Diagnosis and Control of Infection

• Epidemiology is the science that studies when and where diseases occur and how they are transmitted in a population. Epidemiologists collect information to determine:

  • the identity of the pathogen causing the disease (aetiology)
  • the predisposing factors that enable susceptible populations to be identified, such as age, sex, and lifestyle.
  • the number of individuals acquiring the disease in a given time (incidence).
  • the mode of transmission
  • and the public health policy and prevention required.
• The traditional epidemiologic model, the epidemiological triad, holds that infectious disease result from the interaction between the agent, host, and environment. In this model, transmission occurs when the agent leaves its reservoir or host through a portal of exit and enters through the appropriate portal of entry to infect another susceptible host. This sequence is called the chain of infection.
• The chain of infection is a concept used to explain how a patient can acquire an infection from another person. It's used by epidemiologists and clinical microbiologists to develop strategies to prevent and control epidemics. The 6 components of the chain of infection, and these will vary depending on the microbe and the disease it causes. For an infection to occur, all the links in the chain need to be connected. Transmission of any given infection is stopped when one or more of these links are broken.
• The spread of infection is the final requirement for a successful pathogen. There are 2 main factors that affect the spread of infection:

  • Reservoirs of infectious organisms, which refer to places where pathogens can grow and accumulate.
  • Modes of transmission, which refer to the various ways in which pathogens move from place to place.
• The reservoir of an infectious agent is the habitat in which the infectious agent normally lives, grows, and multiplies. It's usually from reservoirs that infectious diseases are transmitted to a susceptible host.

  Reservoirs include humans, animals, and the environment/non-living reservoirs. Human reservoirs may not show the effect of illness due to the infectious agent within them, and in these cases, such individuals are classified as carriers. Humans are the only reservoir for the smallpox virus.
• Many of the diseases that have animal reservoirs are transmitted from animal to animal with humans as incidental hosts.

  Zoonosis refers to an infectious disease that is transmissible, under natural conditions, between animals and humans.
• Plants, soil, and water in the environment are also reservoirs for infectious agents. For instance, many fungi live and multiply in the soil.
• Emerging disease describes an unrecognised infection, or previously recognises infection, that has expanded into a new ecological niche. It's often accompanied by a significant change in pathogenicity. Many emerging diseases are zoonotic; animals act as the reservoir for the infectious agent, which only occasional transmission into the human population.
• Emerging diseases can be caused by:

  • newly identified species
  • newly identified strains that have evolved from an already known infection.
  • ecological changes that alter the composition and size of the reservoirs.
  • spread to a population in a new area of the globe
  • re-emerging infection
  • nosocomial (hospital-acquired) infections.
• Once an infectious agent leaves a reservoir, it must be transmitted to a new host if it's to multiply and cause disease. The route by which an infectious agent is transmitted from a reservoir to another host is called the mode of transmission. Prevention and control measures differ depending on the mode of transmission.
• The different modes of transition are:

  • contact transmission
  • indirect transmission via a vehicle or vector
  • horizontal vs vertical
• Contact transmission is when a healthy person is exposed to pathogens by either touching or being too close to an infected person/object.

  • It can be direct if the pathogen is transmitted by person to person without an intermediate object.
  • It can be indirect if the microbe is transferred via a non-living object or fomite, such as towels, eating utensils, stethoscopes, and needles.
  • Droplet transmission is when microbes are spread in mucus droplets that travel short distances (less than a metre). It can occur through sneezing, coughing, or talking.
• Vehicle transmission is the transmission of disease via a medium such as water, food, air, blood, bodily fluid, and intravenous fluids.

  • Waterborne transmission is usually caused by water contaminated with sewage.
  • Airborne transmission is due to inhaling small pathogens and particles, such as bacteria and fungal spores, that are suspended in the air and can travel long distances.
  • Foodborne transmission is typically due to bad sanitation practice leading to contamination of food with pathogens.
• Vector transmission via animals that carry disease from one host to another. Insects are the most important animal vectors:

  • Mechanical transmission is the passive transport of pathogens on the vector's body. Flees are the most common vector. Most of the diseases can also be contracted more directly through contaminated food, water, air, hands, and person-to-person contact.
  • Biological transmission is when pathogens spend part of its life cycle in the vector and transmit to the host through a bite.
• Transmission from mother to child is called vertical transmission, and can occur in utero across the placenta, at the time of delivery, or during breast feeding.

  Person-to-person transmission that is no between mother and offspring is called horizontal transmission.
• There are 4 ground rules in identifying the causative agent of a particular disease. These rules are called Kock's 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 grown in pure culture.
  3. The cultured microorganism should cause disease when introduced into a healthy individual.
  4. The microorganism must be reisolated from the inoculated, diseased experimental host and identified as being identical to the original causative agent.
• Koch's postulates don't apply to all diseases:

  • exceptions to the 1st postulate - asymptomatic or subclinical infection carriers are a common feature of many infectious diseases.
  • exceptions to the 2nd postulate - some microbes cannot be grown in vitro, or there are no susceptible animal species.
  • exceptions to the 3rd postulate - not all organisms exposed to an infectious agent will acquire the infection.
• Laboratory tests are carried out to detect either the presence of microorganisms and their products, or the patient's immune response to infection. Blood and other body fluids are takes from the patient are examined for presence of antibodies against a pathogen.

  A fundamental step in any diagnosis is the choice of an appropriate specimen that will be collected from the patient. Thus, specimens selected for examination should reflect the disease process.
• The two man methods of laboratory diagnostic detections to confirm infection are:

  • direct detection methods - clinical specimens are examined for the 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.

  The specimen to be collected from the patient needs to be selected taking into account the pathogenesis of the infection.
• Usually when a bacterial infection is suspected, the first approach is to culture the clinical samples. This method is used to propagate microorganisms by allowing them to grow in a pre-determined culture medium under controlled lab conditions (temperature, air supply, light, pH). The medium used for the culture is dependent on the microorganism, but mainly, a solid nutrient media, such as agar plates, are used to produce colonies.

  A colony is composed of thousands of bacteria growing on the surface that originates from a single cell.
• Types of media used to culture bacteria:

  • Defined medium - the exact chemical composition is known.
  • Enriched medium - only specific bacterial species can use the medium due to the presence of a particular component only they can use
  • Selected medium - a culture medium designed to support the growth of only specific microorganisms. The selection is done by adding antibiotics or lacking amino acids.
  • Differential medium - distinguishes closely related microorganisms growing on the same medium based on colony appearance due to the presence of dyes or chemicals added to the medium.
• Choosing the appropriate culture medium is important when culturing bacteria because all species of bacteria have their own unique requirements to enable optimal growth and colonization. There is no universal culture medium that will support the growth of all species of bacteria.

  Initially, a medium called blood agar (agar that contains erythrocytes) is used because it supports the growth of many bacteria.