iii.org). In addition, according to a previous estimate, 361 million dogs were estimated to live in Asia, 102.2 million in Latin America and the Caribbean, and 87.6 million in Africa (Hughes and Macdonald, 2013).

Although many humans derive great benefits from dogs, these canids can also serve as reservoirs for a wide range of zoonotic pathogens, some of which are of major public health importance (Macpherson et al., 2013; Otranto et al., 2017). In many cases, stray dogs might harbour more infectious agents than pet dogs, such that they represent a significant risk to human populations (Otranto et al., 2017; Paoletti et al., 2015); nevertheless, direct contact of pet dogs with people is a significant risk factor (Dubná et al., 2007; Lucio-Forster et al., 2016; Overgaauw and van Knapen, 2013). Of the important zoonotic pathogens, ascaridoid nematodes of the genus Toxocara, including Toxocara canis and T. cati, are causative agents of human toxocariasis with a worldwide distribution (Aghamolaie et al., 2018; Fisher, 2003; Ma et al., 2018). Toxocariasis can cause four major clinical syndromes in humans, including visceral larva migrans (VLM), ocular larva migrans (OLM), neurotoxocariasis (NT) and common/covert toxocariasis (CT) (Aghaei et al., 2018; Ma et al., 2018; Mohammadzadeh et al., 2018; Rostami et al., 2019). Canids, including dogs, foxes, wolves and jackals serve as definitive hosts of T. canis, releasing eggs into the environment, where, under favourable conditions, they can embryonate and eventually become infective. Infective eggs containing third-stage (L₃) larvae can be accidentally ingested by humans and other paratenic hosts via contaminated soil, unwashed raw vegetables, water, or occasionally from the fur of dogs (Ma et al., 2018).

In the past decades, numerous studies have assessed the prevalence of T. canis infection in dogs, often in relatively small geographic regions. Consequently, there are significant gaps in the knowledge of the epidemiology of Toxocara infection/toxocariasis in many countries. Considering the public health implications of toxocariasis, a more comprehensive understanding of the occurrence and prevalence of Toxocara infection would be valuable. This paper provides the first systematic review and meta-analysis of the available data on the global prevalence of T. canis in dogs.