The ectromelia virus, or mousepox, is a contagious and potentially lethal viral disease affecting mice. It belongs to the Orthopoxvirus genus, which includes smallpox and monkeypox viruses. First identified in 1930 in laboratory mouse colonies, wild rodent populations in Europe are suspected to naturally carry the virus. Ectromelia poses a significant threat in laboratory settings, as outbreaks can severely disrupt scientific studies.
How the Virus Transmits
Ectromelia virus spreads efficiently among mice, primarily through direct contact with infected animals or indirectly through contaminated materials. Natural infection often occurs through abrasions in the skin, allowing the virus to enter the host. The virus can also be transmitted via contaminated bedding, food, or water.
Infected mice can shed the virus in their feces, urine, saliva, and from skin lesions, contributing to widespread contamination of their environment. The virus can persist in scabs and feces for up to 16 weeks post-infection, which facilitates its spread between cages, particularly through handling of infected mice. While direct and indirect contact are common, airborne transmission is also a potential route, further contributing to its spread within mouse populations.
Signs of Infection
The clinical signs of ectromelia virus infection in mice vary based on the specific viral strain and mouse genetic background. In susceptible strains, such as A, C3H, and BALB/c, acute disease often leads to high mortality rates, sometimes as high as 80-90%. These mice may exhibit general signs like ruffled fur, a hunched posture, conjunctivitis, and facial swelling. Death can occur rapidly, sometimes before other clinical signs develop.
For mice with intermediate susceptibility or those surviving the acute phase, the disease can manifest in a subacute to chronic form. Characteristic external signs include cutaneous vesicular rashes or lesions on the skin, particularly on the head, tail, and limbs. These lesions can progress to swelling, necrosis, and eventually the sloughing of extremities like toes or the tail, which is how the virus earned the name “ectromelia,” meaning limb amputation. Internally, the virus can cause severe necrosis in organs such as the liver, spleen, thymus, and lymph nodes, which may appear as white spots on the liver.
Controlling and Preventing the Virus
Controlling and preventing ectromelia virus outbreaks in laboratory mouse colonies relies on a combination of diagnostic techniques and strict biosecurity measures. Serologic testing, such as Multi-plex Fluorescent Immunoassay (MFIA) or Immunofluorescence Assay (IFA), helps screen mouse colonies for the presence of infection. Polymerase Chain Reaction (PCR) can be performed on skin lesions and spleen samples to confirm the diagnosis. Histological examination revealing intracytoplasmic inclusion bodies in epithelial cells surrounding skin ulcers can also aid in confirmation.
Biosecurity protocols are essential in preventing the spread of the virus. This includes immediate quarantine of any suspected infected animals and rigorous sanitation of animal housing facilities. Disinfection with agents like gaseous formalin or vaporized hydrogen peroxide is effective, as the virus can survive for about 11 days at room temperature in blood. All contaminated materials, such as bedding, should be discarded as hazardous waste or autoclaved, as autoclaving and common disinfectants effectively inactivate the virus. Vaccination of mice against ectromelia virus has also been explored as a preventive measure to protect valuable mouse colonies.
Is Ectromelia a Human Concern?
Ectromelia virus is not considered a threat to human health. It is a mouse-specific orthopoxvirus, meaning its natural host is the mouse, and it has only been observed in mouse colonies, particularly those used for research. There is no known evidence of ectromelia virus causing disease in humans.
The primary significance of ectromelia virus lies in its impact on scientific research. Outbreaks in laboratory mouse colonies can lead to substantial financial losses and significant delays in experiments, as affected animals may need to be culled to prevent further spread. This disruption highlights the importance of stringent biosecurity and prevention strategies within research facilities that utilize mouse models.