The Disease

Introduction
Aquaculture is the world’s fastest growing food sector and tilapia represents much of that growth. The popularity of tilapia continues to skyrocket. Global production has nearly tripled since the beginning of the decade with an estimated output of 3.7 million tons in 2010, according to the Food and Agriculture Organization (FAO) of the United Nations. No other fish species has displayed such aggressive and sustainable growth, year after year.

Tilapia can now be considered a commodity item with a stable supply, demand and price. However, the cost of raw materials is increasing, which is increasing the cost of production and reducing profit margins. Further advancements in production efficiency are required to improve profitability, and this trend is continually driving the industry toward consolidation and intensification.

Experience has shown that most intensive fish farming operations suffer from between six and eight major production diseases and that these must be prevented or controlled before the industry can become truly sustainable. In tilapia, we have so far identified four major bacterial diseases:Streptococcus agalactiae,Streptococcus iniae, Flavobacterium columnareandFrancisellaspp.


Streptococcus: An established pathogen
By far the most important diseases economically are those due to streptococcus. In many instances, streptococcosis does not contribute the highest mortality, but it kills large fish and, as a consequence, heavily affects the FCR, reduces marketable product and damages production and processing efficiency (Table 2).


Etiology
Streptococcus iniae(S. iniae)is a significant fish pathogen causing disease and mortality in many cultured fish species in both tropical and sub tropical environments. There is a large body of literature on the pathogenesis of this organism in a variety of fish species. Much less information is available for fish-pathogenicStreptococcus agalactiae(S. agalactiae), another significant species. Although more commonly associated with disease in human and bovine hosts, fish-pathogenicS. agalactiaehas been documented from as early as 1966, when a non-hemoltyic Group B streptococcus was identified as the cause of two epizootics in golden shiners (Notemigonus crysoleucas).1With the intensification of aquaculture,S. agalactiaehas been found to be an increasingly significant cause of mortality and morbidity in both marine and fresh water cultured species, particularly in tilapia.


Prevalence
Based on the data collected from the countries sampled by MSD Animal Health,S. agalactiaeis the more prevalent of the two streptococcal species (Table1).

Table 1. Prevalence of Streptococcal Isolations from Tilapia

Table 1. Prevalence of Streptococcal Isolations from Tilapia




Detailed analysis of the tilapiaS. agalactiaeisolates shows two distinct clusters which differ in a variety of phenotypic and biochemical characteristics. These can be referred to as different Biotypes and on this basis they can be differentiated between beta-haemolytic ‘classical’S. agalactiae (S. agalactiaeBiotype 1) and non-beta-haemolyticS. agalactiae (S. agalactiaeBiotype 2). These strains were also classified for some time asS. difficile/difficilisbut has recently been re-classified as non-haemolytic variants ofS. agalactiae.2,3,4

To date the epidemiological studies undertaken by MSD Animal Health have shown (Table 2) that 56% of all streptococcal isolates were identified asS. agalactiaeBiotype 2 and only 26% were Biotype 1. The significance of these findings on vaccine development was explored further by MSD Animal Health.


Table 2. Prevalence of Streptococcal Isolations from Tilapia
Table 2. Prevalence of Streptococcal Isolations from Tilapia



When the data are cross-referenced with the site locations of the contributing tilapia operations, features emerge which confirm the prevalence of individual biotypes in specific regions. It should be noted that this is not a true epidemiological study since some sites were visited on multiple occasions and therefore over represented in the data set. Nonetheless, this represents a detailed analysis of the impact of streptococcal disease in tilapia.

In the key production areas of Central and Southern America and ChinaS.agalactiaeBiotype 2 (&S. iniae) predominate.

Biotype prevalence varies in Asia with biotypes linked to specific countries. In China and Indonesia, we seeS. agalactiaeBiotype 2, whereasS. agalactiaeBiotype 1 is the dominant streptococcal pathogen of tilapia in Thailand, Malaysia and Singapore.

S. iniaeis often found in association withS. agalactiaeBiotypes 1 or 2 in China, Ecuador, Honduras, Indonesia the Philippines and Vietnam. There are only a few countries where all three streptococcal strains have been identified, namely Vietnam and the Philippines.

There are currently no obvious geographical, physiological or environmental explanations for the country-to-country distribution ofS. agalactiaeBiotypes 1 and 2. However it is likely that the distribution will change in time possibly through trade in live fish. MSD Animal Health continues to monitor the situation, collecting and analysing strains from new disease outbreaks.






1Robinson JA and MeyerFP. Streptococcalfish pathogen.Journal of bacteriology1966; 9: 512.
2Eldar A, et al. Experimental streptococcal meningo-encephlitis in cultured fish.Veterinary Microbiology1995; 45: 33-40.
3Vandamme P, et al.Streptococcus difficileis a non hemolytic Group B, Type 1b Streptococcus.International Journal of Systemic Bacteriology1997; 24: 63-85.
4Kawamura Y et al. High genetic similarity ofStreptococcus difficilis:S. difficilisEldar et al. 1995 is a later synonym ofS. agalactiae.Lehmann and Neumann 1896 (approved lists 1980) InternationalJournal of Systemic and Evolutionary Microbiology2005; 55: 961-96.



Disease reprinted courtesy of OIE Diagnostic Manual for Aquatic Animal Diseases, OIE (World Organisation for Animal Health), Paris, France.