infectious diseases
TRANSCRIPT
Dr Adam Ibrahim MBBS
Amoud University
Infectious Diseases
Introduction of infectious disease
Overview Classification of disease Stage of infectious disease Epidemiological aspects of infectious disease
(in a population and individual level) Pathogens Disease Laboratory diagnosis of infectious disease
Overview
¨ Infection is the term used to indicate the presence of an infectious agent in an individual or population.
¨ Infectious diseases are the invasion of a host organism by microbes.
¨ Microbes that cause illness are also known as pathogens.
¨ The most common pathogens are bacteria and viruses; some kinds of fungi and protozoa, also cause disease.
¨ An organism that a microbe infects is known as the host for that microbe.
¨ Infectious diseases remain a leading cause of death and disability.
¨ Advances in the control of diseases such as tuberculosis (TB), malaria, and smallpox after the Second World War led to hopes that the threat from infectious disease would diminish.
¨ Factors such as increasing international travel and trade, the breakdown of public health systems in some countries due to war or economic decline and growing resistance to existing antibiotics have meant that infectious diseases continue to be a significant risk to health
¨ There has been ongoing progress in controlling some vaccine-preventable childhood diseases such as polio, diphtheria and invasive Haemophilus influenza type b infection (Hib).
¨ However, new previously unknown diseases have appeared in the past 25 years such as acquired immune deficiency syndrome (AIDS) and hepatitis C.
¨ Well-known diseases such as measles have become more common again after a period of decline.
Human activities that have led to the emergence and spread of new diseases
Encroachment on wildlife habitats. The construction of new villages and housing developments in rural areas brings people into contact with animals--and the microbes they harbor.
Changes in agriculture. Destroying rain forests.
Uncontrolled urbanizationModern transport.High-speed globe-trotting. With
international jet-airplane travel, people infected with a new disease can carry the disease to the far side of the world before their first symptoms appear.
Koch’s Postulates
¨ Koch developed four criteria to demon- strate that a specific disease is caused by a particular agent.1. The specific agent must be associated with every
case of the disease.2. The agent must be isolated from a diseased host and
grown in culture.3. When the culture-grown agent is introduced into a
healthy susceptible host, the agent must cause the same disease.
4. The same agent must again be isolated from the infected experimental host.
Infectious vs. noninfectious disease
¨ Any disease caused by a pathogen is an infectious disease; any disease not caused by a pathogen is a non-infectious disease
¨ Not all infectious diseases are spread from host to host.
¨ Not all infectious diseases are associated with pathogen infection of the host (e.g., intoxications can result from exposure to secreted toxin rather than the secreting organism).
Classification Of Disease
¨ Diseases may be classified in a number of ways, many of which we will subsequently discuss,
¨ including:– Infectious disease– Communicable disease– Noncommunicable disease– Contagious disease
Communicable disease
¨ A communicable disease is an infectious disease that may be passed from individual to individual.
¨ Yet, not all infectious diseases are considered communicable
¨ A more narrow definition of communicable disease is one that is infectious during the incubation or recovery (convalescence) periods of a disease
Noncommunicable disease¨ A noncommunicable disease is an infectious disease
that is not spread from individual to individual but instead is acquired from an inanimate object or other species
¨ Noncommunicable infectious disease may be differentiated into three categories: Infections caused by opportunists arising from an
individual’s own microflora
Poisonings following ingestion of secreted exotoxins
Infections acquired from organisms found in the environment (other than from individuals of one's own species
Contagious disease
¨ A communicable diseases that is easily passed from individual to individual is said to be contagious
¨ Yes, not all communicable disease are considered contagious (though certainly all contagious diseases are considered communicable)
TYPES OF INFECTIOUS DISEASE
¨ Infectious disease can be described as being
Acute, chronic, sub acute, latent, or as an in apparent (subclinical) infection
Local, focal, or systemic
Septicemia, bacteremia, viremia, or toxemia
A primary/ secondary infection, super infection, or a mixed infection
Classification of Infectious Disease¨ By duration
– Acute – develops and runs its course quickly.– Chronic – develops more slowly and is usually less severe, but may
persist for a long, indefinite period of time.– Latent – characterized by periods of no symptoms between outbreaks
of illness.¨ By location
– Local – confined to a specific area of the body.– Systemic – a generalized illness that infects most of the body with
pathogens distributed widely in tissues.¨ By timing
– Primary – initial infection in a previously healthy person.– Secondary – infection that occurs in a person weakened by a primary
infection.
Phases of Infectious Disease
¨ Diseases caused by infectious agents usually run a standard course that is associated with different signs and symptoms.
¨ Before we describe these different phases of infectious diseases, we must first define the terms, “signs” and “symptoms.”
¨ Signs of an infectious disease are characteristics of a disease that can be observed by examining a patient.
¨ They include things such as fever, coughing, rash, vomiting, and diarrhea.
¨ Symptoms, on the other hand, can be felt only by the patient. They include pain, headache, and nausea.
STAGES OF INFECTIOUS DISEASE
¨ Infectious diseases tend to occur in stages including (in typical order) Incubation period
Prodromal phase (not typical)
Invasive phase
Decline phase
Convalescence period
Sequelae (not typical)
Phases of Infectious Disease
¨ Incubation period – time between infection and the appearance of signs and symptoms.
¨ Prodromal phase – mild, nonspecific symptoms that signal onset of some diseases.
¨ Clinical phase – a person experiences typical signs and symptoms of disease.
¨ Decline phase - subsidence of symptoms.¨ Recovery phase – symptoms have disappeared,
tissues heal, and the body regains strength.
Infection in Individuals – definition
¨ Depending upon the host-microbe interaction, infection can be: Silent (inapparent, asymptomatic, subclinical),
or
Overt, causing a disease of infection.¨ Silent or asymptomatic infections, such as
HIV infection during the early phase, can still be infectious.
¨ A carrier is a person who is infected with an organism but shows no evidence of disease, although disease may have been present earlier
¨ Acute infection implies a 'short-lived' infection, such as influenza, with or without symptoms; the period of infectivity is also short.
¨ Chronic infection refers to a 'long-standing' condition during which the pathogen continually replicates and the patient may be persistently infectious to others, e.g., hepatitis B infection.
¨ Latent infection refers to a persistent infection with the possibility of intermittent shedding of pathogens, e.g., varicella zoster virus causing shingles or herpes simplex virus causing cold sores or genital herpes
Infection in Populations
¨ Endemic infection refers to infection or disease that occurs regularly at low or moderate frequency.
¨ Pandemics are global epidemics. The size of 'outbreaks is dependant upon factors such as the ratio of susceptible to immune subjects, period of infectivity, population density, etc.
Spread of Infection
¨ With respect to the spread of infection, people can be divided in to : Those who are susceptible,
Those who are infected but are not yet infectious,
Those who are infected and infectious, and
Those who are immune.
Recovery from infection usually gives
immunity.
Means of transmission
¨ Contact requires direct or indirect contact ( fomites, blood, or body fluids)
Food ingestion of contaminated food or water water
Airborne inhalation of contaminated air
Vector- borne dependent on biology of vector as well as infectivity of organism
Perinatal similar to contact infection, however, the contact may occur in uterus or during the delivery
Robin Cochran-Dirksen (Many slides from BioEdOnline Baylor Christine Herrmann PhD)
Transmission of Infectious Diseases
¨ Agents that cause infectious diseases can be transmitted in many ways.– Through the air– Through contaminated food or water– Through body fluids– By direct contact with contaminated
objects– By animal vectors such as insects, birds,
bats, etc.
Courtesy of VOA
Chinese students wearing masks during a
SARS outbreak
Courtesy of CDC
Aedes aegypti mosquito Known to transmit
Dengue fever
Incubation Period
¨ The incubation period is defined as the time between exposure to the onset of symptoms or other signs of infection.
¨ Diseases have incubation periods that vary widely
Infectivity
Ability of agent to cause infection
Number of infectious particles required
In person-to-person transmission, secondary attack rate is a measure of infectivity
In person-to-person transmission, the higher the infectivity, the more secondary infections. For example, the infectivity of tuberculosis is low and the infectivity of smallpox is high.
Pathogenicity
Ability of a microbiological agent to induce disease
Because the smallpox virus can produce disease relatively easily, it is considered to have high pathogenicity.
Polio has low pathogenicity.
Immunogenicity
Ability of an organism to produce an immune response that provides protection against reinfection with the same or similar agent
Can be life long or for limited periods
Important information for development of vaccines
Herd immunity
When the number of immune persons is high enough that it is unlikely that a susceptible person will have contact with an infected person
Level of immunity required is dependent on the characteristics of the infectious disease
Pathogenic mechanism
¨ Direct tissue invasion¨ Production of a toxin¨ Immunologic enhancement or allergic
reaction¨ Persistent or latent infection¨ Enhancement of host susceptibility to
drugs¨ Immune suppression
Pathogen
¨ The pathogens fall into five main groups:
1. Viruses
2. Bacteria
3. Fungi
4. Protozoa
5. Helminthes
Definitions
¨ Commensalism: Literally `eating at the same table'! This refers to a neutral situation where the host and bacteria live together, but have no effect on each other's life cycle - either positive or negative..
¨ Parasitism: This describes an unequal relationship where one organism clearly benefits from an association to the detriment of another.
¨ Opportunistic infection: This occurs when the normal human defences are so weakened that it allows infection to take place by organisms that would not generally be able to cause infection in a healthy human.
¨ Nosocomial infection: These are infections that are transmitted in hospitals.
¨ Some of these may be opportunistic infections mentioned above affecting seriously ill patients, others, for example infections with Methicillin-Resistant Staphylococcus aureus (MRSA), may occur because of the special nature of the hospital environment
Bacterial pathogenesis
¨ Host susceptibility¨ Bacterial infectivity¨ Host resistance¨ Genetic and molecular basis for virulence¨ Host mediated pathogenesis¨ Intracellular growth¨ Virulence factors
Disease
¨ Disease is a disturbance in the state of health (of the host) wherein the body cannot carry out all its normal functions
¨ When an infection causes disease, the effects of the disease range from mild to severe
¨ Disease, or illness, is characterized by changes in the host that interfere with normal function
Production of Disease
¨ Once a bacterium has entered the body, there are a number of ways in which it can cause disease.
Local Effects
Distant Effects( Endotoxin and exotoxins)
Evasion of Host Defences
Virulence factors
¨ Properties of a pathogens that allow them to cause disease are termed virulence factors
¨ Virulence factors may distinguish a pathogenic microorganism from otherwise identical non-pathogenic microorganisms by allowing pathogens to invade, adhere to, and colonize a host, and then harm the host
Types of virulence factors
¨ The following are types of virulence factors: Adherence Factors: Many pathogenic bacteria
colonize mucosal sites by using pili (fimbriae) to adhere to cells.
Invasion Factors: Surface components that allow the bacterium to invade host cells can be encoded on plasmids, but more often are on the chromosome.
Capsules: Many bacteria are surrounded by capsules that protect them from opsonization and phagocytosis.
Endotoxins: The lipopolysaccharide endotoxins on Gram-negative bacteria cause fever, changes in blood pressure, inflammation, lethal shock, and many other toxic events.
Exotoxins: Exotoxins include several types of protein toxins and enzymes produced and/or secreted from pathogenic bacteria. Major categories include cytotoxins, neurotoxins, and enterotoxins.
Laboratory diagnosis of infectious disease¨ Is based one or more of the following:
Direct examination of the specimen by microscopic examination or antigenic technique.
Isolation of organism in culture Sero – diagnosis.Molecular genetic detection.
Laboratory diagnosis of bacteria and fungal infection¨ Microscopy : gram stain remain extremely
useful diagnostic technique because it is a rapid and inexpensive method. Isolation and identification of organism: blood culture , CSF culture . urine/stool/genital/throat/respiratory culture.
Laboratory diagnosis of viruses infection By electronic microscopy Antigenic detection Viral isolation in culture Serologic detection Detection of viral genome by molecular
biology technique
Laboratory diagnosis of parasitic infection.¨ Most parasites are detected by
– Microscopic examination of clinical specimen: – e.g. plasmodium, bobesai spp; leishmania spp;
helminthes egg, etc.– Blood smear – Fecal specimen.
¨ Serological diagnosis are good for trichinella, toxoplasmosis, and strongyloidiasis.
Principles of Antibacterial Therapy
¨ age-appropriate antibiotic dosing and toxicities must also be considered.
¨ Specific antibiotic therapy based → microbiologic diagnosis → supported by antimicrobial susceptibility testing.
¨ Common practice is based on a clinical diagnosis with empirical use of antibacterial agents.
Empiric use of antibacterial agents
¨ Appropriate empirical use of antibacterial agents in infants and children is essential.
¨ A complete history + physical examination+ appropriate laboratory + radiographic studies= specific diagnoses= empiric antibacterial use.
¨ Antibiotic action is related to achieving therapeutic levels at the site of infection.
The Patterns Of Antimicrobial Resistance¨ Resistance to penicillin and cephalosporin
antibiotics.¨ Antimicrobial resistance occurs through
many modifications of the bacterial genome
Thank you for your attention