Diagnosis and control of infection Flashcards
(38 cards)
What is the chain of infection?
-concept used to explain how a patient can acquire an infection from
another person
-transmission of infection is stopped by breaking one or more of the links
What 2 main factors affect the spread of infection?
Reservoirs of infectious organisms
-places where pathogens can grow and accumulate
Modes of transmission
-the various ways in which pathogens move from place to place
What are disease reservoirs?
-Habitat in which an infectious agent normally lives, grows, and multiplies
-Usually the source from which it is transmitted to a susceptible host
-Zoonotic diseases spread between animals and people
-The natural reservoir of some
diseases remains unknown
What are emerging diseases?
-Unrecognised infection, or a previously recognised infection that has expanded into a new ecological niche, often accompanied by a significant change in pathogenicity
-Many emerging diseases are zoonotic - an animal reservoir incubates the organism, with only occasional transmission into humans
Examples of emerging diseases
-Newly identified species (e.g. HIV and AIDS)
-Newly identified strains that have evolved from a known infection (e.g. influenza)
-Ecological changes that alter the composition and size of reservoirs (e.g. Lyme disease)
-Spread to a population in a new area of the globe (e.g West Nile fever)
-Re-emerging infections like drug resistant tuberculosis
-Nosocomial (hospital-acquired) infections, such as Methicillin-resistant S. aureus (MRSA)
What are the 3 main modes of transmission?
-Contact transmission
-Indirect transmission - vehicle or vector
-Horizontal (vs vertical)
What is contact transmission?
-Healthy person is exposed to pathogens by touching or being close to an infected person or object
-By touching, kissing, sexual intercourse
-E.g. Hepatitis A, Smallpox, Staphylococcal infections
What is indirect contact transmission?
-Microbe is transferred via a nonliving object or fomite, such as towels, eating utensils, thermometers
-E.g. Herpes simplex virus, Giardia, Impetigo
What is droplet transmission?
-Microbes are spread in mucus droplets that travel short distances
-Can occur through sneezing, coughing, or talking
-E.g. Typical of respiratory viruses: influenza, adenovirus
What is vehicle transmission?
-Transmission of disease via mediums such as water, food, air, blood, body fluids
-Airborne transmission is not the same as droplet transmission
-It’s caused by inhalation of small pathogens and particles (e.g. bacterial and fungal spores) that are suspended in air
E.g. Anthrax, tuberculosis, salmonella
What is vector transmission?
-Transmission of disease via animals that carry disease from one host to another
-Insects are most important animal vectors
-Biological transmission is when pathogens spend part of their life cycle in the vector and transmission to the host is through a bite
-E.g. Malaria, Zika virus, Dengue fever
Horizontal vs Vertical transmission
Vertical transmission
-transmission from mother to child
-can occur across placenta, at the time of delivery or during breast feeding
E.g. HIV, Rubella, Toxoplasmosis
Horizontal transmission
-person-to-person transmission that is not between mother and offspring
What were Koch’s 4 postulates
1) The microorganism must be found in abundance in all organisms suffering from the disease, but not in healthy organisms
2) The microorganism must be isolated from a diseased organism and grow in pure culture
3) The cultured microorganism should cause disease when introduced into a healthy organism
4) The microorganism must be reisolated from the inoculated, diseased experimental host and identified as being identical to the original specific causative agent
3 exceptions to Koch’s postulates
1st) Asymptomatic or subclinical infection carriers are a common feature of many infectious diseases
E.g. cholera, typhoid, polio
2nd) Some microbes cannot be grown in vitro or there are no susceptible animal species
E.g. syphillis, leprosy, wart viruses
3rd) Not all organisms exposed to an infectious agent will acquire the infection E.g. resistance to malaria conferred by possessing at least one sickle cell allele
What are the 2 types of detection methods used in a diagnostic laboratory to confirm infection?
Direct detection methods
-clinical specimen is examined for presence of a microbe or its products.
-these include culture, microscopy and molecular methods
Indirect detection (serological)
methods
-blood and other body fluids are examined for the presence of antibodies against a pathogen
Explain the culture of bacteria
-Method used to propagate microorganisms by allowing them to grow in predetermined culture media
under controlled laboratory conditions (e.g. temperature, air supply, light, pH)
-Uses solid nutrient media (agar plates) to produce colonies
-A colony is composed of thousands of bacteria growing on the surface that originate from a single cell
What are the 4 types of medium in bacterial culture?
Defined medium:
-if the exact chemical composition is known
Enrichment medium:
-contains some component that permits the growth of specific types or species of bacteria, usually because they alone can utilize the component from their environment
Selective medium:
-culture media designed to support the growth of only specific microorganisms (e.g. selection done by adding antibiotics or lacking amino acids)
Differential medium:
-distinguishes closely related microorganism growing on the same media on a difference in the colony appearance due to the presence of certain dyes or chemicals in the media
Explain the culture of viruses
-Propagation requires cell cultures as viruses only replicate in living cells
-Cells that support viral replication are called permissive
-Multiplicity of infection (MOI) refers to the number of virions that are added per cell during infection
-A virus-infected cell can show dramatic changes in appearance or cytopathic effects (CPE) such as cell rounding and detachment from surface, cell fusion (syncytia) or
inclusion bodies
Describe microscopy and whta it is used for
-Microscopy and molecular techniques based on detection of nucleic acids provide a rapid indication of microbial infection in a matter of hours
-Used either with wet or fixed samples
-Can be used in combination with stains
-Can study bacteria morphology and staining characteristics
-Used to determine virus-induced CPE. To see virus morphology electron microscopy is needed
Describe the methods of detection of nucleic acids
-Molecular tests can detect the presence of bacterial and viral DNA or RNA in a patient specimen
-Can be used to detect organisms that are slow or difficult to grow in the laboratory, antibiotic resistance genes or virulence factors
-Very sensitive and specific
What are the 2 main types of detection of nucleic acids?
-Hybridisation techniques with
nucleic acid probes
-Polymerase chain reaction (PCR)
What are serological tests used for?
-To determine the presence of antibodies in serum or microbial antigens in tissue or body fluids
Describe the process of serological tests
-Titre refers to antibody concentration in the sample and
is associated with the number of times one can dilute a sample and still detect the antibody
-Paired sera samples need to be collected during acute phase (5-7 days after onset of symptoms) and
convalescence phase of infection (after 2-4 weeks).
-At least a four-fold rise in specific antibody titre between acute and convalescent samples (seroconversion) must be found for a diagnosis to be made
-Presence of IgM but not IgG antibodies can be an indication of current active infection
-Does not distinguish between previous or current infection
-Serological methods are retrospective
Advantages/disadvantages of direct/indirect methods to confirm infection
