Streptococcosis has become a major problem for tilapia farmers and there is still no effective commercial vaccine available that can be used to prevent streptococcosis in tilapia. Tilapia growers must therefore focus on prevention and treatment of the disease. Streptococcosis can cause mass death in tilapia farms, and unlike many other tilapia diseases it will affect even large and otherwise healthy fish. Fish weighing at least 100 grams are actually more susceptible to streptococcosis than small fish.

Streptococcosis outbreaks are known to take place primarily when the fish has been subjected to some form a stress, e.g. due to overcrowding, improper water chemistry or changing water temperatures. If you want to decrease the risk of streptococcosis in your tilapia, the best course of action is therefore to minimize the amount of stress that your fish is forced to deal with. Keep your tilapia in a suitable, stable environment and feed them nutritious food. A tilapia can catch streptococcosis from the environment as well as from an infected fish.

Streptococcosis can be acute or chronic. Acute streptococcosis normally occurs during the warm season when the water temperature is high and will typically result in peaks of mortality that goes on for 2-3 weeks. Chronic streptococcosis occurs when the water temperature is lower and does not cause any peaks of mortality. The mortality rate will be low but the amount of dead fish will become high in the long run since the mortality level tends to be really persistent.

What causes streptococcosis in tilapia?

In farmed tilapia, the major cause of streptococcosis is Streptococcus agalactiae (including the previously described Streptococcus difficilis  / difficile, now reclassified as Streptococcus agalactiae). The closely related bacterium Streptococcus iniae can also cause streptococcosis in tilapia, but is normally not as lethal as Streptococcus agalactiae.

All species of Streptococcus are gram-positive, non-acid fast, non-motile, oxidase-positive, catalase-negative cocci.

Symptoms of Streptococcosis in tilapia

The clinical of signs of streptococcosis are very similar regardless which species of Streptococcus that is behind the disease. An infected fish will seldom display all the clinical signs listed below and it is therefore advisable to examine at least 10 fishes.

If your tilapia is suffering from streptococcosis, gram-positive cocci will be visible when impression smears of internal organs (such as liver, spleen, kidney or brain) is stained with Gram stain and examined under light microscopy at X1000 resolution.

External signs of streptococcosis in tilapia

• Infected tilapia will often develop eye lesions, e.g. endophthalmia and exophthalmia. Eye haemorrhages are common and streptococcosis can also lead to unilateral (one eye) or bilateral (both eyes) opacification of the eye. It is however possible for a tilapia to be infected with Streptococcus spp. without developing any eye lesions.
  
  
• The eyes are not the only part of the fish body where the disease can cause haemorrhages; multi-focal pin-point haemorrhages on the skin can also occur. These haemorrhages are normally located near the mouth and the base of the fins. Some tilapias become red around the anus and on the genital papilla.  
  
  
• Streptococcosis will often lead to 2-3 mm abscesses in infected tilapia. These abscesses are symmetrically positioned on the inferior jaw. In most cases, the abscesses are short lived and will soon burst and turn into haemorrhagic ulcers. Two parts of the tilapia body are prone to even bigger abscesses filled with purulent material: the base of the tail and the base of the pectoral fins. Even if the tilapia survives the streptococcosis, these big abscesses will normally remain.
  
  
• Streptococcus spp. affects the central nervous system of tilapia and infected fish can therefore engage in various forms of abnormal behaviour, such as lethargy, disorientation, swirling, and bending of the body. 
  
  
• During an acute outbreak of streptococcosis, ascites is commonly observed in tilapia. Ascites is an accumulation of fluid in the peritoneal cavity (the general abdominal cavity). Ascites is known under several different names, such as peritoneal fluid excess, hydroperitoneum or abdominal dropsy. In infected tilapia, ascites is often accompanied by a protruding anus.  

Internal signs of streptococcosis in tilapia

• Tilapia infected with Streptococcus spp. will normally refrain from eating and the stomach and gut of the fish will therefore be empty or nearly empty. When the fish is no longer digesting any food, it is normal to find a big gall bladder. Loss of appetite is however a common symptom of a long row of different health problems that can affect tilapia and an empty stomach and gut and a big gall bladder is therefore by no means a sure indication of streptococcosis. It should also be noted that pond living tilapia sometimes continue to filter feed during the early stages of a streptococcosis outbreak. 
  
  
• During acute streptococcosis, streptococcus spp. will reach the blood system of the fish and travel to all the internal organs. This will lead to septicaemia and you can therefore expect to see the normal internal signs associated with septicaemia, such as haemorrhages and inflammation in the eye, spleen, kidney, liver, intestinal tract, heart, and brain. In many fishes, both spleen and kidney will become bigger than normal.
  
  
• In severe cases of streptococcosis, the internal organs can stick to each other and to the walls of the peritoneal cavity.
  
  
• In severe cases of streptococcosis, fibrinous material can sometimes be seen in the peritoneal cavity.
  
  
• When a fish is seriously weakened by streptococcosis, opportunistic bacteria can take advantage of the situation and attack. It is therefore not uncommon to find bacteria such as Aeromonas spp. (freshwater) or Vibrio spp. (brackish water) in infected tilapia.
  

What should I do?

Avoid crowding
It may be tempting to crowd your growing unit with as much tilapia as possible, but crowding will increase the risk for disease and can therefore affect the survival rate of your fish. Finding the optimal stocking density is difficult and you should be prepared to adjust it over time. If you suspect that your tilapia is infected with streptococcosis, a lowered stocking density will decrease the number of pathogens and make the fish less stressed (which in turn makes them more capable of fending of bacteria).

Avoid oxygen scarcity
If the oxygen level drops it can worsen the disease problem dramatically. It is therefore important to keep the oxygen levels up at all times. You can for instance use paddle wheels to aerate the water.

Lower the water temperature
In aquariums and recirculation systems, it is often possible to control the water temperature. Streptococcus spp. thrives in warm water and it is therefore a good idea to lower the water temperature, if possible. A high water temperature is not only beneficial for Streptococcus spp., it is also stressful for the fish.

If you grow tilapia in a pond, you might be able to achieve at least a minor drop in temperature by using sunscreens and water sprinklers with cold water. Using paddle wheels during the night when the air is cooler can also work.  

Cut down on food
Feeding your fish nothing or just small amounts of food can help control or reduce the mortality rate during an outbreak of streptococcosis. Exactly why this works is still not fully understood. (It should also be noted that most tilapias suffering from streptococcosis will loose their appetite.)

Antibiotics?
Antibiotics can be used to kill Streptococcus spp., but you should think twice before using it to treat outbreaks of streptococcosis in tilapia. Many farmers have reported how antibiotics helped them to (partially) control the mortality rates, but only during the period of application. As soon as they stopped administering antibiotics, the mortality rate increased again. This might force you into a downward spiral where you have to administer antibiotics for longer and longer periods of time, and this will in turn increase the risk of involuntarily culturing bacteria resistant to antibiotics. Administering antibiotics for larger periods of time is also expensive and labour intensive and you should be aware that your tilapia might become less interesting for the food market due to meat residue concerns.

If you decide to use antibiotics, you should keep in mind that antibiotics are only effective in treating streptococcosis in tilapia when applied during the really early stages of the disease. You should also be aware that infected fish usually loose their appetite, which makes oral antibiotics hard to administer.

If you are an aquarist keeping tilapia as pets, the situation is of course a bit different. It might be feasible for you to force-feed your tilapia with antibiotics and you do not have to worry about how the food market will react to your medicated fish. It might also be easier for you to spot an outbreak early on and start administering medicine during the initial stages of the disease.