Infectious and Parasitic Diseases PDF

INFECTIOUS AND PARASITIC DISEASES INTRODUCTION Microorganisms, namely bacteria, viruses, fungi and parasites, are present everywhere—in the soil, water, atmosphere and on the body surfaces, and are responsible for a large number of infectious diseases in human beings. CHAIN IN TRANSMISSION...

INFECTIOUS AND PARASITIC DISEASES INTRODUCTION Microorganisms, namely bacteria, viruses, fungi and parasites, are present everywhere—in the soil, water, atmosphere and on the body surfaces, and are responsible for a large number of infectious diseases in human beings. CHAIN IN TRANSMISSION OF INFECTIOUS DISEASES i) Reservoir of pathogen. ii) Route of infection. iii) Mode of transmission. iv) Susceptible host. MODE OF ENTRY Microorganisms causing infectious diseases may gain entry into the body by various routes e.g. through ingestion (external route); inoculation (parenteral method); inhalation (respiration); perinatally (vertical transmission); by direct contact (contagious infection); and by contaminated water, food, soil, environment or from an animal host (zoonotic infections). FACTORS RELATING TO HOST i) Physical barrier. A break in the continuity of the skin and mucous membranes allows the microorganisms to enter the body. ii) Chemical barrier. Mucus secretions of the oral cavity and the alimentary tract and gastric acidity prevent bacterial colonisation. iii) Effective drainage. Natural passages of the hollow organs like respiratory and gastrointestinal provide a way to drain the excretions effectively. iv) Immune defense mechanisms. These include the phagocytic leucocytes of blood ,phagocytes of tissues and the immune system. DISEASES CAUSED BY BACTERIA, SPIROCHAETES AND MYCOBACTERIA PLAGUE Plague is caused by Yersinia (Pasteurella) pestis which is a small Gram-negative coccobacillus. It has been a great killer since 14th century. Is a zoonotic disease and spreads by rodents, primarily by rats. Infection to humans occurs by rat-flea or by inhalation. After the organisms enter the bloodstream, they reach the draining lymph nodes , rather than being phagocytosed by phagocytic cells, they proliferate rapidly giving lymphadenopathy. CONT. This occurs within 24-48 hours of infection and is accompanied by chills, fever, myalgia, nausea, vomiting and marked prostration. If untreated, death occurs from disseminated intravascular coagulation (DIC) within 1 to 2 days with development of widespread petechiae and ecchymoses leading to gangrene, and hence the name black death. ANTHRAX Anthrax is a bacterial disease of antiquity caused by Bacillus anthracis that spreads from animals to man. The disease is widely prevalent in cattle and sheep but human infection is rare. The disease occurs as an exogenous infection by contact with soil or animal products contaminated with spores. CONT. Depending upon the portal of entry, three types of human anthrax is known to occur: i) Cutaneous form by direct contact with skin and is mostcommon. ii) Pulmonary form by inhalation, also called as “wool sorters’ disease” and is most fatal. iii) Gastrointestinal form by ingestion and is rare. WHOOPING COUGH (PERTUSSIS) Whooping cough is a highly communicable acute bacterial disease of childhood caused by Bordetella pertussis. The use of DPT vaccine has reduced the prevalence of whooping cough in different populations. The causative organism, B. pertussis, live in the brush border of the bronchial epithelium, proliferate and stimulate the bronchial epithelium to produce abundant tenacious mucus. There is low grade fever, rhinorrhoea, conjunctivitis and excess tear production. Paroxysms of cough occur with characteristic ‘whoop’. The condition is self limiting but may cause death due to asphyxia in infants. STAPHYLOCOCCAL INFECTIONS Staphylococci are gram-positive cocci which are present everywhere—in the skin, umbilicus, nasal vestibule, stool etc. Three species are pathogenic to human beings: Staph. aureus, Staph. epidermidis and Staph. saprophyticus. Most staphylococcal infections are caused by Staph. aureus. Staphylococcal infections are among the commonest antibiotic-resistant hospital- acquired infection in surgical wounds. A WIDE VARIETY OF SUPPURATIVE DISEASES ARE CAUSED BY STAPH. AUREUS WHICH INCLUDES THE FOLLOWING : 1. Infections of skin. 2. Infections of burns and surgical wounds. 3. Infections of the upper and lower respiratory tract. 4. Bacterial arthritis. 5. Infection of bone (Osteomyelitis). 6. Bacterial endocarditis. 7. Bacterial meningitis. 8. Septicaemia. 9. Toxic shock syndrome. STREPTOCOCCAL INFECTIONS Streptococci are also gram-positive cocci but unlike staphylococci, they are more known for their non suppurative autoimmune complications. The following groups: 1. Group A or Streptococcus pyogenes, also called β-haemolytic streptococci, are involved in causing upper respiratory tract infection and cutaneous infections. 2. Group B or Streptococcus agalactiae produces infections in newborns. 3. Group C and G streptococci are responsible for respiratory infections. 4. Group D or Streptococcus faecalis, also called enterococci are important in causation of urinary tract infection, bacterial endocarditis and septicaemia. CONT. 5. Untypable α-haemolytic streptococci such as Streptococcus viridans constitute the normal flora of the mouth and may cause bacterial endocarditis. 6. Pneumococci or Streptococcus pneumoniae are etiologic agents for bacterial pneumonias, meningitis and septicaemia. CLOSTRIDIAL DISEASES Clostridia are gram-positive spore-forming anaerobic microorganisms found in the gastrointestinal tract of herbivorous animals and man. These organisms may undergo vegetative division under anaerobic conditions, and sporulation under aerobic conditions. These spores are passed in faeces and can survive in unfavourable conditions. CLOSTRIDIAL DISEASES : 1. Gas gangrene by C. perfringens 2. Tetanus by C. tetani 3. Botulism by C. botulinum 4. Clostridial food poisoning by C. perfringens 5. Necrotising enterocolitis by C. perfringens. DISEASES CAUSED BY FUNGI  MYCETOMA Mycetoma is a chronic suppurative infection involving a limb, shoulder or other tissues and is characterised by draining sinuses. The material discharged from the sinuses is in the form of grains consisting of colonies of fungi or bacteria. Mycetomas are of 2 main types: 1. Mycetomas caused by actinomyces (higher bacteria) comprising about 60% of cases. 2. Eumycetomas caused by true fungi comprising the remaining 40% of the cases. CANDIDIASIS Candidiasis is an opportunistic fungal infection caused most commonly by Candida albicans and occasionally by Candida tropicalis. In human beings, Candida species are present as normal flora of the skin and mucocutaneous areas, intestines and vagina. The organism becomes pathogenic when the balance between the host and the organism is disturbed. MORPHOLOGIC FEATURES Candida produces superficial infections of the skin and mucous membranes, or may invade deeper tissues as described under: 1. Oral thrush. 2. Candidal vaginitis. 3. Cutaneous candidiasis. 4. Systemic candidiasis. DISEASES CAUSED BY VIRUSES INTRODUCTION Viral diseases are the most common cause of human illness. However, many of the viral infections remain asymptomatic while others produce viral disease. Viruses are essentially intracellular parasites. Depending upon their nucleic acid genomic composition, they may be single-stranded or double-stranded, RNA or DNA viruses. YELLOW FEVER Yellow fever is the oldest known viral haemorrhagic fever restricted to some regions of Africa and South America. Is characterised by the following clinical features: Sudden onset of high fever, chills, myalgia, headache, jaundice, hepatic failure, renal failure, bleeding disorders and hypotension. DENGUE HAEMORRHAGIC FEVER (DHF ) The word dengue is derived from African word ‘denga’ meaning fever with haemorrhages. Dengue is caused by virus transmitted by bites of mosquito Aedes aegypti; the transmission being highest during and after rainy season when mosquitos are numerous. Dengue occurs in two forms: 1. Dengue fever or break-bone fever in an uncomplicated way is a self-limited febrile illness affecting muscles and joints with severe back pain due to myalgia (and hence the name ‘break-bone’ fever). CONT. 2. Dengue haemorrhagic fever (DHF), on the other hand, is a severe and potentially fatal form of acute febrile illness characterised by cutaneous and intestinal haemorrhages due to thrombocytopenia, haemoconcentration, hypovolaemicshock and neurologic disturbances. Dengue virus infects blood monocytes, lymphocytes and endothelial cells. This initiates complement activation and consumptive coagulopathy including thrombocytopenia. The entire process takes place rapidly and may evolve over a period of a few hours. If patient is treated appropriately at this stage, there is rapid and dramatic recovery. But in untreated cases, dengue shock syndrome develops and death occurs. VARICELLA ZOSTER VIRUS INFECTION Varicella zoster virus is a member of herpes virus family and causes chickenpox (varicella) in non-immune individuals and herpes zoster (shingles) in those who had chickenpox in the past. Varicella or chickenpox is an acute vesicular exanthem occurring in non-immune persons, especially children. The condition begins as an infection of the nasopharynx. Herpes zoster or shingles is a recurrent, painful, vesicular eruption caused by reactivation of dormant varicella zoster virus in an individual who had chickenpox in the earlier years. The condition is infectious and spreads to children. HERPES SIMPLEX VIRUS INFECTION HSV-1 causes vesicular lesions on the skin, lips and mucous membranes. The infection spreads by close contact.The condition is particularly severe in immunodeficient patients and neonates while milder attacks of infection cause fever-blisters on lips, oral mucosa and skin. HSV-2 causes herpes genitalis characterised by vesicular and necrotising lesions on the cervix, vagina and vulva. Like HSV-1 infection, lesions caused by HSV-2 are also recurrent and develop in non-immune individuals. RABIES Rabies is a fatal form of encephalitis in humans caused by rabies virus. The virus is transmitted into the human body by a bite by infected carnivores e.g. dog, wolf, fox and bats.The virus spreads from the contaminated saliva of these animals.The organism enters a peripheral nerve and then travels to the spinal cord and brain. A latent period of 10 days to 3 months may elapse between the bite and onset of symptoms. Since the virus localises at the brainstem, it produces classical symptoms of difficulty in swallowing and painful spasm of the throat termed hydrophobia. Other clinical features such as irritability, seizure and delirium point towards viral encephalopathy. Death occurs within a period of a few weeks. DISEASES CAUSED BY PARASITES AMOEBIASIS Amoebiasis is caused by Entamoeba histolytica, named for its lytic action on tissues. The parasite occurs in 2 forms: a trophozoite form which is active adult form seen in the tissues and diarrhoeal stools, and a cystic form. The trophozoites as well as cysts are passed in stools but the trophozoites fail to survive outside or are destroyed by gastric secretions. MALARIA Malaria is a protozoal disease caused by any one or combination of four species of plasmodia: Plasmodium vivax, Plasmodium falciparum, Plasmodium ovale and Plasmodium malariae. While Plasmodium falciparum causes malignant malaria, the other three species produce benign form of illness. These parasites are transmitted by bite of female Anopheles mosquito. The disease is endemic in several parts of the world, especially in tropical Africa, parts of South and Central America, India and South-East Asia. CONT. Major complications occur in severe falciparum malaria which may have manifestations of cerebral malaria (coma), hypoglycaemia, renal impairment, severe anaemia haemoglobinuria, jaundice, pulmonary oedema, and acidosis followed by congestive heart failure and hypotensive shock.