UNIT-3 INFECTION Akshaya Blessy Definitions and Terms • DISEASE– a pathological condition of body parts or tissues characterized by an identifiable group of signs and symptoms. • INFECTIOUS DISEASE– disease caused by an infectious agent such as a bacterium, virus, protozoan, or fungus that can be passed on to others. • INFECTION – occurs when an infectious agent enters the body and begins to reproduce; may or may not lead to disease. • PATHOGEN – an infectious agent that causes disease. • HOST– an organism infected by another organism. • VIRULENCE – the relative ability of an agent to cause rapid and severe disease in a host. AGENTS THAT CAUSE INFECTIOUS DISEASES Transmission of Infectious Diseases Agents that cause infectious diseases can be transmitted in many ways. • Through the air • Through contaminated food or water • Through body fluids • By direct contact with contaminated objects • By animal vectors such as insects, birds, bats, etc. • Micro organisms How Infectious Agents Cause Disease • Production of poisons, such as toxins and enzymes, that destroy cells and tissues. • Direct invasion and destruction of host cells. • Triggering responses from the host’s immune system leading to disease signs and symptoms. • Derangement of normal physiological functions of organs or organelles. Microorganisms classified depending on their relationship with their hosts Mi cro o rgan isms S a p ro p hytes P arasites P at h o g e n s Commensals 11 SAPROPHYTES • S a p ro p hy t e s are free-living m i c roorg a n i s m s that exist on dead and d e c aying o rga n i c matter, soil,and w ater. a n i m p o rtant role • T h ey play in t h e d e g ra d a t i o n of orga n i c are m a terials b u t inc a p ab le of m u l tiplying in living tissues. 6 Parasites, Pathogens AND Commensals parasites are m i c ro b e s that c a n m u l t i p ly inside hosts.Parasites c a n b e of twotypes: p a t h o g e n s a n d commensals. P a t h o g e n s are d i s e a s e - c a u s i n g mic robe s, that is, th ey are c a p a b l e of c a u s i n g diseases in t h e host. C o m m e n s a l s are m i c ro b e s that live co m p l e te l y w i t h i n t h e h o st b u t d o n o t c a u s e a ny h a r m to it. C o m m e n s a l s are largely fo u n d in t h e b o d y a s n o r m a l bacterial flora. 7 PHASES OF INFECTIOUS DISEASES • Incubation period – time between infection and the appearance of signs and symptoms. • Prodromal phase – mild, nonspecific symptoms that signal onset of some diseases. • Clinical phase – a person experiences typical signs and symptoms of disease. • Decline phase - subsidence of symptoms. • Recovery phase – symptoms have disappeared, tissues heal, and the body regains strength. CLASSIFICATION OF INFECTIOUS DISEASES BY DURATION • Acute – develops and runs its course quickly. • Chronic – develops more slowly and is usually less severe, but may persist for a long, indefinite period of time. BY LOCATION • Local – confined to a specific area of the body. • Systemic – a generalized illness that infects most of the body with pathogens distributed widely in tissues. BY TIMING • Primary – initial infection in a previously healthy person. • Secondary – infection that occurs in a person weakened by a primary infection. BY ORIGIN Based on whether the source of infection is from the host’s own body or is external, infections areclassified into two types, namely, endogenous and exogenous • Endogenous : originates from inside the body • Exogenous :originates from outside the body Focal infection: A condition where infection at localized sites (such as tonsils and appendix) produces generalized effects is known as focal infection.Itisalso known asfocal sepsis. Cross infection: A cross infection occurs when a new infection isset up in a patient already suffering a disease from another host. Cross infections occurring in hospitals are called nosocomial infections. Iatrogenic infection: Physician-induced infections or infections resulting from investigative or therapeutic procedures are known as iatrogenic infections. • Based on the clinical effects,infections areclassified asfollows: 1. Inapparent infections:These areinfections in which clinical effects arenot apparent.Theyare alsoknown assubclinical infections. 2. Atypical infections:These areinfections in which the characteristic clinical manifestations of a particular infectious disease arenot present. 3. Latent infections: These are infections that occur when some microbes, following an infection, remain in the tissuesin ahidden form proliferating and producing clinical disease when the host resistance is lowered. TYPES OF INFECTIOUS DISEASES • Infectious diseases can be viral, bacterial, parasitic or fungal infections. There’s also a rare group of infectious diseases known as transmissible spongiform encephalopathies (TSEs). • Viral infections. Viruses are a piece of information (DNA or RNA) inside of a protective shell (capsid). Viruses are much smaller than your cells and have no way to reproduce on their own. They get inside your cells and use your cells’ machinery to make copies of themselves. Eg:COMMON COLD ,FLU • Bacterial infections. Bacteria are single-celled organisms with their instructions written on a small piece of DNA. Bacteria are all around us, including inside of our body and on our skin. Many bacteria are harmless or even helpful, but certain bacteria release toxins that can make you sick. • Eg:TB,UTI • Fungal infections. Like bacteria, there are many different fungi. They live on and in your body. When your fungi get overgrown or when harmful fungi get into your body through your mouth, your nose or a cut in your skin, you can get sick. Eg:Candidiasis , athlete's foot • Parasitic infections. Parasites use the bodies of other organisms to live and reproduce. Parasites include worms (helminths) and some single-celled organisms (protozoa). Eg:hookworms ,pinworms • Transmissible spongiform encephalopathies (TSEs/prion diseases). TSEs are caused by prions — faulty proteins that cause other proteins in your body, usually in your brain, to become faulty as well. Your body is unable to use these proteins or get rid of them, so they build up and make you sick. Prions are an extremely rare cause of infectious diseases. TRANSMISSION OF INFECTION 16 • The portal of entry to cause an infection differs with pathogen. • Insome cases,the portals of entry and exit will be the same, whereas in others they are different. • For example, staphylococcal bacteria may escape from a person’s respiratory tract and infect another person’s skin lesion or the bacteria may escape from a person’s skin lesion and contaminate the food, which when consumed by another person can cause food poisoning. common portals of entry are: 1. Respiratory tract:Ifthe pathogen enters through the respiratory tract, the modes of transmission of infection may be airborne droplets and fomites. Diseases caused include measles and common cold. 2. Alimentary tract:The modes of transmission of infection include water, food, flies, and fomites. Diseases caused include typhoid, shigellosis, andpolio. 3. Skin and genital membranes: Transmission occurs through direct contact, fomites, and sexual intercourse. Diseases caused include syphilis andgonorrhea. 4. Ocular mucous: The modes of transmission are flies and fomites. The disease commonly caused is trachoma. 5. Injured skin or skin lesions: Infection is transmitted through blood-sucking arthropod vectors. Diseases caused include yellow fever and malaria. PORTAL OF EXIT 1. Respiratory tract: It is the most important portal of exit and is also very difficult to control. Some of the diseases that use this portal include common cold, influenza, tuberculosis, measles, mumps, rubella, pertussis, and pneumococcal disease. 2. Genitourinary tract: It is involved in transmitting syphilis, gonorrhoea, Chlamydia, and HIV. Parasitic diseases such as schistosomiasis and bacterial infections like leptospirosis also use this route. CAUSES OF INFECTION Poor infection control practices which may spread infection include: • Inadequate hand washing • Failure to wear correct protective clothing • Inadequate precautions where food is prepared/served/eaten • Inadequate procedures in relation to highly infectious individuals • Poor environmental hygiene • Re-using equipment without proper sterilization processes • Poor personal hygiene • Poor ventilation • Overcrowding • Failure to deal appropriately with soiled linen and clinical waste TRANSMISSION OF INFECTION • Transmission of the microorganisms can be grouped into four main routes: Contact-Direct ,Indirect and Vehicle . Airborne and vector borne •The most frequent means of transmission is through contact with an infected host CONTACT TRANSMISSION : There are three types of contact transmission: • Direct contact as in cases of physical contact with an infected person especially in cases of local skin contact • Indirect one as in cases of contact with a contaminated object such as soil which may result in the transmission of organisms such as S. tetani , and droplet contact such as the contact with contaminated secretions from an infected person in saliva or milk as in the transmission of the virus responsible for Herpes cold sores • Vehicle transmission occurs through a transporting agent or medium, such as food, water, or blood as in the case of the transmission of Hepatitis B virus , E. Salmonella or E. coli AIRBORNE TRANSMISSION : • Microorganisms that remain on droplets or dust particles are carried through the air through airborne transmission VECTOR BORNE TRANSMISSION : • A vector is an animal that transfers microorganisms from a reservoir to a host • • A vector picks up disease organisms from a source of infection, such as blood or feces, carries the infection within or on its body, and later deposits them where they infect a new host, directly or indirectly • Mosquitoes, fleas, lice, flies, and ticks are common vectors of disease to humans such as malaria or leishmaniasis • • Sexual contact constitute another important route of transmission when precautions are not taken as in the classical example of HIV virus transmission, in addition to N. gonorrhoae and Hepatitis B infection VERTICAL TRANSMISSION: • vertical transmission of infections from mother to her child either prenatally through the placenta, perinatally during passage of baby through the birth canal, or postnatally through breast feeding, is another important mode of transmission PATHOGENESIS • PATHOGENESIS The ability of microbes, or their components, to cause infection in a host after developing a complex mode of interactions from both hosts and pathogens. • Pathogenecity is referred to the ability of microbial species to produce disease. • Virulence is referred to the ability of microbial strains to produce disease. CHARACTERS OF PATHOGEN : • Bacteria should be able to enter the body. • Organism should be able to multiply in the tissue. • They should be able to damage the tissue. • They must be capable to resist the host defense. STEPS IN THE PATHOGENESIS OF INFECTIOUS DISEASES Steps involved in the pathogenesis of infectious Diseases. • Entry. • Attachment or Adherence • Colonization. • Invasion. • Immune response Inhibitors. • Toxins. 1.Entry • Penetration of Skin.e.g. Bilharzia • Through mucous membranes. Ingestion (Gastrointestinal tract), • Inhalation (Respiratory tract), • introducing pathogen into Genitourinary tract or blood, or • by insects'. E.coli, Malaria, … 2. Attachment (Adhesion) Microorganisms have macromolecules (proteins or carbohydrates) that promote attachment to tissue surfaces. Viruses and many bacteria must first bind to host cell surfaces. • Prevents early clearance. • Pathogens often bind host tissues via surface receptors. e.g. pili in bacteria. Adherence of bacteria 2.COLONIZATION • It is the ability of organism to spread in a host tissue after establishing infection. • Less invasive organisms cause localized lesion. Highly invasive organisms cause generalized infection (septicemia). • Some virulent bacteria produce special proteins that allow them to colonize parts of the host body. • Some Pathogens start multiplication • Some Pathogens compete with normal flora for residence. • Invasion Some virulent bacteria produce proteins that either: a. Disrupt host cell membranes or b. Stimulate endocytosis into host cells. • Endocytosis is the process by which cells absorb material (molecules such as proteins)from outside the cell by engulfing it withtheir cell membrane Immune Response Inhibitors Many bacteria produce virulence factors that inhibit the host's immune system defenses. . Bacterial toxins Many virulence factors are proteins made by microorganisms that poison host cells and cause tissue damage.e.g. Bacteria that produce toxins are called toxigenic. Bacterial toxins Endotoxins: they are part of the outer membrane of the cell wall of bacteria-They are present only by Gram-ve bacteria- Death of the bacteria lead to the release of endotoxins. Exotoxins: they are toxins produced by the bacteria to the surrounding tissue.-They are produced mostly byGram +ve bacteriaE.g. Diphtheria toxins BODY’S DEFENSE T-CELLS AND B – CELLS There are two main types lymphocytes: T cells and B cells. B cells produce antibody molecules that can latch on and destroy invading viruses or bacteria. T cells are direct fighters of foreign invaders and also produced cytokines, which are biological substances that help activate other parts of the immune system.
