Some zoonotic infectious diseases are prevalent among children under 15 years of age in Ardabil Province, northern Iran.

Background: This study sought to investigate the seroprevalence of visceral leishmaniasis (VL), toxocariasis, brucellosis, and salmonellosis, as well as their co-infection and potential cross-reaction, in children under 15 years referred to health centers in Ardabil province, Iran, from 2019 to 2021.

Methods: The current study examined 1,550 serum samples using direct agglutination test (DAT), Toxocara canis ELISA, Wright, and Widal tests to detect antibodies against Leishmania, Toxocara, Brucella, and Salmonella, respectively. We also compared the test results to determine the possibility of cross-reactivity or simultaneous seropositivity in the tested samples.

Results: In general, anti-Leishmania antibodies were positive in 78 samples (5%) at titers of ≥ 1:800, while only 8 cases had titers of ≥ 1:3200, which was considered as positive result. Therefore, the seroprevalence of VL was estimated to be at 5.16 per 1,000 at-risk populations. Meshkin-Shahr city had the highest seroprevalence (7 cases, 87.5%), followed by Ardabil (1 case, 12.5%) (p = 0.03). The highest and lowest seropositivity rates were observed in children aged 1-5 (6 samples, 75%) and 5-15 (2 samples, 25%) years old, respectively (p = 0.02). Anti-Toxocara antibodies were positive in 249 samples, (16.1%, 95% CI: 13.2-18.8), which were primarily males. There was a significant difference in seropositivity to Toxocara infection by city (p = 0.04), and age (p = 0.00). The results of Wright test showed seropositivity of 7.5% (117 samples) with the highest rate in individuals aged ≥ 10 years, males, and urban areas. No significant differences existed between the seropositivity rate and age, sex, residency, or symptoms (p > 0.05). Widal test was positive in 6% (94 samples) of children, with most cases being females (p < 0.05), particularly in those aged ≥ 10 years. Of the 78 DAT-positive sera, only 2 samples with a low titer (1:800) tested positive for anti-Toxocara antibodies, while none of the high titer samples were positive. In addition, samples with a DAT titer of 1:800 were positive for anti-Brucella (1:40: 10.2%, 1:80: 2.5%) and Salmonella (1:40: 3.8%) antibodies. The titers were (1:40: 5.1%, 1:80: 1.3%) for Brucella and (1:40: 2.5%) for Salmonella at a 1:1600 DAT titer. Wright's test on Toxocara-positive sera showed that 1.2% and 0.4% of samples had titers of 1:40 and 1:80, respectively. Furthermore, 2%, 2.8%, and 0.8% of Toxocara-positive samples exhibited anti-Salmonella antibodies at titers of 1:40, 1:40, and 1:80 corresponding to OA and OD antigens, respectively. The Wright (OR:1.099; 95% CI:1.080-1.118) and Widal (OR: 1.078; 95% CI: 1.062-1.094) tests showed cross-reactivity at low titers and minimal co-infection at high titers. Of Widal-positive sera, 11.4% with a titer of 1:40, and 2.7% with a titer of 1:80 were positive for anti-Brucella antibodies (OR:1.078; 95% CI:1.056-1.085).

Conclusion: Given the prevalence of bacterial and parasitic febrile infections among children in the region, and their symptomatic similarity to VL, it is crucial to recognize clinical manifestations, accurately diagnose co-infections, and account for cross-reactivity in serological tests.