This implies that with only the current indoor-based vector intervention , malaria elimination may not be achieved since the remaining exposure to An. arabiensis bites could still occur outdoors and/or indoors before people retire to bed. A study conducted in Tanzania also showed that less than half of all human exposure to An. arabiensis bites occurred at times when using insecticide-treated nets was feasible . Only 28% of human exposure to An. pharoensis bites occurred at times when LLINs would be in use if they were available, indicating that the majority of exposure to An. pharoensis also occurs outdoors and before sleeping hours. For LLIN users, most of residual human exposure to An. arabiensis bites occurred outdoors while 23% occurred indoors before people retire to bed. Similarly, most of the residual exposure to An. pharoesnsis bites occurred outdoors, while 15% occurred indoors before sleeping time. The findings suggest that additional control measures which can protect against outdoor exposure or which target immature stages of vectors are required to complement the current indoor-based vector control interventions to interrupt transmission due to exposure to vector bites occurring outdoors and in the early evening hours. Anopheles arabiensis showed a preference to feed on bovine to humans. The findings of previous studies conducted in different parts Ethiopia showed that the feeding behavior of An. arabiensis varied across different geographical locations. The species exhibited zoophagic behavior in some settings ,cannabis drying racks commercial anthropophagic in other places and anthropozoophilic tendency in some areas . Such inter population variations in feeding behavior might be due to difference in host availability between different settings . Inter population genetic variation in An. arabiensis might have also contributed to the variation in its feeding behavior between different localities.
Sub-population of An. arabiensis with preference to feed on cattle have been shown to correlate with arrangement of 3Ra chromosomal inversion . Such phenomenon could increase the proportion of zoophagic fraction of An. arabiensis in settings where the 3Ra inversion is documented . Similarly, An. pharoensis showed zoophagic behavior, preferring to feed on bovine to other potential hosts available in the study area. The zoophagic behavior of An. arabiensis and An. pharoensis can be considered as an opportunity to introduce complementary vector control intervention such as zooprophylaxis to divert host-seeking mosquitoes from humans . Anthropophilic and endophagic malaria vectors can be controlled by LLINs and IRS, whereas those species predominantly feeding on cattle outdoors could sustain residual malaria transmission despite high coverage of indoor-based vector control interventions. Hence, targeting zoophagic vectors is crucial to achieve malaria elimination. Zooprophylaxis can reduce malaria transmission by pulling mosquitoes toward dead-end hosts and by reducing vector density if cattle are treated with insecticides . The estimated indoor and outdoor EIRs for An. arabiensis were 6.2 and 1.4 ib/p/year, respectively, indicating the contribution of An. arabiensis to both indoor and outdoor malaria transmission. The occurrence of indoor malaria transmission despite high LLIN coverage in the study area might be attributed to the exposure of people to vector bites in the evening before sleeping hours. In addition, An. pharoensis had an estimated outdoor EIR of 3.0 ib/p/year, indicating the contribution of this species to outdoor transmission. Although An. pharoensis has been considered as a secondary vector in Ethiopia, a recent study revealed similar tends of susceptibility of this species to Plasmodium parasite infection as An. arabiensis , indicating that An. pharoensis could also play a major role in outdoor malaria transmission.
Other recent studies have also documented an increasing role of An. pharoensis in malaria transmission in the country .On the other hand, the EIRs of An. arabiensis reported in this study are lower compared to the EIRs of An. arabiensis previously reported from different parts of Ethiopia and elsewhere in Africa . This could be attributed to a relatively higher coverage of LLINs in the study area. The strength of this study is that both vector and human behavior were considered in the calculation of human biting rates and EIRs to better understand where and when exposure to mosquito bites and residual malaria transmission occur. Moreover, this study employed both gold standard method i.e. HLC and alternative methods for vector surveillance to determine vector density, human biting rates and sporozoite rates. The findings of this study suggest that CDC light trap can be paired with HDNT for routine indoor and outdoor malaria vector surveillance as an alternative tool to HLC. The limitation of the study was that the sporozoite infection rates reported in this study were based on ELISA and the positive specimens were not confirmed by PCR. The proportions of human exposure to mosquito bites were estimated assuming no seasonal changes in sleeping habits of people in the study area, hence night to night differences in sleeping time were not tracked in this study. Burning fields to remove crop stubble, weeds and pests occurs worldwide, and California’s estimated emissions from the burning of crop residue ranks fifth nationally . These emissions potentially contribute to particulate matter levels in the San Joaquin Valley, which often exceed standards for ambient air each season of the year . Studies have documented thousands of chemicals in smoke; they can exist in gas, liquid and solid form. During burning, plant matter breaks apart and gases condense on particles or form particles. Most particulate matter in smoke is smaller than 2.5 micrometers in diameter and can be transported over long distances .
The California Air Resources Board estimates annual tons of particulate matter and gases emitted from field, orchard and weed burning for California counties ; their estimates are derived from burns of crop residue in a laboratory . Studies have documented emissions of 14 semivolatile polycyclic aromatic hydrocarbons , the most abundant of which is naphthalene . A respiratory carcinogen , naphthalene is predominantly found in the gas phase of air sampling, with the remainder measured in the particulate phase . Few ambient air monitoring studies have been conducted in the United States during agricultural burns, either adjacent to burns or in towns and communities . Educational efforts for the general public have mostly focused on smoke from wildfires and have included public health recommendations for those exposed to elevated particulate matter and visibility guidelines for those air levels . CARB has also distributed a lengthy educational pamphlet for farmers . However, it was unknown whether health educational outreach efforts specifically targeting agricultural burning were needed. Particulate matter emissions from field burning in Imperial County — a rural desert county in California’s southeast corner — rank among the highest for any county in the state . The agricultural area of Imperial County is anirrigated desert valley,customizable shelf system where a variety of crops including vegetables, hay and grain are grown . Fields of bermuda grass, which is grown both for hay and seed, are burned primarily in the winter, while wheat stubble is burned during the summer. Less than 3% of homes in Imperial County use wood as a house heating fuel . During the winter when night temperatures drop, inversions commonly occur; cooler ground level air, including pollutants, are trapped near the Earth’s surface by an upper layer of warmer air. For fields to be burned, the Air Pollution Control District requires that the estimated inversion layer must be at 3,000 feet or higher, and the burn must be initiated between 10 a.m. and 3 p.m. Farmers who have applied for burn permits are usually notified by the district the day before the targeted burn date that their fields may be burned. Thus, our air monitoring studies required methods that could be rapidly deployed. Our methods and results are described in greater detail in a report to the funding agency .We selected three schools and one church based on their proximity to burns in previous years and installed portable Environmental Beta Attenuation Monitors . We measured hourly average concentrations of PM2.5 and meteorological variables for 69 days starting on Jan. 14, 2009. E-BAM PM2.5 measurements are not recognized as a Federal Equivalent Method or a Federal Reference Method , one of which is required to determine if levels legally exceed air standards. However, E-BAM measurements have proven comparable to FRM measurements in field tests .
A record of agricultural burn events was provided by the Air Pollution Control District. During the E-BAM monitoring period, 15,686 acres were burned on 35 allowable burn days; the acreage burned daily ranged from 0 to 1,400 acres. Average 24-hour PM2.5 concentrations were highest — 12 micro-grams per cubic meter — at the northern station and lowest at the western station . The lower levels in Seeley may have been because the predominant wind direction was from the west, and sources of pollution, including burned fields, were predominantly to the east of the Seeley station. All daily PM2.5 levels were below the federal standard for unhealthy air, 35 µg per cubic meter. However, at the Calipatria station the 95th percentile of 24-hour concentrations was above 16 µg per cubic meter, which corresponds to moderate air quality where “aggravation of heart or lung disease in people with cardiopulmonary disease and older adults” is possible . We also compared 8-hour average PM2.5 concentrations at the four locations. There was little difference during the day , with levels slightly lower on field-burn days compared to nofield-burn days . In contrast, from the early evening to the morning of the next day , average PM2.5 concentrations on field-burn days were 23% higher than on no-field-burn days. Additionally, on days when there was an agricultural burn within 2 miles of the Calipatria station , during the evening-to-morning period the average 8-hour concentrations were 19.5 to 20.7 µg per cubic meter, 170% higher than on days when there were no burns within 2 miles . Following the burns near the Calipatria station, on the subsequent 2 days when there were no additional burns , the evening-to-morning levels remained slightly above levels on days with no burns . Higher particulate matter levels from evening-to-morning hours associated with agricultural burning in Imperial County are consistent with air pollution dynamics. Air pollutants may rise during the day as the Earth’s surfaces are heated and then be brought down to ground level by the descent of an evening inversion layer. The night and next-day accumulation of smoke is described in a CARB pamphlet for farmers .We monitored five specific burns of 65 to 150 acres of bermudagrass stubble during the E-BAM monitoring period. For four burns, ground-level winds were low at 2 to 3 miles per hour , and the plume from the burn rose up to the apparent height of the inversion layer where it was observed to spread out, sometimes in the opposite direction of the ground wind direction. The ground-level plumes dispersed within about an hour, but the upper plumes remained visible, apparently limited by the inversion layer, until sunset. At one of the five burns, the Dunham burn, the wind speed was higher , and the ground-level smoke plume engulfed a house on the same property as the burned field and drifted onto an adjacent field. We deployed portable particulate matter monitors — active-flow and passive personal DataRAM nephelometers — which continuously measured PM2.5 and PM10 , respectively. This monitoring was done at three locations surrounding each of the five burns for 24 to 72 hours. Two locations were near the burns and were places of public access, homes or telephone poles; the other was at the nearest E-BAM, which was farther away . At the 15 locations, field difficulties including power outages, supply delivery problems and apparent equipment or software malfunctions limited monitoring to 11 and 13 locations for the PM2.5 and PM10 nephelometers, respectively. At the 15 targeted nephelometer locations, plus an additional 14 locations near the burns, trained local personnel placed passive samplers to measure particulate matter and naphthalene for 24 to 120 hours and then sent the samplers to our laboratory for analysis. Due to winds shifting from the predicted direction, our samplers were directly downwind only at the Dunham burn. At that burn, although passive samplers were mounted on several telephone poles immediately adjacent to the burned field, only one PM10 nephelometer was successfully deployed.