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Infectious diseases

• Pathogens – bacteria, viruses, protozoans, fungi, and other parasites capable of causing change that dirsrupts homeostasis in the body

• Infectious diseases - Any disease caused by the presence of pathogens in the body

• main sources of pathogens:– soil

– contaminated water

– infected animals/people

Infectious diseases

• Not all microorganisms are pathogenic

• body is host to billions of microorganisms (mostly bacteria)

• have a symbiotic relationship with the body helping to keep harmful bacteria and other microorganisms from growing

Normal microflora

Normal Micro flora & its importance

1. Prevent the growth of pathogens

2. Stimulate the immune system to produce antibodies that cross-react with invading pathogens

3. Aid in digestion of cellulose in ruminants

4. Produce essential nutrients

Koch’s Postulates

1. same pathogen must be present in every case of the disease

2. The pathogen must be isolated from the diseased host and grown in pure culture

3. The pathogen from the pure culture must cause the disease when introduced into a healthy but susceptible organism

4. The pathogen must be isolated from the inoculated animal and be shown to be the original organism.

Koch’s Postulates: modifications

1. Some infectious agents cannot be cultured e.g. prions

2. Some pathogens have non-virulent strains whose presence does not link them to a disease. E.g. non encapsulated Diplococcuspneumoniae

Types of Pathogens

1. Bacteria

a. Gram positive (S. aureus, S. pneumoniae)

b. Gram negative (Y. pestis, Salmonella typhi)

Types of Pathogens

2. Parasites (Eukaryotic Pathogen)

a. Fungi

e.g. Candida, Aspergillus

b. Protozoa

e.g. Plasmodium, Schistosoma

c. Worms

e.g. Ascaris, Taenia

Candida

Plasmodium

Ascaris

Types of Pathogens

3. Viruses

• Are pieces of DNA or RNA surrounded by protein coat.

• Encapsulated e.g. HIV, HBV, measles, mumps, influenza, rabies

• Non-encapsulated e.g. adenoviruses, HPV, Polio

rabies

polio

Modes of transmission

• Direct contact – touching, handshaking, or sexual

intercourse

• Indirect contact – food, water or droplets in air;

• Animal vectors– insect bites in malaria, plague and,

dog bite in rabies

Reservoirs of pathogens

Humans

• carriers are a significant reservoir of infectious disease

• communicate the disease before experiencing symptoms

Animals

• other main reservoir for pathogens, making the eradication of certain pathogens almost impossible

Pathogenesis

Sequence of activities

1. Transmission of causative

agent to susceptible host

2. Adherence of the agent to a

target tissue

3. Colonization and invasion

4. Damage to host by toxins or

other mechanisms

• Pathogenicity - the potential capacity of certain species of microbes to cause a disease (ability to cause disease)

• Virulence - degree of pathogenicity within a group or species of parasites as indicated by case fatality rates and/or the ability of the organism to invade the tissues of the host

Virulence factors - molecules expressed and secreted by pathogens (bacteria, viruses, fungi and protozoa) that allow them to:

• colonize of a niche in the host (this includes adhesion to cells) for example

• evade the host's immune response (Immunoevasion)

• inhibit the host's immune response (Immunosuppression)

• obtain nutrition from the host

Pathogenicity of an organism is determined by its virulence factors

Virulence factors

factors that confer pathogenicity include

• Pili that facilitate attachment

• Capsules that interfere with phagocytosis

• Exotoxins

• Endotoxins

• Proteases that break down antibodies

• Ability to vary antigens to evade antibodies

Endotoxin Exotoxin

• Lipopolysaccharide

• Gram (-)

• Can’t act as enzymes

• part of the outer membrane of cell wall

• less toxic

• Heat stable

• Poor antigens

• Soluble protein

• Gram (+) and (-)

• Can act as enzymes

• are extracellular component

• More toxic

• Not heat stable

• antigenic

Malaria

1. Causative Agent: Plasmodium (4 species)

2. Symptoms (Clinical features): fever, chills, anaemia, headache, nausea, shivering, convulsions (esp. in under 5 yrolds) enlarged spleen.

3. Pathogenesis: site of action of pathogen include: liver, RBC, brain.

4. vector, female Anopheles mosquito, transfer pathogen during feeding

5. Epidemiology: Endemic in 91 tropical and subtropical countries. Invade the liver 1st and move to reproduce in RBCs resulting in their rupture and the associated chills.

6. Incubation Period: 1 – 2 weeks.

7. Lab diagnosis: Microscopy.

Malaria

Infected RBCs

• Reduce the number of mosquitoes; destruction of larvae and adult mosquitoes by biological and chemical control methods

• Avoid being bitten; protective clothing and creams, treated bed nets

• Use of drugs to prevent infection; chemoprophylaxis

• Combination therapy: Artesunate Amodiaquine

Malaria prevention

Emerging infectious diseases

Newly identified & previously unknown infectious agents that cause public health problems either locally or internationally

Re-emerging infectious disease

Infectious agents that have been known for some time, had fallen to such low levels that they were no longer considered public health problems & are now showing upward trends in incidence or prevalence worldwide

Factors contributing to emergence

Infectious agent

• Evolution of pathogenic infectious agents

(microbial adaptation & change)

• Development of resistance to drugs

• Resistance of vectors to pesticides

Factors contributing to emergence

Host

• Human demographic change (inhabiting new areas)

• Human behavior (sexual & drug use)

• Human susceptibility to infection (Immunosuppression)

• Poverty & social inequality

Factors contributing to emergence

Environment

• Climate & changing

ecosystems

• Economic development &

Land use (urbanization,

deforestation)

• Technology & industry (food

processing & handling)

• International travel &

commerce

• Breakdown of public health

measure (war, unrest,

overcrowding)

• Deterioration in surveillance

systems (lack of political will)

Animals to Humans transmission

• More than 75% emerging infections originate from animals-wild & domestic

• Emerging Influenza infections in Humans associated with Geese, Chickens & Pigs

• Animal displacement in search of food after deforestation (Yellow fever, Malaria)

Poverty, Neglect & Weak Health Infrastructure

• Poor populations- major reservoir & source of continued transmission

• Poverty- Malnutrition- Severe infectious disease cycle

• Lack of funding, Poor prioritization of health funds, curative vs. preventive infrastructure

• Failure to develop adequate health delivery systems

Dr. KANUPRIYA CHATURVEDI

Uncontrolled Urbanization & Population Displacement

• Growth of densely populated cities- substandard housing, unsafe water, poor sanitation, overcrowding, indoor air pollution (>10% preventable ill health)

• Problem of refugees & displaced persons

• Diarrheal & Intestinal parasitic diseases

Dr. KANUPRIYA CHATURVEDI

• Unsafe sexual practices (HIV, Gonorrhoea, Syphilis)

• Changes in agricultural & food production patterns- food-borne infectious agents (E. coli)

• Increased international travel (Influenza)

• Outdoor activity

Human behavior

Dr. KANUPRIYA CHATURVEDI

Causes:

• Wrong prescribing practices

• non-adherence by patients

• Counterfeit drugs

• Use of anti-infective drugs in animals & plants

Antimicrobial drug resistance

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Consequences

• Prolonged hospital admissions

• Higher death rates from infections

• Requires more expensive, more toxic drugs

• Higher health care costs

Antimicrobial drug resistance

Dr. KANUPRIYA CHATURVEDI

Examples of recent emerging diseases

Source: NATURE; Vol 430; July 2004; www.nature.com/nature

Dr. KANUPRIYA CHATURVEDI

Dealing with emerging diseases

• Surveillance at national, regional, global level• epidemiological

• laboratory

• ecological

• Anthropological

• Investigation and early control measures

• Implement prevention measures• behavioural, political, environmental

• Monitoring, evaluation

Dr. KANUPRIYA CHATURVEDI

Public health surveillance & response systems

• Rapidly detect unusual, unexpected, unexplained disease patterns

• Track & exchange information in real time

• Response effort that can quickly become global

• Contain transmission swiftly & decisively

Proposed solutions

Dr. KANUPRIYA CHATURVEDI

• Internet-based information technologies– Improve disease reporting

– Facilitate emergency communications &

– Dissemination of information

• Human Genome Project– Role of human genetics in disease susceptibility,

progression & host response

Dealing with emerging diseases

Dr. KANUPRIYA CHATURVEDI

• Microbial genetics– Methods for disease detection, control & prevention

• Improved diagnostic techniques & new vaccines

• Geographic Imaging Systems– Monitor environmental changes that influence

disease emergence & transmission

Dealing with emerging diseases

Dr. KANUPRIYA CHATURVEDI

• Coordinated, well-prepared, well-equipped PH systems

• Partnerships- clinicians, laboritarians & PH agencies

• Improved methods for detection & surveillance

Dealing with emerging diseases

Dr. KANUPRIYA CHATURVEDI

• Effective preventive & therapeutic technologies

• Strengthened response capacity

• Political commitment & adequate resources to address

• underlying socio-economic factors

• International collaboration & communication

Dealing with emerging diseases

Zombies: 28 Weeks Later

Zombie infection: group activity (wine making)

• From the film 28 Weeks Later characterize

• Causative agent• symptoms• Mode of transmission/pathogenesis• Vector• Incubation period• Virulent factors• Epidemiology• Diagnosis method

• What real life infectious agent/s does the one in the movie resemble?

• How would you prevent spread of infection?

• How would you eradicate the causative agent?

Infectious Disease Terms

1. Epidemiology

2. Epidemic

3. Endemic

4. Pandemic

5. Pathogen

6. Opportunist

7. Nosocomial

8. virulence

Yersinia pestis

• Gram-negative rod-shaped coccobacillus

• facultative anaerobe that can infect humans and other animals

• Vector: fleas• Proteins from the bacteria form a biofilm in the proventriculus of

the flea• biofilm contains aggregated bacteria and blood clot• Biofilm prevents the fleas from feeding: ingested blood dislodges

the bacteria in the esophagus and regurgitated into bitten animal (rodents)

• Reservoir host/carriers: rodents

Yersinia pestis

• Fleas to human: bites of fleas carrying Y. pestis allow the bacteria to overcome the skin barrier

• virulence factors are anti-phagocytic in nature

• Has the ability to suppress and avoid normal immune system responses such as phagocytosis and antibody production

• proliferates inside lymph nodes where it is able to avoid destruction by cells of the immune system

Scanning electron micrograph of a phagocyte (yellow, right) engulfing anthrax bacilli (orange, left)

cholera

1. Causative Agent: Vibrio cholerae

2. Symptoms: severe diarrhoea up to 20 liters a day of “rice water stool”, vomiting, muscle cramps caused by loss fluid and electrolytes.

3. Pathogenesis: Vibrio adheres to the small intestinal lining, multiply and produce the enterotoxin choleragen which causes the accumulation of cAMP. An increased secretion of water and electrolyte from the cells results

Cholera

4. Epidemiology: Feacally contaminated

water, crabs and vegetables fertilized

with human faeces. Has been eradicated

most developed countries but a new

strain discovered in 1992 is threatening

another pandemic.

Cholera

5. Incubation period: 12-48 hours

6. Lab diagnosis: Microscopy, culture of

sample from faeces or vomit.

7. Prevention: Purification of water, washing

of hands.

8. Treatment: administration of solution of

glucose and electrolyte orally or

intravenously; tetracycline antibiotic orally

Tuberculosis

1. Pathogen: M. tuberculosis (pulmonary TB);

M. bovis(GI TB)

2. Transmission: airborne droplets (NB MTB is

dessication resistant and survives in dried

sputum); unpasteurized milk.

3. Clinical features: prolonged coughing

sometimes with bloody sputum, shortness of

breath, fever, sweating , weight loss

Tuberculosis

4. No toxin production. Pathogenicty is by invasiveness that produce characteristic lesions in the lungs.

5. Epidemiology: pathogen triggers acute inflammatory response + forms tubercle –giant cells containing MTB and surrounded by epithelial cells. Tubercles heals by fibrosis and calcification. Can desseminate via bloodstream to other internal organs