Host Parasite Relationship and Sources of infections

Host Parasite Relationship and Sources of infections.

Objectives. Definitions of host and parasite Different host-parasite relationships Pathogenesis Source of infection Mode of transmission Route of entry Virulent factors Body’s defence.

Eukaryotes Prokaryotes Fungi Algae Bacteria Protozoa Slime moulds.

Eukaryotes Vs Prokaryotes. The parts of a cell Nucleus Microtubule 2. Mitochondrion 3. Rough endoplasmic 4, retEX11.rn Lysoscrne S Mdo'ilænents 6. 7. Vauole 8. Riboscrnes Golgi oornplex 9. Cell rnernbrano 10. Cenrosome 11. 5 4 3 2 1 6 7 9 11.

Definition of host and parasite. Host - Macro-organism , usually multicellular phylogenetically higher level usually an eukaryote (animal, plant ) able to exist independently Parasite- Microorganism, usually unicellular phylogenetically lower level Living on/in the host → damages → may cause disease obtaining necessities of life from the host prokaryote/ eukaryote.

Symbiosis : living together in more or less intimate association or close union of two dissimilar organisms (macro- and microorganism live together) Mutualism : advantageous for both (reciprocal benefit) Commensalism : one benefits and the other is not significantly harmed or helped Parasitism : one member benefits while the other is harmed damages to macro-organisms by the microorganism (pathogens) → disease of the host endoparasites - live within the host's body ectoparasites - live on host’s surfaces.

Commensalism and mutualism in human as host. Long-term relationship between two different species where both individuals benefit/ one is not harmed or helped Normal microbial flora In skin and mucous membranes , gut and vagina host cell : microbe cell number = 1:10 no entry, no penetration, no colonization inside cells, tissues, organs Bacteria, microscopic fungi - are found as normal flora protozoa and helminths - are not found as normal flora viruses – not found as normal flora, but latent viruses can be shed without symptoms (Human endogenous retroviruses – SDL).

Introduction of a flora. During passage through the birth canal By contact with other people By contact with feeding materials By contact with instruments During nursing \ Ill.

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Advantages of commensalism/mutualism. For microorganisms shelter and food Only colonization, but no entry (penetration) into tissues • For the host (human) to prevent colonization of harmful microbes (pathogens) - Occupying surface and cell receptors - Producing antimicrobial compounds Processing/degrading food components - Producing useful metabolites (vitamin K or B12).

Parasitic microorganisms in human host. By pathogenic mechanisms  damage host cells/ tissues  disease in host Entry  attachment  multiplication  invasion  dissemination spread “pathogenesis”.

Types of parasitism. Obligate parasites: in defined host (range of hosts) always pathogenic, never found in the normal flora Facultative parasites: depending on the condition of both host and microbe and presence of predisposing/ risk factors in host M embers of the normal flora Opportunistic parasites: not pathogenic in normal healthy people take advantage in case of host disorders ( usually immunosuppression) Normal flora/ saprophytes.

Predisposing/risk factors for pathogens. For facultative parasites Physical/mental stress acute diseases, wounds, burns chronic debilitating conditions (diabetes, alcoholism) urinary tract obstruction (Ex. calculi) Medical interventions Nosocomial infections For opportunistic parasites Physiological immune deficiencies (newborn, elderly) Primary immune deficiencies (congenital) Acquired immune deficiencies (infections, medications, tumors).

Characteristics of Parasitism. Encounter: agent meets host Entry: agent enters host Spread: agent spreads Multiplication: agent multiplies Damage: agent, host response, or both Outcome: agent or host wins, or coexist Pathogenesis.

Pathogenesis. Primary replication and innate immune response Invasion Attach Mucosal surface Entry Pathogen Incubation period Shedding Dissemination via blood, lymph, nerve Mild symptoms Disease Secondary sites of infection Secondary replication and adaptive immune response Issue destruction Recovery or death.

Entry. Source of Infection. Susceptible host. Mode of Transmisssion.

Source of Infection. It is defined as the person, animal, object or substance from which an infectious agent passes or is disseminated to the host.

Source of Infection. origin from which a host acquires the infection Exogenous – from outside Humans/ patients Animals/ vectors Objects/ surfaces Food, water, Contaminated medical equipment, contaminated surfaces, soil Endogenous – from own flora.

SOURCES OF INFECTION. Human Beings Patients/ case Carriers Healthy Convalescent Temporary Chronic Contact/ mechanical (hands).

Human reservoir Case Clinical illness Subclinical cases Latent infection Carrier Type Duration Portal of Exit JRC.

Humans patient as a source. Case/ patient (having a overt disease) Clinical case (with symptoms) Mild/ severe Typical/ atypical symptoms Subclinical case Asymptomatic / or mildly symptomatic which does not alert the patient Latent infection Asymptomatic infection capable of showing symptoms under some circumstances if activated..

Humans carrier as a source. Carriers (continue to harbor and shed the pathogens) Incubatory carriers Shed during incubation period (Ex. Measles) Convalescent carriers Shed during convalescent period (Ex typhoid ) clinical recovery does not coincide with bacteriological recovery Healthy carriers Carrier state without suffering from overt disease, but shed (Ex. Meningococcal meningitis) Temporary carriers Are those who shed infectious agent for short periods of time. All the three incubatory, convalescent and healthy are included . Chronic carriers: A carrier who excretes the infectious agent for indefinite period. E.g. typhoid fever, hepatitis B Mechanical carriers – by hand.

Features of a Carrier. Presence of specific microbes in the body Absence of apparent symptoms and signs Shedding of micro-organisms in the discharges or excretions As a source of infection to others.

Animals/ Insects as a source. Zoonosis Infectious diseases transmitted from animals to man Bacterial , rickettsial , viral, fungal, protozoal, helminths Mechanical vectors By mechanical – ex. Dysentery or typhoid bacilli Biological vectors pathogen multiplies in the body of vector- ex. Aedes mosquito in dengue Reservoir Some animals and insects can harbor organisms and act as reservoir of infection Lives, multiplies and transmitted to a susceptible host – ex. Pigs in JE.

Objects as a source. Soil Water Food Medical devices Ex. Catheters, cannulas – cause HAI Surfaces BP cuff, beds/ furniture - HAI.

Inhalation – ex: RTI (pneumonia, COVID-19) Ingestion ( faeco -oral)– ex: gastroenteritis Food borne – food poisoning Water borne – Hepatitis A Contact – ex: chickenpox, COVID-19 Blood transfusion/ body fluids – ex: Hep B, HIV Sexual contact – ex: gonorrhoea, Hep B Trans placental (mother to foetus) - rubella, CMV Vectors – ex: Dengue Iatrogenic/ lab acquired – Ebola.

By dust By respiratory droplets By contarninated water By contarninated food By contact with contarninated objects By arthropod bites By injection Of contarninated soil By contact with anirnals.

Respiratory Tract - most common portal Gastrointestinal - via food, water contaminated fingers Penetration through skin Parenteral Route – through blood/ body fluid Genitourinary tract Trans placental/ Birth canal Multiple portals of entry HIV – parenteral, genitourinary tract, trans placental, birth canal, breast milk.

Pathogenicity/ virulence factors Attachment Invasion Toxin production Multiplication and spread.

Body’s response By immune system Barriers, Cells, Tissues, Secretions Prevent entry of microorganisms failing that, to seek out and destroy them Microorganisms have various mechanisms to evade the immune response If the microbes win the battle  infection of the host.

Characters of pathogens. Organism 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. They must be capable to be shed in excretions/ secretions and to spread to other susceptible hosts.

Stages of infectious disease. Incubation period – no symptoms. Prodromal period – mild and generalized symptoms (fever, weakness, headache). Invasive stage – symptoms specific to the disease. Decline stage – symptoms subside. Convalescence – no symptoms, health returns to normal..

Other terms/ types of infections - SDL. Bacteraemia / Viraemia / fungaemia Septicaemia / Sepsis/ Septic shock Localized/ superficial/ generalized/ deep seated/ disseminated Primary/ reinfection/ superinfection/ secondary/ atypical Community acquired/ hospital acquired Endemic/ Epidemic/Pandemic/ Outbreak.

Summary. Definitions Different host-parasite relationships Pathogenesis Source of infection Mode of transmission Route of entry Virulent factors Body’s defence.

References. Demystified . . . Human endogenous retroviruses; P N Nelson, P R Carnegie,J Martin Infectious diseases epidemiology ;Mauricio L Barreto, Maria Glória Teixeira, and Eduardo Hage Carmo; J Epidemiol Community Health 2006 Mar; 60(3): 192–195.