Introduction
Several diseases have been identified in tilapia farming (see Intervet AAH Newsletter no. 11). Streptococcosis is considered to be the most devastating disease as it can cause massive kills of large size fish and is responsible for heavy economic losses. In this article, we discuss Streptococcosis in tilapia including the various clinical signs observed during Streptococcus outbreaks.
Causative agent
Streptococcus agalactiae (including the previously-described S. difficilis/difficile, now reclassified as S. agalactiae) is the major cause of streptococcosis in farmed tilapia. S. iniae also causes mortality but to a lesser extent. Streptococcus spp. are Gram positive, non-acid fast, non-motile, oxidase-positive, catalase-negative cocci. There is no obvious difference in the clinical signs induced by one or other of the Streptococcus species.
Clinical signs
External signs:
Abnormal behaviour: Due to the tropism of the bacteria for the central nervous system, swirling behaviour, lethargy, bent bodies and disorientated fish are commonly observed. Eye lesions: Sick fish often have eye lesions such as endophthalmia or exophthalmia. Unilateral or bilateral opacification of the eye can also be associated with the previous signs and eye haemorrhages are common. However, not all fish affected with streptococcus necessarily have eye lesions.
Abscesses: In Streptococcus-infected fish, it is common to find 2- to 3-mm abscesses symmetrically positioned on the inferior jaw. In general, these abscesses quickly burst and become haemorrhagic ulcers which do not heal. Bigger abscesses of approximately 5 mm can also be observed at the base of the pectoral fins. The base of the tail is a common site for large abscesses (10-20 cm). These abscesses contain purulent material. When fish survive a Streptococcus infection, these abscesses usually remain and are often found at the time of processing.
Skin haemorrhages: Streptococcosis causes external haemorrhages. In general, these multi-focal pin-point haemorrhages are seen around the mouth or at the base of the fins. Sometimes, it is possible to observe a reddish pigmentation around the anus or on the genital papilla.
Ascites: Presence of abdominal fluid is common during acute outbreaks of streptococcosis. This ascites is often seen in association with a protruding anus.
Internal signs:
The internal signs correspond to the general characteristic signs of septicaemic infections.
Off-feed: Generally, dry feed is not present in the stomach or gut of sick fish. However, in pond-cultured fish, some sick fish at the beginning of an outbreak might still be filter feeding. In relation with an empty gut and stomach, it is common to observe a big gall bladder, a typical sign of the absence of digestive activity.
Septicaemia: During acute infections, bacteria rapidly reach the blood system and are disseminated to all internal organs. Major clinical signs associated with this septicaemic condition are haemorrhages and inflammation in the liver, spleen, kidney, heart, brain, eye and intestinal tract. The spleen and kidney are often enlarged.
Peritonitis: In severe infections, adhesions of the internal organs together and with the peritoneal cavity walls are common. Moreover, the presence of fibrinous material can be observed in the peritoneal cavity.
In cases of severe infections, dual infection with other opportunistic bacteria in the environment, such as Aeromonas spp. in freshwater and Vibrio spp. in brackish water, can be observed.
Epidemiology
- Outbreaks usually take place when fish have been exposed to stress, such as an increase in water temperature, suboptimal oxygen levels in the water or overcrowding for a long period of time.
- The disease is transmitted horizontally from fish to fish (via cannibalism, skin injuries, etc.), and also from the environment to the fish.
- Streptococcosis can theoretically affect all fish sizes. However, bigger fish (from 100 g to market size) are usually more susceptible to the disease.
- The disease can be acute, with peaks of mortality lasting 2-3 weeks during the high water temperature season. However, it may also be chronic, when water temperature is lower, causing a low but persistent level of mortality.
Diagnostic methods
On-farm diagnosis:
- Observation of the previously described clinical signs is useful to indicate a possible case of streptococcosis. However, as it is very rare that one fish carries all clinical signs, the presumptive diagnosis based on clinical signs will only be meaningful if 5 to 10 fish are examined.
- Impression smears of the internal organs (brain, liver, spleen and kidney) can be stained with Gram stain. Their examination under light microscopy at X1000 resolution will reveal the presence of Gram-positive cocci in true cases of streptococcosis.
Laboratory diagnosis: - The identification of the specific Streptococcus species responsible for the mortality requires bacterial isolation and identification in the laboratory.
Control and treatment
Decrease feeding: During acute Streptococcus outbreaks, partially or totally reducing the feed can help to control or reduce the mortality rate. One of the hypotheses to explain this phenomenon is that the bacteria are present in the water and their uptake is facilitated by feeding.
Decrease stocking density: High productivity in tilapia farming is achieved by balancing a high stocking density with a good survival rate. When mortality increases, lowering the stocking density helps to lower both the stress level and the pathogen load within the population. Keeping optimal oxygen levels: The use of paddle wheels to aerate the water is recommended in order to avoid suboptimal oxygen levels that will aggravate the disease situation.
Lower the water temperature: High water temperature is both a stress for the fish and a preferential condition for the bacteria to develop. Therefore, lowering the water temperature can be an approach in recirculation systems where temperature is controllable. In small-size pond farming, the utilization of sunscreens on top of ponds and water sprinklers can be used but with limited success. Activating paddle wheels at night when the air temperature is lower is an additional method that can help reduce the water temperature.
Antibiotic treatment: Antibiotics are only effective in treating a Streptococcus outbreak if treatment is applied very early during the course of the disease. However, in most cases, oral antibiotics are ineffective as infected fish have a reduced appetite. Moreover, it is reported by many farmers that antibiotics are only able to (partially) control mortality rates during the period of application. However, once the course of antibiotic is over, mortality usually increases again. This phenomenon also leads to non-sustainable behaviour; as mortality increases again after a normal antibiotic course, farmers are tempted to increase the duration of application to longer periods or apply higher doses than recommended.
This in turn increases selection pressure toward resistant bacteria. Especially for export-oriented producers, the negative consequences of using antibiotics, such as antibiotic resistance and meat residue concerns, must be carefully evaluated.
Vaccination: Presently, there are no effective commercial vaccines to prevent Streptococcus outbreaks in tilapia. However, in the near future, vaccination will be the key preventive technique to combat this disease and decrease its devastating economic impact on tilapia farming.
Further Information
Columnaria In Tilapia
Diseases of Tilapia - An Introduction
June 2006
