Greening the City

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The hidden face of biodiversity

Biodiversity has many benefits for city-dwellers. Nonetheless, a number of species present in the urban environment can be harmful to human health, as we’ve been reminded recently by two examples that received huge media coverage: the avian flu virus, transmitted by birds, and the chikungunya virus, carried by mosquitoes of the genus Aedes. But apart from these well-publicized cases, many other animals in cities are disease carriers. Pigeons, for instance, can transmit certain microbes to humans, which can cause diseases such as salmonellosis, chlamydiosis, toxoplasmosis, ornithosis, or even acute fungus diseases such as cryptococcosis. This problem is being investigated by several scientists involved in the research program ‘Urban pigeons: reconciliation ecology’, in which the Ecologie, systématique et évolution Laboratory (1) is taking part. In addition, their nesting sites can swarm with parasites such as ticks, mites, bugs and fleas, causing allergic reactions. According to the World Health Organization, direct infection of people through contact with urban birds or their habitats has been recorded for seven different diseases.
Still, birds aren’t the only ones to blame. For instance, several studies have shown a clear relationship between growing domestic exposure to allergens carried by cockroaches, mice and mites, and an increased risk of severe asthma. Of even greater concern is the fact that the fleas of some rodents can transmit murine typhus. Because they can be infected by a wide range of parasites and pathogens, rats (Rattus norvegicus, Rattus rattus) and mice (Mus musculus) are also a threat to people in poor health. As for novel household pets, which are becoming increasingly fashionable, they can carry viral, bacterial or parasitic zoonoses. For example, plague can be transmitted by certain wild rodents, salmonellosis by reptiles, tuberculosis by some birds, herpesvirosis B by Asian macaques, rabies by other primates, etc.
Turning to plants, the pollen of ragweed, various grasses, cypress, birch, etc can cause allergic reactions such as rhinitis, conjunctivitis and asthma. This is a health problem that mainly affects the more sensitive city-dwellers rather than country people, whose immune system is boosted by direct contact with the rural environment. On top of this, pollution makes people more sensitive to allergies and also stresses plants, which pollinate even more as a result.
Over the next few decades, it is unfortunately probable that climate change will lead to the emergence of new infectious and parasitic diseases, due to the introduction of disease-carrying tropical species into France.

1 - CNRS laboratory / Université Paris 11, France