Diagnosing Streptococcus agalactiae in Tilapia: A Practical Sampling-to-Confirmation Approach

Diagnosing Streptococcus agalactiae in Nile tilapia requires a systematic approach, beginning with the outbreak history and ending with identification of the causative organism. Clinical signs and lesions can raise suspicion, but laboratory confirmation is important because different Streptococcus species can produce similar manifestations^[1]. 

Start with the outbreak history 

When mortality or disease occurs, begin by documenting the outbreak history and collecting fish from the affected location. Sampling should include sick fish showing obvious clinical signs as well as healthy fish from the same pond, cage, or production unit. 

Moribund fish are particularly useful for diagnostic investigation and can be collected and promptly transported to the diagnostic laboratory on ice^[1]. 

Clinical examination remains an important first step. Nervousness, aberrant behaviour, disorientation, erratic swimming, spiraling, and spinning can indicate neurological involvement. Other reported findings include anorexia, lethargy, melanosis, dorsal rigidity, altered body curvature, corneal opacity, unilateral or bilateral exophthalmia, peri-orbital or intraocular haemorrhage, diffuse haemorrhage and ascites^[1]. 

Choose tissues strategically 

GBS can infiltrate and reproduce in multiple organs, producing septicemic disease. The brain is considered a primary target because infection can cause meningoencephalitis^[1],[2]. 

For GBS detection, nerve tissue and highly vascularised organs such as the kidney, liver, and spleen are important sampling sites^[1],[3],[4]. The intestine, heart, and eyes can also be used for diagnosis^[1]. 

Venipuncture and kidney aspiration have also been described as safe, feasible and non-lethal approaches for obtaining blood and kidney samples from O. niloticus^[1]. 

Culture provides the first laboratory identification 

Aseptically collected tissue swabs can be streaked onto standard media including blood agar, Todd–Hewitt agar, brain heart infusion agar and tryptic soy agar^[1]. Selective media and enrichment broths can also be used for suspected GBS infections^[1],[5]. 

After incubation, bacterial colonies can be evaluated for morphology, Gram staining, haemolysis, Lancefield group antigen and biochemical characteristics such as catalase and oxidase reactions. Commercial identification kits such as RAPID32 and API20 Strep have also shown applicability for phenotypic GBS identification^[1]. 

However, phenotypic and biochemical methods can sometimes result in misidentification or lack species-level resolution^[6]. 

Molecular confirmation adds specificity^[1] 

Molecular methods can provide more specific identification. PCR, species-specific PCR and qPCR, multiplex PCR, nested PCR, LAMP, and 16S rRNA gene sequencing have all been used for GBS detection. 

GBS-specific PCR and 16S rRNA gene sequencing have been extensively used for definitive identification. qPCR has also demonstrated higher detection frequency than conventional PCR in experimentally infected tilapia, detecting GBS in 95.3% of infected fish compared with 51.1% using the other technique. 

Practical diagnostic focus 

A robust diagnostic pathway therefore moves from outbreak history → appropriate fish selection → targeted tissue sampling → bacterial isolation and identification → molecular confirmation when required. 

Clinical signs and lesions establish suspicion, while laboratory identification determines whether S. agalactiae is responsible for the outbreak.

References:

1. Abdallah ES, Metwally WG, Abdel-Rahman MA, Albano M, Mahmoud MM. Streptococcus agalactiae infection in Nile tilapia (Oreochromis niloticus): A review. Biology. 2024 Nov 11;13(11):914. https://www.mdpi.com/2079-7737/13/11/914
2. Ferrari NA, Favero LM, Facimoto CT, Dall Agnol AM, Gaeta ML, de Oliveira TE, Gonçalves DD, Lopera-Barrero NM, Pereira UD, Di Santis GW. Clinical and histopathological evolution of acute intraperitoneal infection by Streptococcus agalactiae serotypes Ib and III in nile tilapia. Fishes. 2024 Jul 13;9(7):279. https://www.mdpi.com/2410-3888/9/7/279
3. Abdallah ES, Metwally WG, Bayoumi SA, Abdel Rahman MA, Mahmoud MM. Isolation and characterization of Streptococcus agalactiae inducing mass mortalities in cultured Nile tilapia (Oreochromis niloticus) with trials for disease control using zinc oxide nanoparticles and ethanolic leaf extracts of some medicinal plants. BMC Veterinary Research. 2024 Oct 15;20(1):468. https://link.springer.com/content/pdf/10.1186/s12917-024-04298-z.pdf
4. Rahman MM, Rahman MA, Monir MS, Haque ME, Siddique MP, Khasruzzaman AK, Rahman MT, Islam MA. Isolation and molecular detection of Streptococcus agalactiae from popped eye disease of cultured Tilapia and Vietnamese koi fishes in Bangladesh. Journal of Advanced Veterinary and Animal Research. 2021 Mar 5;8(1):14. https://pmc.ncbi.nlm.nih.gov/articles/PMC8043335/pdf/JAVAR-8-14.pdf
5. Teo F. Group B Streptococci in sushi and Sashimi. Annals Academy of Medicine Singapore. 2017 Feb 1. https://www.academia.edu/download/103602105/annals-acadmedsg.pdf
6. Assis GB, Pereira FL, Zegarra AU, Tavares GC, Leal CA, Figueiredo HC. Use of MALDI-TOF mass spectrometry for the fast identification of gram-positive fish pathogens. Frontiers in Microbiology. 2017 Aug 9;8:1492. https://www.frontiersin.org/journals/microbiology/articles/10.3389/fmicb.2017.01492/pdf