Genus Toxocara belongs to sub-family Ascaridoidea. The genus contains species of significant importance for human and animal health (Despommier, 2003; Lee et al., 2010; Rubinsky-Elefant et al., 2010) . Various species of the genus Ascaris, Toxocara canis, Toxocara cati and Toxocara malaysiensis, are clinically important to cause diseases in mammals that are usually canids and felids (Fisher, 2003; Lee et al., 2010). Toxocara canis and T. cati are of zoonotic importance. (Fillaux et al., 2013).
This parasite has a fecal-oral type of transmission. Dogs are its definitive host and humans are its accidental hosts. Its eggs are non-infective when eliminated with feces of dogs. Eggs then develop into infective embryonated stage. The duration of embryo development depends upon different environmental factors such as temperature, humidity and type of soil (Overgaauw et al., 2013). Soil of the parks, streets and other public places polluted with the feces of stray dogs and cats also play a great role in the transmission of toxocariosis to humans. Toxocara spp. have the ability to reproduce and survive for long time periods outside their host. In this way the lands of parks, streets and other public places remain infected for long times. This contaminated soil with Toxocara eggs poses a high risk of infection to humans. Specially children under 5 years of age are the key set affected because they play in playgrounds and by chance can swallow the eggs of these intestinal parasites from contaminated soil (Dado et al., 2012; Thomas et al., 2014).
Infection commences both in man and dogs by ingestion of embryonated eggs, which are then hatched to liberate larvae. Humans get infected by the accidental ingestion of infective eggs with contaminated food or water. Eggs hatch in the stomach and larvae move to the small intestine. From small intestine these larvae migrate to visceral organs via blood causing a series of to retina via retinal artery and impair eye vision (Nicoletti, 2013). In dogs, these larvae mature within the small intestine. Then, mating takes place and new non-embryonated eggs are formed (Despommier, 2003). Humans can acquire infection by infective embryonated infections (Rubinsky-Elefant et al., 2010). Clinical signs of the infection varies from asymptomatic to a range of severe infections i-e. Visceral organ damage (Fillaux et al.) by the migration of larvae of T. canis and T. cati. These larvae cause Ocular larva migrans by the migration of larvae eggs from adulterate soil, dirty hands, un-cooked vegetables and by direct exposure to pets (Good et al., 2004; Wolfe et al., 2003). Nearly 100% of pups are infected in utero by reactivated somatic larvae from day 42 of the gestation period. This trans-placental path and intra uterine infection is the most important means of transmission in pups (Overgaauw et al., 2013).
Zoonotic prevalence is higher in tropical region than in temperate regions. Rural populations consume more risks of infection as compared to urban population of same area (Rubinsky-Elefant et al., 2010). Worldwide surveys show that the prevalence of Toxocara infection ranges from 86% to 100% in pups and 3% to 81% in adult dogs. Infection rate is higher in pups between age of 1-6 months as compared to dogs more than 7 months old (Itoh et al., 2004). In Lahore, prevalence of Toxocara infection is 49% in stray dogs and 30% in pet dogs (Chattha et al., 2009). Various diagnostic methods are employed for diagnosis of Toxocara species. Coproscopical techniques are classical techniques which are extensively used to identify eggs and larvae in fecal samples based upon morphological characteristics, coproscopic techniques have been extensively used to diagnose larvae and eggs in fecal samples (Sweeny et al., 2011). But identification by these coproscopic techniques has limitation in the differentiation of different species of Toxocara (Li et al., 2007a). Immunodiagnostic approaches like ELISA have also been utilized by using excretory/secretory antigens of larvae (Fan et al., 2004). But it has drawback that there is chance of cross reaction between anti-toxocara serum and antigen from other Toxocara spp. (Lozano et al., 2004). Alternatively, the DNA based molecular approaches are being employed for the accurate diagnosis of toxocariosis at species level. This technique may overcome the drawbacks of traditional methodologies and help in molecular epidemiological screening because genetic variation can be studied. Molecular methods based on DNA are more sensitive and specific as compared to classical methods (Jacobs et al., 1997; Ndao, 2009). In Pakistan, DNA based diagnosis of Toxocara spp. has not been employed yet. The present study was designed to achieve the goal of accurate diagnosis of prevalent species of Toxocara in dogs by using effective DNA based diagnostic technique.