Describe the dynamics of disease transmission.

The Chain of Infection: Source and Escape

• The reservoir is where the agent normally lives and multiplies, and on which it depends for survival
• The reservoir is not the same as the source, which is the person, animal or object from which the infection actually passes to the host. They may be the same, but often are not — in typhoid the reservoir is a human carrier while the source may be contaminated water
• The types of reservoir:
• Human — either cases or carriers; man is the reservoir for measles, polio, typhoid, diphtheria, tuberculosis and HIV
• Animal — giving rise to the zoonoses: rabies, plague, brucellosis, leptospirosis, anthrax and Japanese encephalitis
• Non-living (reservoir in soil or inanimate matter) — tetanus, botulism, anthrax spores, histoplasmosis and hookworm
• The carrier state deserves careful treatment because carriers are epidemiologically more dangerous than cases — a case is recognised and isolated, whereas a carrier is well, unrecognised, unrestricted, and continues to move freely in the community
• The classification by duration — temporary, further divided into incubatory (shedding before symptoms appear, as in measles, chickenpox and hepatitis A, which is why these spread so effectively), convalescent (shedding after recovery, as in typhoid, cholera and diphtheria); and chronic, shedding for months or years, as in typhoid (the gall bladder carrier), hepatitis B, and dysentery
• The classification by clinical state — healthy (asymptomatic) carriers, who never develop disease, as in poliomyelitis and meningococcal infection; and convalescent carriers
• The classic example is typhoid MARY (Mary Mallon), a healthy chronic gall bladder carrier who infected dozens over years while remaining perfectly well
• The portals of exit — the respiratory tract (much the commonest); the intestinal tract; the genitourinary tract; open lesions; and mechanical escape through biting arthropods or needles
• The infective period is the time during which the agent may be transferred to a new host, and determines how long isolation is needed
• The incubation period is the time from entry of the agent to the appearance of the first sign or symptom
• Its practical uses are several and are examined — it indicates the probable time and source of exposure when counted back from the onset of cases; it determines the period of quarantine, which equals the maximum incubation period; it identifies the type of epidemic from the epidemic curve; it guides the timing of prophylaxis, which must be given within it; and it helps evaluate control measures
• The generation time — the interval between entry of the agent and maximum infectivity, which may be shorter than the incubation period; and where it is, transmission occurs before the case is recognised, which makes control by isolation alone impossible
• The serial interval is the gap between onset in a case and onset in the person they infected

Modes of Transmission, Host Factors and Control

• Direct transmission:
• Direct contact — touching, kissing or sexual contact, as in STIs, leprosy, scabies and skin infections
• Droplet spread — large droplets (above 5 micrometres) travelling only about 1 metre and settling rapidly, produced by coughing, sneezing, talking and singing; the route for influenza, meningococcus, pertussis and diphtheria
• Direct contact with soil; inoculation into skin or mucosa (rabies, hepatitis B); and transplacental (vertical) transmission (rubella, syphilis, HIV, toxoplasmosis, cytomegalovirus, hepatitis B)
• Indirect transmission:
• Vehicle-borne — through water, food, milk, blood or blood products
• Vector-borne, which subdivides importantly into mechanical transmission (the agent is simply carried on the body of the vector without multiplying, as flies carry cholera and typhoid) and biological transmission, in which the agent multiplies or develops within the vector
• Biological transmission subdivides further — propagative (the agent merely multiplies, as plague bacillus in the flea); cyclo-propagative (it both changes form and multiplies, as malaria parasite in the mosquito); and cyclo-developmental (it develops but does not multiply, as the filarial larva)
• Air-borne — by droplet nuclei (under 5 micrometres, the dried residue of droplets) which remain suspended for hours and travel long distances, as in tuberculosis, measles and chickenpox; and by dust
• The droplet versus airborne distinction IS the one that determines infection control practice — droplet precautions need a surgical mask and about 1 metre of distance; airborne precautions need an N95 respirator and a negative-pressure room
• Fomite-borne — through contaminated inanimate articles; and unclean hands and fingers, which remain the commonest single vehicle of infection in hospitals
• The portals of entry mirror the portals of exit — respiratory, alimentary, genitourinary, skin, and vertical
• The susceptible host is the final link, and susceptibility depends on age, sex, immune status, nutrition, genetic constitution, comorbidity and behaviour
• The measures of an agent’S capacity, which are regularly confused and should be defined separately:
• Infectivity — the capacity to enter, survive and multiply in a host
• Pathogenicity — the proportion of infected persons who develop clinical disease
• Virulence — the proportion of clinical cases that are severe or fatal, measured by the case fatality rate
• The secondary attack rate = the number of exposed contacts developing disease within the incubation period divided by the total number of susceptible contacts exposed. It is a direct measure of infectivity and of the effectiveness of control measures
• The basic reproduction number (R0) is the average number of secondary cases produced by one case in a completely susceptible population
• Its interpretation is simple and powerful — if R is above 1 the epidemic grows; if below 1 it dies out. All control measures are, in effect, attempts to push R below 1
• Herd immunity follows directly — the proportion that must be immune is approximately 1 - 1/R0, so measles with an R0 of 12 to 18 requires about 95% coverage while diseases with lower R0 require less
• Herd immunity does not operate where there is a non-human reservoir or NO person-to-person spread — which is why vaccinating a population does not protect the unvaccinated against tetanus, whose spores live in soil
• Control follows from the chain — measures against the reservoir or source (early diagnosis, notification, isolation, treatment, carrier detection, culling of infected animals); against transmission (water, sanitation, food hygiene, vector control, hand hygiene, air handling); and to protect the host (immunisation, chemoprophylaxis, nutrition, personal protection)
• Isolation separates the ill; quarantine restricts the well who have been exposed — a distinction frequently confused, and the period of quarantine is the maximum incubation period of the disease
• Break any one link and transmission stops, so control becomes a question of which link is cheapest to break
• Carriers are more dangerous than cases, being well, unrecognised and free to move about the community
• Isolation separates the ill and quarantine the exposed, and the quarantine period is the maximum incubation period
• Droplets fall within a metre and nuclei float for hours, which is the distinction that determines mask policy
• Airborne precautions need an N95 and negative pressure, while droplet precautions need only a surgical mask
• Count back one incubation period to find the exposure, which is the commonest practical use of that interval
• Generation time may be shorter than incubation, in which case isolation alone cannot control the disease
• R above one grows and below one dies out, and every control measure is an attempt to push it below one
• Herd immunity threshold is one minus one over R nought, which is why measles needs about ninety-five per cent coverage
• Herd immunity does not protect against tetanus, whose spores live in soil rather than in other people
• Pathogenicity concerns the infected and virulence the ill, and the two are constantly interchanged in error
• Secondary attack rate measures infectivity directly, and also measures whether control measures are working
• Mechanical transmission involves no multiplication, unlike biological transmission where the agent develops in the vector
• Cyclo-propagative means change plus multiplication, as the malaria parasite does within the mosquito
• Incubatory carriers explain why measles spreads, since shedding begins before anyone knows the child is ill
• The gall bladder harbours the typhoid carrier, which is why cholecystectomy was once used to clear chronic carriage
• Unclean hands remain the commonest vehicle of infection in hospitals, despite every technological advance
• The reservoir may be soil or an animal, not only another person, which determines whether eradication is even possible
• Vertical transmission crosses the placenta in rubella, syphilis, HIV and hepatitis B, and is preventable in each
• The infective period determines how long to isolate, and differs from both the incubation period and the illness
• Notification starts the public health response, and is the link between the individual case and community action
• Susceptibility depends on nutrition as much as immunity, which is why malnourished children die of measles