Etiology

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 SV and can stay persistently infected.
In the laboratory, where rats are often free of specific pathogens, SV is not as dangerous to rats as it is in the pet population.

SV 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 37°C (96.8°F), 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 to 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 SV infection.

SV is a descending respiratory infection. It begins in the nasal passages, and moves through the trachea into the lungs. Sendai 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 to 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 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.

Diagnostics

Recommended Testing Facilities:

PCR/non serological testing
http://www.radil.missouri.edu/info/DiagTesting/services/molecularbiology.asp

Charles River Laboratories, Wilmington, MA: (800) 338-9680

Note: Since the SV 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.

Treatment

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

Nursing Care

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

Outcome

Prevention

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 SV 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

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.

Breeders

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.

References

2. Viruses, Respiratory System
http://ourworld.compuserve.com/homepages/TheBroons/path2.htm#sv

3. Implications of infectious agents on results of animal experiments.
Society for Animal Science
http://www.utexas.edu/research/arc/misc/GVSOLAS.pdf

Posted on July 21, 2005, 17:53, Last updated on October 8, 2007, 13:18 | Viruses