Eating carcasses of livestock treated with antibiotics is wreaking havoc on the health of Spanish vultures, a new study carried out in Spain suggests. The researchers say that the practice of feeding vultures livestock carrion–although promoted by bird lovers–may actually threaten the species and should be ended.

Spain is the European stronghold of vultures; it’s home to the griffon vulture (Gyps fulvus), the cinereous vulture (Aegypius monachus), and the endangered Egyptian vulture (Neophron percnopterus). Traditionally, Spanish vultures have lived off dead livestock, deposited by farmers at dumping sites called muladares. Mad cow disease worries triggered a European Union ban on that practice in the 1990s, leading to vulture population declines. The ban was lifted in 2005 after fierce protests by amateur ornithologists, and although some Spanish regions still haven’t reinstated the muladares, they have become a part of official vulture-management programs in other areas.

But Jesüs Lemus and Guillermo Blanco of the National Museum of Natural Sciences in Madrid have long worried that antibiotics, used widely in livestock farming to fight infections and promote growth, might accumulate in vultures. In a study published last year, they showed that eating carrion from farms led to high levels of quinolones, a group of persistent antibiotics, in vultures’ blood and that these were associated with higher risk of bacterial and fungal disease. Several studies in people and animals have hinted that overuse of antibiotics can suppress the immune system, and the researchers hypothesized that this happens in vultures as well.

The new study, published online this week in the Proceedings of the Royal Society B: Biological Sciences, suggests that their hunch was right. Lemus and Blanco climbed trees to access vulture nests in central Spain, where scavengers depend largely on livestock carrion, and took tiny blood samples from a total of 71 nestlings of all three species. They then measured a series of parameters related to the immune system–such as white blood cells and immune signaling molecules–and compared these with samples taken from areas in southern and western Spain, where vultures primarily feed on wild prey and where fewer antibiotics are used because livestock aren’t kept in barns.

Both the cellular and humoral immune systems of the central Spain vultures were suppressed, they found. Lab studies are needed to confirm that antibiotics cause the immune suppression, the duo writes–but in the meantime, putting out dead livestock should be “rejected as a management tool in conservation programs.”

“This is a very interesting paper,” says environmental chemist Alistair Boxall of the University of York, U.K., who has shown that antibiotics used in agriculture can end up in food crops as well. It’s not the first time vultures have become victims of drugs, Boxall notes: Previously, an anti-inflammatory drug called diclofenac was linked to vulture deaths in India and Pakistan (ScienceNOW, 1 October 2004). The Spanish study shows that scientists need to be much more aware of unexpected effects of antibiotic use high up the food chain, Boxall says.