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 the gills.
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
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. (dimilin)
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
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!