Streptococcus is a major problem for tilapia in aquacultures and can rapidly spread throughout the farm. It can lead to mass-death and unlike many other diseases a streptococcus infection can be fatal even for large and otherwise healthy fishes. (Fish weighing less than 100 grams actually seem to be less susceptible to streptococcus than larger fish.) Weakened fish is especially susceptible to the illness, but can Streptococcus occur even in fish farms where the tilapias are kept in ideal conditions and given a suitable diet.
The best way of handling streptococcus is to prevent outbreaks from occurring in the first place. Streptococcosis typically appears in farms where the fish has been subjected to some type of stress which has weakened their immune system. Stress can for instance be caused by improper water chemistry, too much waste products in the water, or a change in water temperature. Crowding can also be very stress full for fish and makes it harder for the farmer to keep the water quality up. The best way of preventing streptococcus infections is therefore to strive to keep your fish in an environment as free from stress as possible. Food is another important aspect of disease prevention, since a nutritious and varied diet will strengthen the immune system of fish.
A tilapia can catch streptococcosis from an infected fish as well as from the environment. In aquacultures, streptococcosis is usually caused by the bacterium Streptococcus agalactiae (including the previously described Streptococcus difficilis / Streptococcus difficile which has now been reclassified as Streptococcus agalactiae). Streptococcosis caused by Streptococcus iniae also occurs in fish farms, but such outbreaks are usually not as lethal as those caused by Streptococcus agalactiae.
Just like all other species of Streptococcus, Streptococcus agalactiae and Streptococcus iniae are both gram-positive, non-motile, non-acid fast, oxidase-positive, catalase-negative cocci.
Streptococcosis can be acute or chronic in tilapias. Acute streptococcosis typically occurs during the warm season when the water temperature is high, while chronic streptococcosis normally occurs in cooler water. Acute streptococcosis will typically show in the form of mortality peaks that goes on for 2-3 weeks, while chronic streptococcosis does not cause any peaks of mortality. Even though the mortality rate might seem low for chronic streptococcosis, the fish farmer may eventually loose a lot of fish since the mortality level tends to be really persistent with this form of streptococcosis.
Regardless of which Streptococcusbacterium that is causing the problem, the symptoms are normally very similar. It is recommended that you examine at least ten fishes, because it is uncommon for one single fish to display more than a few symptoms.
Fish infected with Streptococcus will often suffer from eye lesions. e.g. endophthalmia and exophthalmia. Eye hemorrhages are common in infected tilapia and the disease can also cause unilateral (one eye) or bilateral (both eyes) opacification of the eye. It is however important to keep in mind that even though this is a very common symptom, some tilapias never develop any eye problems despite being infected.
An infected tilapia may display 2-3 mm abscesses symmetrically positioned on the inferior jaw. Within short, the abscesses normally burst and develop into haemorrhagic ulcers. Even larger abscesses sometimes occur on the base of the tail and the base of the pectoral fins. Even if the fish survives the infection, the abscesses will still be there.
Infected fish may develop multi-focal pin-point haemorrhages on the skin, especially around the mouth and at the fin bases. The area around the anus and genital papilla may become red.
Ascites is an accumulation of fluid in the peritoneal cavity (the general abdominal cavity). It is known under many different names, e.g. abdominal dropsy, peritoneal fluid excess, or hydroperitoneum. The anus often protrudes in tilapias that develop this symptom of streptococcosis.
The bacteria can attack the central nervous system of the tilapia and lead to all sorts of erratic behaviour. The fish may seem disorientated and swim in a swirly fashion, or it may just rest apathetically without showing any signs of activity. Some tilapias suffer from bending of the body when the CNS becomes infested by bacteria.
If you examine an infested tilapia, you will usually find an empty or nearly empty stomach and gut, since sick fish tend to avoid eating. The gall bladder is usually larger than normal, which is also caused by the lack of food. Loss of appetite is however a common symptom for virtually all tilapia diseases and the abovementioned findings will therefore not tell you much about the cause of the problem. It is also important to keep in mind that some fishes may continue to eat even when ill.
If your fish is suffering from acute streptococcosis, bacteria will get into the blood stream and spread to all internal organs. This causes septicaemia and you can expect to see all the normal signs associated with septicaemia, i.e. the eyes and virtually all inner organs can show signs of inflammation (spleen, kidney, liver, heart, brain, intestinal tract). The spleen and kidney often becomes enlarged.
If the tilapia survives long enough for the disease to develop into severe streptococcosis, the internal organs can begin to 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 of the fish.
A tilapia weakened by streptococcosis can be attacked by all sorts of opportunistic pathogens. It is not uncommon to find bacteria such as Aeromonas spp. (in freshwater) or Vibrio spp. (in brackish water) in the body of infected tilapias.
Examine smears of internal organs (kidney, liver, spleen, brain etc) stained with Gram stain under light microscopy at X1000 resolution. If the fish has streptococcosis, you will be able to see gram-positive cocci.
For many fish farmers, it is tempting to fill their ponds with as much fish as possible to make ends meet, but crowding will increase the risk of poor health in tilapia and can end up causing massive financial losses. Finding the optimal stocking density is always complicated and the farmer should be prepared to adjust stocking density over time.
If you suspect that streptococcosis is occurring among your fish, a lowered stocking density is one way of making the fish less stressed and lower the number of pathogens.
Avoid oxygen scarcity
Low oxygen levels are stressful for fish and stress is known to weaken the immune system. Also, in a population where streptococcosis is already occurring, a lowered oxygen level can worsen the situation dramatically. To keep the oxygen level up, many fish farmers use paddle wheels or similar.
Lower the temperature
Streptococcus bacteria prefers warm water and lowering the water temperature can therefore decrease a Streptococcus problem, provided of course that you don’t drop the water temperature down to levels where the tilapia suffers. Also keep in mind that already ill fish might not handle sudden changes well.
In aquariums and recirculation systems the water temperature can often be meticulously controlled, while pond farmers might have to resort to using sunscreens and water sprinklers to achieve any decrease. Using paddle wheels during the night when the air is cooler is also known to affect the water temperature in tilapia ponds.
Keeping your tilapias on a suitable diet that contains all necessary nutrients in fitting amounts will boost their immune system and make them more resilient towards all kinds of diseases. When it comes to feeding, it is also important to keep an eye on the water quality since over-feeding can cause foul water.
Refraining from feeding the fish or only feeding small amounts is known to reduce the mortality rate during streptococcosis outbreaks. Exactly why is still not fully understood.
Antibiotics have its pros and cons. Streptococcus bacteria are sensitive to antibiotics and antibiotics can therefore be used to threat streptococcosis in tilapia. The problem is that antibiotics only work as long as you administer them. When you stop medicating your fish, the outbreak might start all over again. This means that you may become trapped in a situation where you are forced to administer antibiotics for extended periods of time, which in turn increases the risk of antibiotic resistance in the pond. Antibiotics aren’t free and you also have to factor in the cost of administer them to the tilapias. Tilapias suffering from streptococcosis typically loose their appetite which can make it virtually impossible to use oral antibiotics. As if this isn’t enough, many consumers prefer to stay away from heavily medicated fish for various reasons, such as meat residue concerns.
Antibiotics are normally only an effective way of treating streptococcosis in tilapia if given during the early stages of the disease. Once a fish has developed a severe case of streptococcosis, it is often too late to help it using antibiotics.
For those who keep one or a few tilapias as pets, the situation is of course different. A pet keeper is probably not interested in selling the fish on the food market, and it is also feasible to force-feed a small group of tilapias to make sure they ingest the antibiotics. In addition to this, it is easier to spot an outbreak early on in an aquarium than in a large aquaculture. Administering antibiotics during the early stages of streptococcosis greatly increases it effectiveness.