Determination of lethal dose of Aeromonas hydrophila RTMCX1 and In vitro efficacy of oxytetracycline hydrochloride in golden mahseer, Tor putitora (Hamilton, 1822)

Aeromonas hydrophila is considered as a major bacterial pathogen to fish. It's wide host susceptibility has caused huge economic losses in aquaculture. In the present study, the lethal dose (LD₅₀ 96 h) of Aeromonas hydrophila, RTMCX1 and its control through oxytetracycline hydrochloride was appraised in fry of golden mahseer, Tor putitora.

LD₅₀ 96 h of A. hydrophila, RTMCX1 in fry of golden mahseer was calculated through Probit model analysis. The total microbial load in muscle tissue of infected fry was estimated using standard microbial technique. The effect of oxytetracycline hydrochloride treatment was assessed at varied concentrations; 20, 40, 60 and 80 mg L⁻¹ in experimentally infected fry at 10⁸ CFU mL⁻¹ of A. hydrophila.

LD₅₀96 h of A. hydrophila, RTMCX1 in fry of golden mahseer was calculated as 10⁸ CFU mL⁻¹. The total microbial load in muscle tissue of infected fry was ranged from 10³-10⁷ CFU mL⁻¹. The post infection by the test bacterium demonstrated 25% of cumulative mortality in golden mahseer fry. The abnormal behaviour of the experimental fry was become normal and feed intake increased (scale 3) within 96 h of successive oxytetracycline hydrochloride bath treatment 1 h every day at 20 to 80 mg L⁻¹. Post 96 h oxytetracycline hydrochloride treatment at 40–80 mg L⁻¹ also demonstrated significant survival percentage (75–80%) in fry as compared to the group treated with 20 mg L⁻¹ and positive control (p<0.05). The present study determined the LD₅₀ of A. hydrophila at 10⁸ CFU mL⁻¹ in golden mahseer fry and its effective control through administration of oxytetracycline hydrochloride bath treatment 1 h at 40 mg L⁻¹ for 4 consecutive days.