Communicable disease 1

Communicable disease 1 Mark Stevenson EpiCentre, IVABS, Massey University, Palmerston North M.Stevenson@massey.ac.nz Roadmap • Natural history of disease • Time course of clinical disease • Epidemiological triads – the iceberg principal – host, agent, and environment – individual, place, and time • Individual, place, and time • Temporal patterns of disease 2 of 57 Natural history of disease • Infectious disease epidemiology – the occurrence of infectious disease in a given host is dependent on the presence of disease in other members of the population and the length of time that infected hosts are able to transmit disease to others – understanding these characteristics of a disease allow us to develop rational measures to control disease 3 of 57 Natural history of disease TIME Death Infection Susceptible host No infection Clinical disease Recovery Incubation period Latent Exposure Infectious Onset Non-infectious 4 of 57 Natural history of disease • Latent period the time interval from infection to development of infectiousness • Infectious period the time during which time the host can infect another susceptible host • Non-infectious period the period when the host’s ability to transmit disease to other hosts ceases • Incubation period the time interval between infection to development of clinical disease 5 of 57 Natural history of disease • Chicken pox – an infectious disease caused by the varicella-zoster virus – signs: a blister-like rash, itching, tiredness and fever – the latent period for chicken pox is shorter than the incubation period, so a child with chicken pox becomes infectious to others before developing symptoms – in the event of an outbreak, does it make sense to keep children with chicken pox away from school? 6 of 57 Natural history of disease TIME Death Infection Susceptible host No infection Clinical disease Recovery Incubation period Latent Exposure Infectious Non-infectious Onset 7 of 57 Natural history of disease • Chicken pox – an infectious disease caused by the varicella-zoster virus – signs: a blister-like rash, itching, tiredness and fever – the latent period for chicken pox is shorter than the incubation period, so a child with chicken pox becomes infectious to others before developing symptoms – in the event of an outbreak, does it make sense to keep children with chicken pox away from school? probably not: children with symptoms of chicken pox are no longer infectious to others 8 of 57 Natural history of disease • Other examples? – HIV (AIDS) • latent period relatively short • infectious period occurs (many years) before the onset of symptoms 9 of 57 Natural history of disease TIME Death Infection Susceptible host No infection Clinical disease Recovery Incubation period Latent Exposure Infectious Onset 10 of 57 Natural history of disease • Malaria – – – – caused by protozoan parasites of the genus Plasmodium human to human transmission via mosquitoes signs: haemolytic anaemia the stages of the parasite that are infective to mosquitoes occur about 10 days after the development of symptoms – latent period is around 10 days longer than the incubation period, so early treatment of symptoms could have an important effect on transmission 11 of 57 Natural history of disease TIME Death Infection Susceptible host No infection Clinical disease Recovery Incubation period Latent Exposure Onset 12 of 57 Infectious Roadmap • Natural history of disease • Time course of clinical disease • Epidemiological triads – the iceberg principal – host, agent, and environment – individual, place, and time • Individual, place, and time • Temporal patterns of disease 13 of 57 Time course of clinical disease • Terms used to describe the time course of clinical disease: – peracute extremely rapid onset and progress (24 - 48 hours) – acute rapid onset and progression (days) – sub-acute slower onset and progression (days to weeks) – chronic prolonged onset and progression (months to years) 14 of 57 Roadmap • Natural history of disease • Time course of clinical disease • Epidemiological triads – the iceberg principal – host, agent, and environment – individual, place, and time • Individual, place, and time • Temporal patterns of disease 15 of 57 Epidemiological triads DEATH Clinical disease SEVERE DISEASE MILD ILLNESS The ‘iceberg’ principal of disease in populations Subclinical disease INFECTION WITHOUT CLINICAL ILLNESS EXPOSURE WITHOUT INFECTION 16 of 57 Epidemiological triads Table 1: Health outcomes resulting from dog attacks in the USA (2001). Dog bite related health outcomes Death Hospitalisations Emergency department visits Other medically treated Non-medically treated Number affected 20 13,360 334,000 451,000 3.7 million 17 of 57 Epidemiological triads • The risk of disease in an individual can be explained by: – host factors – agent factors – environmental factors – individual factors (above) – spatial factors – temporal factors AGENT • The risk of disease in a population can be explained by: DISEASE HOST ENVIRONMENT 18 of 57 Roadmap • Natural history of disease • Time course of clinical disease • Epidemiological triads – the iceberg principal – host, agent, and environment – individual, place, and time • Individual, place, and time • Temporal patterns of disease 19 of 57 Individual, place, and time • Individual factors that influence patterns of disease: – – – – age structure of population genotype immunity of population any others? diet lifestyle occupation presence of existing disease 20 of 57 Individual, place, and time • ‘Place’ (spatial) factors that influence patterns of disease: – proximity to pollutants – proximity to infectious agents – proximity to disease risks 21 of 57 Individual, place, and time • ‘Time’ factors that influence patterns of disease: – calendar time influenza in humans, bovine ephemeral fever in cattle – time relative to certain events milk fever in dairy cattle, gestational diabetes in humans – communicate temporal risks to others via epidemic curves … 22 of 57 Individual, place, and time • It may be useful in some cases to consider ‘space’ and ‘time’ together … 23 of 57 Roadmap • Natural history of disease • Time course of clinical disease • Epidemiological triads – the iceberg principal – host, agent, and environment – individual, place, and time • Individual, place, and time • Temporal patterns of disease 24 of 57 Temporal patterns of disease • Terms used to describe the temporal pattern of disease in a population: – endemic disease occurs at expected frequency – epidemic disease occurs at greater than expected frequency – pandemic huge epidemic (international) – sporadic single case or a cluster of cases 25 of 57 Temporal patterns of disease • Endemic – – – – disease occurs at expected frequency disease present in population or region at all times level of disease usually low and predictable examples: • lameness in dairy cattle • long bone fractures in < 10 year olds 26 of 57 Temporal patterns of disease • Epidemic – – – – – – disease occurs at greater than expected frequency incidence exceeds expected usually infectious disease or poisoning in animals, occasionally referred as epizootic disease may be point source or propagated examples: • influenza in humans • avian influenza • foot-and-mouth disease 27 of 57 Temporal patterns of disease • Sporadic – – – – single case or cluster of cases infrequent disease occurrence irregular and unpredictable examples: • Legionnaire’s disease • food poisoning 28 of 57 Temporal patterns of disease • Temporal patterns of onset can provide insight into nature of epidemic – point source – propagated 29 of 57 Temporal patterns of disease • Point source epidemics – disease arises from a single source of exposure to a causal agent – epidemic curve shows a steep initial rise in case numbers and then a rapid falling off in the tail – examples: • batch of contaminated feed causing an outbreak of salmonellosis in feedlot cattle • milk vacuum problem causing an outbreak of clinical mastitis in a herd of dairy cows • foodborne disease outbreaks 30 of 57 Temporal patterns of disease • Propagated epidemics – occur when the agent is transmitted through the population from host to host (typically infectious conditions) – nature of epidemic depends on • characteristics of agent (virulence) and host (susceptibility) • contact rate • population density – examples: • influenza in humans • foot-and-mouth disease 31 of 57 Temporal patterns of disease • Other uses for temporal displays of disease data: – identify seasonal trends – identify long-term trends 32 of 57 Temporal patterns of disease • How to work out the estimated date of exposure from an epidemic curve – method 1 • identify last case and count back maximum incubation period • identify first case and count back minimum incubation period – method 2 • count back the median incubation period from the peak of the epidemic • difficult to do if there is no distinct peak to epidemic curve 33 of 57 Summary • Natural history of disease • Time course of clinical disease • Epidemiological triads – the iceberg principal – host, agent, and environment – individual, place, and time • Individual, place, and time • Temporal patterns of disease 34 of 57