Vaccination as a Disease Management Tool

Determining the Cost/Benefit Ratio

From both health and economic perspectives, it is far more beneficial to prevent a disease from happening than it is to treat the disease after it strikes. infectious disease outbreaks not only cause losses of valuable fish, they can affect appetite, which reduces the effectiveness of oral therapeutics, and may reduce growth. In addition, survivors may be low-level carriers of the pathogen, capable of infecting healthy fish in the population, and potentially perpetuating the disease cycle.

Farm operators should keep a running history of infectious disease outbreaks and water temperatures on their farms. New farms, or farms that have no disease history, can check with regional fish health specialists to find out what to expect in the way of infectious diseases in their area.

Knowing when the fish will first be exposed to a disease is important, because that will determine when to vaccinate. The ability of a fish to develop immunity to a disease by vaccination appears to be dependent on three main factors: (1) the size of the fish; (2) the water temperature; and (3) the method of vaccination. Environment and hygiene can also affect immunity. If fish health is compromised at the time of vaccination, immunity may be impacted.

The immune response to vaccination increases with fish size and with increasing water temperature, so timing of vaccination before exposure to a disease is important. Immunity develops better if the smallest fish weighs over 2.5 grams. When water temperatures are lower than 6°C, it is generally accepted that fish should not be vaccinated by the immersion method, as immunity may not develop. At 8°C, it will take about 5 weeks for immunity to develop, compared to 2 weeks at 12°C.

There are three basic methods of vaccinating fish: immersion, oral (in the feed), and injection.

If the fish will only be exposed to a disease for 3 to 12 months, and if they weigh less than 50 grams, immersion vaccination may be the easiest and least expensive method. it is also fast. The fish are dipped into a diluted, aerated vaccine solution for up to 60 seconds. Two people can vaccinate up to one-half million 5 gram fish per day. The cost is low, about $0.001 to $0.01 per 5 gram fish. For 50 gram fish, the cost would be ten times this, as the amount of vaccine used depends on the weight of the fish. The effectiveness is good, and it generally produces a 40% to 80% improvement in survival relative to unvaccinated fish.

Oral vaccines are used in the feed, and in most cases are intended as a "booster" following immersion vaccination. Oral vaccination is appetite dependent, and must be administered over a number of feedings. The cost is moderate, about $0.01 to $0.05 per 10 gram fish, but the duration of immunity may be only 2 to 4 months. The effective protection is fair, and a typical response is a 20% to 50% improvement in survival relative to unvaccinated fish.

Vaccination by injection (typically into the peritoneal cavity) is generally reserved for high value fish that risk exposure to one or more diseases over a long period of time (12 to 24 months). Injection is an extremely effective method of vaccinating fish, as it can be done in cold water, it can cover a multitude of diseases with a single injection, and it has a high efficacy, with a 70% to 99% improvement in survival relative to unvaccinated fish. It costs approximately $0.10 to $0.20 per fish to injection vaccinate, and, typically, one worker can vaccinate 5,000 to 10,000 fish per day. Injection vaccination can be more cost effective than any other disease prevention program. Many millions of doses injected over recent years have borne this out.

This bulletin could not possibly cover all of the scenarios for a successful vaccination program. Fortunately, technical help from an PHARMAQ team is as close as our toll-free telephone number.

Twenty-five years of experience as a world leader in developing and producing vaccines for farmed fish has challenged PHARMAQs scientists to develop a wide range of products to prevent or reduce the devastating effects of infectious diseases.

Regardless of the method of vaccination, or of the vaccine used, a vaccine should demonstrate a positive cost to benefit ratio. Farmers can determine this by comparing (1) the costs of vaccination and therapeutic agents combined with the normal expected mortality, against (2) the expected benefits.

COSTS

• value of each fish to be vaccinated
• number of fish to be vaccinated
• weight of each fish (for immersion and oral vaccination)
• volume of vaccine needed vaccine
• cost per liter
• vaccination rate in fish per person per hour (does not apply to oral vaccination)
• hourly wage
• post-vaccination handling mortality
• miscellaneous equipment

SAVINGS

• value of unneeded therapuetics
• value of lesser number of fish needed for stocking, due to increased survival
• Relative Percent Survival (RPS) ***
• increased number of fish available for harvest
• value of increased number of fish to harvest
• difference in value of feed used under conditions, versus post-vaccination

*** RPS = 1 - (% vaccinated mortality / % normal mortality) x 100