Élodie Monchâtre-Leroy, UniLaSalle
THE ESSENTIALS
- Rats, which are ubiquitous in cities, have been linked to certain major epidemics throughout history, including the plague.
- Studying their actual abundance and movements, and tracking the infectious agents they harbor, remains challenging.
- Analyzing wastewater could provide a better understanding of these elusive populations and the risks they pose to human health.
Perfectly adapted to both urban and agricultural environments, the brown rat (Rattus norvegicus) has long been associated with our species. Native to Asia, it has now colonized the entire world, primarily through human transportation.
Despite this longstanding coexistence, we still know very little about each other. This explains why debates about the number of rats in cities resurface regularly. While it is generally believed that their populations are increasing in cities, there is no robust data to confirm or refute this perception.
These gaps in our knowledge of urban rat populations are problematic, as these animals carry various microorganisms that can pose a danger to human health, such as the infectious agents responsible for leptospirosis, hantavirus diseases, and hepatitis E.
To better assess the health risks posed by rats, a growing number of scientific projects are attempting to overcome the challenges involved in studying them.
Rats and Health Risks
Like many rodents, rats are potential reservoirs for numerous pathogens (viruses, bacteria, and parasites), some of which can be transmitted to humans. As we have mentioned, this is the case, for example, with the bacteria that cause leptospirosis, various hantaviruses, and the hepatitis E virus.
Leptospirosis is caused by bacteria belonging to the genus Leptospira. Excreted in rat urine, these bacteria contaminate water or wet environments. In humans, the infection can result in a variety of symptoms, ranging from flu-like symptoms to multiorgan failure—that is, the failure of multiple organs (such as the kidneys and liver). Each year in mainland France, 600 to 700 cases are diagnosed. The incidence of the disease is even higher in the overseas territories.
Depending on the studies and the populations tested, 10% to more than 70% of rats carry the bacterium. In France, rats carrying Leptospira have been found in all the cities studied.
With regard to hantavirus infections, the brown rat is a host to a virus specific to that species, the Seoul virus. Human infection is most often indirect: it results from contact with the rat’s excretions, which are infectious. The resulting illness may be asymptomatic or cause flu-like symptoms. In some people, it can lead to hemorrhagic fever with kidney damage.
Since 2012, 16 cases have been reported in France, all of them severe (asymptomatic or flu-like infections go unnoticed). The prevalence of Seoul virus infection among urban rats in France varies, but there are few available studies: it is estimated to range from 0 to 30 percent, depending on the city and neighborhood.
Finally, the Rocahepevirus ratti virus appears to have emerged in recent years. It belongs to the Orthohepevirinae subfamily, which also includes the human hepatitis E virus. It was initially thought to be restricted to rats, but since 2018, it has been identified as the cause of a growing number of human liver infections (notably, two cases have been diagnosed in France). Depending on the city and neighborhood, the prevalence rates of this virus range from 0 to over 50%.
The viral genome is found in large quantities in sewage where rats live; however, the mode of transmission to humans has not yet been elucidated. Rats infected with these pathogens do not develop disease. This is one of the characteristics of the concept of a reservoir; this tolerance is the result of a long history of coevolution between the infectious agent and its host. The situation is different for humans, and diseases resulting from infections with these microorganisms are closely monitored in France, as they can have serious consequences.
In addition to these three examples, rats harbor many other zoonotic agents (a zoonosis is an infectious or parasitic disease that is transmitted between animals and humans), some of which are present in most cities around the world. The fact that rats serve as such reservoirs means that their populations can sustain the circulation of pathogens and contribute to human infections. These infections can occur through direct contact (which is relatively rare, as we rarely encounter rats in our daily lives) or, more frequently, indirectly through contamination of our environment.
Early detection of rat pathogens and their fluctuations over time and space is therefore essential to preventing the emergence of broader public health problems. Unfortunately, little is known about rat populations.
The sewer rat: a little-known neighbor
Recent research conducted in the United States clearly illustrates the limits of our knowledge regarding the number of rats inhabiting urban environments. The authors studied rat populations in 16 U.S. cities and concluded that these populations were growing rapidly.
However, the data used for this assessment were indirect: they were based on complaints from city residents, not on a count of the rodents themselves. Admittedly, an increase in urban rat populations is highly likely. However, the human perception of a rise in their numbers could also be explained in other ways. It could be due to a change in the rats’ behavior: if, for example, they have become less fearful and more active during the day than in the past, it is possible that residents are spotting them more frequently.
While there is a lack of information regarding the number of rats living in our cities, little is known about their territorial organization as well. It is difficult to know, for example, whether the rats observed in a city’s various habitats—whether sewers, streets, green spaces, or inside buildings—belong to distinct, isolated populations or whether, on the contrary, there is continuity among them.
The answer to this question may seem trivial, yet it is essential: understanding the structure of rat populations makes it possible, for example, to determine how resistance to rodenticides spreads within a city.
If the populations are isolated from one another, resistance to rodenticides will remain limited to the areas where these products have been used. On the other hand, if there is interaction between populations, this resistance can spread and be present in certain areas even before any rodenticides have been used.
These demographic data are also important for understanding the spread of infectious agents within rat populations. A few studies already available suggest that different rodent populations live separately from one another. Indeed, for a given pathogen, the level of contamination varies greatly from one population to another.
We now need to determine the reasons for these disparities. Is this segregation simply due to the existence of insurmountable barriers, such as major roads, waterways, etc.? Or is it the result of the rodents’ behavior? But monitoring rat populations is no easy task.
Why are rats hard to monitor?
Monitoring urban rat populations is highly complex. The traditional monitoring method, which involves capturing the animals using traps, can be cumbersome to implement, especially if multiple sites per city must be considered to account for different rat populations. Access to some of their habitats, such as sewers, is particularly challenging.
Furthermore, rats learn quickly, which means that the effectiveness of trapping declines rapidly, thereby limiting the effectiveness of monitoring. In addition, the individuals captured do not represent the entire population: they are generally neither the youngest nor the oldest, which can lead to biases in the data.
In particular, it is known that with regard to Leptospira infections, prevalence increases rapidly among rats of a certain age (corresponding to a weight greater than 300 grams).
Faced with these limitations, scientists are exploring other approaches. One such approach, for example, involves using ferrets, which facilitate capture by clearing out burrows and driving the rats toward the exits, where they can be caught by placing traps at the various exits. This approach provides a more representative picture of the population at a given site. However, although effective, this technique is complex to implement.
Scientists are therefore seeking to develop methods that would allow them to monitor rats and their health simultaneously without having to capture them systematically. In this context, wastewater analysis is a particularly promising avenue.
Wastewater as an indicator?
It is known that the composition of wastewater reflects certain human-derived contaminants. Analysis of wastewater was particularly useful during the COVID-19 pandemic: viral RNA molecules present in wastewater were quantified to predict waves of the epidemic.
Since certain rat populations are abundant in sewers, monitoring wastewater contamination could, once again, serve as a method for indirectly assessing their population sizes and the pathogens they harbor.
The RatSWIM project was launched to evaluate the relevance of this approach. Specifically, rat trapping will be conducted in the sewers, and wastewater samples will be collected at the same sites to assess the correlation between rat contamination and wastewater contamination. Additionally, methods for quantifying rat DNA will be developed to obtain information on the size of the rodent populations frequenting the sampling sites.
In addition to the three pathogens described above, the project should also explore rat contamination by other microorganisms of concern to humans, such as poxviruses (the family to which mpox and cowpox virus belong).
If the proof of concept is validated, this method should facilitate the early detection and temporal monitoring of pathogens carried by urban rats. It would also make it possible to identify potential seasonal variations and to map the areas at highest risk in each city. Ultimately, this knowledge could strengthen prevention and public health strategies.
The RatSWIM (Rodent Assessment Through Sewers and Sewage Water Impact on Mammals reservoir – ANR-24-PEPZ-0002 RATSWIM) project is one of the projects under the Priority Research Program and Equipment (PEPR) PREZODE, funded by France 2030 through the ANR. The ANR’s mission is to support and promote the development of basic and applied research across all disciplines, and to strengthen the dialogue between science and society. For more information, visit the ANR website.
Élodie Monchâtre-Leroy, Lecturer and researcher in infectious diseases, UniLaSalle
This article is republished from The Conversation under a Creative Commons license. Read the original article.
_also_called_Norway_Rat_or_Common_Rat_-with_food_container_-_Mathias_Baldwin_Park,_Philadelphia,_Pennsylvania,_USA.jpg){kind=link}
.jpeg){kind=link}