FISH AND SHELLFISH DISEASES Introduction Pathology is defined

FISH AND SHELLFISH DISEASES

Introduction Pathology is defined as study of disease and their causative agents. Disease is due to the change in existing environment i. e. something change from the normal metabolism or activity of the animal. Incidence of disease is due to the breakdown of the delicate balance between host, pathogen and environment (Snieszko, 1970)

Pathogen Types Obligate pathogen • Organism which will cause disease because of the mere presence in the culture system • This is mainly due to the interaction between animal and environment. Aeromonas salmoniclda Opportunistic pathogen • Organisms which will not cause diseases even if it is present in the culture system until other stressors limit the capacity of animal. Eg. Vibrio sp in shrimps

Stress Is the sum of all the physiological responses by which an animal tries to maintain or re-establish a normal metabolism in the face of a physical or chemical force (Selye , 1950).

General Adaptation Syndrome (GAS) • The changes which occur in response to environmental stress • animal ties to adopt its original condition is said to be General adaptation Syndrome (GAS). Steps involved in GAS • Alarm reaction • Stage of resistance • Stage of exhaustion

Alarm reaction • This is due to physical change i. e. animal escapement due to external stimuli, physiology. If oxygen is lost in bottom layers of water column animal comes to top. • chemical change i. e. change in p. H or dissolved oxygen or amount of ammonia. Animal cannot escape. Stage of resistance • In response to Physical, Biochemical or microbiological strssors animal will produce counter effect to maintain the normal condition to achieve homeostasis. • This stage helps to know whether the animal gets the disease or not.

Contd…. . Stage of exhaustion • When the animal is unable to achieve homeostasis i. e. resistance / change. The animal gets exhausted. If it reaches this stage leads to disease. • When an animal course these three stages and comes back to the normal condition, the animal does not have disease. • If the animal does not come back to the normal condition prone to disease.

Characteristics of GAS • The changes occurring during the GAS are neither species specific nor stressor specific: anoxia, infection, fright, forced exercise, anesthesia and many other stressors provoke similar responses in higher and lower vertebrates. • Totality of response to each individual stressor will vary, and be composed of some reactions specific to that particular stressor, and others which are common changes of the GAS regardless of the nature of stress.

Adaptation of animals in GAS • The hypothalamus senses the stress by HPI axis – Hypothalamic pituitary Interrenal axis. environmental stimuli carried by means of sensory neuron to the hypothalamus which in turn activates pituitary. • Further adaptations to adverse conditions is mediated by secretion of hormones such as Cortisol, ACTH (Adrenocorlicotiophic hormone) by pituitary and Thyroxine by downstream activation of thyroid gland.

Factor or parameter causing stress Temperature • As fishes are cold blooded animal (Poikilothermic) it can change the body temperature depending upon surrounding and this has profound influence on animal health status. • As the animal approaches the temperature extremes for its survival chances for pathogen invasion increase. • Sudden temperature change rather than temperature per se thatis the stressor.

Inflammation In: Higher vertebrates • Inflammation – It is a change happening in animal due to a stressor. The cardinal signs of inflammation in the higher vertebrates are • Calor (heat) - This is due to increased physical activity • Rubor (Redness) - This is due to the dilation of capillaries due to the increase in the blood flow into the area of infection. • Tumor (swelling) –increased dimaeter of capillary fenestrae allow the largest serum protein molecules such as fibrinogen and immunoglobulin to exude into the tisssue.

Contd…. • Doloro (pain) -The nerve encircling the area of assault will be stretched which carry message and cause pain. • Et functio laeso (Loss of function)- It happens when the integrity of the system is damaged or broken.

Inflammation in vertebrates • As fishes are cold blooded animal except capacity to regulate body temperarture all the factors are found in vertebrates. • In vertebrates inflammatory response happens through multiple chemical changes at tissue level. • The Pharmaco dynamic amine (histamine and 5 hydroxytryptamine) is a chemical compound produced from mast cells. • These amines will have a cascading effect. • It also produces Vasoactive amine (related to blood capillaries) that starts dilation. • The cells like lymphocytes and Thrombocytes (Platelets) are accumulated at the site of Inflammation.

contd… The cells which leave the blood include. • Neutrophils (Polymorphonuctear leucocytes) • Monocytes (Mononuctecu macrophages) • Lymphocytes • Thrombocytes

Types of Inflammation • Acute Inflammation • Chronic Inflammation Acute inflammation • These happen for short duration. There are three response normally happening during the process. – Resolution – Exudates / Exudation – Necrosis

Contd… Resolution The cause of inflammation is countered rapidly return to normal. Exudates/ Exudation continuous production of the products of the inflammatory response. Based on the site and stimulus may be suppurative, catarrhal, fibrinous, serous or diptheretic.

Necrosis • • Necrosis is the death of the cell. Resolution and Exudation is reversible. But necrosis is not reversible. This is caused due to prolonged inflammation. At this stage the inflammatory response ends.

Types of necrosis Liquefactive necrosis It is due to the release of enzymes at particular site resulting in enzymatic digestion of cells. Mediated by either the host cell itself such as neutropils or lysosme or by lytic toxins produced by infecting bacteria.

Contd… Coagulative Necrosis • When the group of blood vessels supplying blood to tissue is cut off the cell will due to the loss of blood supply (ischaemia) to an area. • In sections recognised as an acidophilic are where cell nuclei are destroyed but outlines still visible. • When the coagulative necrosis area invaded by saprophytic bacteria capable of digesting the dead tissue, the necrosis is said to be gangrenous.

Contd… Fat necrosis The area having fat is digested by enzymes produced by pathogens. Lipase are produced and fat is digested which lead to the formation of soap like material.

Sages of necrosis Best characterised in terms of changes taking place in nucleus of cell. 3 stages have been identified 1. Pyknosis 2. Karyohexis 3. Karyolysis Pyknosis Here the nucleus is shrunken and become very dark. Karyohexis characterized by rupture of the nuclear membrane and fragmentation of the nuclear chromatin.

Contd… Karyolysis • As the nucleic acid are hydrolyzed, they lose their basophilia and the whole tissue assumes a shiny pink colour. • Due to persistence nature of strands of fish chromatin nucleus appear as very dark, haematoxylinophilic irregular tadpole or trypanosome shaped structure usually against a pale shiny structureless back ground.

Chronic Inflammation Chronic inflammation: - In case acute inflammation is not resolving quickly, then Chronic inflammation results characterized by contemporaneous inflammation and proliferation of neighbouring support tissues. Granuloma formation: - Chronic inflammation many a times lead to a condition called as granuloma. It is a white to yellow lesion which may have a cheesy or hard consisitency or even may be calcified. May be caused by silicaceous diatoms, bacteria or fungi.

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