Investigating infectious bacterial diseases affecting farmed Nile tilapia (Oreochromis niloticus) from the Philippines

Abstract

Tilapia production in the Philippines has been stagnating if not decreasing for the past decade and several factors have been implicated including infectious diseases. To investigate the reports of disease and mortality outbreaks in 16 tilapia farms in the country, a field survey was performed 2018-2019 where the husbandry and health management practices of tilapia farms were described and biological samples were collected simultaneously, for disease diagnosis and confirmation. The majority of the tilapia farms were freshwater intensive production systems, where analysis of the farm level data showed, a tendency to overstock the systems initially, resulting in low survival levels post-stocking. Although the farming systems varied in their production capacity and management, nearly all farms used commercial feed with little or no comprehensive biosecurity practises being applied. From the survey interviews of this study, data showed that farmers recognised fish diseases using clinical signs and reported mortality outbreaks occurring during March to June, which coincides with increased water temperature.

The fish samples collected exhibited a wide range of gross clinical signs, compatible with bacterial septicaemia. Bacterial isolation and identification recovered Gram-positive Streptococcus agalactiae and S. iniae strains as well as Gram-negative memebers of the motile Aeromonas spp. dominated by A. veronii. Histopathological analysis revealed varied pathologies consistent with bacterial infection. These include haemorrhage, congestion, infiltration of inflammatory cells with necrosis of the cells and inflammatory reactions. The observed pathologies supported the observed clinical signs exhibited by the diseased fish samples.

The Gram-positive bacterial recovery data showed a higher prevalence for S. agalactiae compared with S. iniae, although both species were identified from diseased fish in the farming systems. The higher prevalence of S. agalactiae was similar to findings from other Southeast Asian countries. There were two serotypes of S. agalactiae identified, namely serotype Ia and serotype Ib which were both circulating within the tilapia systems investigated. Main histological lesions found in tilapia samples infected with S. agalactiae and S. iniae were meningitis and encephalitis. The kidney, spleen, heart, and liver were filled with Gram-positive cocci with numerous infiltration of inflammatory cells indicative of bacteremia. The streptococci isolates were all susceptible to globally licensed antibiotics except for oxolinic acid which they have an intrinsic resistance. Additional antibiotic resistance genes were not detected in any strains investigated in this study. Virulence genes necessary for adhesion, invasion and immune evasion were all present in the isolates indicating their virulent nature.

As expected, several species of motile aeromonads were recovered and identified from moribund tilapia. The motile Aeromonas spp. were positively identified as A. veronii, A. caviae and A. dhakensis. The histological lesions in fish where the motile aeromonads were recovered were typical of motile aeromonas septicaemia as described in previous studies. The isolates were intrinsically resistant to all β-lactam antibiotics. Resistance to oxytetracycline, tetracycline, erythromycin and sulphamethoxazole-trimethoprim were observed mostly in A. veronii. The antbiotic resistant genes tetE and sul1 together with the class 1 integron gene IntI1 were detected in several A. veronii strains. Multpile virulence factors were detected in the isolates but were dominated by the cytotoxic enterotoxin act and the aerolysin aer which are important in establishment of MAS in fish. There were several genospecies based on presence of virulence genes showing the varied mechanism employed by these pathogens supporting the complexity of the pathogenesis of MAS in fish.

The findings of the present study highlight the contribution of infectious bacterial disease in the stagnating tilapia production in the Philippines. This study established the aetiological agents namely S. agalactiae, S. iniae and motile Aeromonas spp. especially A. veronii as the main cause of reported disease and mortality outbreaks.