Research | New information about Helsinki rats: They do not pose as great a danger to humans as their species in other parts of Europe

The people of Helsinki rats do not pose a great disease risk to city dwellers, as they have few pathogens, much less than their fellow species in the rest of Europe.

This is evident from the University of Helsinki’s just-published study, in which they mapped the viruses, bacteria and parasites carried by the city’s rats that could potentially cause disease in humans.

“From the point of view of infectious diseases, Helsinki rats are healthier than elsewhere in Europe”, research director, academy researcher Tuomas Aivelo tells.

Researchers received a total of 288 rat carcasses from pest control companies and ordinary citizens to analyze.

“We found almost all the infectious diseases and parasites we were looking for in the Helsinki rats.”

The findings were published Eurosurveillance journal.

Leptospira-parasite is the most feared pathogen spread by rats. Mainly in tropical and subtropical countries, the disease caused by the parasite kills tens of thousands of people every year, according to Aivelo.

Leptospira also infects dogs and cats.

“In New York a few years ago, there were a couple of deaths that were related to the fact that a dog had hunted a rat and the owner had been infected by the dog.”

When the prevalence of this parasite was last investigated in Helsinki in the 1950s, it was found in almost every second rat. In the new survey, only one in a hundred had it.

In the rest of Europe, the dangerous parasite is still abundant.

“In Vienna Leptospira the prevalence is around 40 percent and in different places in Denmark 20-40 percent,” Aivelo compares.

Others too parasites are rare in Helsinki’s rat population.

Common in Europe Hymenolepis diminuta – tapeworm was found in only one rat.

In the past, trichinosis was also common in Finland, ie Trichinella spiralis was not found at all. Even in the 1990s, one in five rats living in the city’s landfills had it.

According to Aivelo, the decrease is due to the fact that trichinosis has become considerably rarer on pig farms and is no longer transferred from them to wild animals at all.

“Different in wild animals of Trichinella spiralis since the 90s, the numbers have decreased regularly. The same can now also be seen in rats.”

When mapping pathogens, researchers are also looking for lungworms and heartworms, which are more southerly species and which are believed to be able to spread north as the climate warms. They were not found.

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Rat-borne diseases may be underdiagnosed.

Viruses Aivelo and partners found out by looking for their antibodies.

Antibodies to hantavirus and smallpox virus were found in only about five percent of the Helsinki rats each. In the rest of Europe, there have been even more than half of them.

Campylobacter, which causes intestinal infection and diarrhea in humans, was present in less than seven percent of the rats in Helsinki.

Helsinki rats have few pathogens probably because the population is small.

“When there are few rats, it is reflected in the low prevalence of pathogens. We assume that rat populations will decrease significantly during the winter. Winters are also bottlenecks for pathogens, which can disappear locally entirely because of them.”

Proven only three Finns have contracted a disease from a rat. All infections have been obtained from a rat bite.

However, Aivelo thinks that diseases transmitted by rats may be underdiagnosed. For example, the Soul virus, which belongs to hantaviruses, can be transmitted through the air from the feces of rats, like mole fever from the feces of other rodents.

“During the diagnosis stage, it is not possible to distinguish whether it is mole fever or a fever caused by the Soul virus.”

It is also possible that some of the stomach problems caused by campylobacter may be due to infections from rats.

Aivelo and his colleagues will start studying antibodies against pathogens in people who may be exposed to rats at work. These include, for example, pest controllers, waste handlers and cleaners.