Antecedent Avian Immunity Limits Tangential Transmission of West Nile Virus to Humans

Background: West Nile virus (WNV) is a mosquito-borne flavivirus maintained and amplified among birds and tangentially transmitted to humans and horses which may develop terminal neuroinvasive disease. Outbreaks typically have a three-year pattern of silent introduction, rapid amplification and subsidence, followed by intermittent recrudescence. Our hypothesis that amplification to outbreak levels is contingent upon antecedent seroprevalence within maintenance host populations was tested by tracking WNV transmission in Los Angeles, California from 2003 through 2011. Methods: Prevalence of antibodies against WNV was monitored weekly in House Finches and House Sparrows. Tangential or spillover transmission was measured by seroconversions in sentinel chickens and by the number of West Nile neuroinvasive disease (WNND) cases reported to the Los Angeles County Department of Public Health. Results: Elevated seroprevalence in these avian populations was associated with the subsidence of outbreaks and in the antecedent dampening of amplification during succeeding years. Dilution of seroprevalence by recruitment resulted in the progressive loss of herd immunity following the 2004 outbreak, leading to recrudescence during 2008 and 2011. WNV appeared to be a significant cause of death in these avian species, because the survivorship of antibody positive birds significantly exceeded that of antibody negative birds. Cross-correlation analysis showed that seroprevalence was negatively correlated prior to the onset of human cases and then positively correlated, peaking at 4–6 weeks after the onse

Quantifying sociodemographic heterogeneities in the distribution of Aedes aegypti among California households.

The spread of Aedes aegypti in California and other regions of the U.S. has increased the need to understand the potential for local chains of Ae. aegypti-borne virus transmission, particularly in arid regions where the ecology of these mosquitoes is less understood. For public health and vector control programs, it is helpful to know whether variation in risk of local transmission can be attributed to socio-demographic factors that could help to target surveillance and control programs. Socio-demographic factors have been shown to influence transmission risk of dengue virus outside the U.S. by modifying biting rates and vector abundance. In regions of the U.S. where Ae. aegypti have recently invaded and where residential areas are structured differently than those in the tropics where Ae. aegypti are endemic, it is unclear how socio-demographic factors modify the abundance of Ae. aegypti populations. Understanding heterogeneities among households in Ae. aegypti abundance will provide a better understanding of local vectorial capacity and is an important component of understanding risk of local Ae. aegypti-borne virus transmission. We conducted a cross-sectional study in Los Angeles County, California during summer 2017 to understand the causes of variation in relative abundance of Ae. aegypti among households. We surveyed 161 houses, representing a wide range of incomes. Surveys consisted of systematic adult mosquito collections, inspections of households and properties, and administration of a questionnaire in English or Spanish. Adult Ae. aegypti were detected at 72% of households overall and were found indoors at 12% of households. An average of 3.1 Ae. aegypti were collected per household. Ae. aegypti abundance outdoors was higher in lower-income neighborhoods and around older households with larger outdoor areas, greater densities of containers with standing water, less frequent yard maintenance, and greater air-conditioner use. We also found that Ae. aegypti abundance indoors was higher in households that had less window and door screening, less air-conditioner usage, more potted plants indoors, more rain-exposed containers around the home, and lower neighborhood human population densities. Our results indicate that, in the areas of southern California studied, there are behavioral and socio-demographic determinants of Ae. aegypti abundance, and that low-income households could be at higher risk for exposure to Ae. aegypti biting and potentially greater risk for Zika, dengue, and chikungunya virus transmission if a local outbreak were to occur

The numbers of House Finches or House Sparrows that ever tested positive (pos) or negative (neg) for WNV antibody transformed by ln(y+1) and regressed as a function of time retained within study areas grouped in 10 week intervals.

<p>All slopes were significant (P<0.001) when tested by ANOVA. LL and UL are the lower and upper 95% confidence limits about the slope; slopes with non-overlapping limits were significantly different (P<0.05). R² is the coefficient of determination. Survivorship was estimated as was the backtransformed slope and measured retention within the study, with losses due to mortality and emigration. Mean age was expressed as weeks remaining within the study area.</p

Number of House Finches (HOFI) and House Sparrows (HOSP) ever testing positive (POS) or negative (NEG) for West Nile virus antibodies transformed to ln(y+1) and plotted as a function time retained within our study area grouped into 10 week intervals.

<p>Number of House Finches (HOFI) and House Sparrows (HOSP) ever testing positive (POS) or negative (NEG) for West Nile virus antibodies transformed to ln(y+1) and plotted as a function time retained within our study area grouped into 10 week intervals.</p

Mean number of House Finches and House Sparrows collected per month during each year.

<p>Mean number of House Finches and House Sparrows collected per month during each year.</p

Total numbers of House Finches and House Sparrows collected per month and the proportion banded or recaptured (recap).

<p>Total numbers of House Finches and House Sparrows collected per month and the proportion banded or recaptured (recap).</p

Inverse of plaque reduction neutralization test (PRNT) titers per mL plotted as a function of enzyme immunoassay positive over negative well optical density ratios (EIA P/N) for WNV experimentally infected and uninfected House Finches and House Sparrows (n = 44).

<p>Inverse of plaque reduction neutralization test (PRNT) titers per mL plotted as a function of enzyme immunoassay positive over negative well optical density ratios (EIA P/N) for WNV experimentally infected and uninfected House Finches and House Sparrows (n = 44).</p

Number of times recaptured birds tested negative, EIA positive, and EIA and PRNT positive.

<p>Data are shown for 6 House Finches and 6 House Sparrows collected on multiple occasions.</p

Proportion of after hatching year (AHY) and juvenile/hatching year (JUV/HY) House Finches and House Sparrows positive for antibodies against WNV based on EIA results.

<p>Seroprevalence was cumulative and based on all birds regardless of recapture status.</p

Quantifying sociodemographic heterogeneities in the distribution of Aedes aegypti among California households.

The spread of Aedes aegypti in California and other regions of the U.S. has increased the need to understand the potential for local chains of Ae. aegypti-borne virus transmission, particularly in arid regions where the ecology of these mosquitoes is less understood. For public health and vector control programs, it is helpful to know whether variation in risk of local transmission can be attributed to socio-demographic factors that could help to target surveillance and control programs. Socio-demographic factors have been shown to influence transmission risk of dengue virus outside the U.S. by modifying biting rates and vector abundance. In regions of the U.S. where Ae. aegypti have recently invaded and where residential areas are structured differently than those in the tropics where Ae. aegypti are endemic, it is unclear how socio-demographic factors modify the abundance of Ae. aegypti populations. Understanding heterogeneities among households in Ae. aegypti abundance will provide a better understanding of local vectorial capacity and is an important component of understanding risk of local Ae. aegypti-borne virus transmission. We conducted a cross-sectional study in Los Angeles County, California during summer 2017 to understand the causes of variation in relative abundance of Ae. aegypti among households. We surveyed 161 houses, representing a wide range of incomes. Surveys consisted of systematic adult mosquito collections, inspections of households and properties, and administration of a questionnaire in English or Spanish. Adult Ae. aegypti were detected at 72% of households overall and were found indoors at 12% of households. An average of 3.1 Ae. aegypti were collected per household. Ae. aegypti abundance outdoors was higher in lower-income neighborhoods and around older households with larger outdoor areas, greater densities of containers with standing water, less frequent yard maintenance, and greater air-conditioner use. We also found that Ae. aegypti abundance indoors was higher in households that had less window and door screening, less air-conditioner usage, more potted plants indoors, more rain-exposed containers around the home, and lower neighborhood human population densities. Our results indicate that, in the areas of southern California studied, there are behavioral and socio-demographic determinants of Ae. aegypti abundance, and that low-income households could be at higher risk for exposure to Ae. aegypti biting and potentially greater risk for Zika, dengue, and chikungunya virus transmission if a local outbreak were to occur