UN­DER­STAND­ING DROPSY

If the body of your fish swells to the point that it takes on a pine cone ap­pear­ance, you can be pretty sure it has dropsy. So, what causes it and can it be treated?

Practical Fishkeeping (UK) - - Welcome - WORDS: DAVE HULSE, TECH­NI­CAL CON­SUL­TANT AT TETRA

One of the fish dis­eases dreaded by most fish­keep­ers is dropsy — an ac­cu­mu­la­tion of fluid in the ab­dom­i­nal cav­ity. Ex­pe­ri­ence tells us that once a fish ex­hibits th­ese symp­toms, re­ver­sal and cure is un­likely and you’ll need to pre­pare for the worst.

In or­der to un­der­stand what causes dropsy, and how it might be treated and pre­vented, we need to first ex­plore fish anatomy and phys­i­ol­ogy of our fish.

So, grab your note­books — we are go­ing back to the class­room!

What is dropsy?

Let’s keep things sim­ple and fo­cus on fresh­wa­ter fishes, where dropsy is a much more com­mon con­di­tion than in their ma­rine coun­ter­parts. Th­ese fish have many elec­trolytes such as Na+, K+ and Cl- dis­solved in their tis­sue flu­ids and an ef­fect of th­ese, in con­junc­tion with plasma pro­teins, is to give the fish an os­motic po­ten­tial greater than the fresh­wa­ter in which they swim. There­fore, wa­ter mol­e­cules are drawn into the body of the fish across any per­me­able sur­faces by os­mo­sis and elec­trolytes can be lost to the en­vi­ron­ment via dif­fu­sion.

The vast ma­jor­ity of the body sur­face is im­per­me­able; skin is thick­ened and of­ten cov­ered in scales or bony plates. How­ever, two or­gans must re­tain a per­me­abil­ity with the wa­ter; the gills, where per­me­abil­ity is es­sen­tial for gas ex­change and ni­troge­nous waste ex­cre­tion, and the gut, where nu­tri­ents are up­taken.

Th­ese two or­gans are the holes in the os­moreg­u­la­tory ar­mour of the fish. For fresh­wa­ter fish, adapt­ing to this per­sis­tent salt loss and wa­ter in­flux across the gills and gut is es­sen­tial. It is one of the func­tions of the kid­ney to ex­crete this wa­ter, hence fresh­wa­ter fishes pro­duce co­pi­ous amounts of a very di­lute urine.

There­fore, if the gills are dam­aged, ad­mit­ting more wa­ter than the kid­neys can fil­ter out, or if the kid­neys fail, wa­ter be­gins to ac­cu­mu­late and will col­lect in the ab­dom­i­nal cav­ity where it is known as as­citic fluid or as­cites (from the Greek askos, mean­ing sack or bag).

How­ever, fail­ure of the kid­ney to

os­moreg­u­late is not the only rea­son fish can ac­cu­mu­late as­citic fluid. Dam­age to the liver can lead to re­duced blood flow through this large or­gan, much of which is from the por­tal vein car­ry­ing nu­tri­ent-rich blood from the in­tes­tine. In hu­mans, this com­pro­mised blood flow raises blood pres­sure in the por­tal veins lead­ing to flu­ids weep­ing out of small blood ves­sels and col­lect­ing in the ab­dom­i­nal cav­ity — a sim­i­lar mech­a­nism is pre­sumed in fish; liver dam­age due to in­fec­tion or pol­lu­tants is of­ten as­so­ci­ated with as­citic fluid, even in ma­rine fish.

So, it is ap­par­ent then that dropsy is ac­tu­ally a symp­tom rather than a prob­lem in its own right. There­fore, we must find the orig­i­nal cause of the liver or kid­ney dam­age if we wish to cor­rect the con­di­tion — and this is usu­ally eas­ier said than done!

There are many fish dis­eases of bac­te­rial or vi­ral ori­gin that have as­citic fluid as a com­mon symp­tom, but all seem to in­clude the liver, kid­ney or the spleen as tar­get or­gans of in­fec­tion. Crudely, if an in­ter­nal or­gan with a large blood sup­ply be­comes in­fected or necrotic, blood flow through the or­gan is hin­dered, which raises blood pres­sure in the sup­ply­ing ves­sels lead­ing to ac­cu­mu­la­tion of as­citic fluid.

What are the symp­toms?

Dropsy is a sim­ple and ob­vi­ous con­di­tion to recog­nise, but it is pos­si­ble to con­fuse with other con­di­tions. The build-up of as­citic fluid places pres­sure on the ab­dom­i­nal wall caus­ing the belly to dis­tend out­wards. As the con­di­tion pro­gresses and more fluid ac­cu­mu­lates, those fish with scales or bony plates (such as the ar­moured cat­fish), will show pro­tru­sions of th­ese scales or plates, im­part­ing a grotesque ‘pine cone’ like ap­pear­ance to the fish. A fish in this con­di­tion re­ally is very poorly as this is a clear sign of ma­jor or­gan fail­ure.

Other causes of ab­dom­i­nal dis­ten­sion to rule out in­clude in­ter­nal tu­mours or the fish sim­ply be­ing a gravid (full of eggs or fry) fe­male. Tu­mor­ous growths rarely lead to a sym­met­ri­cal dis­ten­sion of the ab­domen, usu­ally a pro­tru­sion to one side or bi­ased to the front or rear of the fish will be ap­par­ent, also rarely will the scales pro­trude. Gravid fe­males can have very round bel­lies, again rarely with pro­trud­ing scales, but will not show other signs of dis­ease such as loss of colour, loss of ap­petite, fin ero­sion, or res­pi­ra­tory dis­tress. If a fish with dropsy is suf­fer­ing ma­jor or­gan fail­ure, there will be other clear be­havioural and phys­i­cal symp­toms of dis­ease. How­ever, the only way to be cer­tain a fish has ex­ces­sive as­cites is to ob­serve the con­di­tion post-mortem. If we can rule out other causes of ab­dom­i­nal dis­ten­sion we have to try to deal with the dropsy. Any fish dis­ease in­ves­ti­ga­tion be­gins with a thor­ough wa­ter qual­ity ap­praisal, tak­ing into con­sid­er­a­tion the en­vi­ron­men­tal re­quire­ments of the live­stock in the tank or pond. Us­ing Tetra’s Test 6in1 along­side its free wa­ter test­ing app will pro­vide a clear anal­y­sis of wa­ter pa­ram­e­ters. Also con­sider the re­cent his­tory of the en­vi­ron­ment. Have new fish, plants, or in­ver­te­brates been added? Have fish been han­dled lately or has vi­tal hard­ware failed re­cently?

Can it be dealt with?

As­sum­ing only a sin­gle fish is af­fected, it is vi­tal to iso­late it; re­mem­ber there are nu­mer­ous pathogens that can cause mul­ti­ple or­gan fail­ure and the af­fected fish is al­most cer­tainly a reser­voir of pathogen. Iso­la­tion tanks must be housed well away from the main stock and have ded­i­cated, fil­tra­tion, nets, and syphons. The wa­ter chem­istry and tem­per­a­ture in the iso­la­tion tank should equal that of the main tank and the decor of the iso­la­tion tank must be min­i­mal, though a sim­ple refuge for the fish must be pro­vided.

Once in iso­la­tion, a treat­ment regime is es­sen­tial. Ide­ally, the agent of the in­fec­tion must be iden­ti­fied and this can only be done by a ve­teri­nar­ian. Mori­bund (nearly dead), tank or pond mates may be sam­pled or biop­sies may be taken from the in­di­vid­ual con­cerned to iden­tify the causative pathogen and find an an­tibi­otic to which it is sus­cep­ti­ble. This is likely to be ad­min­is­tered to the iso­lated fish in ad­di­tion to the pop­u­la­tion in the tank or pond. Clearly this ser­vice is go­ing to be ex­pen­sive and is likely to only be un­der­taken on high value fishes such as Koi, ma­rine fish or brood­stock Dis­cus for ex­am­ple.

Non-vet­eri­nary treat­ment in­volves iso­la­tion, wa­ter qual­ity im­prove­ment and pos­si­bly the use of a broad-spec­trum aquar­ium an­ti­sep­tic such as Tetra Med­ica Gen­eral Tonic. Cer­tain fresh­wa­ter fish can ben­e­fit from an ad­di­tion of table salt to the iso­la­tion tank wa­ter. Salt­ing to 3 g/l will re­duce by one third the os­motic in­flux of wa­ter into the fish, and so if the prob­lem is due to kid­ney fail­ure, this lessens the load on the strug­gling kid­ney, pos­si­bly fa­cil­i­tat­ing a self-cure. If the as­cites is due to liver or splenic dam­age, salt­ing will have lit­tle ef­fect. Al­ways in­ves­ti­gate whether your fish are able to tol­er­ate a salt regime in their wa­ter — as a rough rule of thumb, soft wa­ter species

are much less tol­er­ant.

If a fish’s kid­neys fail, wa­ter be­gins to col­lect in the ab­dom­i­nal cav­ity, caus­ing swelling.

Viewed from above, it’s easy to see the classic ‘pine cone’ ap­pear­ance of dropsy in this gold­fish.

Heav­ily gravid live­bear­ers will be very swollen, but won’t show any signs of ill health.

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