but the number of lice is usually low due to intensive surveillance and prophylactic treatments. Maximum agreed counts
of parasite numbers trigger a treatment, to help diminish
infestations and this is supplemented with cleaner fish to
remove lice (wrasse species and lumpsucker). However, atypical strains of Aeromonas salmonicida have been isolated
from wrasse, and recent testing has established that wrasse
species can harbour viral haemorrhagic septicaemia virus.
Sea lice undergo ten developmental stages on the fish
with a moult between each stage. The infective, free-living
stage is called the copepodid and newly attached chalimus
larvae are typically found on the ventral surface and the fins,
while older stages can occur anywhere on the body. Adult
individuals typically locate in scale-less areas such as the
neck and head, around the anal fin and on the ventral side of
the caudal peduncle (Figs. 9.48 and 9.49). If untreated, both
chalimus and the adult lice may occur in substantial numbers
and cause severe skin lesions. Early gross signs are seen as
light grey patches on the head, neck and the perianal area.
Much of the damage caused by copepods is associated with
attachment to the host and feeding behaviour. Lice graze on
host tissues ranging from mucus, epidermal, dermal or subcutaneous tissues, causing skin ulceration, petechiae and
resultant hyperpigmentation. Microscopically, lesions may
be variable depending on fish species but include mechanical
Table 9.2 ‘Laxe-Luus’
Peder Clausson Friis (1545–1614) and Erik Pontoppidan (1698–1764) were a great scholar and bishop respectively
in Norway, and both showed a great interest in biology and natural history. They are probably the first to describe
sea-lice in migrating salmon. Friis (ca.1600) accurately describes the parasite without naming it: ’søge strax op i
Elffuen oc Fosser at de kunde afftoe i Fosser og paa Steen affscraabe store Lus aff sig, som sider i hans Nache’ (‘it
moves quickly up in the river and waterfalls to wash off in the falls, and to scrape off on stones, large lice attached
to its neck’). Furthermore: ‘De Lus som eer paa Laxen er som stuore Edderkopper, dog lenger, haffuer en lang Neb,
sidder haart paa hannom, bider igjennom den sterche oc sej Hud indtill Blod gaar ut’ (‘the lice on the salmon are
the size of great spiders, but longer, with a long beak, firmly attached to it (i.e. the salmon), it bites through the
strong and tough skin until blood emerges’.
Later, Pontoppidan in 1753 states that ‘da den i store Flokke kommer fra Havet og søger op i Elverne, deels for at
forfriske sig i det ferske Vand, deels for at afgnie og afskylde, ved skarpe Strømmes og Fossers Fald, et slags
grønagtig Utøy, kaldet Laxe-Luus, som sette sig imellem Finnerne og plage den i Foraars Varme’. (‘great schools of
salmon moving from the sea into fresh water, partly to refresh themselves, and partly to rid themselves by rubbing
and washing in the swift currents and waterfalls, of a kind of greenish vermin called Laxe-Luus, attached between
the fins, plaguing it in the heat of spring’).
From: Berland and Margolis (1983)
Fig. 9.48 Severe skin lesions caused by sea lice, Lepeophtheirus
salmonis, on farmed Atlantic salmon
144
9 Metazoa
of parasite numbers trigger a treatment, to help diminish
infestations and this is supplemented with cleaner fish to
remove lice (wrasse species and lumpsucker). However, atypical strains of Aeromonas salmonicida have been isolated
from wrasse, and recent testing has established that wrasse
species can harbour viral haemorrhagic septicaemia virus.
Sea lice undergo ten developmental stages on the fish
with a moult between each stage. The infective, free-living
stage is called the copepodid and newly attached chalimus
larvae are typically found on the ventral surface and the fins,
while older stages can occur anywhere on the body. Adult
individuals typically locate in scale-less areas such as the
neck and head, around the anal fin and on the ventral side of
the caudal peduncle (Figs. 9.48 and 9.49). If untreated, both
chalimus and the adult lice may occur in substantial numbers
and cause severe skin lesions. Early gross signs are seen as
light grey patches on the head, neck and the perianal area.
Much of the damage caused by copepods is associated with
attachment to the host and feeding behaviour. Lice graze on
host tissues ranging from mucus, epidermal, dermal or subcutaneous tissues, causing skin ulceration, petechiae and
resultant hyperpigmentation. Microscopically, lesions may
be variable depending on fish species but include mechanical
Table 9.2 ‘Laxe-Luus’
Peder Clausson Friis (1545–1614) and Erik Pontoppidan (1698–1764) were a great scholar and bishop respectively
in Norway, and both showed a great interest in biology and natural history. They are probably the first to describe
sea-lice in migrating salmon. Friis (ca.1600) accurately describes the parasite without naming it: ’søge strax op i
Elffuen oc Fosser at de kunde afftoe i Fosser og paa Steen affscraabe store Lus aff sig, som sider i hans Nache’ (‘it
moves quickly up in the river and waterfalls to wash off in the falls, and to scrape off on stones, large lice attached
to its neck’). Furthermore: ‘De Lus som eer paa Laxen er som stuore Edderkopper, dog lenger, haffuer en lang Neb,
sidder haart paa hannom, bider igjennom den sterche oc sej Hud indtill Blod gaar ut’ (‘the lice on the salmon are
the size of great spiders, but longer, with a long beak, firmly attached to it (i.e. the salmon), it bites through the
strong and tough skin until blood emerges’.
Later, Pontoppidan in 1753 states that ‘da den i store Flokke kommer fra Havet og søger op i Elverne, deels for at
forfriske sig i det ferske Vand, deels for at afgnie og afskylde, ved skarpe Strømmes og Fossers Fald, et slags
grønagtig Utøy, kaldet Laxe-Luus, som sette sig imellem Finnerne og plage den i Foraars Varme’. (‘great schools of
salmon moving from the sea into fresh water, partly to refresh themselves, and partly to rid themselves by rubbing
and washing in the swift currents and waterfalls, of a kind of greenish vermin called Laxe-Luus, attached between
the fins, plaguing it in the heat of spring’).
From: Berland and Margolis (1983)
Fig. 9.48 Severe skin lesions caused by sea lice, Lepeophtheirus
salmonis, on farmed Atlantic salmon
144
9 Metazoa