MICR 3268 Lecture 7 - Entry, Exit and Transmission PDF

MICR3268 MICROBIAL PATHOGENESIS Lecture 7 – Entry, Exit and Transmission Dr. Kelly Brathwaite  By the end of this lecture you should be able to: ❑ Describe the portals pathogens use to gain entry and establish themselves o...

MICR3268 MICROBIAL PATHOGENESIS Lecture 7 – Entry, Exit and Transmission Dr. Kelly Brathwaite  By the end of this lecture you should be able to: ❑ Describe the portals pathogens use to gain entry and establish themselves on or in a human host. Learning ❑ Identify defenses at the portals of entry and ways that pathogens may Objectives overcome them. ❑ Explain the modes of transmission of pathogens from one host to another. ❑ Discuss factors that influence the transmission of infectious diseases. ❑ Discuss the concept of disease communicability on the basis of the modes of transmission of infectious diseases. 2 The stages of pathogenesis Exposure to pathogen (entry) Adherence to skin/mucosa (colonization) Evasion of host defenses Multiplication & spread in the body (localized or systemic) Damage to host → DISEASE Shedding from body (exit) and transmission to new host  Host-pathogen interaction The host-pathogen interaction during infection is complex Pathogen – Aim: To successfully infect host Depends on their ability to evade or overcome host defenses Need to be in sufficient numbers to cause infection Need to be transmissible to new host Host – Aim: To prevent infection Must have useful defense mechanisms Depends on susceptibility to infection  Portals of entry Sites through which pathogens can enter the body Major portals of entry: skin, mucous membranes or intestinal epithelium (surfaces that normally act as microbial barriers) http://onlineimmunology.blogspot.com/2010/09 https://courses.lumenlearning.com/microbiology/chapter/how-pathogens-cause-disease/ /invasion-entry-of-pathogens-into-body.html  Entry via the skin Wounds, abrasions Microbes may Protective or burns → cause a localized skin infection or Pathogens: defenses: common sites of infection infection elsewhere Streptococci, Dry, relatively Leptospira and Hep B impermeable outer virus can enter skin layer with only small break Several types of fungi Fatty acids and (the dermatophytes) sweat infect non-living (skin: pH 5.5) keratinous structures of the skin eg nails and hair Peptides formed by keratinocytes Some parasites, (eg protect against hookworms) or their invasion by group larvae (Ancylostoma, A streptococci Schistosoma) can penetrate unbroken skin Chemicals produced by Some bacteria enter normal flora hair follicles or sebaceous glands → styes and boils  Microorganisms that infect via the skin Microorganism Disease Comments Arthropod-borne viruses Various fevers 150 distinct viruses, transmitted by bite of infected arthropod Rabies virus Rabies Bite from infected animals Human papillomaviruses Warts Infection restricted to epidermis Staphylococci Boils Commonest skin invaders Rickettsia Typhus, spotted fevers Infestation with infected arthropod Leptospira Leptospirosis Contact with water containing infected animals’ urine Streptococci Impetigo, erysipelas Concurrent pharyngeal infection in one-third of cases Bacillus anthracis Cutaneous anthrax Systemic disease following local lesion at inoculation site Treponema pallidum and T. pertenue Syphilis, yaws Warm, moist skin susceptible Yersinia pestis, Plasmodia Plague, malaria Bite from infected rodent flea or mosquito Trichophyton spp. and other fungi Ringworm, athlete’s foot Infection restricted to skin, nails, hair Ancylostoma duodenale (or Necator Hookworm Silent entry of larvae through skin of, e.g. americanus) foot Filarial nematodes Filariasis Bite from infected mosquito, midge, blood- sucking fly Schistosoma spp. Schistosomiasis Larvae (cercariae) from infected snail penetrate skin during wading or bathing Goering et al (2008). Mims’ Medical Microbiology. 4th Ed  Entry via breaches in the skin Cuts/wounds/abrasions Biting arthropods – mosquitoes, fleas, ticks Can penetrate skin during feeding and introduce infectious agent or parasites into host Essential part of lifecycle of microbe Animal bites - rabies Burns  Entry via the conjunctiva Protective Pathogens Pathogens: defenses: require: Kept clean by Efficient continuous Eg Chlamydia attachment flushing & gonococci mechanisms action of tears and wiping action Decreased of eyelids lacrimal gland secretion Lysozyme Damage to conjunctiva or eyelid  Entry via respiratory tract Protective defenses: Cleansing mechanisms Hair in nostrils Turbinate baffles Mucociliary escalator Mucus-secreting cells http://www.virology.ws/2009/05/21/viruses-and-the- Ciliated cells respiratory-tract/ Mucus-secreting glands Alveolar macrophages  Airway clearance in normal lung https://bronchiectasis.com.au/physiotherapy/principles-of-airway-clearance/airway-clearance-in-the-normal-lung https://basicmedicalkey.com/infections-of-the-lower-respiratory-system/ Microbial attachment in the respiratory tract Pathogens entering via respiratory tract Interfere with Avoid alveolar cleansing macrophages mechanism Firmly attach to surface of cells of Avoid phagocytosis the mucociliary sheet Inhibit ciliary Avoid destruction activity, after phagocytosis, eg M. tuberculosis eg B. pertussis, survive in H. influenza macrophages https://bronchiectasis.com.au/physiotherapy/ principles-of-airway-clearance/airway- clearance-in-the-normal-lung  Entry via gastrointestinal tract Protective defenses: Acid Mucus: o Mechanical barrier to infection o Contains chemicals that bind to microbial adhesins and block attachment to host cells o Microbe-specific secretory IgA antibodies Stomach enzymes Bile Peristalsis Intestinal transit time: 12-18 hrs o Not enough time for significant number of microbes to grow Cleansing mechanisms: o None except maybe diarrhoea and vomiting  G I Tract: Devices for attachment Pathogens require: Specific receptors on gut epithelial cells eg Vibrio cholera, rotavirus Tropism o Shigella (large bowel), Salmonella (small intestine) Mechanical devices o Protozoa eg Giardia lamblia has molecules for adhesion to microvilli of epithelial cells as well as microvillar sucking disk o Hookworms have large mouth capsule containing hooked teeth that allow them to attach o Other worms such as Ascaris “brace” themselves against peristalsis while tapeworms adhere to mucus of intestinal wall  Giardia lamblia http://cleansingherbs.com/hookworm.gif http://idsc.nih.go.jp/kansen/k00-g15/k00_12/0012-1.jpg http://www.microbe.org/art/tapeworm.jpg  Entry via urogenital tract Potential sites of entry: o Vagina o Urethra o Bladder Microbes can easily spread from one part of tract to another due to continuity between these sites  Entry via the vagina Protective Pathogens defenses: require: pH ~5 due to Specific attachment presence of mechanisms for lactobacilli → vaginal/cervical mucosa lactic acid Presence of Injury to mucosa during commensals intercourse Continued exposure to potential pathogens during intercourse Impaired defenses - estrogen imbalance, presence of tampons/menstrual cups Entry via urethra & bladder Protective Pathogens defenses: require: Secretory Specialized attachment antibodies mechanism to avoid being washed out eg Gonococcus, Chlamydia → parasite- directed endocytosis Mucus Spermine in prostatic ❑ Males: STIs more common in secretions uncircumcised males than circumcised ❑ Females: UTIs common due to short Flushing urethra and proximity to anus action of (constant source of intestinal bacteria) urine  Entry via oropharynx Protective Mechanisms defenses: of invasion: Flushing action Gum infections of saliva secretory IgA Attachment to antibodies mucosal or tooth (sIgA) surface Decreased salivary Lysozyme flow between meals Competition Dehydrated from normal patients → flora overgrowth of mouth flora Leukocytes on mucosal surface and in saliva  Summary Microorganisms must attach to, or penetrate body surfaces of the human host in order to cause an infection Various defense mechanisms exist at these surfaces which invading pathogens must overcome Successful pathogens therefore possess efficient mechanisms for attaching to, or penetrating these body surfaces  Exit and Transmission Various mechanisms present to ensure that pathogens exit host and can be transferred from one host to another → transmission Effective transmission is dependent on 4 main factors: Infectious dose – Number of varies greatly Stability in microbes between Genetics of the shed from microbes and microbe environment host route of infection also important Goering et al (2008). Mims’ Medical Microbiology. 4th Ed Transmission from respiratory tract Effective shedding/transmission aided by: Crowded indoor spaces Increased nasal secretions, with coughing and sneezing Generation of aerosols A sneeze can contain up to 20, 000 droplets Droplet size Largest droplets fall to the ground Droplets ~10 µm diameter can be trapped on nasal mucosa Droplets 1-4 µm in diameter likely to make it through nose and reach respiratory tract Air-conditioning Handkerchiefs, hands and surfaces (fomites) objects likely to carry infection http://students.washington.edu/grant/random/sneeze.jpg  Transmission from the GI Tract Transmission primarily by faecal-oral route – Contaminated groundwater – Contaminated food – Flies may carry faecal matter onto food – Eg Salmonella spp., Vibrio cholera, Giardia lamblia https://en.wikipedia.org/wiki/Waterborne_diseases#/media/File:Groundwater_Contamination_Latin_America_Sm.png  Transmission from the GI Tract Spread facilitated by: – Lack of public health system or poor hygiene – Shedding of large numbers of pathogens in faeces – Proximity of susceptible individuals Transmission from the urogenital tract Often sexually transmitted UTIs common → MOST NOT SPREAD VIA URINE Table shows some infections spread via urine Hooton, 2010. Comprehensive Clinical Nephrology (Fourth Edition)  Sexually transmitted diseases (STDs) Intimate contact Presence of discharge increases transmission → Some microbes induce production of a discharge (Eg chlamydia, gonococci) Transmission from mucosal ulcers/sores Dependent on social and sexual activity Transmission by semen  Sexually transmitted diseases (STDs) Transmission facilitated by: High number of sexual partners Use of oral contraceptives Limited availability of treatment for STDs (in some countries) Genito-oro-anal contact Transmission from the oropharynx Often spread via saliva Contamination of fingers and objects by saliva Deep kissing Table 13.9 Human infections transmitted via saliva  Transmission from the skin Spread by shedding or direct contact Dermatophytes Staphylococci Streptococcus pyogenes HSV, VZV, HPV Leishmania tropica http://dmatxi.com/wp-content/uploads/2013/03/How- to-eliminate-ringworm-naturally-300x218.jpg  Transmission in milk Microbes are rarely shed in human milk → Exceptions: HIV, cytomegalovirus, HTLV1 Milk from cows, goats, sheep may carry pathogens eg Brucella, Coxiella burnetii Contamination after collection and pasteurization also possible  Transmission from blood Microbes can be spread by blood-sucking arthropods or through needles (transfused blood or contaminated needles) Intravenous drug misuse known to facilitate spread Pathogens in blood eg HIV, hepatitis viruses Vertical and horizontal transmission  Transmission from animals Depends on: Type of environment Urban vs. rural Tropical vs. temperate Level of sanitation Zoonoses Invertebrate vs. vertebrate vectors Close contact with pets/livestock Transmission from invertebrate vectors Passive carriage Biological transmission Blood-suckers Shellfish & molluscs http://www.arbovirus.health.nsw.gov.au/areas /arbovirus/mosquit/aedes_notofront.jpg Snails  Transmission from vertebrates Spread by: Direct contact Inhalation Bites Scratches Contamination of food or water Occupational risks Geographical location – dietary preferences Close contact with domestic pets  Disease communicability Communicability: ability of an infectious disease to spread from an infected host to a susceptible host either via direct or indirect transmission In contrast, non-communicable diseases (NCDs) are not spread from person to person  Why is it important to know the routes/modes of transmission of an infectious disease? QUESTIONS?