Aspergillomycosis in cultured tilapias

Abstract

A study of aspergillomycosis in cultured tilapias was initiated following the observation that high mortalities associated with Aspergillus species occurred in an intensive tilapia culture system. Extensive virological and bacteriological exanimation of clinical specimens failed to reveal any evidence of any other pathogen; however, histopathological examination indicated the presence of a fungus. The organism was isolated and identified. Aspergillus flavus Link ex Fries; A. niger van Tieghen var phoenicis (Corda) Al- Musallam; A. terreus Thom; and japonicus Saito were isolated from such infected tilapia tissues as well as from tilapia pelleted rations; and A. clavatus Desmazieres; A. chevalieri Thom and Church; A. repens (Corda) Sacc; A. candidus Link ex Fries and A. sejunctus Bainier auid Sartory were isolated from tilapia feeds but not the host fish. Experimental pathogenesis, established a definite cause-effect relationship of the various Aspergillus isolates. The infective Aspergilli were consistently re-isolated from the visceral organs of experimentally infected tilapias. The experiments provided positive support for the speculation that under aquaculture conditions, Aspergillus infections occur via concinnated food, subsequent to which systemic spread occurs. Water temperature and fish confinement were important factors, among others, in the elucidation of the epizootiology of the disease in cultured tilapias.

Antibody production against A. flavus, A. niger and A. japonicus was detected by Ouchterlony gel diffusion-precipitation technique from tilapias infected with A. flavus organisms, but was shown to be absent from uninfected tilapias. The indirect fluorescent antibody technique (FAT) gave a specific fluorescence with digested tissue sections from Aspergillus infected tilapias. Aspergillus antigens in formalin fixed tissues were suitable for such tests. Appropriate control measures include prevention by good husbandry practices and surveillance.