大通河流域植物区系

大通河流域共有野生种子植物77科、368属、1154种。分别占本区所属的青藏高原植物亚区唐古特地区总科数的85.56％、总属数的72.44％、总种数的50.50％。区系特征如下：(1)本区是唐古特地区一个值得提倡大力保护的植物模式标本的集中产地,种类相对较丰富。(2)属的分析表明,本区属于以北温带成分,特别是欧亚大陆温、寒地带典型成分为优势的,兼具温性、寒温和高寒类型的温带区系性质。(3)区系具有年轻和衍生的性质。具体表现在木本类型较少,多年生草本多,缺乏古老的和原始的类群,一些中国特有属种为衍生类群。(4)作为青藏高原的边缘地带和过渡区系,本区系在受到我国华北区系和西南高山区系的双重影响的同时,保持了其高原、高山类型的区系性质。(5)为我国黄土高原植物区系向青藏高原植物区系过渡的代表区系之一。本文的分析表明,在中国种子植物区系分区中,本区属于青藏高原植物亚区中的唐古特地区。     

Inter-epidemic Transmission of Rift Valley Fever in Livestock in the Kilombero River Valley,Tanzania: A Cross-Sectional Survey

Background

In recent years, evidence of Rift Valley fever (RVF) transmission during inter-epidemic periods in parts of Africa has increasingly been reported. The inter-epidemic transmissions generally pass undetected where there is no surveillance in the livestock or human populations. We studied the presence of and the determinants for inter-epidemic RVF transmission in an area experiencing annual flooding in southern Tanzania.

Methodology

A cross-sectional sero-survey was conducted in randomly selected cattle, sheep and goats in the Kilombero river valley from May to August 2011, approximately four years after the 2006/07 RVF outbreak in Tanzania. The exposure status to RVF virus (RVFV) was determined using two commercial ELISA kits, detecting IgM and IgG antibodies in serum. Information about determinants was obtained through structured interviews with herd owners.

Findings

An overall seroprevalence of 11.3% (n = 1680) was recorded; 5.5% in animals born after the 2006/07 RVF outbreak and 22.7% in animals present during the outbreak. There was a linear increase in prevalence in the post-epidemic annual cohorts. Nine inhibition-ELISA positive samples were also positive for RVFV IgM antibodies indicating a recent infection. The spatial distribution of seroprevalence exhibited a few hotspots. The sex difference in seroprevalence in animals born after the previous epidemic was not significant (6.1% vs. 4.6% for females and males respectively, p = 0.158) whereas it was significant in animals present during the outbreak (26.0% vs. 7.8% for females and males respectively, p<0.001). Animals living >15 km from the flood plain were more likely to have antibodies than those living <5 km (OR 1.92; 95% CI 1.04–3.56). Species, breed, herd composition, grazing practices and altitude were not associated with seropositivity.

Conclusion

These findings indicate post-epidemic transmission of RVFV in the study area. The linear increase in seroprevalence in the post-epidemic annual cohorts implies a constant exposure and presence of active foci transmission preceding the survey.