Sendai Virus (SeV)


A single stranded RNA virus, family Paramyxoviridae, genus Paramyxovirus, species parainfluenza that replicates in the respiratory tract.

Clinical Signs

With Sendai virus (SeV) you may see any of the following:

  • Sneezing
  • Hunched posture
  • Respiratory distress
  • Labored breathing
  • Porphyrin discharge from eyes and/or nose
  • Lethargy
  • Prolonged pregnancy
  • Partial or full litter loss
  • Failure to thrive in surviving babies and young rats
  • Anorexia


Rats, mice, and hamsters are the exclusive natural hosts of Sendai virus (SeV). SeV is immunosuppressive and can have an immediate, as well as a long-term effect, on the rat’s immune system. SeV infection may cause high morbidity (illness) and mortality rates when combined with bacterial pathogens such as Mycoplasma pulmonis and CAR (cilia associated respiratory) bacillus. The secondary infection may also be caused by bacteria that are non-pathogenic until the immune system is depressed, such as, pasteurella.

Infection is most devastating to very young, elderly, and immunocompromised rats who may develop a more severe pneumonia and in which the virus may persist for a longer amount of time. Athymic rats (rnu/rnu) are more susceptible to SeV and can stay persistently infected.

In the laboratory, where rats are often free of specific pathogens, SeV is not as dangerous to rats as it is in the pet population.

SeV is highly contagious. Natural infection occurs by way of the respiratory tract. Transmission can be contact and airborne. Airborne transmission can occur over a distance of 5–6 feet as well as through air handling systems. Transmission by fomites can be reduced by strict hygienic practices. The Sendai virus is inactivated by UV (ultraviolet) light, temperatures above 37oC. (96.8oF), and lipid solvents (such as alcohol).

Viral replication occurs in the respiratory tract for approximately one week after initial exposure. It is rare for the virus to become a generalized infection in the blood.
The virus titer peaks at 5–6 days, then decreases to undetectable levels throughout the respiratory tract around day 14 post infection. There is no carrier state and cessation of breeding in the colony will eventually naturally eliminate SeV infection.

SeV is a descending respiratory infection. It begins in the nasal passages, and moves through the trachea into the lungs. Sendai virus causes necrosis of the respiratory epithelium (thin layer of cells on the surface of the organs). In the first few days of infection epithelium necrosis is mild. As the disease progresses the necrosis becomes severe and usually peaks around day 5. By day 9 the regeneration of respiratory tract surface cells occurs. Focal interstitial pneumonia occurs, and inflammation and lesions of varying degrees can develop on the lungs.

In uncomplicated infection the respiratory system shows evidence of healing within 3 weeks although there may be residual lesions, inflammation, or permanent scarring.

Serum antibody appears at 6–8 days and remains detectable for approximately 1 year depending on sensitivity of the test used. The virus is self-limiting and in a single rat it sheds for a period of two weeks.

There are two basic types of infection that can affect a rat colony/group:

Enzootic infection (sub clinical)
Often this is the pattern in laboratories or breeding colonies. In an enzootic infection the adult rats have developed active immunity (antibodies) from a previous exposure to the virus, but no longer are actively infected or carry the virus. Maternal antibodies protect the newborn rats for approximately 4–8 weeks after birth. When the maternal antibodies wear off the rats then become infected with the virus. In a lab setting the recovery is usually quick with minimal sickness or loss of life. In a pet colony where there are additional pathogens to complicate the disease there may be clinical signs of illness and even death. Constant breeding perpetuates the virus within the colony.

Epizootic infection (clinically apparent)
When Sendai virus is introduced into a colony for the first time it is usually clinically apparent. The virus spreads quickly and affects the colony much worse than in an enzootic infection. Signs of illness will be and without treatment the mortality rate can be severe. Cessation of breeding and proper quarantine will eliminate active Sendai infection.

If breeding continues through the illness the viral infection becomes sub clinical yet remains active as long as it is perpetuated by the constant addition of new babies/rats.


The standard test for Sendai is the ELISA (enzyme-linked immunosorbent assay).

IDEXX RADIL offers a newer test: MFI (Multiplex Fluorescent Immunoassay) which is more sensitive. Have your veterinarian contact the testing facility for instructions on collecting and sending the blood sample.

During a viral event broad-spectrum antibiotics are given to the exposed rats to treat the often fatal, secondary opportunistic bacterial infections. Usually that approach works well. However, there are times when the bacteria involved is resistant. A culture and sensitivity test can ensure that the antibiotics given are appropriate.

Recommended Testing Facilities:

Note: Since SeV is self-limiting, rats that are not actively infected or carrying Sendai can test positive for antibodies from a prior infection for up to one year. A titer level can help indicate whether the infection is currently active, recently active, or from past exposure.


There is no treatment for SeV. However, it is essential to treat immediately for secondary bacterial infections with broad-spectrum antibiotics as soon as an SeV infection is noticed.
For a listing of antimicrobials and broad-spectrum antibiotics see the Rat Medication Guide. Also refer to listings in the Rat Health Guide articles: Mycoplasmosis or Pneumonia, for additional therapies based on severity of signs.

If the rat is not responding, then switching to a different antibiotic(s) or increasing dosages may be necessary.

Nursing Care

In the event that respiratory signs surface:

  • Give Probiotics such as Bene-Bac or yogurt with live active cultures when using antibiotics, to prevent normal gut flora from being destroyed.
  • Maintain rigid environmental hygiene.
  • Provide additional warmth to maintain body temperature within normal limits. It is essential that the rat does not become overheated or dehydrated. The rat should also be able to move away from the heat source if it becomes uncomfortable. If the rat is unconscious or immobile extreme care must be taken to keep the heat low and stable.
    • You can use an isothermic product that is heated in the microwave such as SnuggleSafe®. Make sure to follow the product directions carefully and wrap in a towel before placing in the cage. SnuggleSafe® will provide heat for 12 hours before needing to be reheated. Other similar types of products may vary in re-heat time. Check directions for individual product.
    • If using a heating pad (good for long term use) use only the low heat setting, put a thick towel in between the pad and the cage bottom, and place beneath a corner of the cage.
    • If none of these options are available you can use a plastic bottle filled with hot water, and wrapped in a towel, in the corner of the cage.
  • Provide additional nutritional supplements to help maintain strength.
  • Provide fluids to prevent dehydration (orally or warmed SQ fluids if necessary).
  • Make sure food and water are easily accessible.
  • Provide humidification to loosen secretions. Provide nebulized breathing treatments if indicated.
  • Contact veterinarian to discuss changes in treatment options if condition does not seem to be improving.
  • If condition continues to deteriorate and precludes further comfort or quality, discuss euthanasia with veterinarian.

*Note: For more in-depth nursing care information refer to the Rat Health Guide articles: Mycoplasmosis or Pneumonia.


  • Reduced severity of signs by treating with appropriate antibiotics
  • Disease is eradicated from colony
  • Virus is not spread
  • Quality of life is not severely compromised


Quarantine is the single most effective way to keep your rats safe from viral diseases. Quarantine must be done in an area where there is a separate air supply.

In the event there is an outbreak, quarantining the entire colony will ensure that the disease is not spread to other rats and/or colonies.

If you have been around (not necessarily handling) rats you may want to avoid going straight back to your own colony. SeV does not survive for more than 3 hours away from its host. Upon returning it may help to remove your clothing, clear your nasal passages, and shower before spending time with your own rats.

If you get new rats or your rats have been exposed to other rats two-week quarantine at a separate location will be helpful lessening any risk of your colony being infected with SeV.

If an outbreak occurs it is often useful to contact others whose rats may have been exposed so that they may treat immediately and not perpetuate the virus.

Note: A formalin-killed SeV vaccine was developed in the 1980s. It involved repeated dosing to show any effectiveness at all. For the most part the research has not been continued. Sendai is rarely an issue in many labs at this time.

Impact on Pet Owners/Breeders

Sendai can devastate a pet or breeding colony. It moves quickly and the complications from secondary infections can virtually wipe out a group. Immediate antibiotic treatment is essential. During a Sendai outbreak a strict quarantine must be observed to ensure that the virus runs its course and is eliminated. No rats should enter or leave the group during this time. The only exception being expedited quarantine in a breeding colony. (Expedited quarantine is the removal of litters to another location to enable the infection to run its course without persistent infection from juvenile rats.)

Pet Owners

Quarantine for a single rat would be 3-4 weeks. For a non-breeding group, the quarantine would be 6-8 weeks after recovery.


In a breeder colony the quarantine is much longer. The involvement of babies (nursing at onset of infection) increases out the time frame. These babies will be protected by maternal antibodies (4–8 weeks) and may become infected later. When babies are involved the quarantine time is roughly 3–4 months. During this time there should be no additional breeding, no rats in, and no rats out.

It is possible to expedite the quarantine to a 2–month time frame by removing and pet placing, in rat-free homes only so as to not spread the disease, all young babies and pregnant females. All breeding must be suspended, and no rats should enter the colony as they will not only be susceptible to the illness but will perpetuate the disease.

  1. (1999). Implications of infectious agents on results of animal experiments. Report of the Working Group on Hygiene of the Gesellschaft für Versuchstierkunde–Society for Laboratory Animal Science (GV-SOLAS). Lab Anim, 33(1), S39-87. Retrieved December 23, 2008, from
  2. Baker, D. (1998). Natural pathogens of laboratory mice, rats, and rabbits and their effects on research. Clin Microbiol Rev, 11(2), 231-66. Retrieved December 23, 2008, from
  3. Institute for Laboratory Animal Research. (1991). Infectious Diseases of Mice and Rats. Washington, D.C.: National Academies Press.


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