Systematic granulomas caused by a rickettsia-like organism in Nile tilapia, Oreochronuis niloticus (L.), from southern Taiwan

. A rickettsia‐like organism was observed in diseased Nile tilapia, Oreochronuis niloticus (L.), from southern Taiwan. Most affected fish were pale and lethargic with haemorrhages and ulcers on the skin. The most significant gross pathological changes were varying degrees of ascites, and enlargement of the spleen, kidney and liver. Marked white nodules (as ring‐shaped foci), varying in size, were found in these organs. A Gram‐negative, rickettsia‐like organism (RLO) frequently appeared as inclusions or within host cell intra‐cytoplasmic vacuoles. Fibrin thrombi, perivascular necrosis, chronic inflammatory cells with hypertrophy and RLO‐laden cells were characteristic of the disease. White nodules induced in experimental fish were similar to naturally infected cases and RLOs were reisolated in tissue culture using a CHSE‐214 cell line. This provides evidence that the systemic granulomas in Nile tilapia were caused by RLOs.