Washington, DC, has a rat problem. According to the Centers for Disease Control and Prevention (CDC), this has led to the emergence of the first two official cases of hantavirus in humans in the city.

Wildlife has its own share of viruses and pathogens to deal with, as do people. Sometimes, however, when these two groups live in close proximity, pathogens can evolve to cross from one to the other. When this takes place from wildlife or livestock to humans, this is known as zoonosis. The coronavirus pandemic started as a zoonosis.

One genus of viruses that can comfortably infect both rodents and humans is known as orthohantavirus, or simply hantaviruses. These are widespread through rodent populations such as city rats, where they cause asymptomatic infections. However, people can become infected with these viruses as well, most commonly through exposure to rat urine or feces, although saliva or bites can also transmit the virus.

Washington DC’s rat problem has led to the emergence of two known cases of hantavirus infection, the CDC reported Thursday. Transmission from one infected person to another is almost unheard-of with hantaviruses, so concerns about brewing epidemics are far from the CDC’s mind. The infections were recorded in 2018 and have been successfully treated.

Still, the situation poses a risk for the health of people in Washington DC, who should take steps to protect themselves from the rodents.

Vermin threat

“Although extremely rare, the two SEOV cases presented in this report highlight the importance of physicians including hantavirus infection in their differential diagnoses in patients with compatible symptoms and history of animal exposure or travel and underscore the importance of reporting notifiable infectious disease cases to health departments for investigation and response,” the CDC’s report explains. “These cases also serve as a reminder to the public to minimize risk for infection by following recommended hygiene practices.”

Hantavirus infection in people can lead to a host of respiratory and hemorrhagic diseases which can easily become fatal. Fortunately for the cases recorded in DC, the strain identified in the two infected individuals is a milder “Old World” strain called the Seoul virus. Old World hantaviruses cause a disease called hemorrhagic fever with renal syndrome (HFRS). In contrast, “New World” hantaviruses, which are present in the Americas, cause a much more severe respiratory infection known as hantavirus pulmonary syndrome (HPS)– which is much deadlier.

HFRS starts out as a generic infection with fever, chills, nausea, and headache, but can then progress to low blood pressure, acute shock, vascular leakage, and acute kidney failure, the CDC notes. The severity of HFRS varies by the strain of hantavirus that causes it, and can reach up to 15% fatality. In the case of the Seoul virus, fatality rates are around 1%. Both individuals reported-on by the CDC made a full recovery.

HPS also begins as a generic infection with fever, chills, and aches, but quickly progresses to an acute, life-threatening phase after about a week. The patient’s lungs and heart are affected; the lungs fill with fluid, and patients require hospitalization and ventilation within 24 hours. HPS is fatal in about 38% of cases, according to the CDC. The deadliest such virus, the Sin Nombre virus, spread by the deer mouse, has a fatality rate of about 50%.

HPS and the Sin Nombre virus first came in the crosshairs of US health officials following an outbreak of deadly respiratory disease in the Four Corners region. In total, 48 cases were identified in that year, 27 of which were fatal. The CDC finally tracked the virus down to rodents in the area, and it gained the moniker of Sin Nombre virus (the virus with no name) during this process.

The Seoul virus has a much lower prevalence in the US, and spreads from the common brown rat, which travelled to all corners of the world on European ships (hence, the virus is known as an “Old World” virus). The virus is present worldwide but was first described in Korea, near Seoul. It is considered a rare pathogen among humans.

This is what makes the two cases reported-on by the CDC notable. Patient A was a healthy male, a 30-year-old maintenance worker who had “frequent rodent sightings at his workplace”. He contracted the disease in May 2018 and made full recovery after receiving treatment. Patient B was an unrelated case. This 37-year-old man with chronic kidney disease who worked as a dishwasher and plumber’s assistant contracted the disease in November 2018; it is unclear from what source. The CDC notes that he did not own any pets, had not recently travelled outside of the US, and was unaware of exposure to rodents at any point in his daily life. He also made a full recovery after receiving treatment.

The CDC believes these two cases were caused by the city-wide rat problem in Washington DC, which they explain has been worsening for years now.

Rodent overpopulation in DC is well documented by increased complaints via the Citywide Call Center to the Rodent Control Program, and the DC Department of Health has amplified efforts to address this public health threat,” the CDC explains.

The cases serve as a reminder of the dangers of rat infestation in our cities, and should motivate the public to follow recommended hygiene practices to insulate themselves from the risk. Meanwhile doctors should keep in mind that the virus is active in the area and look out for signs of hantavirus infection in their patients.