And what are the symptoms:
"Have symptoms similar to those in the fish you examined been identified before?Some of the symptoms - like protruding eyes, fluid in the abdominal cavity or lethargic behaviour - are very unspecific and are seen in a variety of infections. White flocculent material in the eyes has been described for Myxobolus albi infections, and Kudoa islandica is a differential diagnosis for white nodules in the muscle. However, externally visible “blisters” with milky content, which are also seen on internal organs, make the clinical presentation of Tetramicra quite unique. On histology, the xenomas can show characteristic features like a microvillous surface, and the spores themselves show a distinct posterior vacuole with an inclusion body when viewed on wet mounts."
And how many diseases do lumpfish get? A whole lot:
"What other diseases have been found in lumpfish to date?A wide variety of bacteria and parasites as well as some fungi and viruses have been found in lumpfish to date. Bacteria have so far been the most problematic in terms of mortality and Pseudomonas anguilliseptica, Vibrio anguillarum, Vibrio ordalii, Tenacibaculum spp., Pasteurella sp., Piscirickettsia salmonis, atypical Aeromonas salmonicida subspecies and Moritella viscosa are examples of species that have caused mortality in lumpfish. Amoebic gill disease (Neoparamoeba perurans infection) has caused high mortality in lumpfish both at sea and in land-based facilities. N. perurans is also the only pathogen that has been proven to be transmissible between lumpfish and salmon to date.
Caligus sp., scuticocilliates, Gyrodactylus sp., Trichodina sp., Kudoa islandica and Myxobolus albi are other examples of parasites that have been problematic in lumpfish. Known fungal pathogens of lumpfish include Nucleospora cyclopteri (microsporidia) and Exophiala spp.. Two viruses have been described in lumpfish to date, a ranavirus of yet unknown significance and a flavivirus to which significant mortality has been attributed in Norway. New diseases are being described regularly in lumpfish, though diagnosis of outbreaks in sea pens is often difficult and it must be assumed that many disease outbreaks are going undiagnosed."
There are lots being used, millions in fact:
"How widely used are lumpfish in the salmon industry?The use of cleaner fish for sea lice control has grown exponentially in the last years, and is expected to continue doing so, with an estimated 26 million stocked in Norway alone in 2015 and an estimated 50 million, predominantly hatchery reared lumpfish, being needed in 2020 according to Powell. The development of non-medicinal lice control strategies, which include the commercial production of cleanerfish, has been singled out as the area in most urgent need of research and is a major priority for the industry and governments.
As in other European salmon farming countries, sea lice are one of the most significant challenges facing the salmon industry in Ireland and cleaner fish are widely used."
"What are the main areas of research that the FVG are likely to be concentrating on in the wake of the discovery?New diseases are regularly appearing in lumpfish and the research focus of FVG has shifted away from T. brevifilum to other pathogens previously unknown in cleanerfish, such as Piscirickettsia salmonis in lumpfish, piscine myocarditis virus in wrasse and the newly identified ranavirus in lumpfish. However, as cases of Tetramicra brevifilum have been diagnosed during routine visits and will likely reappear in the future, this species will likely return into the focus of research. Future research needs include optimizing a screening protocol, investigating possible treatments and modes of transmission, and comparing isolates from different origins."
And the kicker is they shouldn't be used:
"Does the discovery strengthen the argument to close the lumpfish rearing cycle? [Answer:]Yes,"