Lactococcus garvieae

Lactococcus garvieae is a known fish pathogen affecting saltwater fish in the Far East, specifically in rainbow trout, Japanese yellowtail, and grey mullet (Mugil cephalus). This bacteria causes lesions in the vascular endothelium, leading to hemorrhages and petechias at the surface of internal organs.As few as 10 bacterial cells per fish can cause an infection. L. garvieae is isolated in saltwater fish in the Far East and specifically in European Rainbow Trout. Lactococcus garvieae is usually identified within aquatic species. However, it has also been found in subclinical intramammary infections in cows, subclinical mastitis in water buffalos, poultry meat, raw cow's milk, meat products, porcine blood from industrial abattoirs and from cat and dog tonsils. Lactococcus garvieae was first discovered in rainbow trout raised on a Japanese fish farm in the 1950s. In 1988, L. garvieae was identified in the rainbow trout from Spanish fish farms as well. In later years, L. garvieae was isolated from several septicemic processes in fish and phenotypical and molecular taxonomic studies confirmed the same agent as E. seriolicida. This species was reclassified as a junior synonym of L. garvieae. Lactococcus garvieae is a facultatively anaerobic, non-motile, non-spore forming, Gram-positive ovoid coccus, occurring in pairs and short chains. It can produce α-hemolysis on blood agar (BA). It has the ability to grow at 4-45 °C in media containing 6.5% sodium chloride (NaCl) at pH 9.6. Its optimal growth temperature is 37 °C for a 24‑hour period, while at 4 °C it needs between 12 and 15 days for premium growth. It also grows rapidly in rich media, such as brain heart infusion agar (BHIA), trypticase soy agar (TSA), BA, trypticase soy broth (TSB), and bile esculin agar (BEA). Conversely, L. garvieae does not grow on McConkey agar or Enterococcus agar. Optimum conditions for colony growth exists within brain heart infusion broth (BHIB) where conditions include a pH range of 7-8 and a temperature range of 25-30 °C. In fish models, all clinical forms of lactococcosis show an absence of clinical symptoms and mortalities in fish weighing under 80 grams. Smaller fish can be infected experimentally. In a follow-up study of the pathogenicity of L. garvieae, younger fish at 50 grams underwent a higher mortality than older fish at about 100 grams and the acute period of the disease was reported to be longer in young fish. Numerous investigations of L. garvieae pathogenicity have confirmed that capsulated strains, commonly classified as serotype KG−, are more virulent than non-capsulated strains, commonly classified as serotype KG+. The appearance of the disease is affected specifically by factors of the aquatic environment such as temperature and water quality. Water temperature affects the disease seasonally as the climate changes, specifically when the water temperature is over 18 °C (64 °F). Infection is linked to water temperatures over 18 °C, although acute outbreaks have been discovered in water temperatures of 14–15 °C. Low water quality caused by poor sanitary conditions has been shown to influence evolution of infection. The disease becomes more distinct when the immediate aquatic environment is poor, and oxygen deficiency increases virulence of the agent. Likewise, excessive ammonium concentration causes an increase in mortality of fish.