18.104.22.168 Other infectious diseases
Recent investigations of plague foci in North America and Asia with respect to the relationships between climatic variables, human disease cases (Enscore et al., 2002) and animal reservoirs (Stapp et al., 2004; Stenseth, 2006) have suggested that temporal variations in plague risk can be estimated by monitoring key climatic variables.
There is good evidence that diseases transmitted by rodents sometimes increase during heavy rainfall and flooding because of altered patterns of human–pathogen–rodent contact. There have been reports of flood-associated outbreaks of leptospirosis (Weil’s diseases) from a wide range of countries in Central and South America and South Asia (Ko et al., 1999; Vanasco et al., 2002; Confalonieri, 2003; Ahern et al., 2005). Risk factors for leptospirosis for peri-urban populations in low-income countries include flooding of open sewers and streets during the rainy season (Sarkar et al., 2002).
Cases of hantavirus pulmonary syndrome (HPS) were first reported in Central America (Panama) in 2000, and a suggested cause was the increase in peri-domestic rodents following increased rainfall and flooding in surrounding areas (Bayard et al., 2000), although this requires further investigation. There are climate-related differences in hantavirus dynamics between northern and central Europe (Vapalahti et al., 2003; Pejoch and Kriz, 2006).
The distribution and emergence of other infectious diseases have been affected by weather and climate variability. ENSO-driven bush fires and drought, as well as land-use and land-cover changes, have caused extensive changes in the habitat of some bat species that are the natural reservoirs for the Nipah virus. The bats were driven to farms to find food (fruits), consequently shedding virus and causing an epidemic in Malaysia and neighbouring countries (Chua et al., 2000).
The distribution of schistosomiasis, a water-related parasitic disease with aquatic snails as intermediate hosts, may be affected by climatic factors. In one area of Brazil, the length of the dry season and human population density were the most important factors limiting schistosomiasis distribution and abundance (Bavia et al., 1999). Over a larger area, there was an inverse association between prevalence rates and the length of the dry period (Bavia et al., 2001). Recent studies in China indicate that the increased incidence of schistosomiasis over the past decade may in part reflect the recent warming trend. The critical ‘freeze line’ limits the survival of the intermediate host (Oncomelania water snails) and hence limits the transmission of the parasite Schistosoma japonicum. The freeze line has moved northwards, putting an additional 20.7 million people at risk of schistosomiasis (Yang et al., 2005b).