Streptococcus In Tilapia part 1

Published by Intervet - The theory that tilapia is a hardy and disease-resistant fish species is no longer correct. Producers, scientists and processors have now become aware that diseases may well be the number one threat for the future of this industry. Introduction Several diseases have been identified in tilapia farming (see Intervet AAH Newsletter no. 11).
Nội dung trích xuất từ tài liệu:
Streptococcus In Tilapia part 1 Streptococcus In TilapiaPublished by Intervet - The theory that tilapia is a hardyand disease-resistant fish species is no longer correct.Producers, scientists and processors have now becomeaware that diseases may well be the number one threat forthe future of this industry.IntroductionSeveral diseases have been identified in tilapia farming (seeIntervet AAH Newsletter no. 11). Streptococcosis isconsidered to be the most devastating disease as it cancause massive kills of large size fish and is responsible forheavy economic losses. In this article, we discussStreptococcosis in tilapia including the various clinicalsigns observed during Streptococcus outbreaks.Causative agentStreptococcus agalactiae (including the previously-described S. difficilis/difficile, now reclassified as S.agalactiae) is the major cause of streptococcosis in farmedtilapia. 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 isno obvious difference in the clinical signs induced by oneor other of the Streptococcus species.Clinical signsExternal signs:Abnormal behaviour: Due to the tropism of the bacteria forthe central nervous system, swirling behaviour, lethargy,bent bodies and disorientated fish are commonly observed.Eye lesions: Sick fish often have eye lesions such asendophthalmia or exophthalmia. Unilateral or bilateralopacification of the eye can also be associated with theprevious signs and eye haemorrhages are common.However, not all fish affected with streptococcusnecessarily have eye lesions.Abscesses: In Streptococcus-infected fish, it is common tofind 2- to 3-mm abscesses symmetrically positioned on theinferior jaw. In general, these abscesses quickly burst andbecome haemorrhagic ulcers which do not heal. Biggerabscesses of approximately 5 mm can also be observed atthe base of the pectoral fins. The base of the tail is acommon site for large abscesses (10-20 cm). Theseabscesses contain purulent material. When fish survive aStreptococcus infection, these abscesses usually remain andare often found at the time of processing.Skin haemorrhages: Streptococcosis causes externalhaemorrhages. In general, these multi-focal pin-pointhaemorrhages are seen around the mouth or at the base ofthe fins. Sometimes, it is possible to observe a reddishpigmentation around the anus or on the genital papilla.Ascites: Presence of abdominal fluid is common duringacute outbreaks of streptococcosis. This ascites is oftenseen in association with a protruding anus.Internal signs:The internal signs correspond to the general characteristicsigns of septicaemic infections.Off-feed: Generally, dry feed is not present in the stomachor gut of sick fish. However, in pond-cultured fish, somesick fish at the beginning of an outbreak might still be filterfeeding. In relation with an empty gut and stomach, it iscommon to observe a big gall bladder, a typical sign of theabsence of digestive activity.Septicaemia: During acute infections, bacteria rapidlyreach the blood system and are disseminated to all internalorgans. Major clinical signs associated with thissepticaemic condition are haemorrhages and inflammationin the liver, spleen, kidney, heart, brain, eye and intestinaltract. The spleen and kidney are often enlarged.Peritonitis: In severe infections, adhesions of the internalorgans together and with the peritoneal cavity walls arecommon. Moreover, the presence of fibrinous material canbe observed in the peritoneal cavity.In cases of severe infections, dual infection with otheropportunistic bacteria in the environment, such asAeromonas spp. in freshwater and Vibrio spp. in brackishwater, 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 fora long period of time. The disease is transmitted horizontally from fish tofish (via cannibalism, skin injuries, etc.), and also from theenvironment to the fish. Streptococcosis can theoretically affect all fish sizes.However, bigger fish (from 100 g to market size) areusually more susceptible to the disease. The disease can be acute, with peaks of mortalitylasting 2-3 weeks during the high water temperatureseason. However, it may also be chronic, when watertemperature is lower, causing a low but persistent level ofmor ...