Epidemiology of Vector-Borne Diseases Laura C. Harrington, PhD

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1 Epidemiology of Vector- Borne Diseases Associate Professor Department of Entomology Cornell University 1 Before we begin Review lectures on transmission, arboviruses and malaria Focus on biologically transmitted agents Epidemiology of mechanically transmitted agents and other modes will not be covered 2 Outline Introduction to the field of epidemiology Determinants of disease Epidemiological measures Field based epidemiology i of vector borne diseases Review and references 3 The screen versions of these slides have full details of copyright and acknowledgements 1

2 Goals for this lecture Understand how epidemiology is defined, why it is important? Consider the complexity of factors (determinants) that influence vector-borne disease transmission Know some of the key epidemiological measures and how to calculate them Understand the process of epidemiological investigations of vector borne disease outbreaks 4 What is epidemiology? The study of the distribution of disease and the determinants of disease The study of the causes of disease The science of public health Source: Rothman, KJ. Epidemiology, An Introduction; Oxford University Press, What is the importance of epidemiology? Disease risk can be predicted Cause of disease can be defined Intervention and control strategies can be evaluated 6 The screen versions of these slides have full details of copyright and acknowledgements 2

Person (or animal), place and time Who gets disease? Where does disease occur?

falciparum positive films in West Harerge, Ethiopia between July 1991 and June 2001 (Ministry of

of the clinical and epidemiologic burdens of epidemic malaria; Am. J. Trop Med. Hyg.

area Epidemic - The occurrence of disease clearly in excess of normal expectancy Pandemic - A

3 Person (or animal), place and time Who gets disease? Where does disease occur? When does the disease occur? 7 Cycles of disease a cases Malaria Recurrent P. falciparum positive films in West Harerge, Ethiopia between July 1991 and June 2001 (Ministry of Health, Ethiopia, unpublished data) Months Source: Kiszewski and Awash Teklehaimanot 2004; A review of the clinical and epidemiologic burdens of epidemic malaria; Am. J. Trop Med. Hyg. 8 Endemic, epidemic, pandemic Endemic - The habitual presence of a disease within a given geographic area Epidemic - The occurrence of disease clearly in excess of normal expectancy Pandemic - A worldwide epidemic affecting an exceptionally high proportion of the global population 9 The screen versions of these slides have full details of copyright and acknowledgements 3

4 Endemic vs. epidemic r of cases Number Endemic Epidemic Time 10 Disease cycles in humans and other animals Epizootic Used for disease of non-human animals including invertebrates Zoonosis Refers to a disease that primarily affects non-human animals, often humans are secondary or tangential hosts Anthroponosis Refers to a disease that primarily occurs in humans 11 Epidemiologic triad Host Vector Agent Environment Vector: an organism that conveys a pathogen or parasite from one host to another 12 The screen versions of these slides have full details of copyright and acknowledgements 4

Determinants of vector-borne diseases (1) Host factors Genetics Several well-documented examples showing the

, water storage practices (dengue) Building mud-brick homes (malaria) Source: WHO/Benoist Carpentier 13 Determinants

rate Tissue trophisms Immature Plasmodium vivax schizont (Source: CDC) West Nile virus infected tissue (Source:

5 Determinants of vector-borne diseases (1) Host factors Genetics Several well-documented examples showing the influence of genetics on the outcome of disease with malaria e.g., Duffy group receptors and p. vivax Glucose-6-phosphate dehydrogenase (G6PD) Culture/behavior e.g., water storage practices (dengue) Building mud-brick homes (malaria) Source: WHO/Benoist Carpentier 13 Determinants of vector-borne diseases (2) Pathogen/parasite factors: Strain type/species Virulence Drug resistance Reproductive rate Tissue trophisms Immature Plasmodium vivax schizont (Source: CDC) West Nile virus infected tissue (Source: Public Health Image Library) 14 Determinants of vector-borne diseases (3) Vector factors: Feeding preference Feeding frequency Survival and longevity Abundance Vector competence Anopheles freeborni Anopheles larvae Source: CDC 15 The screen versions of these slides have full details of copyright and acknowledgements 5

Harrington) Rice paddy habitat (Source: Public Health Image Library) 16 Epidemiological measures Incidence rate Prevalence Sporozoite rate Entomological

6 Determinants of vector-borne diseases (4) Environmental factors: Temperature Rainfall/humidity Altitude Landscape ecology Vegetation An. Dirus habitat in Thailand (Source: L.Harrington) Rice paddy habitat (Source: Public Health Image Library) 16 Epidemiological measures Incidence rate Prevalence Sporozoite rate Entomological inoculation rate Vector competence Basic reproductive rate Vectorial capacity Minimum infection rate Disability adjusted life years (DALYs) 17 Incidence rate (1) = The number of new cases occurring within a set time interval 18 The screen versions of these slides have full details of copyright and acknowledgements 6

Incidence rate (2) IR = # new cases per unit time #

malaria observed over 1 yr, with 105,000 people in the

risk = 0.0879 new malaria cases per person/year (or 87.

incidence Per person per year 0.36-0.53 0.21-0.35 0.

014 No falciparum malaria Source: WHO at www.rbm.who.

gif 20 Age adjustment for incidence is critical for

Females Males Age in years Malaria incidence rate (per

7 Incidence rate (2) IR = # new cases per unit time # persons at risk during time Ex: 9,230 new cases of malaria observed over 1 yr, with 105,000 people in the study area 9,230 new cases IR = 105,000 persons at risk = new malaria cases per person/year (or 87.9 new cases per 1,000 person/years) 19 Global malaria incidence Per person per year No falciparum malaria Source: WHO at 20 Age adjustment for incidence is critical for understanding risk e/1000 person-years Incidence rate Females Males Age in years Malaria incidence rate (per 1000 person-years) by age and gender in East Shoa, Ethiopia Source: Yeshiwondim et al., International Journal of Health Geographics : 5 21 The screen versions of these slides have full details of copyright and acknowledgements 7

Prevalence = The number of cases of disease present in a population at a given time Ex: 21,000 existing cases of malaria observed over 1 yr, with 100,000 people in study that time Prevalence = 21,000

8 Prevalence = The number of cases of disease present in a population at a given time Ex: 21,000 existing cases of malaria observed over 1 yr, with 100,000 people in study that time Prevalence = 21,000 = 21,000 malaria cases per 100,000 population 22 Comparing incidence and prevalence Prevalence and Incidence Number of per 100,000 p f cases population Number of new cases per 100,000 popula ation per year Time 23 Sporozoite rate = Proportion of mosquitoes with malaria parasites in their salivary glands Sporozoites in a malaria vector; Source: Public Health Image Library 24 The screen versions of these slides have full details of copyright and acknowledgements 8

9 Entomological Inoculation Rate (EIR) (1) EIR = # of infective bites per unit time EIR = m a s m = Vector density in relation to host a = # of human blood meals per vector per day (m*a can be estimated using human biting collections) s = Sporozoite rate (proportion of mosquitoes with malaria parasites in their salivary glands) 25 Entomological Inoculation Rate (EIR) (2) Anopheles gambiae feeding on a human host Source: Public Health Image Library EIR = mas 15 Anopheles collected per person per night (= m), 78% are human-fed (= a) and 10% are infected with sporozoites (= s) EIR = # mosquitoes x proportion human fed x proportion with sporozoites 15 x 0.78 x 0.1 = 1.17 Each individual receives 1.17 infective bites per night 26 Vector competence = Ability of arthropods to become infected with and transmit a pathogen or parasite Collecting Ae. aegypti saliva in a capillary tube Source: Karine Delroux Steps in determining vector competence: 1. Infect mosquitoes with pathogen parasite (orally or via injection) 2. Wait interval of time and check for disseminated infection (legs) and salivary gland infection 3. Allow infected mosquitoes to feed on uninfected hosts (if model systems are available) 27 The screen versions of these slides have full details of copyright and acknowledgements 9

10 Basic reproductive rate (R 0 ) The average number of secondary cases that a single case may cause in a completely non-immune population 28 R 0 = βcd β = Transmission coefficient - average probability that susceptible will be infected over duration of exposure c = Average # of susceptible infected per unit time D = Average duration of infectiousness 29 Basic reproductive rate (R 0 ) Determines transmission stability of an infection: When R 0 < 1 the infection will die out If R 0 > 1 there is a possibility of an epidemic 30 The screen versions of these slides have full details of copyright and acknowledgements 10

11 Basic reproductive rate and herd immunity Herd immunity = When immunity to infection of the majority of the population (or herd) provides protection to non-immune individuals = Immune hosts = Non-immune hosts Herd immunity lowers R 0 < 1 31 Vectorial capacity C = Number of potentially infective bites received daily by a single host (assuming a totally susceptible host population) 32 C = ma 2 Vp n -log e (p) m = Density of vectors in relation to density of host (3) a = HBR, # of human blood meals per vector per day (0.33) V = Vector competence, probability of acquiring an infection from an infectious person (0.5) p = Daily probability of mosquito survival (0.9) n = Extrinsic incubation period in days (10) 33 The screen versions of these slides have full details of copyright and acknowledgements 11

12 Vectorial capacity spreadsheet 3m = Density of vectors in relation to density of host (3) 0.33a = HBR, no. of human blood meals per vector per day (0.33) 0.5V = Vector competence, probability of acquiring an infection from an infectious person (0.5) 0.9p = Daily prob. of mosquito survival (0.9) 10n = Extrinsic incubation period in days (10) C = ma 2 Vp n -log(p) m*a V*p n Log(p) C = Minimum Infection Rate (MIR) (1) Arthropods are pooled (20-100) by collection date, location and species into groups Pools are analyzed for pathogen or parasite of interest Source: Sylvie Pitcher 35 Minimum Infection Rate (MIR) (2) # infected arthropods (# positive pools) MIR = 1,000 individuals tested Typically MIR is during epidemics 36 The screen versions of these slides have full details of copyright and acknowledgements 12

Minimum Infection Rate (MIR) (3) Source: T.G. Andreadis et al.

overall disease burden which combines mortality and morbidity in one common

premature mortality or from some degree of disability over a period of time One

care priorities, fully assessing the impact disease has on the quality of life

country (per 100,000 inhabitants) no data less than 10 10-50 50-100 100-250

3500 Source: Mortality and Burden of Disease estimates for WHO member states in

13 Minimum Infection Rate (MIR) (3) Source: T.G. Andreadis et al., 2001; Emerg Infect Dis 7 (4): Burden of disease-daly A measure of overall disease burden which combines mortality and morbidity in one common metric Reflects total amount of healthy life lost, to all causes, whether from premature mortality or from some degree of disability over a period of time One DALY is equivalent to one year of healthy life lost Useful in setting health care priorities, fully assessing the impact disease has on the quality of life 38 Age-standardized disability-adjusted life year (DALY) rates from malaria by country (per 100,000 inhabitants) no data less than more than 3500 Source: Mortality and Burden of Disease estimates for WHO member states in 2002 (accessed ) 39 The screen versions of these slides have full details of copyright and acknowledgements 13

Field-based approaches to understanding the epidemiology of vector-borne diseases Vector incrimination New outbreak investigations 40 Steps in new outbreak investigation 1.

14 Field-based approaches to understanding the epidemiology of vector-borne diseases Vector incrimination New outbreak investigations 40 Steps in new outbreak investigation 1. Identification of unusual cases/symptoms 2. Patient interviews and clinical testing 3. Reconstruction ti of events leading to clues of source of infection 4. Diagnostic testing of incriminated sources of infection 41 Vector incrimination = Establishes the relationship between specific arthropods or groups of arthropods and transmission of a specific parasite or pathogen Public Health Image Library 42 The screen versions of these slides have full details of copyright and acknowledgements 14

15 Steps in vector incrimination 1. Demonstrate that species feeds on host or makes contact with host in nature 2. Demonstrate biological association in time and space between vector and infection in host 3. Demonstrate that vectors harbor the infective stage of pathogen 4. Demonstrate efficient transmission of pathogen by vector 43 Review questions Name 3 key aspects of descriptive epidemiology Which measure is more useful in understanding a new outbreak: prevalence or incidence rate? What are some difficult to measure parameters of vectorial capacity? How is vector competence measured? What are the first steps in a new disease investigation? How can environmental factors influence both the vector and host components of a disease cycle? 44 References B.F. Eldridge and J.D. Edman (eds.), A Text Book of Medical Entomology; Human and Animal Diseases Caused by Insects and Other Arthropods; Kluwer Academic Publishers, New York, 697pp. JB Silver 2008; Mosquito Ecology, Field Sampling methods, 3 rd edition; Chapman and Hall; 1477 pp. WC Marquardt (ed.) 2004; Biology of Disease vectors, 2 nd edition Academic Press; 816 pp. D.A. Warrell and H.M. Gillies 2002; Essential Malariology; Hodder Arnold Publication, 4 th edition 384 pp. Rothman, KJ. 2002; Epidemiology, An Introduction; Oxford University, 1 st edition; 240 pp. 45 The screen versions of these slides have full details of copyright and acknowledgements 15

16 Acknowledgements Katherine Connors, Professor John D. Edman Thank you! The screen versions of these slides have full details of copyright and acknowledgements 16

Malaria. Population at Risk. Infectious Disease epidemiology BMTRY 713 (Lecture 23) Epidemiology of Malaria. April 6, Selassie AW (DPHS) 1

Malaria. Population at Risk. Infectious Disease epidemiology BMTRY 713 (Lecture 23) Epidemiology of Malaria. April 6, Selassie AW (DPHS) 1 Infectious Disease Epidemiology BMTRY 713 (A. Selassie, DrPH) Lecture 23 Vector-Borne Disease (Part II) Epidemiology of Malaria Learning Objectives 1. Overview of malaria Global perspectives 2. Identify