CHAPTER EIGHT:
INTRODUCTORY FISH PARASITIC DISEASES AND PATHOLOGY
Kabir Mohammed Adamu Department of Biology, IBB University, Lapai |
& | Usman Naji Gimba Department of Biology, IBB University, Lapai |
8.1 Introduction
Parasites are organisms that live in or on other animals know as hosts. They extract food from the host by sucking blood fluid or eating the tissues. All fish are potential hosts to many different species of parasites. Parasitic diseases of fish pose a significant threat to fish populations. Parasite is found globally and infects freshwater and marine fishes. They attach to the gills and skin of the fish. Among fish parasites, are obligatory parasites, infesting mostly commercially important fishes. Pathological conditions resulting from parasitic diseases assume high magnitude of epidemics under crowded and other unnatural conditions among fish. Fish pathology deals with the diseases and parasites that effects fish ordinary living. Parasites can be internal (endoparasites) or external (ectoparasites). Pathogens which can cause fish diseases comprise:
1. viral infections, such as esocid lymphosarcoma found in Esox species.
2. bacterial infections, such as Pseudomonas fluorescens leading to fin rot and fish dropsy
3. fungal infections water mould infections, such as Saprolegnia sp.
4. metazoan parasites, such as copepods.
5. unicellular parasites, such as Ichthyophthirius multifiliis.
6. Certain parasites like Helminths for example Eustrongylides
8.2 Bacterial Diseases in Fish
Bacteria are responsible for many fatal diseases in fishes like furunculosis, columnaris, fin and tail rot, vibriosis, dropsy, cotton mouth disease and tuberculosis. Some of the bacterial diseases are:
8.2.1 Furunculosis Disease
Furuculosis disease is caused by Aeromonas almonicida in salmon fishes. It is a non-motile, gram-negative bacterium. This disease frequently appears to infect fishes living in the dirty waters containing a large amount of decaying matter. The first symptoms of this disease are appearance of boil like lesions. Others, symptoms are blood-shot fins, blood discharge from the vent, haemorrhages in muscles and other tissues and necrosis of the kidney. Bursting of boils allow the spread of this disease among other fishes and also offer suitable areas for fungus growth. Fishes severely infected with the bacteria die in good number. The disease is controlled on farms by medication or vaccination. Iodine is also used to decontaminate the surface of fertilized eggs to prevent vertical transmission (passage of infection from parent to offspring).
8.2.2 Columnaris Disease
Columnaris disease is caused by Chondroccus columnaris and Cytophaga columnaris in many freshwater aquarium fish. It is a long, thin, flexible, gram-negative slime bacterium (myxobacteriales). This disease is often associated with low oxygen level. Initially it is
marked by appearance of grayish-white or yellowish-white patches on the body. The skin lesions change to ulcerations and fins may become frayed. Gill filaments are destroyed and
eventually lead to the death of the fish. It is caused by infection of Myxosorida. Cysts appear on the body, internal tissues and organs. Fish becomes weak. Scales become weak, perforated and fall off. Give dip treatment in 10% common salt solution.
8.2.3 Fin and Tail Rot Disease
Fin and Tail Rot disease is caused by Aeromonas salmonicid and A. liquefaciens. However, protozoans and fungi may also be involved. It is characterized by appearance of white lines along the margins of fins, the opacity usually progresses towards the base eroding them and causing haemorrhage. The fin rays become brittles first and later break leading to the complete destruction of the fins. The infection may also spread on the body surface. Fin and tail rot are associated with poor sanitary conditions in fish ponds and with water pollution in nature.
Disintegrating fins that may be reduced to stumps, exposed fin rays, blood on edges of fins, reddened areas at base of fins, skin ulcers with grey or red margins, cloudy eyes. It is caused by the bacteria Aeromonas. If tank conditions are not good an infection can be caused from a simple injury to the fins/tail. Tuberculosis can lead to tail and fin rot. Basically, the tail and/or fins become frayed or lose colour. Treat the water or fish with antibiotics. A good antibiotic is Chloromycetin (chloramphenicol) or tetracycline. Treatment of 1% CuSO4 is also effective.
8.2.4 Vibriosis Disease
Vibrio bacteria are the causative agents of vibriosis disease in salmon and many other fishes. This disease may occur in waters with low oxygen. These bacteria are small gram-negative bacilli, characteristically curved. Diseased fishes show large, bright coloured, bloody lesions in the skin and muscles, haemorrhages in eyes, gills may bleed with slight pressure, and inflammation of the intestinal tract. If you suspect your fish is infected with Vibriosis, move them to a hospital or quarantine tank. Test your water chemistry to make sure your levels are within range, evaluate your fish's diet, and consider any aggressive tank mates. Secondary stress goes a long way in making Vibrio infections worse. Over-the-counter "antibiotics" are not effective against Vibriosis. There are many known resistant strains of Vibrio that require the correct treatment. Once a diagnostic culture and sensitivity has been performed, your veterinarian will determine the best course of action. Antibiotics can be given directly as an injection, formulated into a feed, or may require a water-based treatment.
8.2.5 Dropsy Disease
Pseudomonas punctata is the causative agent of this disease. It is characterized by accumulation of yellow coloured fluid inside the body cavity, protruding scales and pronounced exopthalmic conditions. This is known as Intestinal Dropsy. In case of ulcerative dropsy, ulcers appear on the skin, deformation of back bone takes place and show abnormal jumping. This is a fatal disease in culture systems.
Bloating of the body, protruding scales. Dropsy is caused from a bacterial infection (acromonas) of the kidneys, causing fluid accumulation or renal failure. The fluids in the body build up and cause the fish to bloat up and the scales to protrude. An effective treatment is to add an antibiotic to the food. With flake food, use about 1% of antibiotic and carefully mix it. Antibiotics in 250 mg capsules if added to 25 grams of flake food will be sufficient to treat dozens of fishes. A good antibiotic is Chloromycetin (chloramphenicol), or use tetracycline.
8.2.6 Cotton Mouth Disease
The filamentous bacteria, Flexi bacteria are the causative agent of this disease. The main symptom is appearance of fungus like tuft around the mouth. White cottony patches around the mouth. It looks like a fungus attack of the mouth, so it is called mouth fungus. It is actually caused from the bacterium Chondrococcus columnaris. In the beginning a grey or white line appear around the lips and later short tufts arise from the mouth like fungus. This disease may be fatal due to production of toxins and the inability to eat. Hence treatment at an early stage is essential. Penicillin at 10000 units per litre is a very effective treatment. Second
dose should be given in two days, or use Chloromycetin, 10 to 20 mg per litre, with a second dose in two days.
8.2.7 Tuberculosis Disease
Mycobacterium is a disease-causing agent which is difficult to diagnose without pathological examinations. The symptoms are ulcers on body, nodules in internal organs, fin and tail rot, loss of appetite and loss of weight of fish.
Emaciation, hollow belly, possibly sores. Tuberculosis is caused by the Bacterium Mycobacterium piscium. Fish infected with tuberculosis may become hollow bellied, pale, show skin ulcers and frayed fins, and loss of appetite. Yellowish or darker nodules may appear on the body or eyes. The main cause for this disease appears to be over-crowding in un-kept conditions. There is no known and effective treatment for this disease. The best thing to do is to destroy the infected fish and, if un-kept conditions or overcrowding is the suspected cause, it is required to take necessary measures.
8.2.8 Bacterial Gill Disease
This disease is caused by Myxobacteria in salmon fish. Many bacteria are found in swollen gill lamellae which show proliferation of the epithelium, and symptoms are lack of appetite. This disease is transmitted through water from infected fish. Treatment is by removal of the carbon filter from filtration; however, it must be returned 7 days after the last dose.
8.3 Viral Diseases in Fish
Viruses are transmitted from one host to the other through a structure called virion. Viruses are classified mainly based on external structure, shape, size, capsid structure, RNA and DNA nucleic acids. Viruses cause disease by weakening the host tissue or by forming tumors in the host tissues. There is no treatment for viral diseases, only prophylactic measures have to be taken up. Some of the viral diseases are:
8.3.1 Carp Pox
One of the oldest recognized fish diseases, carp pox is caused by Cyprinid herpesvirus-1 (CyHV-1). Pox lesions may be seen on other species of fish and are sometimes referred to as fish pox. Lesions typically are smooth and raised and may have a milky appearance. They are benign, non-necrotizing areas of epidermal hyperplasia. Severe cases may result in development of papillomatous growths, which may be a site of complicating bacterial infection. Generally, lesions are self-limiting and of minimal clinical significance. Carp pox can be a significant problem in koi because their aesthetic quality, and hence market value, is severely compromised.
8.3.2 Koi Herpesvirus:
Koi herpesvirus (KHV), caused by Cyprinid herpesvirus-3 (CyHV-3), was first recognized in 1996. It is widespread in the USA and considered endemic. KHV causes clinical disease in koi and common carp. Goldfish and grass carp are refractory to clinical disease but may serve as carriers. Koi that are exposed, but survive, may also serve as carriers. Clinical disease is seen at water temperatures of 72°–81°F (22°/27°C), with maximal mortality at temperatures of 72°/78°F (22°/25.5°C). Mortality rates can reach 80%/100%.Fish of any age are susceptible, but mortality rates may be higher in younger fish, especially fry. The most obvious lesions are seen on gill tissues, which are severely affected and develop a mottled red and white appearance with obvious hemorrhage in some cases. Affected fish are lethargic, swim at the surface, and may show behavioral signs of respiratory distress. The presence of severe bacterial or parasitic disease may mask the fact that KHV is the primary cause of gill lesions. The disease is transmitted horizontally by exposure to sick or carrier fish and also by exposure to contaminated water, substrate, or equipment. The only effective way to treat Koi that are infected with K.H.V. is with heat therapy. You must have a way to isolate these fish to treat them. You can use aquariums, or smaller isolation ponds to do this. You could set up a temporary isolation pond by using some bricks, back-filling them with dirt, and installing a liner. Make sure you use some bird netting over the top so the Koi will not jump out. Make sure the isolation pond is at least 12 inches deep.
· Remove the Koi from the main pond, into the isolation pond or aquariums making sure that the water you are using in the isolation quarters is the same temperature as the pond water.
· Slowly heat the water up to 86°F (no more than 2°F per hour). If your water is very cool, due this over a two-day period.
· Leave the fish in the heated water for a 7-day period. Do not feed the fish during this time as they will be stressed out, and the last thing you want is to have an ammonia or nitrite problem in the water.
· After the 7-day heat treatment is finished, slowly cool the water down to 80°F. Once you reach this temperature, start an antibiotic treatment with Koi Fix for Food, and Forma-Green in the water...if the fish are still eating. If the fish will not eat, use Koi Fix for Water. The Koi will need to be kept on these antibiotics for a two-week minimum treatment. You may continue these treatments for an extra week if the fish are not quite healed up yet.
8.3.3 Herpes viral Hematopoietic Necrosis
This condition of goldfish is caused by Cyprinid herpesvirus-2 (CyHV-2). It is probably widespread throughout the USA, but it is not a reportable disease. Clinically ill goldfish often are anorectic and exhibit pale gills and ascites. At necropsy, the spleen and kidneys (anterior and posterior) are often enlarged. As is typical for herpesviruses, survivors can be carriers and exhibit clinical signs of disease if subjected to stressors. Water temperatures between 10° and 22°C will result in replication of the virus that can be detected with quantitative PCR.
IHNV can be transmitted through water, by movement of fish, contact with contaminated untreated waste material and by equipment. In areas where IHN is endemic, the number of cases can be controlled by good hygiene, the use of virus-free water supplies and the disinfection of eggs from farms with IHNV. Testing of brood stock is important in identifying carriers; where known carriers are present, the disinfection of eggs with iodine products is recommended.
8.3.4 Herpesvirus of Angelfish
A herpesvirus of angelfish (Pterophyllum spp) has been detected by electron microscopy of skin from moribund angelfish. Affected fish produce copious amounts of skin mucus that gives affected fish a grey sheen. Often, these fish have multiple parasitic infestations and bacterial infections, similar to that of KHV in koi. It is suspected that survivors are carriers.
P. scalare is relatively easy to breed in the aquarium, although one of the results of generations of inbreeding is that many breeds have almost completely lost their rearing instincts, resulting in the tendency of the parents to eat their young. In addition, it is very difficult to accurately identify the sex of any individual until it is nearly ready to breed.
Angelfish pairs form long-term relationships where each individual will protect the other from threats and potential suitors. Upon the death or removal of one of the mated pairs, breeders have experienced the total refusal of the remaining mate to pair up with any other angelfish and successfully breed with subsequent mates. Depending upon aquarium conditions, P. scalare reaches sexual maturity at the age of six to 12 months or more. In
situations where the eggs are removed from the aquarium immediately after spawning, the pair is capable of spawning every seven to 10 days. Around the age of three years, spawning frequency decreases and eventually ceases. The virus can be spread by water and by movement of live infected fish. Controls are based on prevention of movement of fish and equipment from infected farms to uninfected locations, and by appropriate hygiene measures.
8.3.5 Viral Hemorrhagic Septicemia (VHS)
This disease is caused by an unequal shaped fish virus with RNA. This reportable disease is caused by a Novirhabdo virus and is a member of the family Rhabdoviridae. This disease occurs in salmon fishes. Transmission of the disease occurs through the water by a flagellate. Most of the reported hosts are not ornamental fishes, but koi were shown to be susceptible experimentally to genotype IVb. The symptoms are kidney swelling, reduced appetite, obvious distress, erratic spiral swimming, multiple haemorrhages in skeletal muscles, change in body colour, reddish fins. The only control measure is prevention.
8.3.6 Spring Viremia of Carp (SVC)
This acute, virulent, usually hemorrhagic disease of cultured carp is caused by a Vesiculo virus that, like VHS, is a member of the Rhabdoviridae family. Historically, it was reported in Europe and the former USSR; however, several outbreaks have been reported in the USA between 2002–2007, in both wild fish and cultured ornamental koi. SVC is considered a foreign animal disease in the USA and must be reported. It causes disease in common carp, including koi, as well as grass, bighead, silver, and crucian carp. Limited experience suggests that common goldfish may be susceptible. Clinical signs are nonspecific and may include darkening of the skin, exophthalmia, ascites, pale gills, hemorrhage, and a protruding vent with thick mucoid fecal casts. Pinpoint hemorrhage in the swim bladder is indicative of SVC, if present. Coinfection with Aeromonas or other systemic bacteria may obscure the presence of the virus.
The virus can be spread by water and by movement of live infected fish. Controls are based on prevention of movement of fish and equipment from infected farms to uninfected locations, and by appropriate hygiene measures.
8.3.7 Lymphocystis Disease
This typically chronic, viral infection of wild or captive marine and freshwater fish is caused by an icosahedral DNA virus of the Iridoviridae family. Infection may be manifest by benign, cauliflower-like lesions typically located on fins. The disease affects a wide range of fish and is generally considered to have a global distribution. Within the aquarium trade, painted glass fish and marine tropical fishes such as the anemonefishes (Pomacentridae), marine angels (Pomacanthidae), and butterflyfishes (Chaetodontidae) are susceptible. Presumptive diagnosis is based on the presence of enlarged fibroblasts (up to 1 mm), which are easily visualized with a light microscope. Microscopic examination typically reveals the appearance of grape-like clusters of virus-laden cells. Diagnosis is confirmed histologically: feulgen-positive cytoplasmic inclusions and a hypertrophied nucleus are pathognomonic and the disease is usually self-limiting but is of aesthetic concern.
Nodular white swellings (cauliflower) on fins or body. Lymphocystis is a virus and being a virus, affects the cells of the fish. It usually manifests itself as abnormally large white lumps (cauliflower) on the fins or other parts of the body. It can be infectious but is usually not fatal. Unfortunately, there is no cure. Fortunately, this is a rare disease. There are two suggested treatments. One treatment is to remove and destroy the infected fish as soon as possible. The other treatment is to simply separate the infected fish foe several months and wait for remission, which usually does occur.
8.4 Fungal Diseases in Fish
Fungal infections (fungal infections are called mycoses) are among the most common diseases seen in temperate fish. Because fungal spores are found in all fish ponds and create problems in stressed fish. Poor water quality can also lead to an increase in fungal infections in an otherwise healthy fish population. Some of the fungal diseases are:
8.4.1 Saprolegniasis
Saprolegniasis is a fungal disease of fish and fish eggs most commonly caused by the Saprolegnia species called "water molds." They are common in fresh or brackish water. Saprolegnia can grow at temperatures ranging from 32° to 95°F but seem to prefer temperatures of 59° to 86°F. The disease will attack an existing injury on the fish and can spread to healthy tissue. Poor water quality (for example, water with low circulation, low dissolved oxygen, or high ammonia) and high organic loads, including the presence of dead eggs, are often associated with Saprolegnia infections. Saprolegniasis is often first noticed by observing fluffy tufts of cotton-like material coloured white to shades of grey and brown--on
skin, fins, gills, or eyes of fish or on fish eggs. These areas are scraped and mounted on a microscope slide for proper diagnosis. Under a microscope. Saprolegnia appears like branching trees called hyphae. With progression of infection fish usually becomes lethargic and less responsive to external stimuli. So, fish under such conditions is a target to predators.
Tufts of dirty, cotton-like growth on the skin, can cover large areas of the fish, fish eggs turn white. Fungal attacks always follow some other health problems like parasitic attack, injury, or bacterial infection. The symptoms are a grey or whitish growth in and on the skin and/or fins of the fish. Eventually, if left untreated, these growths will become cottony looking. The fungus, if left untreated, will eventually eat away on the fish until it finally dies. Use a solution of phenoxethol at 1% in distilled water. Add 10 ml of this solution per litre. Repeat after a few days if needed, but only once more as three treatments could be dangerous inhabitants. If the symptoms are severe the fish can be removed and treated with small amount of provide one iodine or mercurochrome. For attacks on fish eggs, most breeders will use a solution of methylene blue adding 3 to 5 mg/1 as a preventive measure after the eggs are laid.
8.4.2 Branchiomycosis
Branchiomyces demigrans or "Gill Rot" is caused by the fungi Branchiomyces sanguinis (carps)and Branchiomyces demigrans (Pike and Tench). Branchiomycosis is a pervasive problem in Europe, but has been only occasionally reported by U.S. fish farms.
Both species of fungi are found in fish suffering from an environmental stress, such as low pH (5.8 to 6.5), low dissolved oxygen, or a high algal bloom. Branchiomyces sp. grow at temperatures between 57° and 95°F but grow best between 77° and 90°F. The main sources of infection are the fungal spores carried in the water and detritus on pond bottoms.
Branchiomyces sanguinis and B. demigrans infect the gill tissue of fish. Fish may appear lethargic and may be seen gulping air at the water surface (or piping). Gills appear striated or marbled with the pale areas representing infected and dying tissue. Gills should be examined under a microscope by a trained diagnostician for verification of the disease. Damaged gill tissue with fungal hyphae and spores will be present. As the tissue dies and falls off, the spores are released into the water and transmitted to other fish. High mortalities are often associated with this infection.
Avoidance is the best control for Branchiomycosis. Good management practices will create environmental conditions unacceptable for fungi growth. If the disease is present, do not transport the infected fish. Great care must be taken to prevent movement of the disease to noninfected areas. Formalin and copper sulphate have been used to help stop mortalities; however, all tanks, raceways, and aquaria must be disinfected and dried. Ponds should be dried and treated with quicklime (calcium oxide). A long term bath in Acriflavine Neutral or Forma-Green for seven days helps this condition. Ponds should be dried and treated with quicklime (calcium oxide) and copper sulphate (2-3kg / ha). Dead fish should be buried.
8.4.3 Icthyophonus Disease
Icthyophonus disease is caused by the fungus, Icthyophonus hoferi. It grows in fresh and saltwater, in wild and cultured fish, but is restricted to cool temperatures (36° to 68°F). The disease is spread by fungal cysts which are released in the faeces and by cannibalism of infected fish. Because the primary route of transmission is through the ingestion of infective spores, fish with a mild to moderate infection will show no external signs of the disease. In severe cases, the skin may have a "sandpaper texture" caused by infection under the skin and in muscle tissue. Some fish may show curvature of the spine. Internally, the organs may be swollen with white to grey-white sores. Diseased fish shows curious swinging movements hence the disease is called as swinging disease. Along with liver, particularly severely affected organs are: - spleen (salmonids), heart (herring), kidney (salmonids), gonads, brain (salmonids), gills (salmonids), and musculature and nerve tissue behind the eyes (sea fish).
Salt-like specks on the body fins. Excessive slime. Problems in breathing (Ich invades the gills), clamped fins, loss of appetite. Ich, white spot disease, whatever the name, this is the most common malady experienced in the home aquarium. Luckily, this disease is also easily cured if noticed in time. Ich is actually a protozoa called Ichthyophthirious multifiliis.
There are three phases to the life cycle of these protozoa. Normally, to the amateur aquarist, the life cycle is of no importance. However, since Ich is susceptible to treatment at only one stage of the life cycle, an awareness of the life cycle is important.
Adult Phase is embedded in the skin or gills of the fish, causing irritation (with the fish showing signs of irritation) and the appearance of small white nodules. As the parasite grows it feeds on red blood cells and skin cells. After a few days it bores itself out of the fish and falls to the bottom of the aquarium. In the cyst phase after falling to the bottom, the adult parasite forms into a cyst with rapid cell divisions occurring. Finally, the free-swimming phase occurs after the cyst phase, about 1000 free swimming young swim upwards looking for a host. If a host is not found within 2 to 3 days, the parasite dies. Once a host is found the whole cycle begins a-new.
The drug of choice is quinine hydrochloride at 30 mg per litre (1 in 30000). Quinine sulphate can be used if the hydrochloride is not available. The water may cloud but this will disappear. By reducing the time (with raised temperature) of the phases, you should be able to attack the free-swimming phase effectively. Most commercial remedies contain malachite green and /or copper, which are both effective.
8.5 Spoilage of Fish-Process and Its Prevention
The foods are usually classified as less perishable, moderately perishable and highly perishable in order to understand their perishable nature. Cereals, nuts and grains are included in less perishable and more stable category, vegetables as moderately perishable and seafood's as highly perishable food items. Seafood's are less stable because of their high moisture content and availability of nutrients for the growth of microorganisms. Ambient temperature plays a crucial role to alter the stability of a product. Highly perishable foods like seafood's have low tolerance to ambient temperature, while moderately perishable items like fruits and vegetables have increased tolerance and non-perishable items are least affected.
8.5.1 Causative Factors of Spoilage
Spoilage is the indicative of post-harvest change. This change may be graded as the change from absolute freshness to limits of acceptability to unacceptability. Spoilage is usually accompanied by change in physical characteristics. Change in colour, odour, texture,
colour of eyes, color of gills and softness of the muscle are some of the characteristics observed in spoiled fish. Spoilage is caused by the action of enzymes, bacteria and chemicals present in the fish. In addition, the following factors contribute to spoilage of fish. High moisture content, high fat content, high protein content, weak muscle tissue, ambient temperature and unhygienic handling.
8.5.1.1 Enzyme Action
The Rigor mortis is a physical effect on the muscle tissue of fish caused by chemical changes following the death. In live fish, its movements are controlled by chemical signals which cause the euthymic contraction (stiffing) and relaxation of the muscles. This produces swimming action. After the death, the normal circulatory system breaks down and chemical signals leak into the muscle causing them to stiffen. This process is known as Rigor Mortis. In other words, in live fish the glycogen present in the muscle is converted to carbon dioxide and water after supply of oxygen to the cells. After the death of fish, the blood circulation stops and the supply of oxygen is prevented. The enzymes present in the muscle convert glycogen into lactic acid. The pH of the fish muscle falls. The formation of the lactic acid continues till the supply of glycogen is completely used up.
After the completion of rigor mortis, muscle stiffness gradually decreases accompanied by increase in pH, ending up in softening of muscle. This is followed by breakdown of proteins by enzymes. This process is called as autolysis. Thus, autolysis can be described as an internal breakdown of the structure of the protein and fats due to a complex series of reactions by enzymes. Autolysis of protein starts immediately after rigor and creates favorable conditions for the growth of bacteria. Another important action of the enzymes is that it affects the flavor of fish. The components responsible for the taste and flavor of the fish are changed by the enzymatic action. An example is the progressive, degradation of ATP to AMP and Hypoxanthine. Hypoxanthine is produced by the breakdown of ATP which is a main component of fish muscle nucleotide. The accumulation of Hypoxanthine imparts a bitter taste in the fish muscle accompanied by loss of fresh fish flavor. Thus, the estimation of Hypoxanthine content in fish indicates the degree of freshness. Enzymatic action also causes decomposition in the fish known as belly bursting. The belly bursting is caused by the action of digestive enzymes present in the gut of the fish. The black spot formation in shrimps is also caused by the action of the enzymes on the amino acid. The black colour is due to the
formation of Melanin (Black Pigment) by the action of enzyme tyrosinase on tyrosin present in the shrimps. Black spots present a poor appearance and therefore, are not acceptable.
8.5.1.2 Bacteria Action
The freshly caught fish will be almost free from bacteria but the surface slime, gills and intestine may contain considerable load of bacteria. When the fish is dead, these bacteria start attacking the flesh causing spoilage and produce undesirable compounds. The nature and type of bacteria present in a fish depends upon the water from where it is caught and methods used for handling of the fish after its catch. The important changes brought out by the action of the bacteria in fish are as follows:
1. Reduction of TMAO to TMA: Marine fish contains a small percentage of odourless TMAO which is reduced to an offensive smelling TMA by the action of bacteria.
2. Breakdown of Amino Acids and formation of Primary Amines: the bacterial action of amino acids present in the fish muscle leads to formation of primary amines. Examples are formation of histamine from histidine, arginine from glutamic acid etc. This bacterial action may cause food poisoning in extreme cases.
3. Breakdown in Urea: the high concentration of urea in the flesh of some fishes is degraded to ammonia by the microorganisms. The formation of ammonia is accompanied by an offensive odour.
8.5.1.3 Chemical Action
The most common chemical action which causes spoilage is the oxidative rancidity in fatty fishes. The levels of peroxide value and free fatty acid content both a measure of oxidative rancidity is considered an index of quality of fat fishes.
8.6 Prevention/Reduction of Spoilage.
The activity of organism can be controlled, reduced or even retarded by proper handling and immediate lowering of the temperature. The chilling of the fish immediately after catch and holding the fish at 0oC by proper icing will reduce the spoilage. In case of shrimps, removing head immediately after catch will reduce the rate of spoilage. In the case of big fishes, beheading and eviscerating will reduce the enzymatic actions which cause spoilage.
The spoilage is reduced or prevented in a number of ways like drying, salting, chilling, canning and freezing. Chilling is a means of short-term preservation of seafoods achieved by
the reduction in temperature using ice. Freezing is the most satisfactory method currently available for a long-term preservation of seafood.
It is, in fact by far the best way of preventing fish from spoilage, since fish continues to remain in almost the same natural conditions even after freezing. It is effective for retaining flavour, colour and nutritive value of seafoods. Freezing is a process by which the water in the fish muscle is crystallized into ice. The crystallization will be complete at -40oC. After freezing, the fish must be stored at a temperature maintained constantly at -18oC or below. Fluctuation in this temperature will cause spoilage of products. If there is a wide variation in the temperature recrystallisation takes place.
Dehydration is another important reaction of a physical nature caused by the evaporation of ice due to differences in vapour pressure over the product surface and in the air of the store room. Loss of the moisture by evaporation of ice causes the product surface to dry resulting in dull appearance and even discolouration in some cases. The evaporated water eventually condenses and freezes on the cooling surfaces of the store room and the transfer of moisture from the product will be continuous. Proper glazing and packaging eliminate this evaporation.
8.7 Zoonotic Parasites in Fish
8.7.1 Mycobacterium
Organisms in the genus Mycobacterium are non-motile, acid-fast rods. Two species, M. fortuitum and M. marinum, are recognized as pathogens of tropical fish. Humans are typically infected by contamination of lacerated or abraded skin with aquarium water or fish contact. A localized granulomatous nodule (hard bump) may form at the site of infection, most commonly on hands or fingers. The granulomas usually appear approximately 6-8 weeks after exposure to the organism. They initially appear as reddish bumps (papules) that slowly enlarge into purplish nodules. The infection can spread to nearby lymph nodes. More disseminated forms of the disease are likely in immunocompromised individuals. It is possible for these species of mycobacterium to cause some degree of positive reaction to the tuberculin skin test.
8.7.2 Aeromonas
Aeromonad organisms are facultative anaerobic, gram-negative rods. These organisms can produce septicemia in infected fish. The species most commonly isolated is A. hydrophilia. It is found worldwide in tropical fresh water, and is considered part of the normal intestinal microflora of healthy fish. Humans infected with Aeromonas may show a variety of clinical signs, but the two most common syndromes are gastroenteritis (nausea, vomiting and diarrhea) and localized wound infections. Again, infections are more common and serious in the immunocompromised individual.
References
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