Nicoletti

Human toxocariasis is due to the larval stages of the ascarids Toxocara canis and Toxocara cati, common roundworms of dogs and cats respectively. It is one of the most prevalent helminthiases worldwide, especially in settings where the man-soil-dog relationship is particularly close.[1] T. canis parasites the small intestine of the dog, its main host that can become infected via the placenta or by contact with contaminated feces. The female T. canis produces up to 200 000 eggs per day, releasing them to the environment through feces.[2] Humans can be infected by direct contact with dogs, by the ingestion of contaminated food or soil or by eating infected meat of paratenic hosts.[1] While the large majority of infections are thought to be asymptomatic, visceral larva migrans (VLM) and ocular larva migrans (OLM) are the most common clinical manifestations,[3] even if two less severe syndromes have been described: the covert toxocariasis, which is more common in children, and common toxocariasis that was reported in adults.[4] However, T. canis has also been identified in the Central Nervous System, leading to a wide variety of neurological manifestations collectively termed neurotoxocariasis.[5] In western countries toxocariasis is part of the so-called “neglected infections of poverty” because of its distribution in low income areas of the United States of America (USA)[6] and, for Europe, among eastern countries and the southern regions of European countries, both areas with lower socioeconomic levels, compared with the rest of Europe.[7] Toxocariasis has a seroprevalence of up to 90% in tropical settings and, for western countries, ranging from 35% to 42% in rural areas and between 2% and 5% in urban areas.[8] In Italy, the only two studies have been carried out reporting a seroprevalence of 4.0% in a northern Italian region[9] and a seroprevalence of 1.6% in the Marche region.[10]
The aim of the present study was to describe the seroprevalence of T. canis and the association between demographic and environmental factors in a sample of the adult population in the city of Catania.

The study has been performed in Catania, Italy, a city of the Sicily, which is located at a mean altitude of about 30 m above sea level and has an area of 181 km². Its official population is 293,104 inhabitants.[11] Participants over 14 years old were selected using a multi-stage sampling method. The study is part of a larger case-control study aimed to evaluate the role of both environmental and genetic factors and the risk of Multiple Sclerosis (MS) in the population of Catania. Background and methods have been extensively reported elsewhere.[12]
After enrolment in the study, a blood sample was collected. Samples have been coded and processed to obtain serum aliquots and then stored at -20°C in the laboratories of the “Azienda Ospedaliera Policlinico Vittorio Emanuele.” Serum samples have been shipped in dry ice to the laboratories of the Infectious Diseases Institute (Malattie Infettive e Tropicali, AOU Careggi) of the University of Florence and have been analysed by a biologist blinded to the status of the participants. Specific T. canis IgG has been detected with a commercial ELISA kit (Ridascreen Toxocara IgG; R-Bio farm, Milan, Italy) using excretory/secretory antigens (Toxocara excretory-secretory antigen [TES-Ag]) from second-stage T. canis larvae.[13] A face-to-face semi-structured standardized questionnaire about demographic and environmental factors has been administered to all the participants.
All the analyses have been conducted with the software STATA 12.0. For the prevalence of anti-T. canis antibodies the 95% CI have been calculated. Quantitative variables were described using mean and standard deviation. The difference between means and the difference between proportions were evaluated by the t-test and the chi-square test, respectively.
Unconditional logistic regression analysis was performed, and for each study variable, we calculated OR, 95% CI, and p-value (two-tailed test, α=0.05). Multivariate analysis was conducted to investigate the independent effect of risk or protective factor after adjustment for one or several other factors or to adjust for confounding variables. Parameters associated with the outcome at the univariate analysis with a threshold of P = 0.25 were included in the model. The model was manually constructed using the likelihood ratio test (LRT) to compare the log-likelihood of the model with and without a specific variable. Sex, age, and education have been considered a priori confounder variables. Whenever variables were dichotomized, the cutoffs were derived from the median value of the pooled distribution. Each participant was asked to sign an informed consent. The study has been approved by the Local Ethical Committee (code 64/2018/PO).

Results

At the end of the recruitment process, 300 subjects have been enrolled. After the research of anti-T. canis antibodies in serum, results of 13 participants were deemed unreliable due to incorrect storage of the samples, and thus these subjects have been dropped from the final analysis, with a final sample of 287 participants.
Subjects had a mean age of 48.1±15.6 years, and 193 (67.3%) were women. The majority of them were professionals (n=103, 36.4%) or housewives (n=79, 28.0%). The demographic characteristics of the population are reported in Table 1.

Table 1. Demographic characteristics of the population.

Presence of anti-T. canis antibodies were found in 23 subjects (18 women, 78.2%; mean age 51.1±14.0 years) resulting in seroprevalence of 8.0% (95%CI 5.4-11.7). At the univariate analysis, no association has been found with sex, age, profession or owning pets; a number of siblings were significantly associated with T. canis seropositivity with an OR of 3.46 (95%CI 1.41-8.47) for subjects with more than 3 siblings. A close association was found at multivariate analysis adjusting by age, sex and education (adj OR 3.17; 95%CI 1.09-9.25) as shown in Table 2.

Table 2. Univariate and multivariate analysis of variables associated with the presence of anti-Toxocara canis antibodies.

Toxocariasis is one of the most prevalent helminthiases worldwide and even if the parasite tends to be more prevalent in tropical settings, in urban areas of Western countries seroprevalence ranges from 2 to 5%.[8]
This is the first study carried out to assess the seroprevalence of toxocariasis in a sample of the adult population (over 14 years) in the city of Catania, Sicily. We found a seroprevalence of 8.0%, a value slightly higher with respect to data reported in other urban areas of European countries,[8] as well as to those previously reported in two earlier studies carried out Italy, 1.6 in Central Italy in 2003[10] and 4.0 in Northern Italy in 1990.[9]
The higher seroprevalence found in the city of Catania can be explained by the environmental factors such as the degree of humidity and temperature that increase the overall survival of T. canis eggs in the soil.[14] However, another critical aspect of being considered is environmental contamination due to the presence of infected dog feces that contribute to the dissemination of T. canis eggs. While we have no data about the prevalence of T. canis infected dog feces in the urban areas of Catania, a study conducted in the neighbouring city of Messina, that shares with Catania the same environmental factors, found a prevalence of T. canis eggs of 3.6% in a random sample of dog feces.[15] Furthermore, another contribution to the increased seroprevalence is the increased presence of dogs living in the households, considering the high level of infestation they bear, up to a prevalence of 76% in puppies in a recent study conducted in Italy.[16]
Presence of T. canis IgG was assessed using a TES-ELISA, considered the standard serological test commonly used in epidemiological surveys to determine the seroprevalence of T. canis in a defined population. While we are aware of the limitations of ELISA test, such as the low specificity, we believe that the lack of a confirmatory Western Blot, considered the gold standard procedure,[14] has not influenced our results because the low specificity depends on the frequent cross-reactions with other nematode infections such as Ascaris lumbricoides, filariasis or strongyloidiasis, which are rare in western countries.[14] In our sample T. canis seropositivity was not associated with age , as expected according to recent literature.[17]
Furthermore in our sample pet ownership was not associated with the risk of T. canis seropositivity, probably because the risk of being infected by the direct contact with domestic animals has been showed to be low.[18]
On the other hand, the number of siblings (more than 3) was significantly associated with T. canis seropositivity. Even if we have not a clear explanation for this association, it is possible to hypothesize that a larger number of siblings could act as a proxy indicator of a lower socio-economic level, a risk factor for being infected with T. canis eggs.[17] The size of the study and the selection of a sample from the general population using an equal probability selection method (a multi-stage sampling) represent the main strengths of our study.[12] However, we are aware that the sex and age distribution of the sampled population may not be entirely representative of the general population. It should be underlined in fact that this study is part of a larger population-based case-control study on MS and to this reason, healthy subjects were recruited matched by age and sex with the enrolled MS cases.[12]

Seroprevalence of Toxocara Canis in the city of Catania, Italy