Infection rates in fish farms by season in 2014
This study investigated the state of emaciation infection in 900 olive flounders collected from 60 fish farms in April, May, September, November, and December 2014, in Jeju Island, South Korea, a prime cultivation site of olive flounders. Although there were symptomatic differences in each fish infected with emaciation, which is seriously affecting the olive flounder farms in the Jeju region, the main external symptoms were darkening of the body color and severe emaciation of the abdominal area. Internal symptoms showed that hepatorrhagia and severe kidney nebula were observed (data not shown).
As a result of the 2014 investigation on the state of infection of olive flounders, 26 fish (14.4%) of 13 fish farms in April, 12 (6.6%) of 11 fish farms in May, 44 (24.4%) of 32 fish farms in September, 36 (20%) of 23 fish farms in November, and 78 (43.3%) of 42 fish farms in December showed a positive PCR reaction (Table 4 and Fig. 2). In addition, various rates of infection by season were shown from spring to winter (Table 4). Among these, the highest infection rate occurred from September (fall) to December (winter), when water temperatures began to decrease (Table 4).
Infection rates by size in 2014
Based on the detected genes, the rate of infection according to the size of the fish was investigated. Consequently, in April, according to fish size, 0/14 fish farms (0%) in the < 10 cm, 4/18 (22.2%) in the 11~20 cm, 8/18 (44.4%) in the 21~30 cm, and 1/10 (10%) in the > 31 cm groups, respectively, were infected. The corresponding infection rates in May were 1/7 fish farms (14.2%) in the < 10 cm group, 3/31 (9.6%) in the 11~20 cm group, 6/11 (54.5%) in the 21~30 cm group, and 1/11 (9%) in the > 31 cm group. Thus, in April and May, the rate of infection was low for the group > 31 cm.
In September, the infection rates were 6/6 fish farms (100%) in the 11~20 cm group, 21/38 (55.2%) in the 21~30 cm group, and 5/16 (31.2%) in the > 31 cm group. Infection rates in November were 0/1 fish farms (0%) in the < 10 cm group, 2/2 (100%) in the 11~20 cm group, 16/32 (50%) in the 21~30 cm group, and 5/25 (20%) in the > 31 cm group. Comparatively, high infection rates were detected in December, with 5/5 fish farms (100%), 20/28 (75%), and 16/27 (62.9%) in the 11~20, 21~30 and > 31 cm groups, respectively. Based on the epidemiological survey conducted from April to December, the rate of infection of domestic emaciation was highest in fish between 21~30 cm, but it was evident that all sizes of olive flounders were susceptible to infection (Table 4).
Infection rates in 2015
As a result of the state of emaciation infection by each fish farm in 2015, the number of fish farms detected with emaciation were 30/60 (50%) in March~April, 9/55 (16.3%) in May~June, 47/52 (90.3%) in July~August, and 45/53 (84.9%) in October (Table 5 and Fig. 2). In addition, infection rates by fish size were 0/15 (0%) in the < 10 cm group, 13/23 (47.8%) in the 11~20 cm group, 9/11 (81.8%) in the 21~30 cm group, and 7/11 (72.1%) in the > 31 cm group, in March~April. In May~June, the corresponding values were 0/6 (0%), 3/29 (10.3%), 3/13 (23%), and 3/7 (42.8%) in the < 10, 11~20, 21~30 and > 31 cm groups, respectively (Table 5).
In addition, in July~August, the infection status in the fish farms, based on fish size, were 3/3 (100%) in the < 10 cm group, 19/20 (95%) in the 11~20 cm group, 20/23 (86.9%) in the 21~30 cm group, and 5/6 (83.3%) in the > 31 cm group. Corresponding values in October revealed 1/4 (25%), 14/16 (87.5%), 13/14 (92.8%), and 17/19 (89.4%) in the < 10, 11~20, 21~30, and > 31 cm groups, respectively. Through the epidemiological survey conducted from March–October, the occurrence of infection was equivalent to that of 2014 in all sizes. However, in 2015, the highest infection rate was observed in fish > 31 cm (Table 5). In addition, periodic infection rates throughout the year were found to be the same as 2014 (Table 5), but a higher infection rate was evident in 2015 than in 2014. Among the months, the highest infection rate was observed in July~August, when the water temperature is high, and in October, when the water temperature starts decreasing (Table 5).
Infection rates by weight in 2014 and 2015
Infection rate in accordance with olive flounder weight in 2014 and 2015 demonstrated that olive flounder of PCR positive with the EM-F/R primer set was lighter 30–40% compared to olive flounder of PCR negative (Fig. 3).
Histopathological examination of the diseased olive flounder
Olive flounder with PCR positive showed numbers of 4–9 μm round or egg-shaped spores in the kidneys, some parts of destroyed or metamorphosed plasmodium including glomerulus, and nucleus were observed (Fig. 4b). Normal shape of kidneys was found in olive flounder with PCR negative (Fig. 4a).