Can Horton hear the whos? The importance of scale in mosquito-borne disease.

The epidemiology of vector-borne pathogens is determined by mechanisms and interactions at different scales of biological organization, from individual-level cellular processes to community interactions between species and with the environment. Most research, however, focuses on one scale or level with little integration between scales or levels within scales. Understanding the interactions between levels and how they influence our perception of vector-borne pathogens is critical. Here two examples of biological scales (pathogen transmission and mosquito mortality) are presented to illustrate some of the issues of scale and to explore how processes on different levels may interact to influence mosquito-borne pathogen transmission cycles. Individual variation in survival, vector competence, and other traits affect population abundance, transmission potential, and community structure. Community structure affects interactions between individuals such as competition and predation, and thus influences the individual-level dynamics and transmission potential. Modeling is a valuable tool to assess interactions between scales and how processes at different levels can affect transmission dynamics. We expand an existing model to illustrate the types of studies needed, showing that individual-level variation in viral dose acquired or needed for infection can influence the number of infectious vectors. It is critical that interactions within and among biological scales and levels of biological organization are understood for greater understanding of pathogen transmission with the ultimate goal of improving control of vector-borne pathogens.

Environmental risk factors associated with West Nile virus clinical disease in Florida horses.

The objective of this study was to examine the extrinsic risk factors of West Nile virus (WNV) clinical disease in Florida horses as established from confirmed and negative horses tested within the state from 2001 to 2003. An Arboviral Case Information Form (ACF) was submitted by a referring veterinarian at the time of testing to the Florida Department of Agriculture and Consumer Services on every horse suspected of a viral encephalitis in Florida. A follow-up survey that focused on arbovirus prevention and farm ecology was created and mailed to the owner of each tested horse. Data from the follow-up survey indicated peak WNV prevalence in the late summer months in Florida. Quarter horses were the most commonly affected breed. The WNV vaccine was highly protective and natural water on the property also had a protective association. Factors that increased the risk of WNV to horses were the use of fans and a stable construction of solid wood or cement. Some risk indicators were dead birds on the property and other ill animals on the property. Data from this retrospective study have helped identify factors associated with WNV transmission in equines in Florida. Horses that have not been vaccinated and show clinical signs of arboviral infection from June to November should be tested for WNV. Horses that have been vaccinated and show clinical signs should be tested when the vaccination was administered within 1 month or greater than 6 months prior to the onset of clinical symptoms associated with WN infection.

A simple method for sampling indoor-resting malaria mosquitoes Anopheles gambiae and Anopheles funestus (Diptera: Culicidae) in Africa.

Sampling indoor resting African malaria vectors is traditionally done by hand catches with oral or mechanical aspirators and pyrethrum spray catches (PSCs). In this study, we designed and briefly evaluated an inexpensive but practical alternative by using a cloth resting box or wicker resting basket and a ceiling net. Evaluations were performed in greenhouse and field situations in rural Kenya by comparing capture rates of Anopheles gambiae s.l. and Anopheles funestus (Giles) in these traps to hand collections and PSCs. A resting box and a ceiling net when used together collected more mosquitoes than a single collector using a hand-held aspirator but only one-third the number collected by PSCs. At sites where PSCs are impractical, a resting box and ceiling net can be effectively used as an alternative to hand catches in malaria surveillance.

Chemical odorant of colonial seabird repels mosquitoes.

The crested auklet, Aethia cristatella, emits a class of aldehydes shown to be potent invertebrate repellents when used by heteropterans against their predators. Our aim was to determine the efficacy of these aldehydes against mosquitoes in the laboratory. Synthetic analogues of the auklet odorant were strongly repellent to mosquitoes in controlled laboratory trials. Furthermore, the efficacy was similar to previous reports for commercial mosquito repellents. These results, in combination with a previously published study, show that constituents of the aldehyde odorant are broad spectrum in efficacy against ectoparasitic arthropods of birds. Our report is the first empirical evidence for an endogenous mosquito repellent in birds.

Predicting St. Louis encephalitis virus epidemics: lessons from recent, and not so recent, outbreaks.

St. Louis encephalitis virus was first identified as the cause of human disease in North America after a large urban epidemic in St. Louis, Missouri, during the summer of 1933. Since then, numerous outbreaks of St. Louis encephalitis have occurred throughout the continent. In south Florida, a 1990 epidemic lasted from August 1990 through January 1991 and resulted in 226 clinical cases and 11 deaths in 28 counties. This epidemic severely disrupted normal activities throughout the southern half of the state for 5 months and adversely impacted tourism in the affected region. The accurate forecasting of mosquito-borne arboviral epidemics will help minimize their impact on urban and rural population centers. Epidemic predictability would help focus control efforts and public education about epidemic risks, transmission patterns, and elements of personal protection that reduce the probability of arboviral infection. Research associated with arboviral outbreaks has provided an understanding of the strengths and weaknesses associated with epidemic prediction. The purpose of this paper is to review lessons from past arboviral epidemics and determine how these observations might aid our ability to predict and respond to future outbreaks.

Simulation studies of St. Louis encephalitis virus in south Florida.

Two simulation models were used to investigate the epidemiology of St. Louis encephalitis virus (SLEv) in south Florida, one including sentinel hosts (chickens) and amplification hosts (wild birds), while the other one included age structure in the amplification host population. The overall population size of the vector, Culex nigripalpus, was a major factor in the likelihood of epizootics for both models, but the seasonal dynamics of the vector alone did not explain variation in transmission. Interactions between seasonal factors in the mosquito and reproduction in the wild amplification avian hosts appeared to be important in the likelihood of epizootics. Biased feeding between sentinel and amplification hosts affected the time course of virus prevalence and may have implications for the interpretation of sentinel data. The time of virus introduction strongly affected the timing of outbreaks but did not affect the likelihood of epizootics. In most cases, the outbreak occurred immediately after virus introduction; however, in some cases the outbreak was delayed until the mosquito population increased. This has implications for the timing of control strategies directed against mosquito populations.

Simulation studies of St. Louis encephalitis and West Nile viruses: the impact of bird mortality.

West Nile virus (WNv) has spread through much of the eastern United States following its introduction in 1999, and arrived in Florida in 2001. Prior to its arrival, we anticipated that its transmission cycle was likely to be similar to that of St. Louis encephalitis virus (SLEv). However, high levels of avian mortality have been reported for WNv in the northeastern United States, and it was unknown how this would impact the transmission dynamics of WNv. Simulation models were used to compare the two viruses by considering the impact of bird mortality on the transmission dynamics of arboviruses in south Florida. Transmission models without disease-induced mortality (SLEv) were compared with models including disease-induced mortality (WNv). Disease-induced mortality depressed transmission, eliminating epizootics in two of 14 simulations that were epizootic without the additional mortality. In both models, the most important factor in the likelihood of epizootics was mosquito population size; the mosquito mortality rate was also important. The additional avian mortality altered the factors most important in the size and timing of epizootics, although it did not always directly affect the outcome of the simulations. In some cases, low-level transmission occurred prior to the epizootic peak. When disease-induced avian mortality was included in the simulations, appreciable numbers of dead birds occurred prior to high levels of infection in mosquitoes. This has implications for the use of dead birds as a surveillance tool monitoring the spread and transmission of WNv.

First record of Amblyomma auricularium (Acari: Ixodidae) in the United States.

Amblyomma auricularium (Conil) was collected from an armadillo in Glades County, FL. Two females and 11 males were collected. This is the first record of this species in the United States.

Attractiveness of chickens and bobwhite quail for Culex nigripalpus.

An experiment to determine if wild Culex nigripalpus preferentially enter lard-can traps baited with chickens or bobwhite quail was conducted. A strong preference for the chicken-baited trap was observed. Further development of transmission models for St. Louis encephalitis virus in southern Florida should consider increased biting rates on sentinel chickens and the consequences for the interpretation of sentinel seroconversion data.

Frequency of Saint Louis encephalitis virus in humans from Florida, USA: 1990-1999.

Four general frequencies of human St. Louis encephalitis (SLE) virus (epidemic, focal, sporadic, and no transmission) occurred in Florida between 1990 and 1999. An epidemic with 226 clinical cases and 11 deaths was reported from 28 Florida counties between July 1990 and January 1991. During the autumn of 1993, a focal outbreak was reported from Lee (5 cases) and Collier (3) Counties in southwest Florida. During the autumn of 1997, sporadic transmission to nine humans was reported from five Florida counties (Brevard [1 case], Polk [3], Charlotte [1], Lee [2], and Palm Beach [2]). Human infection with SLE virus depends on a number of variables that drive virus transmission. These include vector, virus, and avian host abundance, and meteorological events, especially rainfall. We monitored the abundance and serological status of wild avian amplification hosts, virus isolations from Culex nigripalpus Theobald females, and SLE virus transmission to sentinel chickens during 1990, 1993, and 1997. The epidemic of 1990 was characterized by conditions that produced an unusual abundance of vector mosquitoes and avian amplification hosts early in the year. We propose that epidemics may result when a specific combination of biotic and abiotic conditions favor SLE virus minimum field infection rates that approach 1:1,000 in Cx. nigripalpus vectors.

Avian serology in a St. Louis encephalitis epicenter before, during, and after a widespread epidemic in south Florida, USA.

Blood and serum from 3,915 wild and domestic birds (2,590 resident, 139 migrant, and 1,186 captive), representing 56 species collected in central Florida from 1989 through 1997, were analyzed for evidence of St. Louis encephalitis (SLE) virus transmission. All sera were tested for SLE hemagglutination inhibition (HI) antibody. Selected sera and bloods were tested for SLE neutralizing (NT) antibody and virus. The reproductive success of resident birds was highest from 1990-1992 and lowest from 1994-1997. Transmission of SLE to resident birds, especially mourning doves (Zenaida macroura), peaked during the summer of 1990, a year during which a widespread SLE epidemic was recorded in central Florida. The SLE antibody-positive resident birds 1st appeared during September of the epidemic year. Some SLE, HI antibody-positive resident birds were captured throughout 1991, but only 5% were yearlings, compared with 36% in 1990. By 1993, wild resident birds expressing HI and NT antibodies to SLE had nearly disappeared. None of the migrant birds tested were SLE-positive. Sentinel chickens maintained in Indian River County during the epidemic year seroconverted to SLE starting in early July with peak seroconversion rates in August, September, and October. High (> or = 50%) SLE seroconversion rates in sentinel chickens preceded those in wild birds by 10 wk and preceded peak human SLE transmission by at least 8 wk. Major SLE epidemics in south Florida depend on abundant wild bird populations, especially during the amplification phase of the transmission cycle. We propose that hard winter freezes along the temperature-subtropical climatic zone interface in central Florida, at approximately 27 degrees 30' North Latitude, opens foraging and nesting habitats for ground-feeding birds, resulting in high reproductive success and an abundance of seronegative individuals that rapidly amplify the SLE later in the year.

Blood feeding and oviposition by Culex nigripalpus (Diptera: Culicidae) before, during, and after a widespread St. Louis encephalitis virus epidemic in Florida.

A widespread epidemic of St. Louis encephalitis (SLE) virus was reported from central Florida from late July through December 1990 with 226 clinical cases and 11 deaths in humans. The abundance of blood-fed and gravid Culex nigripalpus Theobald females, the vector of SLE in Florida, was monitored at a resting site in Indian River County, FL, for the 3 yr before the epidemic, the epidemic year, and for 1 yr following the epidemic. Each study year was divided into four 3-mo periods that depict the subtropical SLE transmission cycle; a January-March maintenance phase, an April-June amplification phase, a July-September early transmission phase, and an October-December late transmission phase. The abundance of blood-fed and gravid females was significantly greater during the amplification, early transmission, and late transmission phases of the 1990 SLE epidemic when compared with the 3 pre-epidemic years. Throughout the study, Cx. nigripalpus blood feeding and oviposition was driven by epic (> 50 mm) rainfall events. The abundance of gravid females decreased, whereas the number of blood-fed females increased following heavy rainfalls. Epizootic and epidemic transmission of SLE to sentinel chickens and humans in Indian River County was greatest immediately following heavy rainfalls that synchronized Cx. nigripalpus oviposition and blood feeding.

A fitness advantage for Aedes aegypti and the viruses it transmits when females feed only on human blood.

Literature on arthropod-borne diseases has traditionally supported the notion that mosquito vectors maintain a feeding duality that includes vertebrate blood meals for egg development and sugar meals from plants for the synthesis of flight and survival energy reserves. Aedes aegypti was found to deviate from that feeding pattern by obtaining a reproductive advantage when feeding only on human blood. Female mosquitoes fed human blood alone had a greater net replacement rate and intrinsic rate of growth during all phases of their reproductive life than conspecifics fed human blood plus sucrose. Feeding frequently on human hosts during each gonotrophic cycle is necessary to avoid death due to starvation and increases exponentially the spread of Ae. aegypti-borne disease. Our results help explain why Ae. aegypti is such an unusually efficient vector of human disease; frequent biting of humans results in a high reproductive rate for vectors as well as the viruses they transmit.

Ultrastructure of the eggs of Culicoides circumscriptus, Culicoides gejgelensis, and Culicoides imicola (Diptera:Ceratopogonidae).

The eggs of Culicoides circumscriptus Kieffer, Culicoides gejgelensis Dzhafarov, and Culicoides imicola Kieffer collected in Israel are described and illustrated by scanning electron micrographs. Eggs of all 3 species were morphologically similar, generally sausage- or cigar-shaped, with slight dorsal-ventral curvature and longitudinal rows of tubercle pillars covered with a thin adhesive layer. Tubercle pillars were scarce on C. circumscriptus and C. imicola eggs, but were common on C. gejgelensis eggs, forming longitudinal plastrons with an associated hydrofuge meshwork. All 3 species had micropyle domes and associated aeropyles at the anterior end of their eggs.

Eastern equine encephalitis transmission to emus (Dromaius novaehollandiae) in Volusia County, Florida: 1992 through 1994.

From May 1992 through October 1994, sera were collected from 204 domestic emus (Dromaius novaehollandiae) at a ranch in Volusia County, FL, and tested for antibody evidence of arboviral infection. Hemagglutination-inhibition (HI) and neutralizing (NT) antibodies to eastern equine encephalitis (EEE) virus were identified in sera collected during each year. In addition, HI and NT antibodies to St. Louis encephalitis virus were detected in 3 naturally infected emus. Isolations of EEE virus were made from emu blood and tissues collected in 1992 and 1994, when EEE-related mortality in emus was 14% and 1%, respectively. A total of 259 mosquito pools was collected and tested for arbo-viruses during the 3-year study. The EEE virus was isolated from 4 of 140 mosquito pools (2 Anopheles crucians and 2 Culex erraticus pools) in 1992 and 3 of 10 pools (all Culex nigripalpus) in 1994. Emus vaccinated against EEE virus showed evidence of short-term HI antibody acquisition. Evidence of EEE antibody transfer from naturally infected hens to their offspring is reported.

Antibodies to arthropod-borne encephalitis viruses in small mammals from southern Florida.

From 1987 through 1991, blood samples were collected from 10 species of small mammals in Indian River Country, Florida (USA). Sera from 1,347 animals were analyzed for hemagglutination-inhibition (HI) antibody to St. Louis encephalitis (SLE) and eastern equine encephalitis (EEE) viruses. Of these, 75 (5.6%) were positive for HI antibody to SLE virus and 121 (9.0%) were positive for EEE antibody. Sera from five mammalian species were tested for neutralizing (NT) antibody to SLE, EEE, Highlands J (HJ a member of the western equine encephalitis virus complex), or Everglades (EVE, a member of the Venezuelan equine encephalitis complex) viruses. By serum neutralization tests, 26 (46%) of 57 had SLE antibodies, 14 (24%) of 58 had EEE antibodies, two (3.2%) of 63 had HJ antibodies, and 9 (14%) of 63 had EVE antibodies. One Sigmodon hispidus and one Peromyscus gossypinus had NT antibodies both to EEE and HJ viruses. Blood samples from 512 mammals were tested for virus. Isolations of one EVE virus and two unidentified arenaviruses were made from P. gossypinus and one EVE virus isolate was made from a S. hispidus.

Transmission patterns of St. Louis encephalitis and eastern equine encephalitis viruses in Florida: 1978-1993.

Sentinel chickens were maintained at field sites in 40 Florida counties for varying periods between 1978 and 1993. For each county, the total number of St. Louis encephalitis virus (SLE) or eastern equine encephalitis (EEE) virus seroconversions were divided by the number of chickens exposed to calculate a mean annual seroconversion rate. These rates were used to evaluate the annual and geographical distributions of these viruses within Florida. For SLE, the rates in counties that reported human SLE cases during a widespread epidemic in 1990 were compared with adjusted mean annual seroconversion rates calculated by excluding the epidemic year seroconversion data. In general, actual rates were higher than adjusted rates in counties where human SLE cases were reported in 1990. Similar calculations were made for EEE virus that was unusually abundant in northern Florida during the spring and summer of 1991. In general, EEE virus was distributed in the panhandle and northern regions and SLE virus in the central and southern regions of the state. However, this distribution was not exclusive, and during years of high transmission, either virus can extend beyond its normal range. The annual distribution of SLE and EEE viruses was sporadic, ranging from years with little detectable activity to years with widespread, high-level transmission.

Effect of wind velocity on suction trap catches of some Florida mosquitoes.

The correlations between wind velocity and suction trap catches of mosquitoes when taken at 15-min intervals during the night were studied at 2 locations. Although normal mosquito flight speeds are approximately 1 m/sec, trap catches were reduced about 50% by winds of 0.5 m/sec and 75% at 1.0 m/sec. Trap catches were inversely related to winds of all velocities and even the lightest winds reduced trap catches. No evidence was found for a threshold below which wind velocity had no effect.

Antibodies to St. Louis encephalitis virus in armadillos from southern Florida.

From January 1990 through March 1991, 189 armadillos (Dasypus novemcinctus) were collected from Brevard and Glades Counties in southern Florida (USA). The sera were analyzed for hemagglutination-inhibition (HAI) antibodies against St. Louis encephalitis (SLE) and eastern equine encephalomyelitis (EEE) viruses. None of the armadillos had detectable HAI antibody to EEE virus, but 59 (31%) had antibodies against SLE virus. Sera from 31 of the HAI-positive armadillos contained significant levels of neutralizing (NT) antibody to SLE virus. Armadillos captured during the 1990 SLE human epidemic in south Florida had a greater prevalence of HAI and NT antibody to SLE virus than did animals captured before the start of the epidemic. This is evidence that armadillos were fed on by mosquitoes infected with SLE virus. We propose that armadillos may be involved in the SLE amplification and transmission cycles in Florida.