Europe's rising rat populations: Causes, effects, and solutions

Göttingen, Germany - an emergency. His symptoms mirror those of a tropical disease: headaches, sudden spikes in temperature and high fever, nausea and utter exhaustion. But the young man hasn't spent time abroad. The doctors are irritated, bewilderment takes root. Suddenly the symptoms disappear, the sick man seems to have recovered. But the infection continues to lie dormant in his body. Six days later he suffers a relapse, one much worse than his first bout. His kidneys fail. For a moment his life hangs in the balance.

The insidious infectious disease is called leptospirosis. Rats and mice can transmit these germs. This forestry student, however, never came into contact with the rodents. How then did he get infected with this pathogen?

The search for clues leads to a forest in southern Lower Saxony. The patient worked in a swamp-like area until shortly before his breakdown. It doesn't take long for the researchers to find something. Leptospirosis pathogens can survive for months in these wet surroundings. The student got infected in a water hole, where he had washed his hands. The rodents presumably contaminated the water with their urine.

In Europe, the number of leptospirosis cases among humans are adding up: 25 percent end fatally. The reason: The climate is getting milder - optimal conditions for rats. To keep these rodents in check, their natural enemies, birds of prey, would have to increase in numbers, too. If they eat the rats, then everything is held in balance. But bird of prey populations are shrinking, so rats are left with few natural enemies and rat populations are skyrocketing.

A dramatic development for human beings: In Germany alone the ratio of rats to inhabitants is 300 million to 80 million. They're intelligent and learn quickly. This is why experts have developed bait with poison that doesn't take effect for a few days. But this alone is not enough, the recipe for the poison must be changed continuously to deceive these clever rodents.

The fight against rats in nature appears hopeless. Luring them with poison could end up harming other woodland creatures or children. The result: rats are multiplying unchecked across Europe - a hidden danger that could represent an increasing threat to humans if birds of prey populations continue to diminish.