September 2005 - Stealth parasite threatens European fish

A new deadly disease, carried by an invasive fish species, is threatening European fish diversity according to a paper published in the journal Nature.

Scientists working for the Centre for Ecology & Hydrology (CEH) and the Centre for the Environment, Fisheries & Aquaculture Science (CEFAS) have found an infectious disease carried by the topmouth gudgeon, Pseudorasbora parva. Although not recorded in the UK, conditions are favourable for its presence and success.

The lead author, Rodolphe Gozlan from the CEH said: “We have found a parasite that may pose a severe threat to some freshwater fish species in Europe. This discovery has major biological and economic implications.”

The parasite they have discovered is a pathogen that infects a variety of freshwater fish. It has been found inside the cells of internal organs within the body of the fish. It is a very small unicellular organism, which is probably a protozoan. The exact nature of the taxonomy of this, and the closely related rosette agent is under some discussion. The rosette agent is an obligate intracellular parasite that causes morbidity and mortality in salmonid fish. It is a strange creature belonging to a strange group and there is some discussion about its actual taxonomic position, whether it is in fact an animal or a fungus.

The report showed that the parasite stops the European sunbleak, Leucaspius delineatus (an endangered species in mainland Europe) from spawning - leading to its rapid decline and possible eventual extinction. The scientists blame the disease for the rapid demise of the sunbleak in parts of Europe following the spread of the Asian topmouth gudgeon. “The new disease is already affecting other freshwater fish such as the fathead minnow and may affect native UK fish species,” added Dr Gozlan.

The researchers found that the parasite does not harm the topmouth gudgeon. Dr Gozlan said: “The topmouth gudgeon is a healthy host for this deadly parasite. It appears that the parasite will infect topmouth gudgeon easily but causes limited or no pathology. The topmouth gudgeon is then a carrier fish spreading the parasite to other species. This parasite could threaten commercial fisheries, including salmon farms.”

The scientists say more work is required to determine the extent of the threat to European fish diversity, but have observed that sunbleak populations have declined dramatically in the last forty years and the species is now on the European list of threatened freshwater fishes. This decline coincides with the rise in numbers of the topmouth gudgeon and its rapid spread throughout Europe since its introduction into Romanian ponds close to the River Danube in the 1960s. First brought to the UK in 1985 for aquaculture, the species had reached only two sites by 2001. But researchers say in the past four years it has invaded 24 sites in the UK, and is bound to spread further.

This work had been carried out in response to an observed stock decline in other European species, straight after introduction of topmouth gudgeon. These species include rudd (Scardinius erythrophthalmus), crucian carp (Carassius carassius), bitterling (Rhodeus sericeus) and gudgeon (Gobio gobio). Although there is a close correlation between these declines and the introduction of the topmouth gudgeon, more work needs to be done to see if an actual link exists with the parasite.

Of further concern is the fact that the researchers believe the parasite is closely related to the rosette agent, identified in the United States for the first time in 1986, which kills salmon and similar species, both farmed and wild. Work is in progress to determine any differences between the present parasite and the rosette agent. At present the work appears to suggest that the parasites are very similar, if not the same agent.

Work in the USA has found high mortality due to the “rosette agent”, Sphaerothecum destruens, in adults of chinook, coho and Atlantic salmon, as well as rainbow, brown and brook trout. External symptoms are not evident even in advanced infections and, even in post-mortem dissections, internal signs of the disease are not always evident. The agent replicates slowly, so the disease does not become apparent immediately. This intracellular parasite ultimately results in host cell death, and once established in the salmonid host the infection causes widespread destruction of various tissues. In wild salmon in the USA, infection levels can reach 100%. It should be said that the European parasite has not been recorded infecting any salmonid species.

The topmouth gudgeon is already considered to be one of the most invasive fish species in Europe. It is currently confirmed present in at least 6 waters in England, although in reality it is likely to be present in many more. Scientists at CEH have so far recorded 24 populations of topmouth gudgeon across England and Wales in rivers, enclosed lakes and sites connected to river networks (river catchments at risk from invasion are the Yorkshire Ouse, Trent, Thames, Medway, Itchen, Test and Severn). It is a small, inconspicuous cyprinid fish and at first glance it is easily mistaken for other small cyprinids, so may be more common than is currently recorded. The presence of topmouth gudgeon in Europe has been related to damage to populations of sunbleak. Ironically, sunbleak or ‘motherless minnows’ have also been regarded as invasive alien species in the UK, after having become established in some waters.
 
The ‘rosette like organism’ is an inconspicuous parasite of an inconspicuous fish species. Most importantly it does not appear to cause direct mortality. The question is, is the parasite present in the UK but unrecorded? Might this parasite explain some of the declines in fish populations that we are seeing? In conclusion this parasite represents a clear threat to European native freshwater fish species.

On another related note a parasitic tapeworm has been discovered in the last 12 months that infects otters. The parasite, known as the bile fluke, is also thought to infect sunbleak or topmouth gudgeon as the intermediate host. This is a previously unrecorded parasite in the UK and is thought to have spread from eastern Europe. Pathology causes liver damage and jaundice in the infected otters although no pathology has been noted in the infected animals in the UK, all of which were killed by road traffic.