Low risk for locally acquired Chagas disease in California: A review of human cases and triatomine submissions, 2013-2023

Abstract

Chagas disease is caused by infection with the protozoan parasite Trypanosoma cruzi, which is carried in the guts of triatomine insects. Transmission typically occurs when infective trypomastigotes in triatomine feces encounter mucous membranes or bite wounds, though it is also possible by food-borne, transplant- and transfusion-mediated, and congenital routes. Most transmission occurs in rural and peri-urban parts of continental Latin America where triatomines often inhabit human dwellings. Triatomines infected with T. cruzi are also present across the southern United States, yet relatively few locally acquired infections have been documented. Rather, most reported cases have plausible exposure in Latin America. In California, the widespread distribution of T. cruzi-infected triatomines suggests a potential risk of local transmission. Here, we summarize triatomine submissions and human case reports made to the California Department of Public Health between 2013 and 2023. Of 226 triatomines tested, 63 (28%) were positive for T. cruzi via PCR; none were linked to any of the 40 human T. cruzi cases reported in the same period. Human cases were assessed for likelihood of local transmission. Country of birth, travel history, and location of primary residence suggested non-local transmission for 31 (78%) cases. Local transmission could not be ruled out for the remaining nine (22%) cases. Information on country of birth and travel history were missing from these case reports and prevented full assessment of local transmission criteria, though most of these patients resided within 400 meters of potential triatomine habitat. Despite the presence of triatomines, T. cruzi, and human cases in California, statewide data indicates the risk for locally acquired Chagas disease is low.

Fig 1. Geographic distribution of triatomine specimens and human case reports submitted to CDPH-VBDS, 2013 - 2023.

(Left) Triatomine submissions include 226 specimens with T. cruzi infection status indicated in red for 190 adult T. protracta. (Right) Human case reports include 52 reports of T. cruzi infection and/or Chagas disease investigated in CalREDIE, of which 40 were classified as cases 2013 – 2023. Basemap sources: Esri, TomTom, Garmin, FAO, NOAA, USGS, OpenStreetMap contributors, CNES/Airbus DS, InterMap, NASA/META, NAS/NGS and the GIS User Community [67].

Fig 2. Seasonality of triatomine submissions and human case reports, 2013 – 2023.

(A) Seasonality and T. cruzi detection in adult T. protracta submissions (n = 190; 62 positive, 128 negative) peaked in the summer months. (B) Temporal distribution of human case reports of T. cruzi infection and/or Chagas disease investigated in CalREDIE (n = 52; 19 confirmed/probable, 21 suspect and 12 negative/not a case) fluctuated by month without a clear seasonal pattern.

Fig 3. Local transmission assessment of Chagas cases reported in California, 2013 – 2023. Five criteria adapted from Lynn et al. [39] were applied to the 40 human case reports that were classified as cases according to the surveillance case definition. Cases classified as confirmed, probable, and suspect were included in the local transmission assessment.