Date of Award

Spring 4-9-2026

Document Type

Thesis

Degree Name

Master of Science (MS)

Department

Biology

First Advisor

Dr. Dave Bachoon

Second Advisor

Dr. Al Mead

Third Advisor

Dr. Daniel Burt

Abstract

This study evaluated the presence of pathogenic Leptospira interrogans, potential carriers, and environmental factors influencing their occurrence in the San Juan Bay Estuary (SJBE) and Río Grande de Loíza (RGDL) watersheds of Puerto Rico from 2024–2025. Detection of this pathogen from water samples was performed using the lipL32 PCR assay. The likely hosts of L. interrogans were determined using fecal source tracking (FST) assays for human (HF-183), canine (BacCan-UCD), equine (HoF597), and rat mitochondrial DNA NADH dehydrogenase subunit 2 gene (RatMt) markers. In 2024, 91 samples were collected (March: 50; June: 41), and in 2025, 124 samples were collected (March: 74; July: 50). Across all years, pathogenic L. interrogans was detected in 15–20% of samples, with higher prevalence in wet months (19.5%) compared to dry months (11.7%), though differences were not statistically significant (χ² = 1.72, p = 0.189). Fisher’s Exact Tests revealed significant associations between L. interrogans and rat markers during the wet season of 2024 (p = 0.042) and in both seasons of 2025 (p < 0.05). No significant correlations were observed with dog or equine markers. These findings reinforce rats as dominant reservoirs, supported by the consistent co-detection of Leptospira and rat mitochondrial DNA markers across multiple years and seasons (e.g., 38% of rat-positive samples in 2025 showed positive for L. interrogans). Human wastewater inputs were subsequently identified as critical contributors, particularly in densely populated watersheds. This is demonstrated by the strong association between L. interrogans and the HF-183 human marker in the 2025 dry season (p = 0.014). Positive detections occurred at 72% of sites with multiple fecal sources indicated by FST. Most sites were prone to rainfall-driven runoff due to their location downstream of urbanized sub-basins.

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