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Recently approved vaccine aids in control of disease
Nile tilapia is a burgeoning industry around the world, including Brazil, but intensified production has brought about stress and the emergence of disease.
One of the major health problems facing the industry is streptococcosis due to the bacterium Streptococcus agalactiae Biotype II, an infection that results in significant losses because it tends to strike fish well into the grow-out period, after considerable resources have been invested, says Rogério Salvador, MV, PhD, a professor at the Agrarian Science Center, North Paraná State University, Campus Luiz Meneghel, Brazil.
The geographic distribution of S. agalactiae includes regions with temperate, tropical weather, where warmwater fish are cultivated. Outbreaks have been reported in countries around the world.
Epidemiologic studies conducted in different regions of the world demonstrate that 56% of streptococci isolates are S. agalactiae Biotype II (see sidebar). In Brazil and the rest of Latin America, serological studies show that S. agalactiae Biotype II has been the only S. agalactiae strain isolated. In addition, the largest economic impact due to S. agalactiae in freshwater-farmed fish species occurs in Nile tilapia, he adds.
“Mortality on one farm can reach 90% at pre-market age,” Salvador says. Besides mortality, signs of the disease include bulging eyes, swimming in circles, a swollen belly and nodular lesions. Significant declines in feed conversion and weight gain are also common in infected fish.
Infection spreads when affected fish — dead or alive, moribund or apparently healthy — release the bacterium into the water, allowing it to colonize the skin of other fish. Invasive infections can also occur, resulting in high mortality. The bacterium can also survive for long periods of time in water, in mud or ponds and even on pieces of equipment used in routine operations, he says.
Complicating matters has been the development of some resistance to multiple antibiotics, which makes the advent of vaccines to control this pathogen desirable, says Salvador, also a veterinarian and director of his university’s veterinary hospital.
Study shows vaccine effective
Toward this end, the professor and colleagues evaluated the efficacy of an inactivated, oil-adjuvanted vaccine called AquaVac® Strep Sa, developed by MSD Animal Health.
The vaccine, which has since been granted regulatory approval for use in Brazil, was developed as an aid in protection against Streptococcus agalactiae infections in tilapia and other susceptible fish species.
When administered intraperitoneally as a single dose to fish weighing no less than 15 grams, AquaVac Strep Sa initiates protection at about 28 days that lasted for at least 30 weeks (with temperatures at 28° C), according to MSD Animal Health.
Salvador’s study with the vaccine involved 180 Nile tilapia juveniles and was conducted at the Fish Immunopathology Laboratory at Paraná State University, where he is the coordinator. Fish weighing about 35 grams were vaccinated with AquaVac Strep Sa, then were challenged with S. agalactiae obtained from natural infections.
Vaccinated fish were protected against challenge and the difference between vaccinates and controls was statistically significant, Salvador says.
‘Important tool’
“Results from this study demonstrated that AquaVac Strep Sa induced effective protection in Nile tilapia experimentally challenged with S. agalactiae. We, therefore, concluded that AquaVac Strep Sa is a safe and highly efficacious vaccine against the disease caused by S. agalactiae Biotype II.
“One single dose of AquaVac Strep Sa, administered as directed, can be an important tool in the prevention and control of streptococcosis in Brazil, since serological results so far show only the presence of Biotype II in this country,” says Salvador, who presented the results of his study at a conference on bacterial disease in warmwater fish held in conjunction with the World Aquaculture Society Conference in June 2011.
Mortality observed in vaccinated fish challenged with the pathogen was significantly lower compared to unvaccinated fish that were challenged (Figure 1) and the vaccine was demonstrated to be safe, he says.
Field trials
Rodrigo Zanolo, MV, MSc, of São Paulo, a veterinarian and technical representative for MSD Animal Health, says that two controlled field trials in Brazil were also conducted at separate commercial tilapia-production sites in the Northwest São Paulo state to evaluate the safety, efficacy and economic benefits of the vaccine.
The trials were conducted in July 2010 as an experimental vaccine program, before approval of AquaVac Strep Sa in Brazil, because both farms had a history of acute mortality and a positive diagnosis of S. agalactiae Biotype II.
On Farm 1, there were 30,000 fish in the study and on Farm 2, there were 32,000 fish in the study. Both farms used similar production systems, based on 18-m³ cages. On each farm, half the fish were vaccinated and the other half were not vaccinated and served as controls. In both trials, the fish weighed an average of 40 to 60 grams, and were vaccinated intraperitoneally with a single 0.05 mL dose of AquaVac Strep Sa, according to label directions.
After vaccination, fish were monitored daily until harvest for their behavior, daily mortality and growth and during the course of the trial, microbiological and serological studies were performed with samples from all experimental groups, Zanolo says.
The use of AquaVac Strep Sa proved to be safe for fish, reduced mortality and improved fish performance (Table 1), he says. Zanolo notes that final average bodyweight was higher in fish from the control group because there was more mortality, which reduced fish density and enabled better growth among surviving fish.
The reduction in S. agalactiae Biotype II (serotype Ib)-associated mortality due was significantly less (p < 0.05) than mortality among controls. Vaccinated fish also had improved biomass gain, higher performance and a lower feed conversion ratio compared to controls (Table 2), Zanolo says.
Furthermore, increased survival, and consequently a decreased feed conversion ratio, resulted in consistently higher economic returns. After discounting vaccination costs, the return on investment (ROI) in Trial 1 was 7.4 times higher when compared to the investment in the vaccine. In Trial 2, ROI for vaccination was 5.3 times higher.
“AquaVac Strep Sa represents a technological breakthrough that is expected to help Brazil’s tilapia producers reduce losses from disease and make production even more sustainable in the future,” he says. “Considering the tremendous economic impact of streptococcosis on Brazilian tilapia farms, AquaVac Strep Sa is an indispensable tool for boosting the productivity, performance and economic viability of tilapia production in Brazil.
Zanolo says that vaccination results should be even more dramatic in commercial production, since protecting large fish populations could result in decreased disease pressure in the field, which can be reflected in overall benefits for intensive tilapia farms.
Proceedings from the conference on bacterial disease in warmwater fish can be obtained from MSD Animal Health representatives.
Most strep isolates found to be S. agalactiae Biotype II
Only 26% of all streptococcal isolates of tilapia were S. agalactiae Biotype I while 56% were S. agalactiae Biotype II, according to epidemiological surveys conducted by MSD Animal Health.
The surveys, initiated in 2000, were conducted in the major tilapia-producing regions of Asia and Latin America. Over 1,000 bacterial isolates from tilapia reared at 74 sites in 14 countries were identified.
As other investigations have found, streptococcal species were the dominant bacterial pathogens, accounting for more than half of all bacteria identified. However, it is interesting that while Streptococcus iniae is the most commonly reported pathogen of fish, MSD Animal Health data show that S. agalactiae is the more prevalent of the two in tilapia.
S. agalactiae biotype prevalence
Detailed analysis of the isolates has revealed two distinct clusters that differ in a variety of biochemical and phenotypic characteristics. These clusters are referred to as biotypes. One type is the typically beta-haemolytical “classical” S. agalactiae (Biotype I) and the other is the typically non-beta-haemolytical S. agalactiae (Biotype II).
S.agalactiae Biotype II is considered the most globally significant of the biotypes, with chronic mortality in many Asian and Latin American countries; S. agalactiae Biotype I is limited to Asia and displays acute mortality peaks, often associated with higher temperatures.
