eChapters

Calcification

• Calcification is abnormal deposition of calcium salts in tissue other than bone.

• Calcium is normally present in blood and deposited in bones.

• Calcium if deposited in an abnormal tissue with normal or abnormal blood calcium level is considered as pathological condition.

Pathological calcifications are

1. Metatstatic calcification

2. Dystrophic calcification

Diseases are among the greatest deterrents to the sustained production in aquaculture. White spot disease (WSD) in penaed shrimp and epizootic ulcerative syndrome (EUS) in fresh and brackish water fishes are the well known devastating diseases with serious impact in aquaculture. Aquaculture medicine which broadly encompasses prevention and management of diseases in cultured aquatic organisms becomes a vital requirement for a sustained industry.

The four K’s essential for scientific aquaculture health management are knowledge about the disease process, knowledge about the pathogen, knowledge about the host and the knowledge about the environment. Disease development process is often complicated and involves host-pathogen- environment interactions. Knowledge about the pathogen, attaching and entering the host, deriving nourishment, reproduction, transmission, overcoming host defence barriers, etc are very essential. Susceptibility of a host to a pathogen is important which depends on host species, age, size, immuno competence and stress response. Besides knowledge about how the temporal and spatial aspects of environment stress the host and favour the pathogen to cause disease is vital

Disease Development Process

There exist a delicate balance between the host, pathogen and the environment. When this delicate balance gets upset, disease can result. Aquaculture environments can stress the host, favour the pathogen and result in disease development. To appreciate the process of disease development understanding the pathogenicity mechanisms of the pathogens, disease resistance mechanism of the host and the role of the environment is essential. Furthermore, role of various pathogens, their adaptive modifications, the functional importance of the target tissue, interaction between pathogen and host at the target tissue level, pathogenicity mechanisms of pathogens, etc are necessary to gain insight into process of disease development. Only when a pathogen can establish on or in the host, proliferate, overcome the non-specific and/or specific defense barriers of the host, produce the pathogenic factors, cause cellular and tissue damage, produce significant pathological changes, impair the function of the target tissue cause clinical disease often with mortality. This process is very complicated which often get accelerated by stress and environmental factors. However, the sequence of disease development will to a large extent depend on the nature and load of the pathogen (parasite, bacteria, fungi, virus), its intensity per unit area or unit weight of the host, size of the host and environmental factors.

Tuberculosis

Tuberculosis is a chronic bacterial disease of cattle, buffaloes, sheep, goat and pigs caused by Mycobacterium sp. and characterized by presence of tubercle nodules in lungs, spleen and lymph nodes (Fig.2.1).

Etiology

• Mycobacterium tuberculosis

• M. bovis

Pathogenesis

Disease spreads through contact with infected animals or their discharges or morbid tissues. The infected animals release the organisms through sputum at coughing and contact animals get infection through droplet inhalation.Besides sputum, faeces, urine, vaginal discharges, semen, milk, lymph and wound discharges may act source of infection. It is also transmitted through contaminated instruments, utensils and beddings. After entry through inhalation or ingestion, bacilli localizes at the point of entry and produce typical tubercle in associated lymph nodes. Mostly pharyngeal and mesenteric lymph nodes are affected. From lymph nodes, infection may extend across body cavities, blood vessels and lymph vessels. The inhaled organism enter in the bronchial tree after lodging in bronchi a tissue reaction takes place. Neutrophilic infiltration is there and when neutrophils are necrosed, macrophages come to destroy the organism but they fail then many macrophages becomes elongated to form epithelioid cells, the hall mark of granulomatous inflammation. Some macrophages are joint together to form giant cells to destroy the acid fast bacilli. As a consequence of this some bacilli are phagocytosed and are destroyed. A zone of lymphocytes, macrophages, epithelioid cells, giant cells and fibrous connective tissues are formed around the central necrosed area.

Salmonellosis

Salmonellosis is caused by G-bacteria Salmonella and characterized by early chick mortality with necrosis and hemorrhage in liver, heart and spleen and enlarged bronze coloured liver with focal or diffuse necrosis, marbled spleen and/or oophoritis and salpingitis in adults (Fig.4.1).

Etiology

• Salmonella enteritidis var. Galinarum (S. Galinarum).

• G-, rod, non lactose fermenter.

• White small colonies on MLA and pink on BGA.

Developmental Anomalies

Persistent right aortic arch

This is a developmental anomaly of aorta in which the aorta develops from right arch present on right side of trachea and oesophagus. The ductus arteriosus forms a ring around trachea and oesophagus by connecting aorta and pulmonary artery. This ring causes partial obstruction of trachea and/or oesophagus.

Patent ductus arteriosus

The ductus arteriosus is a short blood vessel which connects pulmonary artery to aorta in foetal life for diversion of blood. Normally, soon after birth this duct is sealed and remains in the form of a ligamentum arteriosum. But sometimes this ductus arteriosus remains open and blood is continuously shunted between aorta and pulmonary artery, after leading to congestive heart failure, pulmonary hypertension and cyanosis due to mixing of venous and arterial blood (Fig. 3.1).

Chlamydiosis

Chlamydiosis is an infectious disease of animals caused by Chlamydia sp. and characterized by encephalitis, polyarthritis and pneumonia in calves and abortions in cows and sheep.

Etiology

• Chlamydia psittacii

Pathogenesis

Chlamydial infection spreads through inhalation of contaminated dust from infected birds. Organism multiplies in lungs and through blood it spreads to liver, spleen, kidneys, brain, joints and placenta to cause pathology.

Characteristic symptoms

Calves

• Fever, dyspnoea

• Nasal discharge, hypersalivation

• Swelling of joints

Developmental Anomalies

Congenital icthyosis

Congenital icthyosis is scaly epidermis which resembles the skin of fish and occurs due to a simple autosomal recessive homozygous gene in calves. This condition is characterized by scaly, horny, thick epidermis divided into plates by deep fissures. Microscopically, there is thick keratin layer over the epidermis (Fig.1.1).

Epitheliogenesis imperfecta

Epitheliogenesis imperfecta is a congenital defect characterized by discontinuity of epithelium on skin leaving patches without squamous epithelium mostly at feet, claws and oral mucosa. Such defect may occur in calves which succumb to infection after birth or such foetus may abort. This disease condition is inherited as an autosomal recessive trait.

Developmental Anomalies

Epitheliogenesis imperfecta of tongue

Abnormal smooth surface of tongue due to small filiform papillae. It occurs as a defect in autosomal recessive gene and occurs in Holstein-Friesian cattle. This is also known as smooth tongue.

Cleft palate

This is most common congenital abnormality that occurs due to failure of oral nasal cavity to divide leaving cleft. It may also extend towards lips producing ‘harelip’ condition.

Mega colon

There is distention of colon which abruptly terminates in rectum due to mutant gene in dogs.

Pathology of Endocrine System

Pathology of Hypothalamus

The lesions in hypothalamus may cause diabetes insipidus characterized by polydipsia and polyuria with low specific gravity of urine. It occurs due to deficiency of antidiuretic hormone vasopressin.

Etiology/ Occurrence

• Lesions in hypothalamus and/or pituitary.

• Adenoma and adenocarcinoma of pituitary.

• Necrosis of hypothalamic nuclei due to larval migration.

Ringworm

Ringworm is a fungal disease of skin affecting animals characterized by popular skin lesions in circular form discharging serus exudate.

Etiology

• Microsporum canis- dogs

• M. gypseum- dogs, horse, pig

• Trichophyton verrucosum- bovines

• T. equinum- horses

Pathogenesis

The fungi enter in skin through abrasion and the hyphae appear in stratum corneum after germination and invade the hair follicles. The hyphae growth and spore formation in stratum corneum leads to hyperkeratosis and the fungus spreads centrifugally from the point of primary invasion leading to ring shaped lesion (Fig.8.1).

Aspergillosis

Aspergillosis is a fungal disease of poultry caused by different species of Aspergillus sp and characterized by granulomatous nodules in lungs, thickening of air sacs and presence of fungal growth in lungs and air sacs in early age of chicks. It is also known as Brooders pneumonia (Fig.8.1).

Etiology

• Aspergillus fumigatus

• A. flavus

• A. nidulans

• A. glaucus

• A. niger

• A. candidus

Female Genital System

Developmental Anomalies

Agenesis

Absence of ovary, uterus, oviduct and cervix in females. It may be unilateral or bilateral.

Hypoplasia

Complete or partial lack of germ cells in ovaries. Hypoplasia of uterus is related with agenesis of gonads. Ovaries of freemartin are also hypoplastic. Hermaphrodite animal has ovary and testicular tissue both in the gonads.

Hermaphroditism

In hermaphrodites, there is presence of organs of both sexes in same individual animal. Both ovarian and testicular tissue occur in one animal leads to sterility in animal (true hermaphrodite) while in pseudohermaphrodite the gonadal tissue of only one sex is present but there is some degree of development of opposite sex organs.

Developmental Anomalies

Hereditary anemia

Hereditary anemia has been reported in mice due to defects in erythropoiesis or reduced vitality of erythrocytes. Erythropenia along with leucopenia occurs in mouse foetus on 20th day of gestation due to defective autosomal chromosome 4. Sex linked anemia in mouse is hypochromic with deficient bone marrow and occurs in hemizygus males or homozygus females. This anemia occurs due to deficiency of iron as a result of poor absorption from gastrointestinal tract.

Autoimmune hemolytic anemia in foals

It occurs due to incompatible blood group antigens of male and female parents. The mare does not have that blood group antigen but foetus acquires it from father. The foetal blood exposed to dam through placental exchanges leads to induction of antibody production in mares against foetal blood group antigen. These antibodies accumulate in colustrum and when foal suck the milk from mares, they are readily absorbed through G.I. tract of foals in blood and cause destruction of erythrocytes leading to anemia.

Tyzzer’s disease

Tyzzer’s disease is an acute infectious disease of laboratory animals including rats caused by Clostridium spp and Characterized by watery diarrhoea, necrotic hepatitis and death

Etiology

• Clostridium piliforme

• Gram positive, Spore forming, Anaerobe

Pathogenesis

• Horizontal transmission- faecal oral route

• Contaminated feed, water

• Bacteria in gut causing enteritis

Pathology of Muscles

Equine Rhabdomyolysis

It is also known as Azoturia or Monday Morning Disease. The disease occurs in well fed horse after a spell of inactivity. Suddenly after walking a few steps, the horse is unable to move further and feels pain with intense sweating and hardening of muscles.

Etiology

• Accumulation of lactic acid in muscles.

• High glycogen storage.

• Lack of oxygen supply.

Macroscopic features

• Hardening of muscle just like wood.

• Urine is dark brown with myoglobin – myoglobinuria.

• Tonic spasms in muscles.

• Atrophy of affected muscles in chronic cases.

Contagious

Bovine Pleuropneumonia (CBPP) Contagious bovine pleuropneumonia (CBPP) is an infectious disease of cattle caused by Mycoplasma sp. and characterized by cuprous pneumonia, pleurisy and dilation of lymph vessels (Fig.5.1).

Etiology

• Mycoplasma mycoides

Pathogenesis

The infection spreads through inhalation of droplets from infected animals. The recovered animals act as source of infection at least for a period of 3 years. After entering through respiratory tract the organism reaches to bronchioles. It may remain in retropharyngeal gland from where it may spread in whole body. The organism from bronchioles enter into interlobular septa and causes inflammation followed by edema which causes dilation and subsequent thrombosis of lymph and blood vessels prior to development of pneumonic lesions. Death occurs due to anoxia and toxaemia.

Chronic Respiratory Disease (CRD)

Chronic respiratory disease is caused by Mycoplasma gallisepticum and E.coli in birds under poor management and characterized by cloudy air sacs with thickening of their wall, accumulation of cheesy material in air sacs and lungs (Fig.6.1).

Etiology

• Mycoplasma gallisepticum

• E.coli

• Bad management like poor ventilation and inclement weather conditions

• Disease occurs more during winter and rainy season.

Nervous system is composed of brain, spinal cord, and peripheral nerves. The neuron is a basic functional unit of nervous system. Necrosis of neurons in brain is known as encephalomalacia while necrosis of neurons in spinal cord is termed as myelomalacia. If the necrosis occurs in grey matter it is known as polioencephalomalacia while necrosis of neurons in white matter is called as leukoencephalomalacia. There are three types of scavenger cells in nervous system known as microglial, oligodendroglial and astrocytes. Microglial cells surround the necrotic neurons and are known as satellite cells and the process is called as satellitosis. As the neuron dies, it is engulfed by microglial cell and this process is termed as neuronophagia. The necrosis of nerve fibers starts from myelin sheath and this change is called demyelination or Wallerian degeneration.

Nervous system is composed of brain, spinal cord, and peripheral nerves. The neuron is a basic functional unit of nervous system. Necrosis of neurons in brain is known as encephalomalacia while necrosis of neurons in spinal cord is termed as myelomalacia. If the necrosis occurs in grey matter it is known as polioencephalomalacia while necrosis of neurons in white matter is called as leukoencephalomalacia. There are three types of scavenger cells in nervous system known as microglial, oligodendroglial and astrocytes. Microglial cells surround the necrotic neurons and are known as satellite cells and the process is called as satellitosis. As the neuron dies, it is engulfed by microglial cell and this process is termed as neuronophagia. The necrosis of nerve fibers starts from myelin sheath and this change is called demyelination or Wallerian degeneration.

Equine Rhabdomyolysis

It is also known as Azoturia or Monday Morning Disease. The disease occurs in well fed horse after a spell of inactivity. Suddenly after walking a few steps, the horse is unable to move further and feels pain with intense sweating and hardening of muscles.

Etiology

• Accumulation of lactic acid in muscles.

• High glycogen storage.

• Lack of oxygen supply.