Zoonoses Transmitted by Rats

Rats have coexisted with humans since time immemorial, and the same is true of their pathogenic organisms. Bacteria, viruses, parasites… many have even evolved to infect humans. Understanding these zoonoses is essential for better preventing and treating them—and, above all, for learning how to prevent the emergence of new diseases, as was the case with COVID-19.   Humans have coexisted with rats for millennia. Only recently have we begun to study and domesticate them, but they have always been an integral part of human culture and history—and not always for the right reasons. Often associated with filth, pests, and disease, rats have been carriers of numerous diseases that have been transmitted to humans over time. We all remember the Black Death, which killed millions of people in the 14th century and left a lasting scar on history. Among the most serious rat-borne zoonoses are leptospirosis, lymphocytic choriomeningitis, and sodoku. Bites, scratches, contact with urine or feces, or bites from a vector are often enough to cause infection. Understanding these diseases is essential for effectively preventing them and treating them as best as possible. But that’s not all: the current COVID-19 crisis reminds us that understanding zoonoses is essential for managing new, emerging diseases of animal origin. We must not forget that 60% of all emerging human infectious diseases originate in animal populations. And one of the most recent examples involves rats and hepatitis E. An overview of the most well-known rat-borne zoonoses.  

Bacterial Zoonoses

Bacterial zoonoses transmitted by rats are the most common.

Leptospirosis

Of particular note is leptospirosis, commonly known as “rat disease,” which is often overlooked but can be particularly dangerous. It affects 600 people each year in France and particularly impacts workers in marshy areas, such as farmers and sewer workers. The bacteria known as “leptospires” are primarily stored in the kidneys of infected animals and then excreted in their urine. They then contaminate the animals’ fur and their environment, thriving particularly in warm, humid settings (freshwater, muddy soil) where they can survive for several months. The bacteria, which enter the body through the skin or mucous membranes, can cause damage to the liver and kidneys. Vaccination (3 injections followed by a booster every two years) is recommended only for certain limited indications, determined on a case-by-case basis, particularly in the context of occupational medicine and taking into account environmental and individual risks. This vaccine protects only against Leptospira interrogans, which accounts for about 30% of cases, and is associated with frequent side effects.  

Salmonellosis

A relatively common and often serious bacterial disease, salmonellosis of animal origin is also a major zoonosis. While the main cause of salmonellosis in humans in Europe is the consumption of Salmonella Enteritidis in raw eggs and egg products, rats are often the source of contamination in chicken coops or the human environment in urban areas. A resistant bacterium can survive for long periods in the external environment. Transmitted through the digestive tract, when the disease manifests, it primarily affects the digestive system—particularly the intestines—sometimes causing acute symptoms. However, many animals—and humans—can be asymptomatic carriers of the disease and thus capable of transmitting it without showing symptoms. Similarly, a person who has recovered may continue to engage in shedding of Salmonella for several weeks. While treated with antibiotics, prevention and the isolation of sick individuals are essential to prevent an outbreak.  

Rat-bite fever

Another often-overlooked risk is rat-bite fever, which, as its name suggests, is transmitted through a bite. The bacteria primarily responsible, Streptobacillus moniliformis, is a common host of the rat’s oral cavity and is transmitted to humans in up to 10% of bites. The bite wound heals quickly, but after an incubation period, an acute viral syndrome develops, featuring chills, fever, vomiting, headaches, back pain, and joint pain. This disease can quickly become fatal due to sepsis if antibiotic therapy is not initiated early. If left untreated, rat-bite fever has a case fatality rate of approximately 10%.  

Bubonic Plague

One of the most dangerous rat-borne diseases in history is bubonic plague (Yersinia pestis), better known as the “Black Death,” and its variants. The plague, primarily carried by rats, is transmitted to humans through the bites of fleas from infected rodents. Fleas carried by rats are believed to have been responsible for the plague in the Middle Ages, which killed millions of people. Unfortunately, this disease remains a threat today, as it continues to spread in Africa, Asia, and the Americas and is among the emerging diseases worldwide. During the 20th century, the use of antibiotic treatments and the strengthening of public health measures drastically reduced morbidity and mortality from this disease, but did not eradicate it. Nearly 50,000 human cases of plague were reported to the World Health Organization (WHO) between 1990 and 2015.  

Other bacteria

Of course, many other bacteria can be carried by rats, such as several strains of Staphylococcus, Escherichia coli, Mycobacterium, and Campylobacter.  

Viral Zoonoses

The risk of zoonosis from rats can also be viral.  

Hantaviruses

Certain species of rats are known carriers of hantaviruses. Hantavirus pulmonary syndrome is one of the most serious diseases transmitted by rodents, and it has garnered significant attention in recent years in the western United States. The initial symptoms are similar to those of the flu, but they rapidly worsen, affecting the lungs, kidneys, and heart. If left untreated, the disease can quickly have a negative impact on the prognosis. Often requiring hospitalization in intensive care, these infections are severe and fatal in about 40% of cases. Rodents are not affected by the viruses, but they spread them through saliva, urine, and feces.  

The LCMV Arenavirus

Like many rodents, rats can also carry the lymphocytic choriomeningitis arenavirus (LCMV). This is a viral infectious disease transmitted through rat saliva and urine that primarily affects neurological function to varying degrees of severity. Infection during pregnancy has been linked to congenital hydrocephalus, chorioretinitis, and mental retardation in children. Several serological studies conducted in urban areas have shown that the prevalence of anti-LCMV antibodies in human populations ranges from 2% to 5%.  

Cowpox and Others

In France, rats can also be asymptomatic carriers of the cowpox virus, which was formerly used to vaccinate against human smallpox. Or the Puumala, Tula, or Seoul viruses, which cause, respectively, epidemic nephropathy, hematuria, or hemorrhagic fever with renal syndrome. But the list is much longer.  

Parasitic Zoonoses

In France, like many animals, rats are also implicated in parasitic zoonoses. They can carry a nematode, Trichinella spiralis , which causes gastroenteritis when ingested. Like many species, rats can also serve as reservoirs for Toxoplasma gondii , Cryptosporidium spp., or another nematode that is a parasite of the rat’s liver, Capillaria hepatica .   In urban areas, human infection generally results from poor hygiene and primarily affects children who may ingest parasite eggs present in the environment.