Liver Lesions in Estuarine Dolphins in the Indian River Lagoon, Florida: Does Microcystin Play a Role?

Microcystin (MC), a hepatotoxin produced by cyanobacteria, was introduced into the Indian River Lagoon (IRL), Florida, in 2005 through freshwater outflows. Since then, MC has been detected in humans, domestic animals, and wildlife in the lagoon. Potential public health effects associated with MC exposure along the IRL include an increased risk of non-alcoholic liver disease among area residents. Yet, there are limited studies characterizing liver disease, as well as the potential role of MC, in humans and animals in this region. Thus, histopathology reports (<i>n</i> = 133) were reviewed in the stranded common bottlenose dolphin (<i>Tursiops truncatus truncatus</i>) (<i>n</i> = 156, 2005-2024) to describe liver lesions in this important IRL sentinel. Liver and fecal samples (<i>n</i> = 161) from stranded individuals were screened for MC via an enzyme immunoassay (ELISA). These samples were then confirmed via the 2-methyl-3-methoxy-4-phenylbutyric acid technique (MMPB) to evaluate whether liver histopathologic lesions were linked to MC exposure. Minimally invasive MC screening methods were also assessed using respiratory swabs and vapor. Inflammation (24%, <i>n</i> = 32), fibrosis (23%, <i>n</i> = 31), lipidosis/vacuolation (11%, <i>n</i> = 15), and necrosis (11%, <i>n</i> = 14) were the most common liver anomalies observed. These non-specific lesions have been reported to be associated with MC exposure in numerous species in the peer-reviewed literature. Ten bottlenose dolphins tested positive for the toxin via ELISA, including two individuals with hepatic lipidosis, but none were confirmed by MMPB. Thus, this study did not provide evidence for MC-induced liver disease in IRL bottlenose dolphins. Other causes should be considered for the lesions observed (e.g., heavy metals, metabolic disease, and endoparasites). Respiratory swabs require further validation as a pre-mortem MC screening tool in free-ranging wildlife.