ABSTRACT
OBJECTIVE: Cutaneous ulcerative skin lesions in a complex of invasive Gulf of Mexico lionfish (Red Lionfish Pterois volitans, Devil Firefish P. miles, and the hybrid Red Lionfish × Devil Firefish) became epizootic beginning in mid-August 2017. Herein, we provide the first pathological descriptions of these lesions and summarize our analyses to elucidate the etiology of the disease. METHODS: We examined ulcerated and normal fish through gross pathology and histopathology, bacterial sampling, and unbiased metagenomic next-generation sequencing. We tracked prevalence of the disease, and we used biological health indicators (condition factor, splenosomatic and hepatosomatic index) to evaluate impacts to health, while considering sex and age as potential risk factors. RESULT: Typical ulcerative lesions were deep, exposing skeletal muscle, and were bordered by pale or reddened areas often with some degree of scale loss. Only incidental parasites were found in our examinations. Most fish (86%; n = 50) exhibited wound healing grossly and histologically, confirmed by the presence of granulation tissues. A primary bacterial pathogen was not evident through bacterial culture or histopathology. Metagenomic next-generation sequencing did not reveal a viral pathogen (DNA or RNA) but did provide information about the microbiome of some ulcerated specimens. Compared with clinically healthy fish, ulcerated fish had a significantly lower condition factor and a higher splenosomatic index. Disease prevalence at monitored sites through July 2021 indicated that ulcerated fish were still present but at substantially lower prevalence than observed in 2017. CONCLUSION: Although some common findings in a number of specimens suggest a potential role for opportunistic bacteria, collectively our suite of diagnostics and analyses did not reveal an intralesional infectious agent, and we must consider the possibility that there was no communicable pathogen.
Subject(s)
Perciformes , Animals , Gulf of Mexico , Perciformes/physiology , FishesABSTRACT
Based on isolations from naturally infected fish in Florida, we investigated the role of the fungi Aphanomyces invadans, Achlya bisexualis, and Phialemonium dimorphosporum in the etiology of ulcerative mycosis (UM) in striped mullet Mugil cephalus. We injected healthy striped mullet subcutaneously with secondary zoospores of four oomycete isolates: two concentrations (50 and 115 zoospores/mL) of SJR (an endemic isolate of Aphanomyces invadans in American shad Alosa sapidissima from the St. Johns River); two concentrations each of CAL (25 and 65 zoospores/mL) and ACH (1,400 and 2,000 zoospores/mL; endemic isolates of Aphanomyces invadans and Achlyva bisexualis, respectively, in striped mullet from the Caloosahatchee River); and two concentrations of the ascomycete culture MTZ (2,500 and 3,500 zoospores/mL; endemic isolate of P. dimorphosporum from whirligig mullet M. gyrans in the Matanzas Inlet). All fish injected with either concentration of SJR developed granulomatous ulcers after 8 d and died within 21 d. Eighty percent (8/10) of fish injected with the high dose of CAL developed ulcers after 13 d and died within 28 d, but only 30% (3/10) of fish injected with the low dose of CAL developed ulcers. Four of the ulcerated fish died within 28 d, and the remaining fish were terminated after 32 d. Fish injected with zoospores of Aphanomyces invadans developed ulcers that were grossly and histologically similar to those observed in naturally infected striped mullet with UM from several estuaries or rivers in Florida. These hemorrhagic skin ulcers were characterized by myonecrosis and the presence of mycotic granulomas. None of the fish injected with ACH, MTZ, or sterile water developed ulcers. This study fulfilled Koch's postulates and demonstrated that ulcers could be experimentally induced in striped mullet after exposure via injection to secondary zoospores of an endemic Florida strain of Aphanomyces invadans.
