Emergencies preparedness, response

Seoul virus – United States of America and Canada

Disease outbreak news
20 February 2017

On 24 January 2017, the United States Centers for Disease Control and Prevention (CDC), through their Health Alert Network (HAN) publication, reported 8 cases of infection with Seoul virus in the states of Wisconsin (n=2) and Illinois (n=6). The first two cases were reported in early December 2016, when two home-based pet rat breeders in Wisconsin State developed an acute febrile illness, later confirmed as Seoul virus infection. Rats (Rattus norvegicus) at some facilities also tested positive for Seoul virus. Human infection with Seoul virus is not commonly found in the United States; this virus family also includes Sin Nombre virus, which is the most common hantavirus causing disease in the United States. This is the first known outbreak associated with pet rats in the United States.

To date, a total of 11 people have been infected in the states of Wisconsin, Illinois, and Colorado. Two of the individuals were hospitalized. Seoul virus infection was also confirmed in pet rats from ratteries in Wisconsin, Illinois, and Minnesota. Follow-up investigations indicate that potentially infected rats may have been distributed or received in Alabama, Arkansas, Colorado, Illinois, Indiana, Iowa, Louisiana, Michigan, Minnesota, Missouri, North Dakota, South Carolina, Tennessee, Utah, and Wisconsin. All investigations to date have indicated that the affected breeding facilities are limited to the pet rat trade. None of these ratteries supply (or have supplied) rats to research facilities.

In addition, follow-up investigations by the United States CDC and public health officials in Canada indicate that rats have been exchanged between the United States and Canada. According to the Canada IHR national focal point report of 10 February 2017, the Canadian rat breeding facilities under investigation exported rats to the United States and also imported rats from affected United States facilities. As of 10 February 2017, three positive human cases for the Hemorrhagic Fever Renal Syndrome (HFRS) group of hantaviruses, which includes Seoul, Hantaan, Puumala and Dobrava viruses, have been identified by serology in Canada. No serious illness was reported in these individuals. Two of the cases breed rats, and the third had contact with rats. Further laboratory testing and virus characterization is ongoing. Further epidemiologic investigation and testing of rats is planned.

Public health response

Health authorities both in the United States and Canada are implementing the following measures to respond to the outbreak:

Canada

  • Further laboratory testing and virus characterization to confirm Seoul virus exposure in humans.
  • Assessment of associated pet rat breeding facilities.
  • Further epidemiological investigation and testing of rats.

United States of America

  • The United States CDC and State Health Departments are collaborating to investigate the outbreak.
  • Depopulation carried out in some affected ratteries.
  • Investigations regarding the importation and exportation of the rats before the detection of the outbreak ongoing.

Information on Seoul virus

Seoul virus is a type of hantavirus that is transmitted from rats to humans after exposure to aerosolized urine, droppings, or saliva of infected rodents, or after exposure to dust from their nests or bedding. Transmission may also occur from rat bites or when contaminated materials are directly introduced into broken skin or onto mucous membranes. For Seoul virus, the natural host is the Norway rat (Rattus norvegicus) and the black rat (Rattus rattus). This virus has been found in both pet rats and wild rat populations around the world. The incubation period varies from 1 to 8 weeks; however, most individuals develop symptoms within 1 to 2 weeks after exposure. Seoul virus infection symptoms can range from mild to severe. In the severe form of the disease, patients can exhibit bleeding and renal syndromes. Inapparent infections can also occur. Seoul virus infection is not transmissible from human to human. There is no effective treatment available for Seoul virus infection.

WHO risk assessment

Hemorrhagic fever with renal syndrome (HFRS) is the severe form of the infection with Seoul virus. The case fatality rate (CFR) among humans who develop HFRS due to Seoul virus ranges from 1-2%. Of the 11 cases reported in the United States so far, two were hospitalized and none have died.

Although the three HFRS cases in Canada are still under investigation, there is some evidence of an epidemiological link to the United States Seoul virus outbreak.

There is no available information on further distribution of the infected rats outside of the United States and Canada. Rats do not show symptoms of disease when they are infected with Seoul virus. Once infected, rats can continue to shed virus throughout their lives, potentially infecting other rats and humans. The United States CDC is working with state health departments in the United States and others to investigate the outbreak of Seoul virus infections in pet rats and humans, to trace shipments and transport of rats, some of which may be infected with Seoul virus, to better understand how the virus entered the pet trade and to interrupt transmission of Seoul virus to other rats and humans.

Because there is presently no effective treatment for Seoul virus infection, preventing infections in people is important.

If infected rodents have contact with local rat populations, the infection with Seoul virus could spread to non-infected rodents and consequently change the prevalence of this zoonotic disease, both in rodents and in humans.

WHO advice

International pet trade has the potential to spread and cause emerging or re-emerging disease in humans. WHO encourages State Parties to developed and maintain the capacity to detect, and report similar events.