What Is Yersinia ruckeri and What Does It Cause?
An overview of the *Yersinia ruckeri* bacterium, the fish disease it causes, and the scientific principles behind its management in aquatic environments.
Yersinia ruckeri is a bacterium responsible for significant economic losses in aquaculture, particularly in farming salmonid fish like rainbow trout. First identified in the 1950s, this pathogen causes a disease known as Enteric Redmouth Disease, or ERM. The disease affects both farmed and wild fish populations across the globe.
Understanding the Yersinia ruckeri Bacterium
Yersinia ruckeri is a rod-shaped, Gram-negative bacterium from the Enterobacteriaceae family. As a facultative anaerobe, it can survive in environments with or without oxygen. The bacterium is classified into different serotypes and biotypes based on the molecules on its outer surface.
This diversity is significant because different strains exhibit varying levels of virulence, or the ability to cause disease. For instance, the serotype historically associated with the most severe outbreaks in salmonids is O1a. The existence of multiple serotypes complicates disease management, as control measures like vaccines must target the specific type of Y. ruckeri in a fish population.
Enteric Redmouth Disease in Fish
When Yersinia ruckeri infects a fish, it causes a systemic infection called Enteric Redmouth Disease (ERM). The name comes from one of its most visible symptoms: a distinct reddening around the mouth, jaw, and throat from subcutaneous hemorrhaging. Hemorrhages can also appear at the base of fins and within the eyes, sometimes causing a condition known as “popeye” (exophthalmia).
The infection begins as bacteria enter the fish’s body, often through the gills, before spreading into the bloodstream. This leads to septicemia, where the bacteria colonize internal organs like the spleen, liver, and kidney. In later stages, the fish’s lower intestine can become inflamed and filled with a thick, yellowish fluid. Infected fish often become lethargic, lose their appetite, and may swim near the water’s surface.
Transmission and Affected Populations
Yersinia ruckeri affects a broad range of hosts, including commercially farmed species such as Atlantic salmon, turbot, seabass, and seabream. The pathogen has a widespread geographical distribution and has been identified in aquaculture operations in the Americas, Europe, Australia, and Asia. It is found in both freshwater and marine environments.
Transmission occurs mainly through direct contact with infected fish or exposure to contaminated water. Fish that survive an infection can become asymptomatic carriers, shedding the bacteria into the environment and spreading it to other fish. Yersinia ruckeri can also persist in the aquatic environment by forming biofilms, which are communities of bacteria attached to surfaces, making eradication difficult.
Detection and Management of Yersinia ruckeri
A preliminary diagnosis of ERM can be made based on visible signs in the fish population. For a definitive diagnosis, laboratory methods are required, such as culturing the bacteria from tissue samples or using molecular techniques like polymerase chain reaction (PCR) to identify the pathogen’s genetic material.
Preventing and controlling ERM in aquaculture relies on a combination of strategies. Vaccination is a primary tool, with several effective vaccines available to protect fish against specific strains. In addition to vaccination, good husbandry and biosecurity are preferred for prevention. Key management practices include:
- Maintaining high water quality.
- Avoiding overcrowding by managing stocking densities.
- Using effective vaccines developed for specific strains.
- Administering specific antibiotics to treat an outbreak if it occurs.