RESUMO

All known Rift Valley fever(RVF) outbreaks in Kenya from 1950 to 1998 followed periods of abnormally high rainfall. On an interannual scale, periods of above normal rainfall in East Africa are associated with the warm phase of the El Niño/Southern Oscillation (ENSO) phenomenon. Anomalous rainfall floods mosquito-breeding habitats called dambos, which contain transovarially infected mosquito eggs. The eggs hatch Aedes mosquitoes that transmit the RVF virus preferentially to livestock and to humans as well. Analysis of historical data on RVF outbreaks and indicators of ENSO (including Pacific and Indian Ocean sea surface temperatures and the Southern Oscillation Index) indicates that more than three quarters of the RVF outbreaks have occurred during warm ENSO event periods. Mapping of ecological conditions using satellite normalized difference vegetation index (NDVI) data show that areas where outbreaks have occurred during the satellite recording period (1981-1998) show anomalous positive departures in vegetation greenness, an indicator of above-normal precipitation. This is particularly observed in arid areas of East Africa, which are predominantly impacted by this disease. These results indicate a close association between interannual climate variability and RVF outbreaks in Kenya.

Assuntos

Assuntos

RESUMO

To assess how temperature affects fecundity and Venezuelan equine encephalomyelitis (VEE) viral replication in VEE-infected Amblyomma cajennense (F.), adult ticks were inoculated intracoelomically with either VEE virus (I-A variant) or diluent. Ticks were held at 19, 26, or 33 degrees C and tested for VEE virus over 20 d. Up to 10 pairs of ticks (1 female:1 male) from each temperature group were allowed to feed on guinea pigs on day 8 after inoculation. Maximum titers did not differ significantly among infected ticks held at 19, 26, or 330 degrees C (P = 0.59), and ticks maintained high viral titers throughout the sampling period. Likewise, there was no significant difference in the number of eggs oviposited between ticks inoculated with virus or diluent and held at the same temperature, or among groups held at different temperatures (P = 0.48). The percentage of viable eggs from virus-inoculated or diluent-inoculated adults also was not significantly different (P > or = 0.14).

Assuntos

RESUMO

To compare the potential for an enzootic or an epizootic strain of Venezuelan equine encephalomyelitis (VEE) virus to infect Amblyomma cajennense (F.), larval ticks were fed on guinea pigs (strain 13) inoculated with an enzootic viral strain of variant I-E (68U201) or on guinea pigs inoculated with an epizootic strain of variant I-A (Trinidad donkey). Peak viremias were 10(5.2) plaque-forming units (PFU)/ml and 10(7.3) PFU/ml in guinea pigs infected with enzootic and epizootic viral strains, respectively. Ticks feeding on enzootic- and epizootic-infected hosts had viral titers of 10(2.5) and 10(3.9) PFU per tick, respectively, at drop-off. Although epizootic virus was recovered from 98% (127 of 130) of larval ticks up to 16 d after drop-off, enzootic virus was recovered from 95% (19 of 20) at drop-off (mean titer, 10(2.5) PFU per tick), with recovery rates declining rapidly to 2 of 10 (mean titer, 10(1.4) PFU per tick) by 16 d after drop-off. Transstadially transmitted epizootic virus was found in 0.4% (12 of 2,950) of unfed nymphs (mean titer, 10(2.8) PFU per tick) 63 d after drop-off, 1% (5 of 521) fed nymphs 69 d after drop-off, and 1% (4 of 400) of unfed adults (mean titer, 10(3.6) PFU per tick) 106 d after drop-off. No enzootic virus was recovered from 4,600 unfed nymphs tested 63 d after drop-off.

Assuntos

RESUMO

Studies were undertaken to determine whether two species of hematophagous mites could transmit Venezuelan equine encephalomyelitis (VEE) virus to suckling mice. The chicken mite, Dermanyssus gallinae (De Geer), mechanically transmitted this virus to previously uninfected mice up to 16 h after taking a blood meal from a viremic animal. In contrast, Laelaps kochi Oudemans failed to transmit virus to mice in any feeding trials. Virus did not replicate in either species of mite but was detected in 20% of D. gallinae and in 12% of L. kochi specimens tested 48 h after a viremic blood meal. In mites inoculated intracoelomically with virus, recovery rates declined over time, but virus was detected up to 7 d later in 5% of D. gallinae and up to 3 d later in 10% of L. kochi. Biological transmission of VEE virus was not demonstrated in either species of mite.

Assuntos

RESUMO

To assess a possible role of ticks as the maintenance host for epizootic strains of Venezuelan equine encephalomyelitis (VEE) virus, laboratory experiments were conducted to determine if ticks could become infected, maintain, and transmit the virus. Larval and nymphal Amblyomma cajennense (F.) and larval Dermacentor nitens Neumann ticks were exposed to epizootic VEE virus (Trinidad donkey strain) by allowing them to feed on viremic guinea pigs (strain 13). In A. cajennense, transstadial transmission was observed from larvae to nymphs and adults. Horizontal viral transmission to a mammalian host was accomplished by nymphs. Infection rates in nymphs and adults were 2% (42/2,750) and 4% (9/244), respectively, afer ingestion of virus as larvae. Virus was detected in A. cajennense adult ticks for up to 171 d after infection in the larval stage. A cajennense, exposed as nymphs, ingested virus but did not become infected (0/164 after 10 d after taking an infective bloodmeal). No virus was detected in D. nitens 7 d after exposure. These findings suggest that A. cajennense potentially could be involved in an interepizootic maintenance cycle of epizootic VEE viral strains.

Assuntos