Tilapia tolerate adverse water quality and other stressors better than most commercial aquaculture species. Because stress and environmental quality play such important roles in the disease process, tilapia are labeled as being very "disease-resistant." This means that in the presence of pathogens, tilapia are slow to break with disease.
Throughout the development of the global tilapia industry, growers have often leaned on the species' inherent disease resistance instead of implementing proven biosecurity measures developed for industries that grew less disease-resistant fish such as trout and salmon. During the formative years of the industry, there seemed to be no downside.
At one point, it was generally believed that there were few commercially significant tilapia diseases. This changed quickly during the 1990's. The surge in tilapia disease was the direct result of the introduction of novel tilapia pathogens via infected fish and because of the intensification (growing fish at higher production densities) of fish culture methods globally. Tilapia are being reared at higher densities than ever before, and more tilapia are being reared in recirculating systems than ever before. Although tilapia perform exceptionally well in recirculation systems, so do pathogens.
Once a pathogen is introduced into a recirculating system, it is difficult to eradicate. Eradication of a pathogen may involve depopulating, sterilizing, and repopulating the facility. In this process, money invested in the fish can be lost, and even after the "sterilization," the successful eradication of the target pathogens is not a certainty. During our twenty year tenure in the U.S. tilapia industry we've witnessed dozens of producers fail as a direct result of disease. The most serious tilapia disease is caused by two species of bacteria in the genus Streptococcus. We know of only three producers that have successfully rid their systems of this pathogen. The vast majority of the remaining infected producers failed.
In order to prevent disease, one needs to consider how the pathogens that cause disease reach a facility, and how they can then overwhelm the tilapia's natural resistance. A common means of introducing pathogens to a clean facility is by introducing contaminated fish. Once a pathogen reaches a facility, it may be able to multiply geometrically. Recirculating systems are often an ideal environment for the pathogens to multiply - warmth, nutrient-rich water, lots of places to hide, and plenty of hosts.
Water from infected facilities can also carry pathogens. Dripping live-haul trucks that travel from one farm to another is a source of a wide variety of pathogens. Pathogens can enter a facility on the soles of employees shoes, on a live-hauler's dip net that was used at another facility, or on the hands of a truck driver that is allowed to dip his finger in a production tank. While no producer can live in a bubble, by addressing the obvious routes of pathogen transfer (fish, water, employees' hands and shoes), a producer can dramatically reduce the risk of becoming infected.
The first step in disease prevention is to buy fingerlings from a reputable source. A producer can further reduce his or her risk by implementing the following simple methods:
- maintain good fish nutrition
- avoid over-crowding
- maintain good personal hygiene
- hand-washing with antibacterial soaps
- disinfectant foot baths
- live-haul truck disinfection
- limit fish introductions
- use well or municipal water wherever possible
- limit visitors
The clinically significant tilapia pathogens fall into the general categories of viruses, bacteria, and protozoa. Mycotic (fungal) diseases are only significant if the tilapia are under constant stress. In certain systems, metazoan ectoparasites and endoparasites cause problems, but do not significantly impact the tilapia industry.
One of the most significant diseases in tilapia culture worldwide, and particularly in indoor systems, is caused by bacteria in the genus Streptococcus. The two primary strains of streptococcus affecting tilapia producers are Streptococcus agalactiae and Streptococcus iniae. This disease results in the clinical signs of generalized hemorrhagic septicemia such as:
- Lethargy, weakness, loss of appetite, red discoloration at the anus and base of fins, hemorrhagic eyes, gills, internal organs, and muscle, blood tinged abdominal fluid, and swollen kidney, spleen, and liver.
- Streptococcus has additional clinical signs including an erratic spiral swimming motion, a curved body, corneal opacity in one or more eyes, exopthalmia (protruding eyes), and abdominal distention.
Streptococcal infections respond to antibiotic therapy, but since the withdrawal period for all effective antibiotics is longer than it takes for the streptococcal infection to return, the disease cannot be legally controlled with antibiotics all the way to market. Furthermore, it is only a matter of time before strep develops resistance to the antibiotics now used. Streptococcal strains at several facilities have already developed resistance to some antibiotics.
Injectable vaccines are being developed in earnest, and initial results seem promising. However, it is not confirmed that vaccinated fish in infected facilities perform as well as unvaccinated fish in uninfected facilities. Currently, the vaccines have to be custom-developed from the strain of strep at each facility.
Vaccines are also expensive - nearly equivalent to the cost of the fingerling itself - and since the fingerlings can't be vaccinated until they reach 20 grams, they are still vulnerable to strep for their first month on the farm. Each fish must be individually vaccinated by hand.
The cost of not vaccinating fingerlings in a Streptococcus infected facility is even greater. Mortalities of up to 75% have been observed on some farms, although the highest mortality rate that we have heard of in a large commercial operation is 40%. One operation was reportedly losing 4,000 market-sized animals per day during a severe outbreak.
Strep also severely reduces the appetite of the fish, thereby significantly reducing their growth rates. It is not uncommon for a 7-8 month growout in a clean facility to stretch to 10-12 months in an infected facility - and the end products of the two are like apples and oranges. Fish from infected facilities that make it to harvest without coming down with strep don't tolerate live haul as well as healthy fish and have markedly reduced shelf life once they reach the market.
The physical appearance of infected and uninfected fish in the market place can be vastly different. It is not uncommon for infected fish being held in live tanks at Asian stores to be missing one or both eyes, be covered with patches of fungus, and have hemorrhages all over their bodies. Infected fish don't last well, sell well, or market well. A producer's objective is to grow fish from fry to adult as fast and efficiently as possible. Streptococcus can single-handedly alter a producer's ability to control his or her own commercial destiny. It's not worth the risk.
Another bacterial disease that has significantly impacted production at some farms is the disease aeromonad septicemia ("Aeromonas"). This disease is caused by the bacteria Aeromonas hydrophila. Much like Streptococcus, Aeromonas results in the clinical signs of generalized hemorrhagic septicemia such as lethargy, weakness, loss of appetite, red discoloration at the anus and the base of the fins, hemorrhagic eyes, gills, internal organs, and muscle, blood tinged abdominal fluid, and swollen kidney, spleen, and liver. Aeromonas generally affects systems that have systemic poor water quality or over-crowding. In other words, a producer really has to be abusing the fish, or have another nasty pathogen in his or her system, to break with Aeromonas. Aeromonas temporarily responds to antibiotic therapy, but if a farm has Aeromonas, they really need to either change their source of fingerlings, or drastically improve their husbandry, whichever is to blame. As always, avoid getting fish from infected stocks at all costs.
Trichodina, or "Trich", is a protozoan parasite that has severely impacted production at many facilities. Trich can result in extremely high mortality rates, particularly in young fish. The parasites heavily infest the gill and body surfaces of infected fish. Infected fish display flashing (swimming against floors of tanks to scrape parasites off), rapid breathing, weakness, and uncoordinated swimming. Since trich attacks the gills, the gills are less efficient in doing their job - absorbing oxygen, giving off carbon dioxide, excreting ammonia, and maintaining chemical balance between their body and the environment. Trhichodina populations can be temporarily controlled with copper sulfate and salt (forget doing hydroponics) or formalin (bye bye, biofilter). Treated fish remain carriers even after treatment, and much like Streptococcus, it is nearly impossible to eliminate trich from a system once it has been introduced. Any fish that come to you from outdoor ponds should be carefully examined for trich before letting them on your premises.
Columnaris is a disease caused by the myxobacteria, Flexibacter columnaris. In general, tilapia must be significantly stressed before this organism will cause disease. Systems that use outdoor surface water are at particular risk. Outbreaks generally result from temperature fluctuatioins, trauma, and poor water quality. Crowding and poor nutrition further increase the severity of the disease. Infected fish generally show lethargy, anorexia, weak swimming, and mortality. Additionally, raised white patches appear on the skin or fins. These patches may later develop into ulcers. Certain antibiotics, copper sulfate, and potassium permanganate are reported to be effective for temporary treatment.
The only currently significant viral pathogen that we are aware of is an irido-like virus that has been traced to fish from a single fish producer in the U.S. This virus has been credited with massive, synchronized die-offs at infected facilities. A vaccine has been developed that is administered through intraperitoneal injection.
Disease impacts the production of tilapia worldwide. You can dramatically reduce the risk of introducing pathogens by implementing simple biosecurity and management measures. A clean facility should begin with uninfected fish. You also have the right to insist on examining a current health inspection that has taken place within the last six months. The inspection should be performed by a certified pathologist from a USDA accredited aquatic disease diagnostic laboratory for it to be considered valid. It should also specifically indicate that after examining a sample of at least 60 fish, all specimens are free of significant bacterial, parasitological, and certain viral pathogens.
Any health inspection that only states that there is "no indications of disease," or an equivalent statement, is not credible and should not be trusted. Infected fish often do not show clinical signs of disease despite being infected and capable of transmitting a pathogen to your facility.
Don't be shy about specifically asking your fingerling producer if they have ever had Streptococcus, Trichodina, Columnaris, or Aeromonas. If they have, ask them if and how they have gotten rid of it and what measures they have taken to insure that they don't give it to you. Visual veterinary inspections have little clinical significance.