Intro to Diseases PDF

⮚ Disease- any deviation, obvious or latent, from what is believed to be the normal condition in appearance, structure or functions of an organ, a group of organs, or of the body as a whole ⮚ Pathogen- any disease-causing agent ⮚ Diagnosis- involves recognizing the occurren...

⮚ Disease- any deviation, obvious or latent, from what is believed to be the normal condition in appearance, structure or functions of an organ, a group of organs, or of the body as a whole ⮚ Pathogen- any disease-causing agent ⮚ Diagnosis- involves recognizing the occurrence of an abnormality and identifying its cause Disease is defined as an abnormality of structure or function and displayed by living organisms through a specific or non-specific sign or symptom. The occurrence of a disease is the result of the interaction of three factors; 1) susceptible host - cultured animal, 2) pathogen - disease causing agent and 3) environment - increases virulence of the pathogen or decreases the resistance of the host. It is the balance among these three factors that determines the state of health of the cultured organism and a disturbance in the relationship among the three could give rise to disease. Diseases can be classified as 1) microbial diseases – caused by virus, bacteria, fungi and protozoans and 2) nutritional and environmental diseases – caused by nutritional deficiencies, environmental pollutants and adverse environmental changes. Microbial diseases are caused by organisms that are part of the normal micro - flora and fauna in the environment. These organisms are opportunistic pathogens that cause diseases only under conditions that favor them over the host. In crowded conditions, the population of pathogenic organisms increases to overwhelm the defenses of the host. An integrated health management program involves 1) the implementation of appropriately planned guidelines for prevention, control and eradication of diseases and 2) correction of disease-causing and disease spreading conditions in the farm. Prevention is considered the cornerstone of a disease protection program which involves the 1) reliable identification of diseases and its carriers 2) adequate knowledge of the transmission mechanisms 3) development of effective methods of preventing the access of pathogens and their carriers into culture facilities 4) provision of environmental conditions conducive to maintenance of health of cultured species. A relatively new technique in disease prevention in fish is through immunization by vaccines. Vaccines contain antigens that are generally attenuated or killed disease agents. When administered to a host, they stimulate the production of specific antibodies or non-specific resistance to a particular pathogen. The chances of survival of the host, when infected by the pathogen, are greatly enhanced by the immunization achieved through the production of antibodies. Causes of Disease Outbreaks Disease is defined as a deviation from the normal physiological functions brought about by two major causes: a) biological, e.g. microbes such as virus, bacteria, fungi, protozoans and other parasites; and b) non-biological, e.g. toxins, water quality and nutrition. In general, the development of disease is due to the interaction of the three factors: the host (shrimp), environment and pathogen or the disease-causing organisms. The Host The shrimp or the host remains healthy and disease-free so long as it is provided with optimum conditions and proper nutrition. This is because healthy hosts have an efficient natural body defense mechanisms and immune system to resist pathogens. Host defenses become ineffective when it is 1 stressed because of conditions during confinement, poor water quality and improper nutrition which may then lead to disease. The Environment The environment of the stock/shrimp consists of the living (other shrimps and all organisms in the pond) and the non-living (water and soil). Maintaining optimum conditions or keeping the water and soil properties within the shrimp’s safe levels will be of much help in the prevention of disease. Table 1 presents the recommended water physico-chemical levels for sugpo or Penaeus monodon culture. Alterations in environmental parameters beyond tolerable levels due, for example, to the accumulation of wastes and unconsumed food at the pond bottom and water could readily result to stress in stock thereby weakening their resistance to diseases as well as favoring growth of pathogens in the pond. Overcrowding and overgrowth of other organisms in the pond is stressful to the stock not only because of competition with pond resources but also result to high volume of wastes. The Pathogen Disease-causing organisms compose a natural part of the environment although in small quantities and co-exist with the host. It is during conditions favorable for their growth such as the presence of rich organic matter or wastes and when the host is weak when disease occurs. Many of these pathogens especially the microorganisms such as bacteria, protozoans and fungi, use o.m. as food and when in high quantities could readily attack weak hosts. Development of Disease The development of disease involving the above three factors can be illustrated as three corners of a triangle. ENVIRONMENT PATHOGEN HOST During normal conditions a balance is maintained by the host and the pathogen so that no disease occurs. Pressure on any corner of the triangle, e.g. changes in water property, introduction of a large number of pathogens, weakening/stress or improperly fed host, upsets this balance which could either be beneficial or detrimental to the host or pathogen. In many cases the host is adversely affected and becomes vulnerable to disease. Disease Prevention in Ponds Treatment of diseases of the cultured stock in ponds is a matter for serious consideration. For one, the volume of pond water is too large that chemical or antibiotic applications can be economically impractical. Improper treatment such as the administration of insufficient dosage of antibiotics has the risk of producing drug-resistant bacteria that may further complicate the problem. Moreover, once disease has set in, it may not only adversely affect the volume of the produce but the quality of the stock/shrimp as well. Prevention of disease from occurring therefore, remains the most important strategy for a successful operation. This requires sufficient knowledge and consideration of the causative factors and the ability to discern possible problems in the cultures. 2  In the culture of shrimps and other cultured species as well, there are four essential requirements for disease prevention in ponds. 1. Proper Pond Preparation It is during this time of culture operation when manipulation of the environment of the stock such as soil conditioning repairs, pest eradication and growth of natural food, is done and is a very important phase of the culture period. Adequately prepared ponds have significant influence for the best survival and growth of the stock. It is advisable, therefore, to follow recommended procedures. Moreover, pond dimensions should be known to determine areas and water volumes because all inputs in the pond depend on them. 2. Proper Stocking and Monitoring This involves careful selection of shrimp fry for stocking in ponds. They must be disease-free and healthy, collected from reliable sources and properly acclimated prior to or during stocking. Stocking densities should follow recommended rates as overcrowding may result to disease outbreaks and other problems. Monitoring of the stock is important to be able to determine the condition and rate of growth of the stock at certain stages of culture and provide remedial measures whenever problems are observed. 3. Proper Water Quality Management Maintaining good water quality for the entire culture period is one of the keys to a good production. Water supply must be adequate to provide efficient water exchange and keep the physico-chemical parameters of the water at optimum levels for the stock. The quality of the water source is equally important to attain the required water properties. The use of water pumps and aeration systems and monitoring of water properties are very important especially when rearing at high densities. 4. Proper Nutrition It is not enough to provide the required amount of food but the diet must also contain the proper nutrients needed by the stock. Thus, the choice of supplemental or artificial feeds must be considered carefully to provide the stock with the required nutrients for good growth. During culture, recommended feeding rates and frequencies at different stock sizes should be followed to avoid starvation or wastage of feeds. This would also require regular monitoring of feed consumption as conditions in the pond influence feeding by the stock although food is given following recommended procedures and amounts. Some Signs of Possible Disease Occurrence in Ponds When diseases occur among the stock, early detection can help in providing immediate remedial measures and minimize losses. Some behavioral and physical manifestations of probable problems in shrimps are: 1. Erratic swimming of shrimps at the water surface and sides of the pond which is usually caused by low D.O., high water temperatures or polluted waters. 2. Poor feeding response and slow growth. 3. Discoloration of shell. 4. Prolonged soft-shelling. 5. Visible damage on appendage and shell. 6. Dead shrimps at the pond bottom. When disease outbreak has occurred, the following are recommended: 3  1. Submit diseased/dead shrimps to accredited laboratories for examination to determine possible causative factor/s. 2. If mortality is light (less than 10% of the population), apply water quality controls. In intensive cultures feeding is minimized or stopped until water quality improves and feeding response is normal. 3. If mortality is high (over 20% of the population), the culture is discontinued and remaining live shrimps harvested. Dead stock should be disposed properly and pond bottom disinfected. Guidelines for sending specimens for disease diagnosis (Baticados et al., 1992) 1. In the absence of facilities or personnel capable of diagnosing shrimp diseases, samples for diagnosis maybe sent to a disease diagnostic laboratory. If the laboratory is located nearby, live samples may be sent by packing these in clean, aerated culture water in plastic bags. Diseased shrimps must always be separated from normal ones and stocking density during transport must be reduced by at least 25%. For larval and postlarval stages, at least 20 diseased individuals and an equivalent number of normal shrimps are needed to make a diagnosis. All types of examination and diagnostic procedures may be done on live samples. 2. If the diagnostic laboratory is far from the hatchery/farm and there are no facilities for immediate and fast transport, fixed or iced samples may be sent. Specimens are fixed in 5% (for larvae/postlarvae) or 10% (for adults) buffered formalin in plastic or glass bottles. The same number of specimens are sent as for live samples and diseased animals must also be separated from normal ones. Only direct microscopic examination (for parasites and fungi) and histopathological examination may be done on fixed samples. 3. Iced samples are sent by packing adult/juvenile shrimp in plastic bags (separate diseased from normal) and placing these in between layers of ice in a Styrofoam box. Like fixed samples, very limited diagnostic procedures may be done on iced samples. 4. All pertinent data/information must be sent with the samples. 10% Buffered Formalin (1 liter): Formalin = 100 cc Seawater = 900 cc Sodium phosphate, monobasic = 4 grams Sodium phosphate, dibasic, anhydrous = 6 grams Guide Information On The Investigation Of Disease Outbreaks And Related Problems In Ponds History A. Species 1. Source 2. Age B. Stocking Density C. Water 1. Source 2. Salinity 3. Flow and Temperature 4. D.O. and Hardness 5. Gasses 6. NH3 7. pH 4  8. NO2 D. Past Episodes of Disease E. Feed 1. Storage 2. Type 3. Date acquired and source 4. Size and amount fed 5. Frequency of feeding 6. Method of feeding 7. Feeding response F. Sanitation 1. Frequency of pond cleaning/water management 2. Equipment 3. Dead fish/shrimp removal and disposal G. Other Events 1. Pond condition 2. Water temperature changes 3. Climatic condition 5