© Ann Storey MSc FIBMS
Most species of fancy animals have a problem with respiratory infections. Sendai virus, one of the commoner ones which periodically causes outbreaks in fancy and pet rats, is also of interest to mouse and hamster fanciers, as these are also infected with this virus.
Sendai Virus (SV) also called parainfluenza 1, is a paramyxovirus which is normally considered a pathogen of mice. However, it also attacks rats and hamsters. Cavies can become infected with it but in them the virus does not cause disease and it is suspected that this is a different strain of the virus. Laboratory rabbits have been known to get a mild infection of the upper respiratory tract with this virus but it does not spread to the lungs. Gerbils are not affected. This virus tend to cause more severe disease in mice than in rats, however what is said about rats in this article is generally applicable to mice and hamsters as well. Sendai virus is closely related to parainfluenza viruses of human origin. It is named after Sendai, Japan, where it was first isolated from laboratory mice inoculated with human lung suspensions in the early 1950s, and later isolated from naturally infected mice. Because of this, there is some controversy regarding the original host.
The virus is extremely contagious but is easily killed by U.V light, drying and disinfectants.
Transmission is by direct contact or by inhalation of aerosols. Following exposure by inhalation, the virus replicates in the cells in the upper respiratory tract, before proceeding down the rest of the respiratory tract into the lungs. The deeper the virus gets into the lungs before the immune response kicks in, the more severe the disease is likely to be.. Virus can be detected from 1-7 days after infection in adults and up to 12 days in young animals. This indicates that this is the period when the rat is infective for other animals. The exception would be rats with thymus deficiency, who would not necessarily show symptoms immediately but could carry the virus for much longer. Normally, however, rats soon start to produce antibodies, which may be detected for up to 9 months following infection. These antibodies clear the virus from the system.
Like human flu, the clinical illness is caused by the rat's immune system trying to clear the body of the infection. Symptoms are vary variable, from none at all to no obvious symptoms but lung lesions on post mortem, mild snuffles through to severe pneumonia with breathing difficulties, staring coat, lack of appetite, weight loss, decreased litter size and growth retardation in kittens. The effect on litter size is not down to the virus crossing the placental barrier, but due to the doe being so ill that not enough oxygen is getting to the litter in the uterus.
The immune response clears the virus but also produces the disease which is the same as human flu. The level to which the infection extends into the respiratory tract is determined by the rat's (or mouse) genetics. In very susceptible animals and kittens, the virus is allowed to extend deep into the lungs before the animal mounts a very strong response which results in severe disease. This is similar to that which happens in human bird flu and probably what happened in the great flu outbreak of 1918 -1920, which killed more people than bubonic plague and where, according to my grandmother (who didn't get it) people were 'dropping dead in the street'
Those animals which produce subclinical infection because of genetics/previous infection, produce a rapid immune response which prevents lower respiratory tract involvement and thus serious disease.
New born rats are passively protected by maternal antibody until they are 4-6 weeks of age, at which time they become infected. The severity will depend on genetics and whether or not the virus is endemic in the rat population at the time. Obviously epidemics are more likely to produce worse disease than endemic disease (where a disease is always circulating in the population). Once the animal has been infected they normally develop a life long immunity. Adults from populations where this virus is endemic rarely show disease but infection with the virus may give other opportunistic respiratory pathogens which normally aren't doing much, the opportunity they need to cause trouble. It also predisposes rats to develop middle and inner ear disease.
Epidemic disease, where a previously uninfected population becomes infected, usually results in nearly all the animals becoming ill. There may be many deaths, especially in kittens whose immune response is not mature, and in old ones whose immune and respiratory system is not what it was. The virus will persist in a colony as long as as susceptible animals (newborns or animals bought in) are present.
There are many respiratory infections which need to be excluded including Rat coronavirus (SDAV), pneumonia virus of mice (also attacks rats), Streptococcus pneumoniae, Bordetella bronchiseptica etc. The best method is an ELISA blood test for the antibodies.
While antibiotics will not work against viruses, they do help control the bacterial species such as Pasteurella and Mycoplasma who are likely to cause secondary respiratory infections. Both doxycycline and Baytril (enrofloxacine) have been used successfully. Seriously ill rats need fluid replacement and keeping cool. Rats suffering severe respiratory distress are unlikely to survive.
Further Reading:Pathology of Laboratory Rodents and Rabbits 2001 2nd Ed. D H Percy and S W Barthold, Iowa State University Press.
The Biology and Medicine of Rabbits and Rodents 4th ed, Harkness and Wagner. 1995, Williams and Wilkins.