Epidemiology Kept Simple Chapter 3 The Infectious Disease Process Jul-19

Reasons to Study Infx D. Epi Infectious disease control, including emerging and reemerging agents Bioterrorism To illustrate general principals of epidemiology (“One Epi”) Jul-19

What is Infection? Infection ≡ biologic agent living and replicating within host Contamination ≡ agent living on exterior surface of host Silent Infection ≡ infection without disease (commensal) Infectious disease ≡ infection accompanies by pathology Infection ≠ disease! Pathos Jul-19

Components of the Infx Disease Process Agent Reservoir Portals of entry and exit Mode of transmission Immunity Jul-19

1. Agents Helminths (parasitic worms) Fungi & yeast (parasitic plants, lack chlorophyll) Protozoa (eucaryotes; complex life cycles) Bacteria (independent reproduction) Rickettsia (intracellular agents; require Ixodes tick carrier) Viruses (submicroscopic; incapable of multiplication outside of host) Prions (infectious proteins) Jul-19

2. Reservoirs Reservoir ≡ where the agent multiplies and perpetuates Types of reservoirs Symptomatic cases Carriers Animals & insects Inanimate objects Jul-19

Cases Reservoirs Some (not all) cases are contagious Examples of agents with case reservoirs Influenza Measles Smallpox Sexually transmitted diseases Jul-19

Carrier Reservoirs Carrier ≡ contagious individual without discernable signs Types of carriers Inapparent throughout Incubatory Convalescent Jul-19

Animals Reservoirs (Zoonoses) Human disease with an animal reservoir is called a zoonosis. Types of zoonoses Direct zoonosis:  vertebrate animal  human e.g., rabies Cyclozoonoses:  species #1 species #2  human  species #1 (e.g., Echinococcus tapeworm) Jul-19

Types of Zoonoses (cont.) Metazoonoses  vertebrate animal  invertebrate  human e.g., malaria Saprozoonoses vertebrate animal  inanimate object  human e.g. (coccidiomycosis) Valley fever Jul-19

Inanimate Reservoirs Water Food Soil Other Jul-19

Portals of Entry & Exit Respiratory Conjunctiva Urogenital Gastrointestinal Skin Placenta Etc. Jul-19

Transmission by Contact Direct (host  host) Indirect (host  secretion  host) Droplet (airborne, short distance) Nuclei (airborne, suspended) Jul-19

Transmission via Intermediaries Vector (living) Vehicles (inanimate) Vectors types: mechanical, developmental, propagative, cyclopropagative. Jul-19

Transmission Dynamics Common source Serial transmission Jul-19

Transmission: Cycle in Nature Each agent has its unique cycle in nature Example: the blood worm (Shistosoma sp.). Jul-19

Immunity Immunity ≡ all factors that alter likelihood and severity of infection after host is exposed Types of immunity (figure) Jul-19

Innate Immunity Physical barriers: skin, cilia, mucosal, sheaths Chemical barriers: acidity, enzymatic, etc. Non-specific cellular & physiologic responses: phages, polymorphs, inflammation Jul-19

Acquired Immunity Cellular (immunocytes) Lymphocytes Granulocytes Non-cellular (humoral) Antibodies Cytokines Jul-19

Interaction of Innate & Acquired Immunity Jul-19

Immunization Immunization ≡ the act of acquiring immunity Active immunization ≡ host response to exposure (Natural exposure or artificial vaccination) Passive immunization ≡ receipt of immunity products from others (Therapeutic e.g., anti-serums or maternal (transplacental, colostrum) Jul-19

Types of Vaccines Killed vaccine: killed agent, not capable of self-replication Modified live vaccine: attenuated version of agent capable of replication Toxoid: denatured toxin (no agent) Jul-19

Herd Immunity Non-susceptible individuals represented by dark circles. You do not need to vaccinate the entire herd to achieve control Jul-19

Herd Immunity Animation http://www.immunisation.nhs.uk/About_Immunisation/Science/Herd_immunity_-_animation Jul-19