Route and source of infection

INFECTION TRANSMISSION

1. ROUTE AND SOURCE OF INFECTION Dr. Abhishek Kumar Jain Assistant Professor, Dept. of Microbiology JNU IMSRC, Jaipur 1 Monday, 21 September 2020

2. Outline Introduction Classification Source of infection Method of transmission Factor predisposing to microbial pathogenicity Types of infectious disease 2

3. Introduction Infection - Invasion by and multiplication of pathogenic microorganisms in a bodily part or tissue, which may produce subsequent tissue injury and progress to overt disease through a variety of cellular or toxic mechanisms. Disease -Any deviation from a condition of good health and well-being. Infectious Disease - A disease caused by the presence or growth of infectious microorganisms or parasites. 3

4. Relationship with host Saprophytes – Free living microbes subsisting on dead or decaying organic matter. eg . Aspergillus . Parasites – Organism establishing and multiplying themselves in hosts. eg . Plasmodium species. Commensals – Microbes living in complete harmony with host without causing any harm to them. eg . Normal body flora. Pathogens -Microbes capable of causing disease in host.eg. Vibrio cholerae . 4

5. Classification of infection Acute Infection - an infection characterized by sudden onset, rapid progression, and often with severe symptoms. eg . Carbuncles, Furuncles. Chronic Infection - an infection characterized by delayed onset and slow progression. Eg . Tuberculosis. Primary Infection - Initial infection that develops in an healthy individual. Secondary Infection - a n infection that develops in an individual who is already infected with a different pathogen. 5

6. Reinfection - subsequent infection with same pathogen in the same host . Eg . Urinary tract infections. Localized Infection (focal) - an infection that is restricted to a specific location or region within the body of the host. Eg . Carbuncles, furuncles. Systemic Infection - an infection that has spread to several regions or areas in the body of the host. Eg . Sepsis. Clinical Infection - an infection with obvious observable or detectable symptoms. Eg . Upper respiratory tract infections. 6

7. Subclinical Infection - a n infection with few or no obvious symptoms. Eg . Anicteric Hepatitis A infection. Atypical infection - typical clinical features of particular infection is not present . Eg . Viral pneumonia. Cross infection - Already suffering from disease and acquire new disease from another host or another external sources. Nosocomial infection - Hospital acquired infection. Eg . Ventilator associated pneumonia. 7

8. Iatrogenic infection- Physician induced infection during therapeutic or investigative procedures. Eg . Injection abscess. Latent infection - after following an infection the pathogen remain in tissues in a hidden or latent form which produce clinical disease when the host resistance is lowered. Eg . Herpes Zoster. Opportunistic infection - an infection caused by microorganisms that are commonly found in the host’s environment. This term is often used to refer to infections caused by organisms in the normal flora. Eg . Candida infections. 8

9. Endogenous Endogenous Infections are due to organism of the normal flora which turns pathogenic due to certain circumstances, like decreased immunity. Example – Viridans Streptococci are the normal flora of mouth but when there is abnormality of the heart like RHD, an injury to the oral cavity like tooth extraction, these organism turn pathogenic. 9

10. Exogenous Due to the organisms derived from a source outside the body. Example – by drinking contaminated water with Vibrio cholerae bacteria. 10

11. SOURCES OF INFECTION 11 Human Beings Patients Carriers Healthy Convalescent Temporary Chronic Contact Paradoxical Animals Zoonosis Insects Mechanical Biological Soil & Water Food Fomites

12. Patients – Individuals suffering from disease. Carrier - An individual who carries an infectious agent without manifesting symptoms, yet who can transmit the agent to another individual. Convalescent carrier - Individual who recovered from infectious disease but continues to harbor the pathogen. Eg . Typhoid. Temporary carrier – Harbor the pathogen for short time (days, weeks). Eg . Chronic carrier - Harbor the pathogen for long time (months, years, life). Eg . HCV. Contact carrier – Acquires the pathogen from a patient. Eg . Chicken pox. Paradoxical carrier - Acquires the pathogen from another carrier. Eg . Rhinoviruses. 12

13. ANIMALS ZOONOSES – infectious disease transmitted from animal to human being are called zoonoses . EXAMPLES – plague from rat, rabies from dog, toxoplasmosis from cat etc. 13

14. INSECTS Vectors - an animal (nonhuman) that can transmit an infectious agent to humans. Two types:- Biological vectors : The infectious agent must incubate in the host as part of the agent’s developmental cycle; eg , the transmission of malaria by infected mosquitoes. Mechanical vectors : The infectious agent is physically transmitted by the vector, but the agent does not incubate or grow in the animal; eg , the transmission of bacteria sticking to the feet of flies . 14

15. SOIL – some pathogen can survive in soil for very long periods. Ex. Spores of tetanus bacilli. WATER – the source of infection either due to contamination with pathogenic microorganism (cholera vibrio , infective hepatitis virus) or due to presence of aquatic vectors ( cyclops in guineaworm infection) FOOD – contaminated food may act as source of infection. 15

16. MODES OF TRANSMISSION OF INFECTION 16 Human host may acquire microbial agents by various means which are known as Method/Modes of transmission. Ex - Contact – infection acquired by contact with contaminated object. term Contagious disease has been used for disease transmitted by contacts.

17. Inhalation – respiratory infection such as influenza and tuberculosis are transmitted by inhalation of pathogen. Ingestion – intestinal infection are generally acquired by ingestion of food or drink contaminated by pathogens. 17

18. Inoculation – pathogen in some instances may be inoculated directly into the tissue of host. ex tetanus spores. Insects – insect may act as mechanical or biological vectors of infectious diseases. Congenital – some pathogen are able to cross the placental barrier and infect the fetus in utero, this is known as vertical transmission. 18

19. Iatrogenic – infections transmitted during clinical procedures. Laboratory acquired – laboratory personnel handling infectious material are risk of infection, such infections are called laboaratory acquired infections. 19

20. Factors Predisposing to Microbial Pathogenicity Pathogenicity - The ability of a microbe to cause disease. This term is often used to describe or compare species. Virulence - The degree of pathogenicity in a microorganism . This term is often used to describe or compare strains within a species. Exaltation -Enhancement of virulence. Induced by serial passage of a strain in susceptible host. Attenuation -Reduction of virulence. Induced by serial passage in unfavourable host/artificial media. 20

21. Determinants of virulence Virulence of a strain is due to combination of various underwritten parameters. Adhesion – Initial event characterised by attachment of bacteria to host cells due to specific surface receptors (Adhesins). Examples- fimbria, pili. Invasiveness – ability of an organism to spread in the host tissue after establishing infection. Toxigenicity – some bacteria produce disease with the production of toxins which include - exotoxins and endotoxins. 21

22. Difference between Endotoxin and Exotoxin 22 Exotoxins Endotoxins source Gm + & Gm – organisms. Integral part of cell wall of Gm – organisms chemistry Protein (polypeptide). lipopolysaccharide Mechanism of release Diffusible, secreted by living cells. Non diffusible, released on cell lysis . Toxicity. Very high toxicity. low toxicity antigenicity. Highly antigenic. P oor a ntigenic

23. Exotoxins Endotoxins Effect of heat Unstable to temperature > 60°C , toxicity destroyed rapidly. Stable at temperature > 60°C for several hours without loss of toxicity. E ffect of formalin. Convert it into toxoid (antigenic but non toxigenic). No effect, can not be converted to toxoid. specificity Every toxin has specific action. Not specific action (all give fever and shock). fever Do not produce fever in the host. Produce fever in the host through release of interleukin-1 from macrophages 23

24. Plasmids - extra chromosomal DNA segments that carry genes for antibiotic resistance. Ex – R factor. Bacteriophages – viruses that infect bacteria. Enzyme production – many species of bacteria produce tissue degrading enzymes. Ex. Coagulase, streptokinase. Communicability – ability of a microbe to spread from one host to another is known as communicability. 24

25. Bacterial appendages – bacterial capsule, etc. eg . Klebsiella pneumoniae . Biofilms – self produced extra-cellular polymer matrix. Infecting dose – adequate number of bacteria is required for successful infections. Dosage may be estimated as Minimum Infecting Dose (MID) and Minimum Lethal Dose (MLD) Route of infection – virulence changes with various routes of entry. 25