Rodent pests and pestilence

HIV/AIDS may be the plague of the modern world but in the mid 1300s, the bubonic plague or 'Black Death' killed 25 million people in Europe, almost a third of the population, in just under five years of the disease spreading from China. The effect on medieval society was severe; whole villages were wiped out and many farms were abandoned. Today, the plague persists not only in China, south Asia and southern Africa, but also in parts of north America. The disease may be now less well-known but its effects continue to have an impact. More importantly, this notorious disease is only one of many serious diseases, which are carried by rodents, that affect humans and livestock and is only part of the reported 're-emergence' of rodent-borne diseases in many countries.

credit: Steve Belmain, NRI

Despite the infamous impact of the plague, the most widespread and prevalent zoonotic disease carried by rodents is leptospirosis, found throughout Europe, the Americas, Asia and Africa. Rats are a natural host for some of the most dangerous varieties of the disease, and the mobility of rats can lead to a rapid contamination of water sources. Pathogenic leptospires live in the kidneys of the natural host and are excreted in the urine. Poor water management strategies, such as failing to cover water containers, can result in infection in livestock and humans. Infected individuals can also excrete leptospires and thus function as an intermediate host. Symptoms range from mild, 'flu-like, to potentially fatal conditions such as Weil's Syndrome, characterised by kidney and liver failure and haemorrhages. However, recovery even from mild versions of the disease can take several months, and because of its variable and indistinct symptoms, it can be confused with other more commonly known diseases such as influenza and malaria. Recent research in Mozambique revealed that 18% of villagers tested positive for anti-leptospira antibodies.

Although much research into the rise in rodent-borne disease remains to be done, there appear to be several likely factors. Rodent populations may be increasing, boosted by intensified agricultural production in both rural and peri-urban areas, and the build up of wastes amongst poorly managed urban settlement. There may also be a greater frequency of human exposure to zoonotic diseases that have previously been confined to wild animal populations. Research in 1995 revealed that rodents are a host to approximately 60 such diseases which can be transmitted from animals to people, and since then a further 25 'new' viruses have been added to that list. Encroachment of human activity and settlement into natural habitats is bringing people, livestock and 'domestic' rats into contact with these wild populations, or their insect vectors, thereby increasing the risks of infection. Improvements in roads and transport allow new infections to quickly enter urban areas, where further spread can be rapid in crowded conditions.

credit: Steve Belmain, NRI

However, the dynamics of rodent-borne diseases are far from understood. Plague, for example, has cyclical outbreaks of variable lengths, but while it is clear that such outbreaks are increasing in virulence, the factors that determine the timing and severity of an outbreak are not clear. Rodent population density, climatic conditions and agro-ecological patterns could all play a part. Practices like the burning of fields and bush areas in preparation for planting may be important, since they bring large numbers of wild rodents onto farmland in search of food. There are also some question marks over the routes of transmission. While the best known route is through fleas, which pass the infection through bites after feeding on an infected host, plague bacilli are known to survive on animal skins for several days, and could be transmitted through frequent handling of rats. This might be important in some villages in Mozambique, for instance, where rodents are eaten more widely than any other source of protein (see Rising above the Rat Trap).

As part of a cross-sectoral project, being led by the UK based Natural Resources Institute, agriculturists and public health workers from seven countries including Mozambique, South Africa, Tanzania and Zimbabwe will be studying the different transmission routes and clinical manifestations of three rodent-vectored diseases, plague, leptospirosis, and toxoplasmosis. The research, which is supported by the European Commission INCO-DEV programme, is the first attempt to comprehensively address a range of issues regarding the risk of disease transmission linked to rats. The focus will include whether changes in rural ecology are impacting on rodent-borne disease levels, and on whether the spread of these diseases is being aided by the increased contact between rural and urban areas, by deteriorating hygiene standards in urban areas, and by the rise in urban rodent populations. The aim is to develop predictive models for the three disease types, in order to anticipate outbreaks. The models will also be used to create risk management strategies, which may include changes to agricultural production, improved land and water management and rodent control methods. Like other current research on malaria (See Points of View) and tuberculosis (See Bovine Tuberculosis: a knotty problem), the study illustrates a growing need for greater collaboration between, among others, health, agriculture and wildlife sectors, in tackling zoonotic diseases, and their consequences.

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