| Background: Hantaviruses are rodent-borne viruses belonging to the Family Bunyaviridae, and can cause two kinds of human diseases, i.e. hemorrhagic fever with renal syndrome (HFRS) in Europe and Asia and hantavirus pulmonary syndrome (HPS) in the Western Hemisphere. Humans usually acquire hantavirus infection by contact or inhalation of aerosols and secretions from infected rodent hosts. Each hantavirus species is predominately associated with a distinct or a few related rodents as its primary natural reservoirs. In China, Hantaan virus (HTNV) and Seoul virus (SEOV), the recognized causative agents of HFRS, are respectively associated with Apodemus agrarius and Rattus norvegicus. HFRS is a significant public health problem in mainland China. The number of incident cases accounts for 90% of the total reported cases in the world. Shandong Province is one of the most severe endemic areas in mainland China. Although integrated control measures including vaccination have been carried out for years, the disease incidence has bounced up and new foci continuously emerged these years. The incidence and geographic distribution of HFRS are known to be mainly associated with the species, population density, distribution and infection rate of rodent hosts, which are usually influenced by the natural habitat structure including the land cover and burrow space of host, etc. Environmental changes in habitat could lead to an increase of virus transmission risk from infected rodents to humans. Studies on HPS indicated that the emergence of hantavirus was associated with weather and climatic events like El Ni?o Southern Oscillation. The prevalence of HFRS in China was determined by various environmental factors such as elevation, precipitation, temperature, vegetation and soil types. Our previous study revealed that precipitation, humidity, and temperature were associated with the seasonal variation of HFRS incidence in Shandong Province. However, the factors highly associated with the risk of rodent infection and the future distribution need further study.Avian influenza is a fulminating infectious disease occurred in bird species with influenza-like symptoms by infect with Influenza A-viruses. Highly pathogenic avian influenza (HPAI) virus subtype H5N1 is an emerging issue of world public health since it was first shown to be the cause of human morbidity and mortality in Hong Kong in 1997. It has caused numerous disease outbreaks in domestic poultry and wild bird populations, and highly threatens human health. According to the latest data of WHO, a total of 533 human cases with 315 deaths around the world was reported from 2003 to Mar. 15th, 2011. In China, the morbidity is 65% with 26 deaths out of 40 cases. Since January 2004, there is a number of HPAI H5N1 outbreak in poultry and wild birds throughout mainland China., occasional trans-species transmission to humans have also been reported Although many control measures had been taken, including culling of more than 140 million poultry, outbreaks have recurred in different parts of the world. Several studies showed that the risk of HPAI H5N1 virus presence was associated with many factors, like wild migratory birds, the free-ranging duck, duck farming, the local abundance of geese, chicken numbers, paddy cultivation, human population density, and so on. And other studies have reported strong influences of other factors on the transmission of avian influenza, like plant phenology and land-surface reflectance. Yet, the transmission mechanism of the virus largely remains as an incomprehensible"black box". Transmission of disease across a spatial and temporal scale can be attributed to the overlapping ecological niches of the vector, host, pathogens and their associated ecological landscape. It has been investigated that the vector-borne diseases like chagas, malaria, dengue and West Nile virus are transmitted due to such niche overlapping. Therefore, it is necessary to characterize the ecologic niche of the HPAI H5N1 virus infected hosts in China and identify the most significant risk factors, then make risk prediction for future and provide scientific clues for precautionary efforts against any future outbreak.Objectives:①To clarify the pattern across time and space of HFRS in Shandong Province over past years, and analyze the changing of epidemic foci type and epidemic trend; identify the most significant factors highly associated with the risk of rodents infection; Increase our understanding of the habitat of hantavirus infected hosts in Shandong Province by construct their ecologic niche model and make risk prediction for future.②Characterize the ecologic niche of the HPAI H5N1 virus infected poultry and wild birds in China and identify the most significant risk factors, then make prediction of risk distribution in future and provide scientific clues for precautionary efforts against any future outbreak.Methods: After located all HFRS human cases from 2002 to 2007 on the map according to their address, we used the space-time scanning cluster analysis to determine the distribution of the hot spots at the town level. We combined Ecologic niche modeling with geographic information systems (GIS) and remote sensing techniques to identify the most significant factors associated with risk of HV virus infections in rodents in Shandong Province and HPAI H5N1 virus infection in poultry and wild birds. Based on the 33 sites positive for HV in rodents in 4 surveillance counties in Shandong Province and 113 epidemic sites of HAPI outbreak, we constructed their ecologic niche model respectively and then made prediction for future risk distribution.Results:①The incidence of HRFS from 2002 to 2007 in Shandong Province showed a decrease trend(except for 2003). Opposite with 2002 and 2003, the fall-winter incidence peak was a little higher than spring peak from 2004 to 2007 based on the statistic result by month. It is thus clearly that the incidence peak has transformed from spring pattern into fall-winter peak, yet all of these years showed a double peak ,indicating that the epidemic foci type of Shandong Province belonging to mixed type. The number of town with HFRS cases reported showed a little decrease, and the hotspots zone was downsized. The result of space cluster analysis on town scale showed that the most likely cluster area situated in the center and southern area of the province, and the secondly and third likely cluster area was not obvious, this result is similar to the result of previous studies, which meant that there was no big change in as to the hotspot extent. The surveillance result of the four hosts'surveillance sites showed that all of these sites belong to a mixed type with Rattus as the predominant species, while, Jvnan and Jiaonan have a higher concentration of Apodemus species than the other two sites. The ranking of hosts density of the 4 couties is Jvnan, Jiaonan, Zhangqiu and Qingzhou, and the indoor hosts density of the four sites is higher than outdoors, obviously, the habitat of the rodents trapped are more often indoors. The result of the ecologic niche models of the infected rodent hosts showed the Land cover, Elevation and NDVI had bigger contribution to the models, suggesting they are more associated with the distribution of hantavirus infected hosts. The response curves indicated nonlinear associations between the probability of infected rodents and variation of each variable. Among all types of land cover,"Rainfed croplands"and"Mosaic Vegetation/Croplands"were highly associated with the presence of hantavirus-infected rodent hosts. The probability of infected rodents increased with elevation, peaked at about 180 m and decreased subsequently. The response curves of NDVI, LSTN and temperature show similar patterns, in which the risk for presence of infected rodent hosts was initially increased, peaked at a certain value and declined thereafter. The risk for presence of infected rodents was highest when RH was around 55%, dropped to the lowest at 66%, increased to the second peak at 71%, and declined again. The average ROC for 10 replicate runs of the model was 0.864 with a standard deviation of 0.074, indicating the model performance was good. The distribution map of predicted risk of hantavirus-infected rodent hosts of Shandong province in 2009 indicated the highest risk area was locate in the center and southern of the province. To validate accuracy of the model, the detection result of hantavirus infected rodents at the field surveillance sites in 2009 were overlaid on the risk map, and all of the sites positive for HV fell on the moderate and high risk areas,indicate a high predicting accuracy. Meanwhile, the result of the human cases was used for validation, and it showed that most of human cases were locate in or around the high risk area, indicate the actual case localities were in accordance with the predicted distribution.②The ecologic niche model result of Highly pathogenic avian influenza(HPAI) showed that among all land cover type,"Closed broadleaved forest permanently flooded (saline-brackish water)","Closed to open vegetation regularly flooded, Irrigated croplands","Closed to open mixed broadleaved and needle-leaved forest"and"Rain-fed croplands"were area of the highest risk. While,"Mosaic Vegetation/Croplands","Water bodies","Closed to open shrub land"and"Closed to open grassland"was less associated with it. From the response curve of soil type, we could find that there were 9 soil types highly associated with the presence of the infected hosts, including fluvo-aquic soil, southern paddy field; irrigate silt fluvo-aquic soil, oasis soil, land-scrub grassland, northern paddy field, red paddy field, red soil, drab soil. The distance to the traffic system (including road, highway, etc.) is strongly associated with presence of HPAI H5N1. The risk of infection increase as the distance to traffic decrease from 30 kilometers. The distance to cropland also affects the risk of HPAI H5N1 virus infection. When below 40 kilometers, the risk increases as the distance decreases. While the distance is above 50 km, the risk decreases to zero as the distance increase. Similarly, the distance to water body has direct effect on the transmission of virus between waterfowl and local poultry, the risk decrease as the distance increase, when the distance increase to 100~150km, the risk decrease slowly to zero. It can be inferred like that the virus could be transmitted across the water body, where the duck, goose, and other water fowl habitat. Therefore, the risk is going to increase when the distance decreases. Population density showed a clear association with the occurrence of HAPI epidemic, the risk will increase as the population density increases until it reach 5000person/km2, then keep steady before it reach 7500 person/km2 , finally decrease as the value rises. Generally, the risk is high in area with low and mid population density and low in area where there has high density or no people. Poultry density is also a one of the risk factors of presence of HPAI. The risk will decrease as the slope increase before it reach 10 degree, then the risk decrease to zero rapidly. This means the area with low slope experience higher risk than those areas with high slope. The response curve of elevation showed that a higher elevation, a lower risk for presence of HPAI. Compound topographic index (CTI, which summarize tendency to pool water), is an indirect index to reflect the moisture of soil. When CTI get higher, the risk will rise as well and then keep steady, as was showed in its response curve.. The risk of HPAI will rise as Normalized difference vegetation index (NDVI) increase in the range of 0~180, and then the risk will decline. Relative humidity has a modest effect on HAPI occurrence, yet, the risk will increase as relative humidity rise from 45% to 75%. In addition, the risk for HPAI presence is also influenced by precipitation, higher annual precipitation is likely to increase the risk, probably because that precipitation would influence the vegetation which determinate the food for the hosts and increase the interaction between them. Annual average temperature has a association with HAPI, too. The risk reaches a peak when the temperature is 25℃, and then decrease to a standstill of 0.6.Conclusions: This study has clarified the distribution pattern of HFRS epidemic in Shandong Province over past years, and analyzed the changing of epidemic foci type and epidemic trend. More importantly, we identified the most significant factors highly associated with the risk of rodents infection, and increased the understanding of the habitat of hantavirus infected hosts in Shandong Province by construct their ecologic niche model and validated the accuracy of model prediction with both human cases and rodent hosts. Meanwhile, this study has characterized the ecologic niche of the HPAI H5N1 virus infected hosts in China, found the most significant risk factors, and then made prediction of the risk distribution in future. |
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