BACTERIA Bacterial Gill Disease I. Causative Agent and Disease predispose fish to infection by these Bacterial gill disease (BGD) is bacteria that are present at low levels in most commonly caused by filamentous the aquatic environment. BGD typically bacteria within the genus Flavobacte- can occur in the spring with the feeding rium (most often F. branchiophilum). In of starter mash that irritates delicate gill previous taxonomy these bacteria were tissues of swim-up fry. The resulting gill known as members of the Myxobacte- hyperplasia (excessive cell division and ria and were first placed in the genus thickness) interferes with normal gas ex- Cytophaga, later changed to Flexibacter change while secondary infections from and now Flavobacterium. The syndrome fungus or other opportunistic pathogens of this disease includes swollen gill may occur. lamellae caused by proliferation of the epithelial cells sometimes resulting in IV. Transmission lamellar fusion. The epithelial prolifera- Transmission occurs horizontally tion is a response to irritation from the through the water from fish to fish. Pre- large numbers of filamentous bacteria disposing factors for epizootic outbreaks found attached to the gill surface. The are sub-optimal environmental condi- thickened epithelial layer results in tions and suspended solids or abrasive decreased gas exchange for respiration feeds. The incubation period can be as triggering explosive epizootics with high little as 24 hours or up to several weeks, fish mortality of up to 25%. most commonly during periods of colder water temperatures below 5°C. II. Host Species All cultured salmonids are suscep- V. Diagnosis tible and the disease is found worldwide. Fish with BGD have pale, swol- In Alaska, sockeye, Chinook and coho len gills, flared opercula, are listless salmon appear to be most susceptible. and do not feed well. Large numbers Adults and yearlings are less susceptible of filamentous rod-shaped bacteria are than fry and fingerlings. found attached to the gills causing epi- thelial hyperplasia and possibly fusion or III. Clinical Signs clubbing of gill lamellae. The causative Fish with BGD show a loss of ap- filamentous bacteria are Gram-negative, petite, orient to the water current for non-motile (or have gliding motility) and increased flow over the gills and exhibit grow on Cytophaga or TYES agars. exaggerated opercular movements. An increase in mucus on the head and upper VI. Prognosis for Host body may also be noted. BGD usually Early intervention in the progression affects fry or fingerling salmonids in of the disease may reduce fish mortality high density culture conditions. There- which can be significant. In a hatchery fore, the disease is often associated with setting external chemical treatments sub-optimal water and environmental with hydrogen peroxide may help control quality such as overcrowding that result the bacteria. If gill tissue is severely in excessive ammonia, low dissolved damaged, fish may not survive the treat- oxygen levels and excess suspended ments. Preventative measures for BGD organic matter. Such stressors can include maintaining the water supply 18 BACTERIA free of fish (especially adults), mud and VII. Human Health Signifcance silt, reducing stress such as overcrowd- The causative bacteria of BGD are of ing, avoiding low dissolved oxygen or no human health concern high ammonia levels and avoiding exces- sive fish handling. Histological section of gill lamellar fusion (arrow) caused by external bacteria, X 100. Higher magnifcation showing flamentous bacteria (arrow) on gill, X 400. 19 .