The pathogenic helminth parasites of eels

Abstract

Although 63 and 55 species of helminths have been reported from each species of Atlantic eel and from 29 to 19 for each species of Pacific eel only the monogeneans Pseudodactylogyrus bini and P. anguillae and the nematode Anguillicola crassus, originally specific to species of Pacific eels, can be considered serious pathogens. None of the three are normally pathogenic to their preferred natural eel host species in the wild. Pseudodactylogyrus spp. only cause serious local gill damage when present on a host in large numbers under optimal conditions that facilitate transmission. This is the case in eel aquaculture, where infections can be controlled by drugs. Anguillicola crassus is only pathogenic to Anguilla anguilla and A. rostrata when Atlantic eels are introduced to the far east or when the parasites have been introduced to Europe. Here the parasite life cycle differs in that A. crassus can infect a wide range of intermediate hosts, employ paratenic hosts and survive as larvae for months in the swimbladder wall. This makes it an excellent colonizer. Its major pathogenic effects on eels result from haemorrhaging in, and thickening of, the swimbladder wall. It reduces the oxygen concentration in the swimbladder, reducing its ability to function as a hydrostatic organ, and increases the stress response of eels. In shallow lakes at warm temperatures this can result in mass mortalities. It is also feared that the parasite affects the ability of eels to migrate to the Sargasso Sea and so contributes to the decline in eel populations. Control by drug treatment is possible in culture, but not in the wild.

Macroparasite communities in European eels,Anguilla anguilla, from French Mediterranean lagoons, with special reference to the invasive speciesAnguillicola crassusandPseudodactylogyrusspp.

“…Second, we intended to characterise the epidemiological trends of the invasive helminth species, the swimbladder nematode Anguillicola crassus and the monogenean Pseudodactylogyrus spp. (Kennedy, 2007), in regard to spatio-temporal dynamics, host biological characteristics and parasite community. …”

Salinity dependence of parasite infestation in the European eel Anguilla anguilla in northern Germany

“…Pseudodactylogyrus spp. appear to cause less pronounced effects in wild eels, but they can cause economically important losses in eel farms (Kennedy, 2007).…”

“…These eel parasites were introduced into western European water bodies as a result of uncontrolled intercontinental transfer of live eels for consumption (Køie, 1991), and they are considered serious pathogens (Kennedy, 2007). Infection with A. crassus reduces the functionality of the swimbladder as a hydrostatic organ and is feared to influence the ability of eels to migrate to their spawning grounds in the Sargasso Sea (Kennedy, 2007). Pseudodactylogyrus spp.…”

Bacterial and parasitic factors diagnosed in eel farmed in Poland in the years 2010-2014

“…U zdrowych ryb dzikich zidentyfikowano dotąd około 55 gatunków pasożytów, m.in. : 4 gatunki przywr zewnętrznych i 16 wewnętrznych, 6 gatunków tasiemców oraz 21 gatunków nicieni (9). W wodach naturalnych liczba pasożytów izolowanych od węgorza zależy od miejsca i okresu pobierania prób.…”