Fig 1.
Map highlighting the study area.
Illinois is located in the midwestern United States, which contains 103 counties.
Table 1.
Distribution of seropositivity of Illinois household cats across seasons, sex, age, and breed.
Fig 2.
Effects of age and season on the seroprevalence of SARS-CoV-2 antibody detection in household cats in Illinois, 2021–2023.
Predicted marginal effects calculated from a multivariable logistic regression model. Seropositivity (yes/no) of cat serum samples (n = 1,715) as the outcome variable, while age and season as predictor variables were included in the model.
Fig 3.
SARS-CoV-2 antibodies in Illinois household cats during 2021–2023.
(A) Point map illustrating the distribution of seroprevalence by county. (B) Isopleth map illustrating the distribution of seroprevalence across Illinois by using the Empirical Bayesian kriging spatial interpolation method.
Fig 4.
Incremental spatial autocorrelation analysis for the seroprevalence of SARS-CoV-2 antibodies in Illinois domestic cats.
Results of the Global Moran’s I statistic. The default incremental distance was selected as the starting distance that signifies the average distance to each county’s nearest neighboring centroid. The color of each point on the graph corresponds to the statistical significance of the z-score values. The peak signal is the distance where the spatial processes influencing clustering are most evident. The zone of indifference conceptualization parameter was used for the analysis. Statistically significant at p ≤ 0.05.
Fig 5.
Hot spot analysis of the seroprevalence of SARS-CoV-2 antibodies in Illinois household cats.
The Getis-Ord Gi* statistic was applied to identify local clusters. Hot spots (red color) signify counties with high seroprevalence surrounded by high seroprevalence counties, while cold spots (blue color) signify counties with low seroprevalence surrounded by counties with low seroprevalence. A Euclidean distance band of 80.7 km, and the zone of indifference conceptualization parameter, were used for the analysis. Statistically significant at p ≤ 0.05.
Fig 6.
Spatial and space-time clustering of seroprevalence of SARS-CoV-2 antibodies in Illinois household cats during 2021–2023.
(A) Purely spatial clusters of SARS-CoV-2 antibodies in Illinois household cats. The relative risk (RR) of counties within a spatial cluster where cats had higher than expected SARS-CoV-2 antibodies is shown. The circle represents the location of the cluster. Within the circle, the color of each dot represents the value range of the RR rate in each county. (B) Space-time clusters of SARS-CoV-2 antibodies in Illinois household cats. Retrospective analysis, scanning for clusters with high seroprevalence, using 50% population at risk and 50% period scanning window, 999 Montecarlo permutation, and the Bernoulli model. The circle represents the location of the cluster, and the period of the cluster is also represented. Within the cluster, the color of each dot represents the value range of the RR in each county. Statistically significant at p ≤ 0.05.
Table 2.
Spatial and space-time clusters of SARS-CoV-2 infection in household cats in Illinois, United States, 2021–2023.
Fig 7.
Space-time clustering of seroprevalence of SARS-CoV-2 antibodies in household cats in the Chicago area during 2021–2023.
Retrospective analysis, scanning for clusters with high seroprevalence, using 50% population at risk and 50% period scanning window, 999 Montecarlo permutation, and the Bernoulli model. The circle represents the location of the cluster, and the period of the cluster is also represented. Within the cluster, the color of each dot represents the value range of the RR in each city. Statistically significant at p ≤ 0.05.
Table 3.
Space-time clusters of SARS-CoV-2 infection in household cats around Chicago city, United States, 2021–2023.
Fig 8.
Prevalence rates of SARS-CoV-2 infections in humans and domestic cats in Illinois during 2021–2022.
The weekly seropositive rate of Illinois domestic cats is shown in the orange line, while the weekly viral nucleic acid test positive rate of reported human cases is shown in the blue line. Note, the peak of human cases during January of 2022, followed by a set of peaks of cat cases starting on February 2022 and continuing for the rest of the study period.