Clinical
Leptospirosis in urban rats: A One Health study
Leptospirosis is a threat to human and animal health the world over, and not just in tropical climates. We caught up with a veterinarian who was part of a multi-disciplinary research team of animal and human health professionals studying transmission and spread of this disease among urban rats and learned how in this case sanitation is more effective than eradication.
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A recent study published in PLOS Neglected Tropical Diseases by a multidisciplinary team representing both veterinary and human a medicine evaluated the presence and spread of leptospirosis among urban rat populations in Boston, Massachusetts. Researchers conducted a multi-year effort to determine the prevalence of Leptospira interrogans and other Leptospira infections in brown rats (Rattus norvegicus and R. rattus) and to learn how strains of these bacteria differ among populations of rats.
One of the principal investigators was Marieke Rosenbaum, DVM, MPH, MS, Assistant Professor at the Cummings School of Veterinary Medicine at Tufts University. She said that rats are typically asymptomatic carriers of leptospirosis and that they can be an important source of exposure to humans in urban environments. Since the organism is shed in the urine, common methods of exposure include direct contact with rat urine or with environments contaminated with rat urine.
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“The risk may not be the same for all segments of the population,” she said. She suspects that unhoused individuals who live in encampments or who engage in outdoor injection drug use and sanitation workers all may be at higher risk for this type of exposure due to increased risk of contact with rat urine.
Noting that greater than 30% of R. norvegicus are thought to be infected with Leptospira spp. globally, investigators in this study aimed to determine the prevalence of leptospirosis infections in rats in an urban setting and how infection is likely to spread between rats in different areas of the city.
About the study
Researchers identified rat populations in 17 different areas of Boston and sampled the kidneys of representatives of each community to look for Leptospira spp. infection. Positive samples were evaluated both serologically and genetically to classify the strain of leptospirosis present. These classifications were then compared within members of the same individual communities and across communities. Samples from a human patient from the Boston area diagnosed with leptospirosis were also evaluated and compared to the rat samples.
Leptospira spp. infections were identified in kidney samples from 59 out of the 328 rats using PCR, which came to an infection rate of 18%. All isolates were considered to be from the Icterohaemorrhagiae serogroup but were divided across 12 different genetic groups (or strains).
Rosenbaum said that individual populations of rats didn’t travel very far from their burrows and tended not to intermingle much with populations of rats three to five blocks away or across busy roads.
“Our research corroborates previous reports that rats live in tight social networks that are well-defined,” she said. “We also found that within each subpopulation of rats, the genetic identity of the leptospirosis they carry is unique to that family of rats and it’s different from the genetic identity of the leptospirosis detected in a family of rats across the city.”
They also found that the strain of leptospirosis that was detected in the human sample was almost identical to the strain detected in a population of rats in the study. “It’s not definitive that this human infection came from this rat group, but it’s highly suggestive of that occurring,” she said.
Serogroup vs genetic classification
Serogroup (a group of serovars) is determined based on the host's immune response to the bacteria, whereas the genetic classification is determined by the bacteria's DNA.
Rosenbaum explained that serogroup and genetic classification do not always line up. This means that two strains that are genetically different may both be of the same serogroup, and two strains that are genetically similar may be classified in different serogroups.
More than 200 different pathogenic leptospirosis serovars have been identified worldwide, and they are spread among 30 different serogroups. While the total number of possible genetic strains of leptospirosis has not been widely reported, one study identified 843 different leptospirosis genomes.
Rat control and effects on leptospirosis transmission
Rosenbaum said that one of the major takeaways from this research is that efforts to eradicate rat infestations, while well-meaning, are often ineffective and could result in the transmission of new leptospirosis strains from one rat subpopulation to another. When pest control efforts aim to trap or otherwise kill rats in a particular location, some rats will survive and/or relocate, carrying the strain of leptospirosis from their original rat group with them to a new location or population. “You can’t get them all,” Rosenbaum said.
And, as soon as the eradication efforts are discontinued, rat populations quickly rebound. Instead, Rosenbaum recommended focusing on sanitation, sharing an old saying in the pest control industry: “The best defense against rats is a good trash can.”
Taking away both their source of food and shelter can help to limit rat populations. Other measures currently being studied include an oral bait that acts as temporary birth control to reduce the size of rat populations.
Rosenbaum said that urban rat infestations are often referred to as a “wicked problem” because they are so multi-factorial and require the coordination of so many different parties to adequately address.
“We need to pay more attention to how our pest control interventions impact rat ecology and the ecology of the diseases that they’re carrying that we’re concerned about,” Rosenbaum said. “We need to start thinking more about the things that actually impact us—like the risks they pose to human health and wellbeing.”
Further reading:
Leptospira: The dawn of the molecular genetics era for an emerging zoonotic pathogen
Urban rats spread deadly bacteria as they migrate, study finds
2022 AAHA Canine Vaccination Guidelines
Update: AAHA guidelines designate Leptospirosis vaccine as core
Photo credit: Silvio Kopp/iStock via Getty Images
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