The pathological Changes in Tilapia Due to Experimental Infection with Streptococcus

Abstract- Tilapia culture is gaining importance as a candidate species for aquaculture in India in recent days. Many countries had started culturing tilapia several years back. As culture becomes more intensified, chances for occurrence of diseases also increase. Although tilapia is a hardy species, it is seen that stress can become a main factor for onset of disease, particularly at higher temperature. Stress leads to other diseases such as bacterial diseases. Occurrence of Streptococcosis has been reported in tilapia from other countries but not yet from India. Since India is a tropical country, chances for occurrence of Streptococcosis by Streptococcus agalactiae is more.

The experiment comprised of four groups of fishes maintained in duplicate. Of these, one group acted as control and other three groups were injected with 106, 107 and 108 cfu/ml of Streptococcus agalactiae inoculum. Clinical signs like cessation of feed, haemorrhage in eyes and body, tail and fin rot, erratic and spinning like movement, fishes coming upto the surface of water and lying near the corner of tanks were observed in all the three treatments that were injected with S. agalactiae. Upon post mortem examination, the fishes in all the three treatments other than control had enlarged and pale coloured spleen, liver and gall bladder. Pale coloured gill and soft brain was also observed. On swabbing of different organs into Brain Heart Infusion agar plates, colonies of Streptococcus agalactiae were reisolated and confirmed as the causative agent for the diseased condition. Haematological parameters were also analysed. Haemoglobin content showed decreasing trend from lower concentration to higher concentration of bacteria in the injected fishes. However, RBC count was found to increase from lower to higher concentration of injected fishes, but the size of erythrocytes was much smaller; a condition typical of hypochromic microcytic anaemia. Observation of the blood smear clearly showed the haemolysis of RBCs, confirming the occurance of haemolytic anaemia. Distinct pathological signs viz; encephalitis, increased vacuolation, congestion and RBC infiltration were noticed in the brain, while in the heart RBC infiltration and slight oedema were observed in histological sections. The liver of infected fishes showed vacuolation, cellular swelling, congestion and RBC infiltration irrespective of the dose.

The Kirby Bauer test showed that Streptococcus agalactiae was sensitive to nine antibiotics, viz; erythromycin , chloramphenicol, kanamycin, azithromycin, gentamicin, nitrofurantoin, tetracycline, polymixin B, amikacin and resistant to nine antibiotics, viz; like ampicillin, ciprofloxacin, streptomycin , trimethoprim , vancomycin , nalixidic acid , penicillin G, methicillin and cefipime out of the 18 antibiotics to which its sensitivity/resistance was studied. In MIC test, among those antibiotics that showed sensitivity in Kirby Bauer antibiotic test four antibiotics viz., gentamicin, amikacin, erythromycin and polymixin B showed sensitivity even at the lowest concentration of 0.01,0.05, 0,25 and 4 mcg respectively. So these antibiotics can be recommended for treating streptococcossis caused by Streptococcus agalactiae, if need arises.