Epidemiology Kept Simple Chapter 3 The Infectious Disease

Epidemiology Kept Simple Chapter 3 The Infectious Disease Process 6/4/2021 1

Reasons to Study Infx D. Epi • Infectious disease control, including emerging agents and bioterrorism • To illustrate general principals of epidemiology 6/4/2021 2

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 6/4/2021 Pathos 3

Components of the Infx Disease Process 1. 2. 3. 4. 5. 6/4/2021 Agent Reservoir Portals of entry and exit Mode of transmission Immunity 4

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) 6/4/2021 5

2. Reservoirs • Reservoir ≡ where the agent multiplies and perpetuates • Types of reservoirs – – 6/4/2021 Cases [symptomatic] Carriers Animals & insects Inanimate objects 6

Carrier Reservoirs • Carrier ≡ contagious without discernable signs • Types of carriers – Inapparent [throughout] – Incubatory – Convalescent 6/4/2021 8

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) 6/4/2021 Source: www. cdc. gov 9

Types of Zoonoses (cont. ) • Metazoonoses vertebrate animal invertebrate human e. g. , malaria (right) • Saprozoonoses vertebrate animal inanimate object human e. g. (coccidiomycosis) Valley fever 6/4/2021 10

Inanimate Reservoirs • • Water Food Soil Other 6/4/2021 11

Portals of Entry & Exit • • Respiratory Conjunctiva Urogenital Gastrointestinal Skin Placenta Etc. 6/4/2021 12

Transmission by Contact • Direct (host host) • Indirect (host secretion host) • Droplet (airborne, short distance) • Nuclei (airborne, suspended) 6/4/2021 13

Transmission via Intermediaries Vector (living) Vehicles (inanimate) Vectors types: mechanical, developmental, propagative, cyclopropagative. 6/4/2021 14

Transmission Dynamics Common source Serial transmission 6/4/2021 15

Transmission: Cycle in Nature Each agent has its unique cycle in nature Example: the blood worm (Shistosoma sp. ). 6/4/2021 16

Immunity • Immunity ≡ all factors that alter likelihood and severity of infection after host is exposed • Types of immunity (figure) 6/4/2021 17

Innate Immunity • Physical barriers: skin, cilia, mucosal, sheaths • Chemical barriers: acidity, enzymatic, etc. • Non-specific cellular & physiologic responses: phages, polymorphs, inflammation 6/4/2021 18

Acquired Immunity Cellular (immunocytes) Lymphocytes Granulocytes Non-cellular (humoral) Antibodies Cytokines 6/4/2021 19

Interaction of Innate & Acquired Immunity 6/4/2021 20

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) 6/4/2021 21

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) 6/4/2021 22

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 6/4/2021 23

Herd Immunity Animation • http: //www. immunisation. nhs. uk/About_Im munisation/Science/Herd_immunity__animation 6/4/2021 24