One Health dynamics: toxoplasmosis in free-roaming (semi-domiciled and stray) dogs and reported human cases over time

Characterized as a wide endemic and distributed zoonosis worldwide, toxoplasmosis has been mostly associated with consumption of contaminated raw or undercooked meat and untreated water. Although cats have played an important role as definitive hosts, free-roaming dogs may be used as sentinels for occurrence and spreading of human toxoplasmosis over time. Accordingly, the present study aimed to assess the seropositivity of free-roaming (stray and semi-domesticated) dogs in the city of Foz do Iguaçu, Paraná, an important tri-border city among Brazil, Argentina and Paraguay, comprising an urban conglomerate of around 800,000 inhabitants. A total of 1125 stray dogs were captured, 1006/1125 (89.4%) blood samples collected in three sequential samplings (6 months apart and lasting 10 days each) between 2018 and 2019, and serum tested by indirect fluorescent antibody test (IFAT). The overall seropositive rate was 387/1,006 (38.5%), with 156/408 (38.2%) stray and 231/598 (38.6%) semi-domesticated dogs, with seropositivity in 37/98 (37.8%), 79/184 (42.9%) and 40/126 (31.7%) stray and 66/198 (33.3%), 77/167 (46.1%) and 88/233 (37.8%) semi-domesticated dogs in each phase, respectively. Human notified cases of congenital toxoplasmosis in the Foz do Iguaçu city were 15, 9, 26, 11, 4, 13, 5, 1, 3 and 7 (total 95) during the 2015-2024 period, as provided by the city secretary of health at Foz do Iguaçu. In addition, notified (gestational, congenital and food/waterborne outbreak) toxoplasmosis cases of Parana state and Brazil were available by the Brazilian Ministry of Health starting in 2019, with 165/2858, 175/3058, 254/3876, 267/4583, 309/6593, 478/7172 during the 2019-2024 period, respectively, with a total of 1648/28140 overall cases in the period. In conclusion, although toxoplasmosis in dogs may not be a significant disease, stray and semi-domesticated dogs, due to their long-distance travel and contact with sources contaminated with T. gondii from cat feces and, potentially, infected rodents and ticks, can play an important role as disease sentinels to human infection due to the possible mechanical protozoan dissemination. However, further random spatial serological surveys may be needed in healthy dog populations and humans in households throughout the city to ensure accurate data prevalence over time.