Soil-Transmitted Helminthic Zoonoses in Humans and Associated Risk Factors - Soil Contamination - page 50

subjects range from 2.4% in Denmark (3247 subjects ≤ 40 years old; Stensvold et al., 2009) to
92.8% in La Réunion (387 subjects> 15 years old; Magnaval et al., 1994).
Although some risk factors for toxocariasis have been identified in human populations,
results remain largely inconsistent. Male gender, for example, was suggested to be
associated with both higher (Alonso et al., 2000; Kanafani et al., 2006; Roldán et al., 2009;
Won et al., 2008) and lower (Abo-Shehada et al., 1992; Magnaval & Baixench, 1993) risk of
infection, whereas several large studies showed no association between gender and risk
(Chieffi et al., 1990; Rubinsky-Elefant et al., 2008. Young age (Fan et al., 2004b; Rubinsky-
Elefant et al., 2008), low socioeconomic status (Campos Junior et al., 2003; Lynch et al.,
1988a; Santarém et al., 2011; Won et al., 2008), low parental education (González-Quintella et
al., 2006; Won et al., 2008), poor sanitation (Alderete et al., 2003; Magnaval et al., 1994) and
playing in sandpits (Paludo et al., 2007) are additional factors contributing to Toxocara
exposure.
Having a dog has been recognized as a risk factor in most (Chiodo et al., 2006; Fan et al.,
2004b; González-Quintella et al., 2006; Jarosz et al., 2010; Won et al., 2008), but not all studies
of human toxocariasis (Ajayi et al., 2000; Rubinsky-Elefant et al., 2008). Discrepancies are not
altogether surprising, especially in tropical settings where dogs roam freely and spread eggs
across large areas. As a result, infection may be acquired, especially in sandpits of childrens
playgrounds, regardless of the presence of pet dogs in the households. The contribution of
cat ownership to Toxocara seropositivity has been less studied. Having cats as pets has been
described, in two recent serosurveys in Brazil, as representing both a risk (Paludo et al.,
2007) and a protective factor (Rubinsky-Elefant et al., 2008). A third survey in Poland (Jarosz
et al., 2010), but not a large nationwide study in the United States (Won et al., 2008) found
cat ownership to be a significant predictor of Toxocara seropositivity.
The antigen used in ELISA (Enzyme-linked immunosorbent assay) contains both species-
specific epitopes and epitopes that are shared between T. canis and T. cati (Kennedy et al.,
1987). If species-specific epitopes predominate, serology would preferentially diagnose
exposure to T. canis . Nevertheless, if cross-reactive epitopes predominate and exposure to T.
cati is frequent, ELISA would be unable to distinguish between exposure to T. canis and T.
cati with both dog and cat ownership emerging as a risk factor for seropositivity.
Positive associations have been described between Toxocara seropositivity and current
infection with other nematodes, such as whipworm (Cancrini et al., 1998) and hookworm
(Rubinsky-Elefant et al., 2008). These results may reflect some degree of cross-reactivity of
antibodies to TES with proteins excreted by other tissue- or lumen-dwelling nematodes.
Although test sera in most laboratories are pre-incubated with an Ascaris suum extract to
prevent cross-reactivity with this common human nematode (Elefant et al., 2006), other
highly prevalent helminths may still elicit cross-reactive antibodies (Lynch et al., 1988b).
Alternatively, Toxocara and other soil-transmitted helminths may co-infect the same host
due to the similar ways of acquiring these infections.
Prevalence and geographic distribution of infections with zoonotic hookworms in humans
and their definitive hosts remain relatively unknown (Bowman et al., 2010). As a rule,
human CLM is more prevalent in children living in regions with warm and humid climates.
U. stenocephala infects dogs and cats in the Americas, Europe, Asia and Oceania, while Anc.
ceylanicum is commonly found in South and Southeast Asia, Australia, and most parts of
South America. Anc. braziliense can be found from the southeastern coast of North America
(but not on the Pacific Coast of United States and Mexico) down to South America, in
African countries and Southeast Asia, but less frequently in Australia.