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Potential Reintroduction of Vector-Borne Diseases by Levone

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									Potential Reintroduction of Vector-Borne Diseases
Rebecca J. Eisen, Ph.D. Division of Vector-Borne Infectious Diseases Centers for Disease Control and Prevention Fort Collins, CO

Risk Assessment for Potential Reintroductions
• Is the pathogen of interest likely to be reintroduced? • What factors are related to the likelihood of the pathogen being re-established following a reintroduction? • Specific examples
– – – – Yellow Fever Dengue Malaria Plague

What are likely routes of pathogen reintroduction?
• Introduction of infectious vectors • Introduction of infectious humans • Introduction of infectious zoonotic hosts

Re-introduction: Transit between the southeastern U.S. and climatically similar disease endemic regions • • • • • Air travel Cargo ships Exotic pet trade Migratory birds Intentional release

Reintroduction: Vector-specific factors
• Preferred breeding habitat of the vector is present at the site of origin • Host preference
– Human-specific for maintenance in humans – Catholic feeder that frequently bites humans (bridging vector)

• • • • •

Vector efficiency Extrinsic incubation period Likelihood of surviving to the second bloodmeal Efficiency of transovarial transmission Ability to remain infectious long-term (e.g., introduction of insects on ships)

Reintroduction: Vertebrate hostspecific factors
• Duration of infectivity • Duration of incubation period • Pathogen load at or above transmission threshold • Degree of virulence

Vectorial Capacity Models
How many potentially infective mosquito bites will ultimately be delivered by all vectors feeding on a single host in 1 day? m x a2 x pn x b -lnp

V=

m = vector density in relation to the host a = probability a vector feeds on a host in 1 day = host preference index x feeding frequency b = the proportion of vectors ingesting an infective meal that successfully become invective p = probability the vector will survive 1 day n = duration of the extrinsic incubation period (in days) 1/(-lnp) = duration of the vector’s life, in days, after surviving the EIP

Sir Ronald Ross

Establishment: Vector-specific factors
• Number of vector species • Density of vectors • Host preferences
– Human-specific for anthroponotic cycles – Catholic feeders for zoonotic pathogens – Specific to non-human hosts to maintain zoonotic cycles – Seasonal host switching

• Seasonal patterns of vector populations • Vector efficiency

Establishment: Vector-specific factors
• • • • • • • Length of the extrinsic incubation period Duration of infectivity (e.g., overwintering) Transovarial transmission Length of gonotrophic cycle Likelihood of surviving to second bloodmeal Infection rate in vector population Co-infections in vectors (and effect of secondary infection on transmission)

Establishment: Host-specific factors
Susceptible Exposed Infectious Recovered

• Density of susceptible hosts (human or zoonotic)
– Influenced by cross-immunity – Duration of immunity – can infection burn out in small host pool? Herd immunity? – Reproductive rate of host (new susceptibles in population)

• Density of infectious hosts (incidence)
– Mortality/virulence

• Likelihood of contact between infectious and susceptible hosts/vectors
– For humans: peridomestic vs. recreational risk? Time of year? – For non-human hosts: seasonality will play a large role

Establishment: Host-specific factors
• Duration of infectivity (overwintering), virulence • Duration of incubation or latency period • Pathogen replication rate and time to transmissible level • Ability of host to support pathogen load at or above the transmission threshold long enough to transmit • Mobility of host during infectious period • Single or multiple host species

Establishment: Host-specific factors • For zoonotic hosts, what taxa or guild?
– Avian
• frequently attracted to peridomestic environments • increased chance of contact with humans

– Non-human primate
• Not likely to establish in the southeastern US

– Livestock – Small mammals

Establishment: climate and landscape factors
• Is the southeastern U.S. climatically and ecologically similar to regions of endemicity?
– – – – Type of breeding environment Vegetation type Growing degree days Mean, maximum, and minimum temperatures

• Is the southeastern landscape suitably connected to promote transmission?
– If livestock serve as amplifying hosts, are agricultural areas connected (via transmission pathways) to urban areas? – Are zoonotic hosts and humans connected in the landscape?

Malaria
 Malaria is a mosquito-borne protozoan infection. Plasomdium falciparum and P. vivax, the two most common species causing disease in humans, are maintained in anthroponotic cycles.  Incidence of disease is highest in sub-saharan Africa  Life cycle is strongly temperature dependent. To complete the life cycle in Anopheles mosquitoes, temperatures must be >20°C (often absent in highlands within endemic regions)  In southeastern U.S. Anopheles quadrimaculatus is a competent vector  Malaria was eliminated from the continental US by 1950
 Population shift from rural to urban  Improved water management, housing and access to medical services  Improved vector control, case finding and treatment

 Malaria is the most commonly imported vector-borne disease with approximately 1,200 reported annually  Reports of limited local transmission
 New Jersey (1991), New York (1993 and 1999), Texas (1993), Michigan (1995), and Georgia (1999)

Dengue
 Mosquito-borne (Aedes aegypti) flavivirus maintained in anthroponotic cycles  Denguelike illness was first reported in North America during an outbreak in Philadephia in 1780.  Locally-acquired cases have been rare in the US in the past 50 years  From 1977 to 1995, more than 2,706 suspect and 584 confirmed dengue cases were reported from travelers to endemic areas  Dengue is endemic in Mexico
 From 1980-1999 62,514 dengue cases were reported from Mexican states bordering texas, whereas only 64 locally-acquired cases were reported in Texas during the same time period (Reiter et al. 2003)

 Incidence of disease affected by human behavior
 Vector abundance was higher in Texas  Air conditioning, evaporative coolers, intact screens, greater distance between homes and fewer residents per household was protective

Yellow Fever
 Mosquito-borne (Aedes aegypti) flavivirus maintained in sylvatic cycles by non-human primates in sub-Saharan Africa and tropical South America
 Occurs seasonally coinciding with vector abundance  Incidence typically higher in Africa than South America likely because in Africa there are more species of competent vectors that frequently bite non-human primates and humans and there is a lower rate of immunity in the human population.

 Epidemics occur when YFV is introduced into human-mosquito cycles in high density settings  Last epidemic in North America occurred in New Orleans, 1905  The virus was eliminated from the US through quarantine and mosquito control.  Only 3 imported cases of Yellow Fever have been reported since 1970 (IHR require proof of YF vaccination for travelers)  Aedes aegypti still abundant in southeastern US, but being displaced by the tiger mosquito Aedes albopictus  Despite reintroductions, probability of re-establishment is low because of the absence of zoonotic hosts and low contact rates between humans and infected vectors

Plague
 Plague is a severe, primarily flea-borne bacterial zoonosis caused by Yersinia pestis  Identified in New Orleans in 1914 (244 of 378,563 small mammals tested positive), following introduction via rat-infested cargo ships (continued maritime re-introductions through 1934)  From 1914-1920, a total of 51 human cases were reported of which 18 were fatal; 1 additional case (stowaway) was reported in October 1924  Xenopsylla cheopis implicated as primary vector (flea index as high as 18 in warmest months and rate of infection in rats increased with the flea index)  Elimination of plague in New Orleans was attributed primarily to intensive rat-trapping and secondarily to rat-proofing and removal or rat harborage  In an attempt to control plague, several ordinances were instated, resulting in several permanent sanitation improvements for the city
 rat proofing, garbage storage and collection, animal-holding regulations, much improved system of wharves including inspection and quarentine

Conclusions
• Although devastating at the time, previous epidemics of Yellow Fever, Dengue, Malaria, and plague led to improvements in sanitation, public health, vector control and quarantine

• Provided such infrastructure is maintained or improved these vector-borne diseases are unlikely to re-emerge in the southeastern US

Acknowledgments
• • • • Ron Rosenberg Brad Biggerstaff Ken Gage Ben Beard • • • • • Lars Eisen Barry Miller Chet Moore Erin Staples Tom Burkot


								
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