Can histology and haematology explain inapparent Streptococcus agalactiae infections and asymptomatic mortalities on Nile tilapia farms?

• Published in: Research in veterinary science Vol. 129; pp. 13 - 20
• Main Authors: Owatari, Marco Shizuo, Jesus, Gabriel Fernandes Alves, Cardoso, Lucas, Lehmann, Nicollas Breda, Martins, Maurício Laterça, Mouriño, José Luiz Pedreira
• Format: Journal Article
• Language: English
• Published: England Elsevier Ltd 01.04.2020; Elsevier Limited
• Subjects: Acclimation; Acclimatization; Animals; Bacterial infections; Blood; Epidemics; Farms; Fish; Fish diseases; Fragments; Hematology; Hemoglobin; Histology; Immune system; Immunity; Immunology; Infection; Infections; Intestine; Lesions; Liver; Mortality; Oreochromis niloticus; Organs; Pathogens; Researchers; Spleen; Streptococcosis; Streptococcus agalactiae; Streptococcus infections; Subclinical infection; Temperature; Tilapia; Veterinary medicine; Brazil; India; Infection; Fish diseases; Oreochromis niloticus; Histology; Streptococcosis; Immunity
• ISSN: 0034-5288; 1532-2661
• DOI: 10.1016/j.rvsc.2019.12.018

The aim of this study was to characterise possible histopathological and haemato-immunological changes after subclinical infection by S. agalactiae S13 serotype Ib. One hundred juveniles of Nile tilapia with average weight of 45 g were distributed in ten 90 L experimental units. After the acclimation period 25 fish were euthanised, and fragments of liver, spleen and posterior mid-intestine tissue were sampled to verify the integrity of the organs and blood samples taken to check the haematological profile. Fifty animals were used to verify the optimal dosage for the challenge. The remaining 25 fish were infected with S. agalactiae S13. After 96 h, tissue fragments from the liver, spleen and posterior mid-intestine and blood samples were collected. The analyses revealed that, 96 h after contagion, S. agalactiae S13 serotype Ib caused subclinical lesions in the liver and spleen that are not commonly described as pathognomonic, in addition to haematological alterations. These results allow a better understanding of sudden recurrent mortalities in Brazilian tilapia farms, since the serotype Ib of S. agalactiae causes inapparent infections and can remain lodged in internal organs and cause irreversible lesions and haemato-immunological alterations, therefore compromising physiological functions vital for the health of fish without revealing external clinical signs in the animals. •“Streptococcus agalactiae causes hematological and subclinical inflammatory changes in the liver, spleen and intestine of juvenile tilapia”.•“Streptococcus agalactiae causes inapparent infections and can remain lodged in internal organs compromising physiological functions vital for the health of fish without observing clinical signs in the animals.”•“Inapparent infection may make tilapia an asymptomatic carrier of Streptococcus agalactiae serotype Ib.”