A Fungus That Gives Fish Anxiety

Pseudoloma neurophilia is a microsporidium – a group of fungus-like single celled parasites that can elicit many types of physiological- and behavior-altering effects on their hosts. This one in particular has a taste for zebrafish brains. This can be a huge problem because zebrafish (scientifically Danio rerio) are a model organism used in many labs all over the globe to study behavior and even human mental disorders. P. neurophilia is one of the most common zebrafish parasites, being found in up to 75% of diagnostic screenings [1]. Outwardly, P. neurophilia leads to an infection with symptoms such as emaciation and inflammation of the brain, spinal cord, and muscles. It primarily infects the muscles and neurons, usually forming “parasitic clusters” in the axons of cells in the brain and spinal cord. The axon’s job is to conduct information between the sensory organs, brain, and muscles in the form of electrical impulses, so damage to these cell regions would have a pretty major effect on the body. This parasite doesn’t stop there, though. Fish with severe infections get multiple parasitic clusters in each cell, which can lead to necrosis in the cell body – the main command center of neurons [2].

P. neurophilia parasitic cluster found in the white mater of the brain (arrow), putting pressure on the nearby grey matter (star). (Spagnoli et al, 2015b)

Damage to these structures can have some pretty obvious consequences: blockages in the motor neurons would cause changes in the fish’s ability to move and react to change in the world around them. In the brain, clusters of P. neurophilia are found in areas associated with anxiety and aversion learning (the medial habenula of the telencephalon and the griseum centrale, if you’re a fan of big words) [3]. All of this inflammation, necrosis, and general grossness in the brain can’t be good for you. These infections are usually considered subclinical – it can be really hard to detect when a population has gotten the Pseudoloma Plague – but though their lives may be long, they’re not necessarily happy. Infected fish show signs of stress and anxiety at a higher rate than their uninfected peers. One study tested the startle response of infected animals compared to uninfected ones by tapping them over and over again to see how fast they jumped away. Over time, uninfected fish habituated – they relaxed and stopped jumping at every little bump – but those with P. neurophilia infections continued to react much more than the control [3]. Another study shows that infected fish shoal closer together – a behavior present in healthy fish who are under a lot of stress from outside factors such as predation risk [1]. All of this suggests that P. neurophilia causes increased anxiety and higher stress levels in their hosts. Physical manipulation of the nerve cells and brain areas seems to be a likely suspect for the way this is caused, the swelling in key areas of the brain could lead to some changes in stress hormone levels, or in the function of the central nervous system as a whole. It’s not always so simple, though. Further studies are needed to know whether neurophilia – like many other well-studied behavior altering parasites – changes its hosts’ brain chemistry by releasing its own rally of hormones. Either way, this disease is probably a major headache for the poor zebrafish and their researchers alike!

[1] Spagnoli, S., Sanders, J., & Kent, M. L. (2016). The common neural parasite Pseudoloma neurophilia causes altered shoaling behaviour in adult laboratory zebrafish (Danio rerio) and its implications for neurobehavioural research. Journal of fish diseases, 40(3), 443-446. https://doi.org/10.1111/jfd.12512
[2] Spagnoli, S. T., Xue, L., Murray, K. N., Chow, F., & Kent, M. L. (2015a). Pseudoloma neurophilia: a retrospective and descriptive study of nervous system and muscle infections, with new implications for pathogenesis and behavioral phenotypes. Zebrafish, 12(2), 189-201. https://doi.org/10.1089/zeb.2014.1055
[3] Spagnoli S, Xue L, Kent ML. (2015b). The common neural parasite Pseudoloma neurophilia is associated with altered startle response habituation in adult zebrafish (Danio rerio): Implications for the zebrafish as a model organism. Behav Brain Res, 291, 351–360. doi:10.1016/j.bbr.2015.05.046

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