Diagnostics: Evaluation of Animal Health

John J. Reddington

DiagXotics, Inc.

27 Cannon Road

Wilton, CT 06897

john@diagxotics.com

Abstract

Financial loss due to disease represents one of the highest costs of operation in aquaculture. These losses can be acute and overt, as in the case of disease outbreaks. Alternatively, the losses can be insidious as when subacute infections decrease feed conversion rates, increase labor costs and decrease carcass value at harvest. Diagnostic monitoring and prevention are some of the most cost effective means of avoiding the economic losses that result from pathogenic organisms. One of the first lines of defense in controlling or eradicating diseases is the identification of the causative pathogen(s). Researchers over the past two decades have been applying sophisticated technologies developed in the human and veterinary health care research field to the problems facing aquatic animal health. Significant advances have been made in diagnostics for some of the primary pathogens of salmonids and shrimp; however, there is still a void in our ability to rapidly and specifically detect the disease causing agents of other farm-raised species.

Key Words: Aquaculture, aquatic animals, disease, diagnostics, health management

Introduction

The ability to prevent or control the establishment of infection, and its progression into disease, is in large part what helps dictate profitability in aquaculture. Aquaculturists are in the business of growing a commodity, and as such need to make decisions and implement management practices that make sense from a cost/benefit perspective. It is with this in mind that this commentary has been written. Extremely virulent pathogens, whether they are introduced into a new geographic location or evolved locally through mutation, can effectively be managed in large part by detection and avoidance. This detection (diagnosis) allows for the potential management by exclusion, destruction of infected stock, or delayed introduction into a population. Early and accurate diagnosis of infectious agents is the first line of defense against the spread and potential catastrophic financial losses that can result when they get out of control and result in widespread disease.

Along with the domestication and intense husbandry of food animals comes an increase in the incidence of infection and potential for disease. The distinction between infection with a pathogen and disease that might result should be established. An example of this distinction can be seen with the agent(s) that cause the common cold (often Rhinoviruses). During the course of a normal winter the vast majority of us are exposed to, and become infected with, the virus. However, only a small percentage of us will actually succumb to the infection and develop clinical disease. Those at greatest risk of breaking with the disease are the very young, because they have not been infected with the virus or a similar virus before, and the old, because they often have failing immune systems. Another category of individuals at risk are those that are stressed. Stress, either physiologic or environmental, is a potent suppresser of the immune system, which in turn can predispose to disease.

The same analogy can be made with potential pathogens of aquatic species. Frequently animals can be exposed to an agent and become infected, but not develop disease. Here too, it is often the young animals that show disease because they are naive in terms of their previous exposure to the agent, and therefore do not have the necessary preexisting immune readiness to keep the pathogen in check following exposure. This holds true for older fish in areas or facilities that have not previously been exposed to a given pathogen. Old animals (most often broodstock under farming conditions) are also the ones that show the greatest incidence of disease due the physiologic stresses that they undergo. In terms of the general population, it is normally those animals that have had their immune systems compromised due to stress that are most likely to have an infection progress into disease. Some of these stressors are crowding, poor nutrition, low levels of dissolved oxygen, high or low salinity or water temperature, algal blooms, or pollution. There might also be some genetic predisposition to certain diseases that may be inherent in some species, or that may have been introduced by selecting for some other desirable trait. This is an important reason for understanding the genetic makeup of, and having genetic markers for, the various domesticated aquatic species.

As an industry, the health care needs of aquaculture have been dramatically under serviced. The health care support, like the industry itself, is similar to what it was for the poultry and swine industries about 30 to 40 years ago. Aquaculture, as a maturing industry, should take advantage of the advances made in these similar food animal industries, and adopt the best, most applicable technologies that they have to offer and learn from their mistakes. In order to prevent or control disease (preventative medicine) it is first necessary to understand the who, why and where of the pathogen. Therefore, there is an acute need to develop a better, more comprehensive, data base for the various infectious agents of aquatic species and the diseases they cause. One way to develop such a data base is through the use of sensitive and specific diagnostic tests and reagents that have been standardized and certified by the aquaculture health care community.

Diagnostic Testing and Its Application to Production

Over the past decade the field of diagnostics has developed ever more sensitive detection systems. For this reason it is important to keep in mind the end goal of your preventative medicine program - control or eradication of disease(s). It is now possible to detect a single infectious unit, but what does it mean in a real life situation? If the animal is not showing signs of clinical disease it simply means that the animal is infected. Yet it is still important to know the number and location of those individuals that are infected in order to combat its spread and progression into disease. It is for this reason that the most sensitive diagnostic procedures have value. What is most important is what one does with the information. Diagnostic tests are valuable management tools with which to make rapid, prudent cost effective management decisions.

There are many stages in the production cycle where diagnostic tests are of utility. The first is in the screening of broodstock to prevent the potential spread of a pathogen from parent to offspring (vertical transmission). Here a non-lethal test sample source is important. In addition, the most sensitive system available is desired if the goal is to establish a Specific Pathogen Free (SPF) breeding population, or even if one wants to curtail as much as possible the likelihood of vertical transmission to offspring. The next place where routine diagnostic testing is of value is in monitoring the very young animals. Generally, these animals are segregated into groups that are raised in small tanks, ponds, or runs, or even further separated in individual free-standing buildings or rooms within a building. It is wise to periodically monitor these groups to make sure that they remain ‘clean’ or that the prevalence of the pathogen is not increasing. It is important to develop this information because if a sample from a tank, pond, or run comes up ‘hot’ (infected or increased level of the pathogen) you want to prevent the potential spread to other tanks that may occur by your normal husbandry practices. For example, if the person feeding or maintaining the facility comes in contact with the infected tank and then goes to other tanks to feed or manipulate the system, they run the risk of spreading the pathogen throughout the facility. Or, in many operations, as the animals grow several tanks may be combined in large runs or bigger ponds and tanks. If you have one hot tank being combined with several clean tanks you will have successfully infected all the animals.

Another critical point for testing is at the time of stocking, or the purchase of stock. It is similar to the principle given above where you are combining several smaller groups of animals to stock a raceway, netpen or pond. You want to know which groups, if any, have the infectious agent present so that you do not mix them with uninfected animals. This act of stocking is typically a very stressful time for the animals as they are being subjected to physical stress such as handling and transport, environmental stress such as different water temperature or salinity, and perhaps even physiological stress such as smoltification or molting. All of these will exacerbate an infection if it is present, and may induce increased shedding of the agent from infected animals. If you are purchasing animals to stock your grow-out facility, it is also a very good idea to have them checked for the presence of various pathogenic agents so that you do not introduce them into your operation. You want to maintain as clean a facility as possible because uninfected animals generally have a better chance of surviving and reaching market size faster.

Routine testing during grow-out is also a good management practice. There are several valuable aspects to this type of testing program. For example, if very early in grow-out it is determined that the pathogen load is escalating dramatically, the management decision may be made to eliminate the population, decontaminate and restock. This has the advantage that if you are producing an aquatic species that is season dependent, you can still restock to take advantage of the warmer weather, photoperiod, etc. It can also be a cost saving measure as you will not continue to feed a population of animals that will have a lower survival to market weight, or that is using the feed energy to combat the infection rather than gaining weight. If, on the other hand, the pathogen load is increasing dramatically late in grow-out, the management decision might be made to harvest early. Although top dollar might not be gained for the product, some value may be recouped from a situation where if harvest was delayed, the entire crop might be lost or it might be of even lesser value. The most difficult situation is where the pathogen load increases during the middle of grow-out. Even here, the earlier you detect an increase in pathogen load, the better your chances for obtaining successful treatments. Animals, like humans, stop eating when they become ill. Therefore, early application of targeted medicated feeds may be cost effective because the animals that need the antibiotic will still be feeding and will consume the feed thereby reducing waste and cost. In addition, the earlier in the disease process that the medicated feeds are applied following culture and sensitivity testing, the lower the amounts of antibiotics that will need to be fed to gain the desired effect. Even in the face of viral infections that may be the cause of disease, secondary bacterial infection with such pathogens as Vibrio may be the ultimate insult that results in the demise of the animals. Here too, with viral infections the early administration of medicated feeds can be of economic benefit. Also, by monitoring on a pond by pond or net pen by net pen basis, the farmer can apply therapeutants or targeted management changes only to the effected ponds rather than a farm-wide approach that might be unnecessary and would be more costly. Other potential management interventions that can be taken in the face of increased pathogen load are to reduce stocking density, administer immunomodulators, improve nutrition, and water quality (oxygen tension, pH, etc.).

Conclusions

In order for the aquaculture industry to continue to grow and keep up with world demand, it must provide a top quality product on a consistent production schedule. A big factor in altering product quality and disrupting production schedules is the outbreak of disease. To prevent and control these disease outbreaks, a broad ranging preventative medicine approach will have to be adopted, vis à vis the poultry and swine industries. An integral part of a preventative medicine program is the use of diagnostic tests at strategic points in the production cycle. Newly commercialized diagnostic tests for use in aquaculture are available to begin to combat some of the more important pathogens, with a great number of new tests and reagents under development. It will be the responsibility of the industry to see that these tests and reagents are standardized and assimilated into a routine management practice. However, diagnostics alone will not win the battle against the spread of infectious diseases. Good sanitary practices must be implemented along side diagnostics as components of a preventative medicine approach. Cross contamination by infected implements, equipment or water carried from area to area within a hatchery or farm by unsuspecting workers is also an important factor in disease transmission. As growers become more familiar with the various test systems and sanitary practices, and the results they provide, they will appreciate the financial benefits that preventative medicine can help generate. These financial benefits will be realized over time when prudent management decisions and practices are made based on a comprehensive data base generated from the most sensitive and specific diagnostic tests available.

Recommendations

Short Term (1-3 years)

1. The USDA should support the development and application of new diagnostic technologies for the detection of the major pathogens affecting US aquaculture.
2. The USDA should support the generation of panels of defined, characterized samples of tissue from aquatic species infected with the various pathogens of interest. These panels of samples should serve as the "gold standards" against which old and new technologies are evaluated, as well as for certifying those technologies and the facilities approved for their use.

Mid Term (4-7 years)

3. The USDA should support the international and domestic standardization and certification of technologies, laboratories and personnel (diagnostic harmonization).
4. The USDA should support risk assessment studies to determine factors that lead to the establishment and spread of pathogenic organisms of aquatic species.