Principles of infectious disease

Definition of infection

a. Complex process of interaction between pathogen

and human body

b. Infection is composed of three factors: pathogen,

host and environment

What is an infection??

Infectious DiseasesInfectious Diseases

Bacterial Gram-negativeGram-positive

Viral DNA virusRNA virusEnveloped vs non-enveloped

Fungal DisseminatedLocalized

Parasitic ProtozoaHelminths

Microbiological Classification of Infectious Diseases

EPIDEMIOLOGY OF INFECTION

PATTERNS OF INFECTION IN DEVELOPED COUNTRIES

• During the last 100 years the incidence of communicable diseases in developed countries has fallen dramatically.

• This has been due to improved nutrition, better sanitation and housing , immunizations and antimicrobial chemotherapy.

• Infections such as diphtheria, poliomyelitis and tetanus

have decreased and in some locations have almost disappeared.

• Smallpox, has been eradicated from the world while another lethal infection, human immunodeficiency virus (HIV), has emerged in pandemic proportions.

INFLUENCES ON PATTERNS OF INFECTION IN DEVELOPED COUNTRIES

The factors responsible for these changes are:Vaccines • Improved uptake of vaccines. • New vaccines, e.g. conjugate vaccines for Haemophilus

influenzae type B, meningococcal type C disease, pneumococcal vaccine

Animal husbandry and preparation of food • Salmonella and Campylobacter infections originating in

poultry and eggs. • Escherichia coli type O157, causing haemorrhagic colitis

and haemolytic uraemic syndrome, associated with beef. • Listeria infections from soft cheeses.

Microbial resistance • Staphylococcus aureus (meticillin-resistant, MRSA; glycopeptide- resistant, GRSA),• Gram-negative bacilli (extended spectrum β-lactamase resistance, ESBL), • Streptococcus pneumoniae (penicillin),• vancomycin-resistant enterococci (VRE) and • multidrug-resistant Mycobacterium tuberculosis (MDRTB)

Sexual behaviour • Increase in HIV infection and other sexually transmitted diseases

International travel • Importation of malaria• Legionnaires' disease from holiday hotels. • HIV infection.

Immunosuppression • Advances in the treatment of malignant disease and in

organ transplantation, leading to infections with opportunistic organisms

Resurgence of infections • Tuberculosis-world-wide, especially in association with

HIV infection • Poliomyelitis in the Netherlands (in a religious sect

refusing vaccines) • Streptococcal infections in the USA (including rheumatic

fever) • Measles in the USA (mainly in immigrants in inner cities) • Diphtheria in the former Soviet Union • Hepatitis A and typhoid fever in the former Yugoslavia

Injection drug addiction

'New' and emerging infections

1. West Nile fever in the USA

2. Bioterrorism.

3. Avian/pandemic 'flu.

4. Swine flu

Killers of children, preventable but variably prevalent

• Measles • Diphtheria • Pertussis • Poliomyelitis • Tetanus • Hepatitis B • Gastroenteritis • Malaria • Meningococcal disease • Acute diarrhoeal illness

Contact Requires direct or indirect contact (fomite, blood, or body fluid)

Food or Water Ingestion of contaminated food or water

Airborne Inhalation of contaminated air

Vector-borne Dependent on biology of vector as well as infectivity of organism

Perinatal

Sexual

Similar to contact infection, however, the contact may occur in utero or during delivery. transmission by sexual intercourse.

Means of Transmission of Infectious Diseases

Factors Influencing Disease Transmission

• Weather

• Housing

• Geography

• Occupational setting

• Air quality

• Food

EnvironmentAgent

Host• G Age

• Sexe

• Behaviour

• Nutritional status

•Health status

• Infectivity

• Pathogenicity

• Virulence

• Immunogenicity

• Antigenic stability

• Survival

Barriers for Defense Against Infection:Infectious DiseasesInfectious Diseases

1. Skin: Prevents entry of infectious organisms, unless injured. Severe burn patients who die are usually killed by infections. So much skin is

damaged they are very vulnerable to infections.

2. Mucus membrane: Mucous is usually rich in enzymes that will kill many pathogens

3. Cilia: These are hair-like structures lining the respiratory tract. They work to sweep

foreign particles out of the respiratory tract. Damaged by smoking, leaving smokers more vulnerable to infections.

4. Coughing:

Helps remove foreign material from respiratory tract.

5. Personal Hygiene

Helps reduce the number of pathogenic organisms on the skin and other

surfaces of the body.

Infection and ImmunityManifestations of infectious process (Infection spectrum):

1) Clearance of pathogen (no infection)

2) Covert infection (subclinical infection)

3) Overt infection (Clinical infection or apparent infection)

4) Carrier states

Health carrier after covert infection.

Convalescent carrier after overt infection.

Incubatory carrier before onset of disease.

According to carrier time : #acute (transient) carrier

#chronic carrier

5) Latent infection.

Infectious DiseasesInfectious Diseases

MICROORGANISM-HOST INTERACTIONS

Infection has many effects on the body, They may be

• acute

• chronic• allergic• toxigenic

CLINICAL EFFECTS OF INFECTION ON THE BODY - Acute :

• Fever; anorexia, protein catabolism, acute-phase protein response, hypoalbuminaemia, low serum iron, sequestration of iron, anaemia, neutrophilia

• Inflammation; pain, dysfunction, tissue damage • Convulsions; especially in children • Confusion; especially in the elderly • Shock.• Haemorrhage; haemolytic anaemia, intravascular

coagulation • Organ failure.

CLINICAL EFFECTS OF INFECTION ON THE BODY- Chronic :

• Weight loss and muscle-wasting • Malnutrition; especially associated with diarrhoea • Retardation of growth and intellect in children • Anaemia; iron sequestration, maturation arrest in

marrow, folate deficiency • Tissue destruction; e.g. lung in pneumonia or

tuberculosis, nerves in leprosy, liver in hepatitis B • Post-infective syndromes; e.g. lactose intolerance,

malabsorption, irritable colon, depression, post-viral fatigue syndrome

CLINICAL EFFECTS OF INFECTION ON THE BODY- Allergic (immune-mediated)

• Rash; e.g. urticaria with helminths, maculo-papular in typhoid and endocarditis, erythema nodosum in tuberculosis

• Arthritis; e.g. in rheumatic fever, Reiter's syndrome • Pericarditis; e.g. in meningococcal infection • Encephalitis; e.g. in measles or following vaccines • Peripheral neuropathy; e.g. in post-infective polyneuritis • Haemolytic anaemia; e.g. in infectious mononucleosis • Nephritis; e.g. in streptococcal infection

Clinical effect of infection on the body- toxic or toxin mediated

• Erythematous rash in streptococcal infection

• Multisystem disturbance in staphylococcal or streptococcal toxic shock syndrome

• Diarrhea; e.g. staphylococcal enterotoxin, Bacillus cereus

• Organ disturbance; e.g. diphtheria • Neurological; e.g. tetanus, botulinum, diphtheria

• The incubation period

is the period between the invasion of the tissues by pathogens and the appearance of clinical features of infection.

• The period of infectivity

is the time that the patient is infectious to others.

INCUBATION PERIODS OF IMPORTANT INFECTIONS

Short incubation periods (< 7 days):

• Bacillary dysentery 1-7 days• Cholera Hours-5 days ( 2-3 hours)• Diphtheria 2-5 days• Gonorrhoea 2-5 days

• Scarlet fever 1-3 days

Intermediate incubation periods (7-21 days)

• Chickenpox 14-21 days• Measles 7-14 days• Mumps 12-21 days• Poliomyelitis 3-21 days• Rubella 14-21 days

Long incubation periods (> 21 days)

• Brucellosis Days-months

• Hepatitis B 6 weeks-6 months

• Rabies Variable

• Tuberculosis Months-years

PERIODS OF INFECTIVITY IN CHILDHOOD INFECTIOUS DISEASES

Disease Infectious period

• Chickenpox 5 days before rash to 6 days after last crop

• Measles From onset of prodromal symptoms to 4 days after onset of rash

• Mumps 3 days before salivary swelling to 7 days

after

• Rubella 7 days before onset of rash to 4 days after

Prevention of infection Any reduction in the reservoirs of infection will naturally reduce the

incidence of disease produced by these organisms. Endogenous reservoirs may be reduced by• chemoprophylaxis• physical isolation of cases whilst infective • physical separation of animal sources from human hosts and• their rapid treatment will reduce zoonoses. Blockage of transmission by source isolation, vector control careful infection control will also halt the spread of infection. Public health measures control spread of infection and• vaccination may improve 'herd immunity' in communities.• Prophylactic immunoglobulin may help some individuals at high risk

of infection

INDICATIONS FOR CHEMOPROPHYLAXIS IN CONTACTS OF PATIENTS WITH INFECTIOUS DISEASE

Infection Antimicrobiol agent Adult dose

• Diphtheria Erythromycin 500 mg 6-hourly for 5 days

• Meningococcal Rifampicin 600 mg 12 hrly 2 days or Ciprofloxacin 500 mg single dose

• Whooping cough Erythromycin 500 mg 6-hourly for 7 days

• Tuberculosis Isoniazid 300 mg daily for 6 months

INDICATIONS FOR PROPHYLACTIC IMMUNOGLOBULINS

Human specific immunoglobulin

• Virus B hepatitis (needle stick injuries, sexual partner) • Tetanus (susceptible injured patients) • Rabies (post-exposure protection)

• Chickenpox (immunosuppressed children, adults and pregnant women)

• Respiratory syncytial virus infection (high-risk infants, e.g. premature-investigational use)

VACCINATION

CHARACTERISTICS OF EFFECTIVE VACCINES

• Safety • Protection• Long-lasting effects

• Cost Inexpensive to produce and deliver

• Administration Easy to deliver with no side-effects

GUIDELINES FOR IMMUNISATION AGAINST INFECTIOUS DISEASE

• The principal contraindication to inactivated vaccines is a significant reaction to a previous dose

• Live vaccines should not be given to pregnant women or to the immunosuppressed, or in the presence of an acute infection

• If two live vaccines are required, they should be given either simultaneously

in opposite arms or 3 weeks apart

• Live vaccines should not be given for 3 months after an injection of human normal immunoglobulin (HNI)

• HNI should not be given for 2 weeks after a live vaccine

• Hay fever, asthma, eczema, sickle-cell disease, topical corticosteroid therapy, antibiotic therapy, prematurity and chronic heart and lung diseases, including tuberculosis, are not contraindications to immunisation

IMMUNISATION SCHEDULE FOR INFANTS RECOMMENDED BY THE WHO EXPANDED PROGRAMME ON IMMUNISATION

Vaccine Birth 6 weeks 10 weeks 14 weeks 9months

BCG *

Oral polio * * * *

Diphtheria, * * * pertussis, tetanus

Hepatitis B * * *

Measles *