Study reveals healthier rat population in Helsinki compared to southern cities

A study conducted by the University of Helsinki has revealed that rats in the Finnish capital are healthier compared to those in many southern cities. Research on the city’s rat population has been ongoing since 2018, focusing on the relationship between rats and humans, particularly regarding the prevalence of viruses, bacteria, and parasites that can be transmitted from animals to humans.

Pest control companies and residents have been providing researchers with rat carcasses, complete with time and location data. Findings indicate that the Helsinki rat population has lower incidences of infectious diseases and parasites than previously thought, with significantly reduced rates compared to other parts of Europe. For instance, only about one percent of Helsinki’s rats carry the Leptospira bacteria, a cause of leptospirosis—a zoonotic disease feared for its potential lethality. In contrast, over 40 percent of rats in Vienna and around 30 percent in Denmark test positive for this bacterium, according to Tuomas Aivelo, the lead researcher of the Helsinki rat study.

The smaller rat population in Helsinki compared to other major cities is likely a contributing factor, as the spread of pathogens correlates with the number of rats. While there is a stable rat population in Helsinki, harsh winters may reduce their numbers, Aivelo notes.

Studying diseases that can transfer from rats to humans presents challenges, as humans may pose a greater risk of transmitting diseases to rats. Aivelo emphasizes that it is relatively uncommon for people to come into contact with rat droppings, while rats often inhabit sewers, making them susceptible to human pathogens. The study continues to explore variations in the rat populations over the years and the distribution of pathogens in different locations.