Rats: in the streets of Lisbon, it is quite possible that you have already crossed paths with these creatures that roam the city quickly, dragging their tails and sniffing around. And even in your own home, hidden in a hole they have turned into a burrow. According to experts, there may be thousands of rodents in Lisbon.

“For every inhabitant, there could be up to four rats. We can certainly estimate thousands of rodents in the city.”

Maria da Luz Mathias

“They are opportunistic animals: they eat everything,” says Maria da Luz Mathias, a professor of Animal Biology at the Faculty of Sciences of the University of Lisbon.

And it is in the city that they find the ideal conditions for their reproduction: “abundant food, places to nest, mild climate,” says Maria da Luz.

They invade ecosystems, competing with native species, occupy city warehouses, contaminating food with their droppings, and transmit dangerous diseases such as “leptospirosis” and “salmonellosis.”

Where are the city rats?

In Lisbon, they proliferate precisely in areas where these factors are present, as explained by the Lisbon City Council: in historic areas, along the riverfront, and in areas with higher degradation, where there are catering establishments, abandoned plots of land, and sewage networks, providing both shelter and food.

The City Council says it conducts regular assessments throughout the territory, promoting awareness programs on proper sanitation practices (proper waste disposal, elimination of food sources and shelter). Additionally, they have a technical team that assesses critical locations and intervenes if necessary by applying rodenticides.

Rodent infestations became a more severe problem last year when Lisbon was without a pest prevention contract for two months, leading to failures in garbage collection.

However, in August of last year, the City Council signed a contract with the company Luthisa for pest prevention and control, worth 552,000 euros, valid for three years. But the problem may worsen again with the construction of the new metro line, which will cause vibrations in the ground.

In 2016, the documentary Rat Film, about the American city of Baltimore, exposed the most attractive factors for rodents: failures in garbage collection, vacant buildings, access to water, and inadequate sewage systems.

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Last year’s pest control and waste collection woes: challenges faced by lisbon revealed in striking. Photo by Nuno Mota Gomes.”

Which rats are found in the city?

There are three rodent species that are most prevalent in the city: the house mouse, the brown rat, and the black rat. The house mouse is the one that is most commonly found in our homes.

In her doctoral thesis on the house mouse, biologist Sofia Gabriel traces the trajectory of this species: they originated in the Middle East about 10,000 to 12,000 years ago, but it was during the Age of Discovery that they truly began to travel and spread globally.

The house mouse is smaller than the other species and less neophobic, meaning it is less afraid of new objects compared to other rodents. “They are primarily animals that live in close association with humans, in buildings,” summarizes Maria da Luz. “In warehouses, supermarkets, and so on.”

The brown rat, also known as “rattus norvegicus,” is heavier with a thicker tail, about the same size as or smaller than its body. It lives at ground level, often in burrows. Maria da Luz says it is common to find them in city gardens, such as the Estrela Garden and Campo Grande Garden.

It is difficult to distinguish between the brown and black rats. “To the ordinary citizen, they look very similar,” explains the biologist. The color of their fur is not always a distinguishing factor, despite their names. “The best way to describe it is that the black rat is a much larger house mouse, with a longer tail compared to its body and a more tapered head.”

Black rats live at higher levels, which is why they are also known as “roof rats.” Like brown rats, they are nocturnal animals, and although brown rats are more aggressive, most complaints are related to black rats. “It has to do with the fact that farmers have more encounters with black rats, and in those immediate situations, they react and may bite or attack,” explains Sofia.

How can they be combated?

The commonly adopted solution for combating an infestation involves the use of anticoagulant rodenticides that allow the animals to consume the poison without immediately associating it with discomfort. This causes them to die away from their burrows. However, taking action only during the infestation phase is not enough.

“Above all, we need to be aware of certain situations,” says Luís Manuel Madeira de Carvalho, a professor at the Faculty of Veterinary Medicine of the University of Lisbon. “It is never too much for people to be informed, to read about the biology of these animals.”

In the 1950s, researcher David E. Davis concluded that the prevalence of rodents in cities was linked to human factors. For him, the best way to control rats would be by controlling garbage collection, vacant buildings, access to water, and sewage systems in order to stabilize rodent populations.

“The beauty of this ecological control method is that it improves living conditions for humans while reducing the rat population.”

David E. Davis, quoted in Rats Film.

A new problem has emerged in the fight against rats: the resistance that rodents have developed to anticoagulant pesticides.

As a result, other alternatives have started to emerge, such as pesticides that cause death due to excess vitamin D. Maria da Luz also points out new traps capable of capturing and killing multiple rodents at once.

“Prevention is the best weapon we have,” says Sofia Gabriel. “Rats are a war that we won’t be able to win anytime soon. There needs to be constant control to ensure that, even if they don’t disappear, populations remain controlled.”

Preventive actions against rodents:

  • Limit access from the exterior to houses.
  • Do not leave uncovered food.
  • Ensure that garbage is inside containers.
  • Keep garbage containers as isolated as possible.
  • Avoid excessive vegetation growth in gardens.
  • Pay attention to small holes, cracks, and gaps in houses.

“If we consider that there can be up to four rodents per inhabitant, if the urban population increases without proper control and prevention, these species tend to increase as well,” says Maria da Luz.

And in other countries?

Rats are not an exclusive problem to Lisbon, of course.

A study published this year in the scientific journal Peer J estimated that between 1930 and 2022, rodents have incurred costs reaching up to 3.6 billion dollars (approximately 3.3 billion euros).

In 2017, the Mayor of New York City, Eric Adams, announced a 32 million dollar plan to reduce the rat population by 70% in three of the most infested neighborhoods – without much of an impact, however.

In some cities, it is an even bigger problem for disadvantaged communities, as highlighted in the film Rats. After the Wall Street crash in 1929, the Homeowners Loan Corporation was established in the United States, which would redefine the city of Baltimore, segregating the population based on race and socioeconomic status.

The result of building four different neighborhoods was predictable: the ones with the worst living conditions would end up with a higher proliferation of rats. This scenario still exists today and may not be much different from Lisbon, a city characterized by poorly constructed buildings.

Eliminate rats or live with them?

There are cities elsewhere that are finding solutions to the problem.

An unprecedented case is the Alberta region in Canada, where the cities of Calgary and Edmonton are located. It is the only area in the world with an urban and rural population that does not have a rat reproduction problem.

This achievement is due, in part, to geography. There are few points of entry for rodents in the region since they cannot survive the cold temperatures of the North or the West, and the southern border is highly mountainous, with a scattered population that makes it difficult for rodents to spread here.

One of the Posters from the 1948 Campaign Against Rodents in Alberta. Source: Provincial Archives of Alberta

Therefore, the only remaining border for rodents is to the East.

Rodents only arrived there in the 1920s, with rats being declared a pest in 1950. In Alberta, poison was used to kill them, and the area along the border was policed in a strip measuring 300 kilometers in length and 20-50 kilometers in width, creating a rodent control zone that still exists today.

An educational campaign was also launched, teaching residents how to distinguish brown rats from native rodents.

If there were 500 rats recorded in 1950, the number dropped to 50 by 1970, and by 1990, the number was between 10 and 20 rats. In 2003, no rodents were registered.

Meanwhile, while Alberta achieved this feat, there are cities where alternative methods are being studied, rather than exterminating rodents using pesticides.

Paris steps back

Recently, the Mayor of Paris, Anne Hidalgo, announced that she would step back from her “anti-rat” plan of 2017, instead focusing on studying new methods that are not aggressive towards rodents.

The plan also involves learning to “coexist” with these animals.

Meanwhile, in the United States, data is already being used to prevent infestations. This is happening in Washington DC, where the director of the rodent control division, Gerard Brown, partnered with data scientist Peter Cassey to develop a data model that predicts infestations.

To create this model, 911 calls were used, along with other data such as the number of restaurants and apartments in a particular area, as well as other landscape-related indicators.

Chicago also adopted this tactic and achieved results: data scientist Daniel Neill’s method proved to be 20% more effective than the traditional method of rat traps.

*The image illustrating this article was created using an artificial intelligence program.

You can read this article in Portuguese here.

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