Bacteria are unicellular, prokaryotic microorganisms that multiply by cell division (fission) and that typically have a cell wall. They may be aerobic or anaerobic, motile or non-motile, free-living, saprophytic or pathogenic.
There are only a few bacterial diseases that are currently listed by the OIE, and none of these cause diseases of finfish.This chapter deals with five bacterial diseases of warmwater finfish, all of which affect mainly common carp and other cyprinids.
Although bacterial infections are often be successfully treated, the misuse of antibiotics in aquaculture, agriculture and human medicine, particularly those antibiotics important to treating disease in man, can result in the development of bacterial strains having
antimicrobial resistance (AMR), a topic of growning global concern, as it may lead the ineffectiveness of antibiotics against bacteria causing human disease.
7.1 Carp erythrodermatitis
Carp erythrodermatitis (CE) or ulcer disease is caused by Aeromonas spp. It occurs mostly in cyprinids. It does not cause a general infection (septicaemia), but evokes acute epidermal ulcers (Figure 9). Mortality is usually low, not reaching 20 percent, but the consequences of secondary infections increase the mortality rate.
Optimal epizootic temperature: Water temperature plays an important role in the development of this disease. Below 12 °C the infection remains without clinical sign;
however, at over 22 °C well-visible clinical signs always appear.
Transmission: In transmitting the disease from fish to fish, blood-sucking parasites might play role. However, epithelial lesions could develop to ulcers when infected water, fish or infected items carry the pathogen to an uninfected fish stock.
Clinical signs: Typical clinical signs are the following: small haemorrhages appear on the skin or fins, at the base of which these develop into deep, round ulcers of about 3 mm in diameter that are margined by a purple-coloured layer of living cells. The ulcers can deepen to the serosa covering the abdominal cavity. Ulcers can also develop in scaled carp, but their shape is not round. The changes start with the elevation of scales due to the accumulation of fluid in the scale pockets, which then is followed by scale loss. Very often exophthalmia, petechial bleedings on the gills and dropsy can also be observed, but these clinical signs seem to be the consequence of a secondary infection.
Figure 9. Ulcer disease of common carp caused by Aeromonas salmonicida forma
achromogenes
Fresh mount picture
Prevention and treatment: Antibiotics mixed in the feed of the fish and fed for 7 days at a dose of 30–70 mg/body weight (BW) give good results (see Annex 3). Before feeding, the antibiotic should be tested for drug resistance.
Pathogen detection: Diagnosis of the causative agent is based on bacterial isolation. Isolation of the atypical Aeromonas strain can only be done from the freshly infected red zone of the ulcer. Only secondarily invasive water bacteria are isolated from other parts of the ulcer.
7.2 Infectious dropsy (septicaemia) of carp
Bacteria causing infectious dropsy occur worldwide, and all freshwater fish are prone to it.
This disease is mostly caused by mesophilic, motil Aeromonas spp., such as A. hydrophila, A.
caviae abd A. veronii.
Optimal epizootic temperature: The disease develops when water temperature is above 10 oC, in most cases as co-infection of another pathogen.
Clinical signs: Septicaemia is marked by bleedings, exophthalmos, oedema in the scale pockets and ascites in the abdominal cavity, as well as petechial haemorrhages on the gills. Internally, there are varying amounts of pinkish-red fluid in the abdomen.
Frequently, there are haemorrhages of the gastrointestinal tract and enlargement of the spleen (Figure 10). Co-infection with Saprolegnia is frequent.
Prevention and treatment: Prevention and treatment correspond to those suggested for CE.
Pathogen detection: Detection includes finding bacteria in wet-mounts upon microscopic examination, isolation in blood agar substrate and identification with biochemical methods.
7.3 Flexibacteriosis or columnaris disease
This disease is common in salmonids but also occurs in stocks of common carp, Chinese major carps and wels catfish. The bacterium which causes this disease occurs in the water, soil and on the surface of fish, especially on the gills.
Optimal epizootic temperature: Outbreaks usually occur at water temperatures above 18 °C.
Figure 10. Septicaemia caused by Aeromonas hydrophila in common carp. Note haemorrhages in tissues
Fresh-mount picture
Clinical signs: The first external sign of the disease is the appearance of greyish-white spots on the body of the fish, often on the head, lips and fins (Figure 11). These spots are very similar to those caused by Saprolegnia. A localized lesion often develops on the gills too. In severe infections, the fish looks as if it is covered with a white cotton-wool layer. Groups of bacteria form white-coloured slopes (columns) on the surface of the skin and gills, which can be easily noticed upon microscopic examination (Figure 12). Unfavourable environmental conditions of keeping and feeding, stress and mechanical injuries support and accelerate the development of infection.
Prevention and treatment: For therapy, antibiotics mixed into the feed of fish are used.
Improving water quality and living/rearing conditions of fish can also ensure a good result.
Pathogen detection: Infections can be easily diagnosed by examination of wet-mounts of tissue samples using a microscope.
7.4 Mucophilosis or epitheliocystis disease of common carp
This is a rarely diagnosed disease, with infection seeming to occur in different freshwater and marine fish. Besides common carp, intensive infection of the gills in silver carp is also known. Infections usually occur during summer.
Clinical signs: In cases involving intensive infection, the epithelial layer of the gill is full of small round spots (Figure 13). Due to proliferation of epitheloid cells, lamellae are no longer seen on the filaments. Changes are caused by Chlamydia-like organisms.
Diseased fish show signs of suffocation.
Figure 11. Flexibacteriosis in common carp;
colonies of bacteria cover the skin on the head
Photo by Gy. Csaba. Fresh–mount picture
Figure 12. Columns built up by Flexibacter bacteria on the skin
Photo by Gy. Csaba.Wet- mount preparation
Figure 13. Mucophilus (epitheliocystis) units in the gill filaments of common carp harbouring thousands of Chlamydia-like org.
Histological section, H & E staining
Prevention and treatment: No treatment is known, but drying and disinfecting the bottom of the pond and transferring the fish into another pond stops the intensification of infection.
7.5 Fish tuberculosis
Fish tuberculosis is a slowly developing chronic disease that is rather common in aquarium and marine fish. There are also reports from infected carps.
Clinical signs: Most fish species manifest few or no external signs of disease, but in advanced stages cachexia, exophthalmia, dermal ulcerative lesions and loss of scales can be observed.
Internal signs include an enlarged and softened spleen, kidney and liver; development of nodules of different size; and formation of granulomas from epitheloid cells surrounded by a thick connective cell capsule.
Prevention and treatment: There is no treatment. As these bacteria can cause zoonosis in the form of skin granulomatosis, infected fish stocks must be destroyed.
Pathogen detection: The finding of bacteria in nodules stained by the Ziehl-Nelsen method supports diagnosis.