Fungal-like Pathogens of fish: what lies beneath?

Diseases in fish can be cause by a wide array of infectious agents, however, currently emerging are increasing issues with diseases cause by true fungal and fungal-like pathogens. Disease caused by such pathogens is increasing in incidence, geographic range and virulence in recent years and so poses a global biodiversity threat. At this time though the underlying causes of this observed increase are not known and so it is vital to further understanding of the ecological factors driving such change.

A review paper by Gozlan et al therefore attempts to collate information on the current understanding of such fungal pathogens, in order to highlight not only what may be causing infections, but also what direction research should take in order to minimise the ecosystem impacts. This is with the hopes that it may aid in the control of the increasingly infectious outbreaks of disease that have, in places, caused local extinctions.

The review states that the majority of infections in fish caused by fungi are caused by species in the phylum Ascmycota and that, most importantly, the majority of these fungi are opportunistic pathogens. In addition to this, within a clade of fungal-like organisms, the Mesomycetozoea, there are some notable pathogens within the clade Dermocystida, such as Sphaerothecum destruens, which can infect a wide range of hosts. Finally there are a number of oomycete parasites of fish which morphologically resemble fungi but are taxonomically distinct.

One of the most important things to take from this study is the generalist nature of such pathogens. It states that all three grouping of  fungi and fungal-like organisms contain pathogens that are true generalist. This ability to infect and cause disease in fishes across often drives high virulence in vulnerable hosts, and is what makes these diseases such a threat to global fish populations. The study of these diseases therefore becomes imperative, especially given that the current method for getting rid of the diseases in aquaculture is through the use of culling. As such, the review proposes the use of up to date molecular methods like Fluorescence in situ hybridisation (FISH) and PCR to examine the microbial species causing disease.

The power of this study lies in its collaborative nature. By bringing together a wide range of current knowledge on the subject of fungal and fungal-like pathogens it is able to highlight just how little epidemiological data there is for fish pathogens. And, since the fisheries sector employs 33.1 million people, these diseases can be considered a significant threat to a vital ecosystem service. When coupling this with their evidently generalist nature, it is not unfair to suggest that such diseases are emerging as a major concern for global fish populations, thus meaning their study is of great importance.

Gozlan, R. E, Marshall, W. L, Lilje, O, Jessop, C. N, Gleason, F. H. and Andreou, D. (2014). Current ecological understanding of fungal-like pathogens of fish: what lies beneath?. Frontiers in Microbiology. 5, Article 62.