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

Reasons to Study Infx D. Epi Infectious disease control, including emerging agents and bioterrorism To illustrate general principals of epidemiology Apr-19

What is Infection? Infection ≡ biologic agent is living and replicating within a host Contamination ≡ agent living on exterior surface of host Silent Infection ≡ infection without disease (“commensal”) Infectious disease ≡ infection accompanied by pathology Pathos Apr-19

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

1. Agents (from large to small) 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) Apr-19

2. Reservoirs Reservoir ≡ where the agent multiplies and perpetuates Types of reservoirs Cases [symptomatic] Carriers Animals & insects Inanimate objects Apr-19

Carrier Reservoirs Carrier ≡ contagious without discernable signs Types of carriers Inapparent [throughout] Incubatory Convalescent Apr-19

Animals Reservoirs (Zoonoses) Zoonosis = an animal disease transmissible to humans Types of zoonoses Direct zoonosis: vertebrate animal  human (e.g., rabies) Cyclozoonoses:  species #1 species #2  human  species #1 (e.g., Echinococcus tapeworm, right) Apr-19 Source: www.cdc.gov

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

Inanimate Reservoirs Water Food Soil Other Apr-19

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

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

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

Transmission Dynamics Common source Serial transmission Apr-19

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

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

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

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

Interaction of Innate & Acquired Immunity Apr-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) Apr-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) Apr-19

Herd Immunity Non-susceptible individuals represented by dark circles. You do not need to vaccinate the entire herd to achieve infection control if the agent meets a lot of dead ends Apr-19

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