- the fish louse
Written by Frank Prince-Iles
a real nasty
A major threat in a pond or tank
Argulus, or fish lice, represent a major threat to fish health; both as
a result of direct tissue damage and secondary infections. Fish lice are
one of the biggest parasites (5-10 mm) and visible with the naked eye.
Argulus feed by first inserting a pre-oral sting which injects
digestive enzymes into the body. They then suck out the liquidised body
fluids with their proboscis-like mouth. (Take a look at the movie
to see the mouth in close-up.) Feeding can take place on the skin or in
||A close up shot showing its eyes at the top of the
picture. The two prominent circular structures near the top are
the suckers used to hold onto the fish while feeding. Just below
and in-between the suckers is the proboscis-like mouth that it
uses for feeding
This feeding activity causes intense irritation. Fish are damaged by
the constant piercing of the skin by the stylet and there is often
localised inflammation. The other danger is that opportunistic bacteria
such as Aeromonas or Pseudomonas sometimes infect these
damaged areas leading to skin ulcers and gill disease. It is also believed
that the stylus may occasionally ‘inject’ viruses and bacteria into
the fish. The various spines, suckers and hooks that lice use for
attachment may also cause additional tissue damage. So all-in-all a
thoroughly nasty parasite!
In addition to physical damage, affected fish are subjected to severe
stress, which often leads to secondary parasite infestations such as
white-spot and Costia. This type of combined attack on stressed and
often weakened fish can result in high numbers of fatalities.
So quite clearly, even finding one louse warrants immediate treatment
and a follow up examination to check for secondary health problems
Biologically, Argulus are crustacean parasites in the subphylum
Crustacea - which means they are grouped along with shrimps, prawns and
water fleas etc. Animals in this group have a rigid or semi-rigid chitin
exoskeleton, which has to be moulted as they grow larger. They are in the
class Branchiura, a group of crustaceans with very similar features; all
branchiurians are fish parasites.
Although it is easy to spot lice when you know they are there, they are
easy to miss in the rush to take skin scrapes. To the naked eye they
appear as very small dark spots that are easy to overlook unless they
move. They are often found in relatively sheltered areas behind the fins
or around the head. They are usually easier to spot on fins rather than
the body, as they tend to show up more against a plain transparent
background. Lice are oval-shaped and flat and capable of moving very
quickly. In an aquarium, they can sometimes be seen swimming as they move
from host to host.
Fish with a heavy lice infestation will show a classic irritation
response such as rubbing and flashing. At a later stage they will become
lethargic. Affected fish may have focal red lesions on their body.
The life cycle of Argulus
As with most fish parasites, they have a high reproductive potential.
Mating takes place on the fish, after which the female swims away and lays
eggs on plants and other submerged objects. When the eggs hatch the
juvenile passes through several metamorphic changes as it develops into an
adult. Around 4 days after hatching, the newly-hatched juvenile actively
seeks a host and continues its development on the fish. The whole cycle
takes between 30 – 100 days depending on temperature. The eggs can
over-winter and hatch in spring as water temperatures increase. Adults can
survive without a host for several days. Any treatment plan has to take
account of emerging juveniles and therefore prevailing temperatures.
The most successful and effective treatments against lice are
organophosphates. Using three treatments over the estimated life cycle of
the parasite almost always eradicates lice. At typical summer pond
temperatures of 20oC or higher, treatments at 10-day intervals will kill
existing adults and juveniles as well as emerging juveniles. The down-side
is that in the UK organophosphates are banned for use as fish disease
treatments! They are still obtainable - but at a sky-high price!
There are no other treatments currently available that are likely to be
totally effective. There is some suggestion that using a chitin inhibitor
such as dimilin will stop the juveniles developing as they moult their
exoskeleton – but there has been no real testing done on this proposal.
More environmentally friendly alternatives are currently undergoing
licensing evaluation tests for use in the food-fish industry. However, the
draw back is again liable to be costs. Initial reports suggest that these
alternatives may be better at controlling rather than eradicating lice.
On a final point. In one incident last year I examined a small aquarium
that was overrun with lice. There had been no new additions of fish or
plants. The only possible source was live Daphnia put in the tank from
time to time!