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What is epidemiology?
Essentially the study of transmission and proliferation of disease and its application to control disease
What factors affect the successful transmission of a microorganism
- 1. Number of microorganisms shed
- most of shed microorganisms die, with few reaching potential hosts alive ∴ more shedding increases the chance of reaching a fresh host
- 2. Microorganism stability in the environment
- thermostability and resistance to drying increase stability (Clostridial endospores, cysts)
- decreased stability creates reliance on close contact, vector, or food/water contamination for infection
- 3. Efficiency of infection (# needed to cause infection)
- varies between organism (10 Shigella dystenteriae vs 106 Salmonella spp) and route of infection (10 gonococci in urethra vs 1000s gonococci in the oropharynx)
- 4. Genetic factors in microorganisms: some strains are more readily transmitted than others
- 5. Utilizing host responses to benefit the microorganism
- Coughing and sneezing can increase fluid secretions and make transmission more efficient
- Diarrhea is a highly effective spreading mechanism
How are the most common infections spread?
How is humidity related to transmission from the respiratory tract?
- Ill-ventilated rooms are more humid, favoring survival of suspended microorganisms
- Air-conditioned rooms tend to be dry which impair mucociliary activity
What are the VERY BASIC 3 steps in fecal-oral spread
- Poor public health and hygiene
- Fecal microbes appear in water and food
- Susceptible individuals are infected
What are the two methods of venereal transmission w/ organisms and description?
- Purulent (pus-like) discharge: N. gonorrhoeae, Chlamydia trachomatisinduce discharge to carry organisms over epithelial surfaces easily for transmission
- Mucosal sores (ulcers): herpes simplex virus
- cold sores are sites that contain virus particles
What are the social factors that facilitate STDs?
- An increased number of sexual partners per person
- Decline of belief that sexual activity is sinful
- Knowledge that STDs are treatable and pregnancy is avoidable
- ***favor of the pill over mechanical barriers for contraception
What are examples of infectious agents of the oropharynx?
- streptococci and tubercle bacilli reach saliva during respiratory tract infections
- paramyxovirus, herpes simplex virus, cytomegalovirus, and human herpesvirus type 6 infect the salivary glands
What are the two methods of transmission from the skin? Which is more common? Example microorganisms?
- Direct contact/contaminated fingers is much more common than release into environment
- Can include staphylococci and human papillomaviruses
What organisms can be shed in human milk?
HIV, cytomegalovirus (CMV), human T-cell lymphotropic virus (HTLV1)
What are the two main paths for transmission from blood?
- Blood-sucking arthropods
- Needles (transfusion, drug use, etc)
What is vertical transmission? Horizontal? Importance of each?
- Vertical transmission: from parents to offspring via sperm, ovum, placenta, milk, or blood
- Only important in small, isolated, communities since there are only a few affected individuals per lifecycle
- Horizontal transmission: from individual to individual
- Cause quick spread amongst populations
What are the most important vectors? Examples of devastating diseases caused by vectors w/ organism.
- Insects, ticks, and mites (the bloodsuckers)
- Yersiania pestis spread by fleas of rats caused the plague
- Rickettsia typhi, Rickettsia prowazekii spread by lice caused typhus
- Plasmodium spread by Anopheles mosquito causes malaria
Aquatic molluscs are necessary intermediate hosts for....?
Schistosomes (blood flukes)
Describe the major infections from domestic pets
- Toxocariasis: (from dogs) caused by roundworms (Toxocara canis) that can be transmitted from dogs OR cats. Can cause blindness and rheumatic, neurologic, or asthmatic symptoms
- Toxoplasmosis: (from cats) caused by protozoan Toxoplasma gondii which affects brain, lung, heart, eyes, or liver