What does it take to cause a disease outbreak?
If a disease agent such as the bacterium Aeromonas salmonicida and a susceptible host, such as the brook trout (Salvelinus fontinalis), or the brown trout (Salmo trutta) are in proximity to one another in the same rearing unit, or the same stream, is disease inevitable? Are there other factors involved that can influence whether or not a disease outbreak occurs when a disease agent and a potentially susceptible host are in the same water?
These are old questions, but they have been given new attention recently after the New Jersey Division of Fish and Wildlife “humanely euthanized” over 200,000 brook trout and brown trout at Pequest Trout Hatchery. Was this action necessary in response to an outbreak of furunculosis, caused by A. salmonicida?
Furunculosis is so-named because of the boil-like “furuncles” on the surface of infected fish.
A generalized septicemia is typically involved and the disease can cause substantial losses in fish hatcheries and fish farms. A. salmonicida has been reported in fish from all continents, except South America. It is thought to an obligate pathogen requiring a live host animal. It is not known to infect any warm-blooded animals, such as domestic livestock, companion animals, or humans.
An aspect of the reports on the outbreak at the Pequest Hatchery that I considered very bothersome was the statement that the fish were euthanized to prevent the infected fish from infecting fish in the wild. However, the suspected source of the infection was thought to be wild fish that birds had been feeding upon. While it is standard practice to not stock hatchery-reared fish that show obvious signs of infection, the primary reason is that anglers would be very upset if they caught a fish covered with furuncles. The disease agent is already present in the wild; although stocking diseased fish would probably increase the incidence of disease-carrying fish in the wild populations.
Almost all, probably all, disease agents known to infect fish have originated in wild fish populations, not hatcheries. Other common disease agents, such as Renibacterium salmoninarum, which causes bacterial kidney disease, have been found in salmonid fishes worldwide, including polar regions. Proximity of potentially susceptible fish and a potential disease agent does not automatically lead to disease. The greater density of fish in raceways, tanks, and ponds, combined with various stress factors, make fish in hatcheries and farms more susceptible to disease, but, the hatcheries and farms are not the original sources; contrary to the beliefs of “hatchery bashers.”
Each disease situation is potentially unique, so it is inappropriate to second guess the New Jersey hatchery managers who decided to destroy the fish and disinfect the facility. Hatchery management and the attending fish pathologist suspect fish-eating birds that frequent the nearby Pequest River brought the pathogen into the hatchery through feces, or possibly from their feet and beaks as they attempted to prey on fish in the hatchery raceways. The published reports made no mention of the hatchery water supply, but the possibility of contaminated water should not be overlooked.
Unless the water supply comes from a well or an enclosed spring it must be considered as a potential source of the disease agent; a long history without serious disease problems notwithstanding. In my experience, water supplies have been more common sources of disease problems than birds or other predators.
Dr. S.F. Snieszko, the preeminent fish pathologist with the US Fish and Wildlife Service throughout the middle half of the 20th century, summarized disease for his many students as an interaction of the disease agent, a susceptible host, and the environment in a complex set of interactions. Very few pathogens are so virulent that they always cause disease in potentially susceptible hosts. An array of stress factors can make hosts more vulnerable to infection.
Fish in wild populations rarely experience a combination of stress factors and exposure to large numbers of highly virulent disease agents; therefore, there are relatively few large scale mortalities from disease agents in wild fish populations. Those fish that do succumb to disease disappear quickly; usually consumed by scavengers before humans see them. An unfortunate result of these natural situations is that many people who do not have complete background information jump to the conclusion that wild fish are always healthy and disease is a hatchery by-product.
The “moral to the story” in the New Jersey situation and others like it where the disease agent is ubiquitous in the wild is that disease outbreaks are rare in clean, well-managed hatcheries. A colleague with extensive fish health management experience once told an assembled group of fisheries administrators that they were the reason their hatchery system had so many disease problems. They had refused, repeatedly, to allocate funds for water supply protection and exclusion of wild predators. The result was dirty water, stressed fish, and easy access for potential disease vectors.
When disease agents are ubiquitous, the best management practices focus on clean water and fully enclosed rearing units. Vaccination may be economically feasible in some situations.
Post-infection treatments, including antibiotics, “humane” euthanasia, and post-disease disinfection are expensive, last resort options that should not be necessary if best management practices are employed. Highly virulent disease agents that are previously unknown in specific areas require different strategies. Quarantines and restrictions on movement of fish can be essential tactics to prevent these pathogens from being introduced into new locations. That was not the situation at the Pequest Trout Hatchery, but potential public relations issues seem to justify the actions that were taken.
One of the oldest “laws of fish husbandry” is: Clean water… few problems: Dirty water… nothing but problems.
— John Nickum