Experimental hexamitiasis in the oyster Crassostrea virginica

In vitro growth characteristics of Hexamita sp. and course of experimental hexamitiasis initiated in oysters via the oral and intracardial route were used in an attempt to analyze the controversial issue of whether Hexamita is an oyster pathogen. In culture, the growth of Hexamita is temperature-dependent; the flagellate multiplies 1000 times at 6° and 12°C in 15 and 6 days, respectively. At 18° and 25° its growth is limited to 100 times to 10 times in 3 to 4 days. When 3.2 × 10 6 Hexamita were injected into the oyster intracardially at 6°C, the flagellates increased in numbers after a lag period of 8 days and all oysters died within 18 days. After injection of 4.0 × 10 5 Hexamita per oyster at 12°C 9 of 10 died within 20 days. In the one survivor, the count of Hexamita in heart blood decreased steadily after the crisis on the 8th day as the oyster overcame the infection. When the dosage injected was reduced to 1.0 × 10 3 Hexamita per oyster and the oysters were kept at 12°C, half survived. At 18°C when 8.8 × 10 5 Hexamita were inoculated into the oyster, not even a patent infection was observed in the 2-week period. No conclusive results were obtained from a series of experiments designed to ascetain whether Hexamita sp. introduced into the oyster via the oral route was able to breach the epithelial barrier. However, implications are discussed. The data demonstrate conclusively that physiological conditions of the oyster, the reproductive capacity and the pathogenicity of the microorganism, and the proper manipulation of experimental conditions (e.g., ambient temperature, size of inoculum, route of entry, etc.) can alter the course of infection.