A recent study examined the diversity of rodents and their bacterial pathogens to better understand how human disease risk changes in a shrinking urban environment. The scientists examined Leptospira bacteria in rodents over a gradient of urban decline in New Orleans to Katrina.

Most people live in urban areas. And while this urban dwelling trend is expected to increase globally over the next few decades, there are cities in the United States, Europe, and East Asia that are experiencing significant population losses. When people leave cities, it leads to a counter-urbanization process in which infrastructure is lost, the availability of living space for non-humans increases and the risk of illness for those left behind increases. Although urban living will increase, there are many regions of the world where understanding the public health impact of counter-urbanization is essential.

A recent study looked at how counter-urbanization affects rodent communities and the bacteria they carry. Research is focused on New Orleans, Louisiana, and specifically on how rodents respond to changes in the city following devastating Hurricane Katrina in 2005. The hurricane caused catastrophic flooding across the city, but varying relocations and recovery have resulted in multiple Niches of pockets counter-urbanization. Unfortunately, but perhaps not surprising, the locations and density of abandoned and vacant lots disproportionately affect disadvantaged communities, perpetuating the disparities that existed prior to Hurricane Katrina.

Rodents harbor a wide variety of pathogens that infect humans and often enjoy living next to or even in human dwellings. The work of the same group has shown that abandoned land and the loss of infrastructure in New Orleans are linked to higher frequency and diversity of rodents, but whether or not they carry more pathogens has not been understood. The focus of this current study was on a group of bacterial pathogens of the genus gen Leptospira.

Two, spiral-shaped, Leptospira Interrogans Bacteria imaged with a scanning electron microscope. Photo credit: CDC / NCID / HIP / Janice Carr (PHIL # 1220).

Rodents can be severely infected without showing any signs of disease. The bacterium infects the kidneys and is transmitted to new hosts through contact with urine or through the urine-contaminated environment (water, soil, or food). In humans, an initial infection can lead to fever, chills, muscle pain, or diarrhea. In some cases, a more chronic infection occurs that can lead to kidney or liver failure. Although treatment is possible, prevention is preferred. Limiting contact with water or soil that may be contaminated with rodent urine can be a very effective form of prevention. However, this requires understanding where and when rodents are infected with Leptospira.

To understand where, when and which rodents have Leptospira infections, the researchers undertook massive trapping attempts (96 locations!) In communities in New Orleans. These sites were divided into areas that reflected a gradient in socio-demographic conditions and property abandonment. They also selected locations in a non-residential area to serve as a “natural” control. Mice and rats were caught with live animal traps from May 2014 to February 2017. Each location was trapped until no new individuals were caught – suggesting that all rodents were collected from the area. The captured individuals were humanely euthanized and samples were taken for diagnosis. Kidneys were used for a sensitive and specific DNA test for Leptospira Infections. These results not only indicate infection, but can also help differentiate the amount of bacteria in each individual that is important for transmission.

During the investigation, the scientists caught 1,472 rodents of five different species. Most rodents were invasive species (Rattus rattus, Musculature, and Rattus norvegicus), but they also found two new native species (Sigmodon hispidus and Oryzomys palustrus). All types (except O. palustrus with only 4 people)were found with Leptospira Infections. At a single trapping site, the prevalence was between 1 and 100% and the prevalence varied depending on the season and species (see figure below). When examining the burden of infection, they found that Musculature had the highest compared to the other species.

Prevalence of Leptospira in species over time (adapted from Figure 3)

For a subset of infected people, the authors also sequenced the bacteria to produce the Leptospira Species. They found three different types: Leptospira borgpetersenii, L. interrogate and L. kirschneri.L. interorgans was most common in R. norvegicus, whereas L. borgpetersenii was the most commonly infected species M. musculus and R. rattus. These two species of rodents are both terrestrial and likely to overlap in some areas and to share bacteria. R. norvegicus is more tree-dwelling – known as the roof rat – which could explain the different infection patterns. There is some evidence that these rodents share pathogens Musculature and Rattus norvegicus probably the most effective hosts. This is aided by a higher prevalence of leptospirosis in species-rich areas that often have these two competent hosts. These higher prevalence locations are also the locations with higher abandonment rates, suggesting that the process of counter-urbanization increases the risk of leptospirosis.

Campaign in the 1970s to improve public health by raising awareness about rodents and promoting waste reduction.

This study shows that the increase in the diversity and abundance of rodents may intensify Leptospira Infections and actually increase the risk of disease. In areas where people have moved and there is minimal support for maintaining infrastructure, rodent reintroduction can increase disease in the remaining residents. Targeted research to identify effective reduction interventions Leptospira Risk in these areas is an important next step. Taken together, this highlights an additional challenge in counter-urbanizing landscapes – an understanding of which is critical in both New Orleans and many other regions where this occurs.

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