Rat Congestion
Microbial Solutions
Regina Kelder

Rat Congestion

Respiratory pathology and pathogens in 711 urban rats could explain why these rodents don't live long.

Those of us who live in urban environments will likely have encountered the common Norway rat, otherwise known as the brown rat, hovering around garbage cans or subway platforms. Black rats are another common periodomestic species that tends to favor warmer climates.

The 'yuck factor' extended to these rodents needs little explanation. Perhaps most notably, rats shoulder the blame for sparking the Black Plague in Europe during the Middle Ages—the bacteria that causes plague is believed to have been spread by fleas living on the backs of black rats. With that said, researchers know surprisingly little today about the pathogens that common rats harbor and could pass on to humans.

Enter the Vancouver Rat Project, so-named because the city's rat population is serving as a kind of laboratory for zoonotic research. Canadian scientists launched the effort several years ago to better understand urban rat ecology, natural rat disease and identifying known and—perhaps more importantly—unknown pathogens that urban rats are carrying, known as zoonoses, which could potentially be passed on to humans.

Eureka wrote about the Vancouver Rat Project about two years ago, describing, in particular, ongoing efforts by Canadian researchers and Charles River Laboratories' Research Animal Diagnostic Services, to analyze gross and histological lesions in the respiratory tracts of a sample of 711 wild urban rats. Testing for known respiratory pathogens included serology and polymerase chain reaction (PCR) of lung samples.

The results of that study appeared online July 13 in Veterinary Pathology.[1] The study was undertaken to assess respiratory pathology in a large sample of live-captured urban rats, assess the occurrence of respiratory disease, and generate new information about the role of disease in the ecology of wild urban rat populations. The specific objective of this study was to describe, categorize, and determine the cause of respiratory pathology in wild urban Norway and black rats (R. norvegicus and R. rattus) from Vancouver.

"The biggest question with urban rats is we don't know why they die," veterinarian Jamie Rothenburger from the University of Saskatchewan's Western College of Veterinary Medicine told Eureka two years ago. "Some interesting studies show their life span in the wild is less than a year of age, but in a laboratory they can live up to three years."

Rothenburger was a lead author of the study, which also included scientists from Charles River Laboratories, the British Columbia Ministry of Agriculture's Animal Health Centre, the University of Washington's Department of Comparative Medicine and the Canadian Wildlife Health Cooperative.

So what did the analysis find?  Grossly evident lesions were rare—identified in just eight of 711 rats—but those that were found were severe.  Moreover, the true prevalence of macroscopic lesions was probably underestimated, researchers believe, because live trapping may favor rats that are healthy enough to be actively foraging.

In contrast, microscopic lesions were very common and mirror other studies of wild Norway rats.  Upper respiratory tract inflammation was evident in 93 of 107 rats tested and and was significantly associated with the presence of cilia-associated respiratory bacillus (CARB)—an unclassified Gram-negative bacterium that intermingles with cilia of large airways and is transmitted through direct contact—Mycoplasma pulmonis, and increased body mass.  Within the lungs, peribronchiolar and/or perivascular lymphoplasmacytic cuffs were present in 152 of 199 rats and were also significantly associated with CARB, M. pulmonis, and increased body mass.  Rats were frequently co-infected with M. pulmonis— which is also an important respiratory pathogen in lab rats—and CARB, and lesions associated with these pathogens were histologically indistinguishable. Pneumocystis sp was detected in 48 of 102 rats using PCR but was not significantly associated with lesions.

Wild rats typically live about a year—compared to lab rats live up to three years. While it's difficult to say conclusively, the prevalence and severity of respiratory lesions found in the Vancouver study suggest that disease may be a major factor contributing to the short life span of wild urban rats, as opposed to poisoning or trapping which limited effect on rat population size. 

This study underscores the importance of understanding natural disease in wild rats in order to develop more ecologically-based rodent control strategies. Which could go a long way toward making that subway ride home a bit more pleasant?


  1. Rothenburger, J.L. et al., Respiratory Pathology and Pathogens in Wild Urban Rats (Rattus norvegicus and Rattus rattus), Vet Pathol, July 13, 2015 

How to cite:

McEnery, Regina. Rat Congestion. Eureka blog. July 20, 2015. Available: https://eureka.criver.com/rat-congestion/