Sphaerothecum destruens or the rosette agent is a unicellular eucaryotic parasite of fish which has caused disease and moralities up to 80% in north American Chinook salmon (Oncorhynchus tshawytscha) and chronic mortalities in cultured salmon (Salmo salar). It was first discovered in the United States in association with invasive species including topmouth gudgeon (Pseudorasbora parva) but was also found to be the causative agent of a disease in the UK affecting other salmonid species such as brown trout. It is thought to pose more of a risk in Europe than in the USA as native species there are more susceptible to the parasite. The disease causes high rates of morbidity and mortality in a number of different salmonid species and can also infect other freshwater fish such as bream, carp and roach.
Researchers at Bournemouth University in the UK are very concerned and this was triggered by a study they conducted and published already last year. They were able to link the rapid population decline of sunbleak (Leucaspius delineatus), a cyprinid fish native to Europe to the spread of the invasive topmouth gudgeon (Pseudorasbora parva). In addition they were able to isolate and culture Sphaerothaecum destruens from sunbleak.
The UK Environment Agency is already attempting to eradicate topmouth gudgeon from UK waterways. They have already removed hundreds of thousands of the fish from English rivers. Despite this, very little is known about how many UK lakes, rivers and fisheries the topmouth gudgeon let alone the parasite are present in. Another question is how wide-spread both invasive species and parasite are in continental Europe.
There are more than four million anglers in the UK alone and the sport generates an estimated £3.5 billion for the country's economy. The use of environmental DNA (eDNA) methods comes to mind. The colleagues from Bournemouth University are certainly looking into the option of detecting the parasite directly in the water instead of going through the lengths of extracting it from an infected fish and culturing it. A smartly designed eDNA probe could help with screening many river systems in the UK and Europe in a far more timely fashion than any traditional approach.