Specific pathogen causing Tilapia Diseases

Specific pathogen causing Tilapia Diseases

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.

Streptococcus

One of the most significant diseases in tilapia culture worldwide, and particularly in indoor systems, is caused by Streptococcus. The primary strain of strep infecting aquaculture facilities is suspected to be Streptococcus iniae, although other strains have been implicated and are in the process of being identified. 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.

Antibiotic Therapy
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.
Vaccines
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. It costs approximately 5¢ to vaccinate a tilapia. That’s over 60% of what it costs to buy the fingerling in the first place. 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. Additionally, each fish has to be individually vaccinated by hand.
The cost of not vaccinating fingerlings in a Strep 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.
Growth Effect
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’ve reached the market.
Market Effect
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 farmer's objective is to grow fish from fry to adult as fast and efficiently as possible. Streptococcus can single-handedly alter a farmer's ability to control his or her own commercial destiny. It's not worth the risk.

Aeromonas

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 base of 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 farmer 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

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 primary job - absorbing oxygen, giving off carbon dioxide, excreting ammonia, and maintaining chemical balance between their body and the environment. Trichodina 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

Columnaris is a disease caused by the myxobacteria, Flexibacter Columnaris. In general, tilapia really have to be significantly stressed to break with this organism. Systems that use outdoor surface water are at particular risk. Outbreaks generally result from temperature fluctuations, 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.

Disease is impacting the production of tilapia at fish farms worldwide, particularly in indoor systems. 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 . It should also specifically indicate that after examining a sample of at least 60 fish, that the fish are free of Streptococcus, Trichodina, Columnaris, and Aeromonas.
Any heath 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. However, they are still infected and very 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 have they taken to insure that they don’t give it to you.