Vector borne disease

1. Vector-Borne
Disease Research
at Tulane
SPHU-1020
Dawn Wesson, PhD
Department of Tropical Medicine

2. Overview
• Introduction
• General Concepts: vectors, types of vector-borne
transmission, vector incrimination, zoonoses,
surveillance
• Specific Pathogen-Arthropod-Vertebrate
Transmission Cycles: dengue virus, West Nile
virus, American trypanosomiasis (Chagas disease)

3. Introduction
• Training for VBD research
• VBD at Tulane (dengue, WNV, malaria,
chikungunya, Chagas dz, Lyme dz)
• Local vs. International research
• Funding sources (NIH, NSF, BMGF,
BORSF, etc.)

4. How do we define a vector?
An arthropod* responsible
for transmission of parasites
among vertebrate hosts.
* Arthropods include insects (mosquitoes, tsetse
flies, sandflies, blackflies, lice, fleas, kissing bugs)
and arachnids (mites, ticks, spiders and scorpions).

5. Aedes aegypti
Dengue and Yellow Fever
viruses
Culex quinquefasciatus
Filariasis, West Nile & other viruses
Anopheles gambiae
Major mosquito-borne
diseases and the most
important mosquito
vector species
Malaria

6. Tsetse fly – African Sleeping Sickness
Kissing bug – Chagas Disease
Sandfly - Leishmaniasis
Blackfly - Onchocerciasis

7. Flea - Plague
Tick – Lyme Disease, CrimeanCongo Hemorrhagic Fever
Body louse – Epidemic Typhus
Horse fly – Tularemia

8. Types of VectorBorne Transmission
Mechanical Transmission – simplest form
(not necessary for pathogen survival – “flying/crawling
hypodermic needle” phenomenon)
Biological Transmission – more complex
(necessary for pathogen survival – results in
amplification or development in vector)

9. General Types of
Biological Transmission
Vertical
Horizontal

10. Vertical Transmission
Trans-stadial
(ticks, kissing bugs)
Trans-generational
Transovarial – female to progeny, including males
(mosquitoes, ticks)

11. Horizontal Transmission
Bite (saliva or gut infection)
(malaria, dengue, filariasis, etc.)
Feces
(Chagas disease)
Crushing Infected
Arthropod
(epidemic typhus, etc.)

12. Horizontal Transmission Types
Multiplicative (Propagative)
(increase in number - Dengue)
Developmental
(change stage - Filariasis)
Cyclopropagative
(change stage and increase in number - Malaria)

13. Pathogen Infection in Arthropods
Extrinsic Incubation Period - EIP
Time interval between vector infection and
potential for pathogen transmission to
vertebrate host (determined by controlled
experimental studies)
If vector dies (is killed) before EIP is
completed, transmission will not occur.

14. Ecological Parameters Affecting
Vector Populations:
Availability of food
Space
(What do they eat, and when?)
(Where do they occur, and when?)
Water (Is their appearance seasonal – wet/dry?)
Temperature (Is their appearance seasonal – warm/cold?)
Predation ( Can we enhance or introduce predators to control them?)
Disease ( Can we identify pathogens to control them?)

15. Vector Incrimination
• Demonstrate an association in time and space
between arthropod and disease
• Demonstrate direct contact between the
arthropod and humans
• Demonstrate natural occurrence of the
infectious agent in the insect
• Demonstrate “transmission” of the agent by the
insect (laboratory-based vector competence)

16. How are these pathogen transmission
parameters determined?
Laboratory Experimentation
(vector competence, EIP)
Field Surveillance
(temporal and spatial association between
pathogen, vector and humans; other factors
affecting disease prevention – insecticide or
antibiotic resistance)

17. Why is understanding vector life cycle important?
Mosquito Life Cycle

18. Specific Pathogen-ArthropodVertebrate Transmission Cycles
Highlighted Transmission Cycles
Mosquito-borne pathogens - arboviruses
(dengue and West Nile virus)
Bug-transmitted pathogens – Chagas disease

19. Arboviruses: ARthropod-BOrne VIRUSES
• Dengue viruses 1-4 – dengue fever, dengue hemorrhagic
fever (DHF) and dengue shock syndrome (DSS)
• West Nile virus - West Nile fever and WN neuroinvasive
disease (encephalitis, meningitis, etc.)
different vertebrate hosts and mosquito vectors
different transmission ecology
different control approaches

20. Worldwide dengue distribution, early 2012.
20

21. Dengue Viruses
• 40% of the world’s population is at risk
for dengue infection
• Each serotype provides specific lifetime
immunity, and short-term crossimmunity
• All serotypes can cause severe and
fatal disease
• Genetic variation within serotypes

22. Aedes aegypti
• Dengue transmitted by female
mosquitoes between humans
• Primarily a daytime feeder
• Lives in and around human
habitation
• Prefers to take blood from humans
• Lays eggs and produces larvae
preferentially in artificial containers

23. Aedes aegypti Aedes albopictus
Yellow Fever mosquito
Asian Tiger mosquito

24. 24

25. 25

26. West Nile Neuroinvasive Disease (NID) in the
United States, 1999 - 2012
Year
WNV NID
Deaths
Mortality Rate
1999
59
7
11.9%
2000
19
2
10.5%
2001
64
9
14.1%
2002
2,946
284
9.6%
2003
2,860
264
9.2%
2004
1,142
100
8.8%
2005
1,294
119
9.2%
2006
1,459
177
12.1%
2007
1,217
124
10.2%
2008
687
44
6.4%
2009
335
30
9.0%
2010
601
45
7.5%
2011
486
46
9.5%
2012
2,734
243
8.9%
Total
15,903
1,494
9.4%
26

27. West Nile virus (WNV) activity reported to ArboNET, by state, United
States, 2012 (as of Dec. 11, 2012)
27

28. West Nile virus (WNV) Neuroinvasive Disease Incidence reported to
ArboNET, by county, United States, 2012 (as of Dec. 11, 2012)
28

29. •
•
•
At least 326 bird species have been reported to
CDC's West Nile Virus avian mortality database
from 1999-present.
At least 60 species of mosquitoes have been
found naturally infected with West Nile virus
since 1999.
40% of unvaccinated equines will die from WNV
infection.
29

30. Relative Vector Competence (Laboratory)
Species
Ae. albopictus
Cx. restuans
Cx. salinarius
Cx. tarsalis
Ae. atropalpus
Ae. japonicus
Cx. nigripalpus
Cx. pipiens
Vector
Competence
+++
+++
+++
+++
+++
+++
++
++
Species
Cx. quinquefasciatus
Ae. sollicitans
Ae. taeniorhynchus
Ae. triseriatus
Ae. vexans
Cq. perturbans
Ae. canadensis
Ae. cantator
Vector
Competence
++
++
++
++
+
+
+
+
30

31. Chagas Disease
(American Trypanosomiasis)
31

32. Chagas’ Disease - General
• Trypanosoma cruzi, the causative agent of
Chagas disease, infects 8–11 million people.
• Triatomine bugs transmit Trypanosoma cruzi while
feeding on vertebrate host; pathogen defecated in
feces while bug feeds - host scratches infective
feces into site of bite or into mucous membrane
(eyes, etc.)
• Any triatomine bug can support development of T.
cruzi, but differing environmental conditions
enhance or suppress transmission to humans
• Important vectors: Triatoma infestans, Rhodnius
prolixus, Panstrongylus megistus, T. dimidiata
32

33. Chagas Disease Control
Plague

34. Non-human reservoirs for Chagas disease
Armadillo
Opossum
Also, rats and mice, squirrels, raccoons, skunks, carnivores,
monkeys, and domestic pets.
34

35. Nature, 2010
Migration routes from Latin America and estimation of the total
number of infected individuals in non-endemic countries.
35

36. Chagas in the United States
• In the United States, the disease exists almost exclusively
as a zoonosis.
• Only six autochthonous insect-borne cases have been
reported in humans. The most recent was documented in
New Orleans in 2006.
• The distribution of Chagas disease in the United States
includes approximately the southern half of the country.
• Twelve species of triatomines are known to occur in the
United States, the most important being Triatoma
sanguisuga in the eastern United States, Triatoma
gerstaeckeri in the region of Texas and New Mexico, and
Triatoma rubida and Triatoma protracta in Arizona and
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California .

37. Nature, 2010
Rhodnius prolixus
Triatoma infestans
37

38. Triatomine bugs transmitting
Chagas disease feed at night,
and pass the parasite to new
hosts via fecal contamination.
Poor house construction
contributes to transmission.
Fumigation and home
improvement are two
successful control methods.

39. Careers in VBD
•
•
•
•
Research (academic, industry)
Vector control programs (municipal)
Pest control operator (industry)
International, national, regional, state
level surveillance, epidemiology, policy
• Monitoring and assessment
• NGO’s
• Funding agencies