Rat Coronavirus

© Ann Storey MSc FIBMS published in Pro-Rat-a 171, May/June 2009.


Sialodacryoadenitis is an infectious disease of rats caused by rat coronavirus (RCV). This infection of rats has been recognised for many years but the causitive agent was not discovered until 1970, when the first virus of this group was isolated. This virus was designated Parker's Rat Coronavirus (PRC) after its discoverer. A second virus, Sialodacryoadenitis virus or SDAV was discovered just two years later. These two viruses are very similar and are now considered to be strains of the same virus, namely Rat Coronavirus or RCV. Since then, several more strains have been reported, including CARS (not the same as CAR bacillus), RCV-BCMM, RCV-W and RCV-NJ. This is a virus that mutates frequently and whose virulence can vary widely, from rat to rat, strain to strain and outbreak to outbreak.

Coronaviruses are widespread among mammals and birds, (one strain is responsible for SARS in humans) but are largely species specific. RCV does not cause disease in other mammals. A related virus MHV, or mouse hepatitis virus is common in mice.

Clinical Signs

The severity of symptoms varies with the strain. Generally if the strain is new to the population then it is likely to produce more severe symptoms, as the population has no resistance to it, this is the same for most infections though.

Symptoms can vary from the subclinical (no detectable illness) to death. Most non immune rats do show some degree of illness however. Early signs, especially in very young rats, include squinting, photophobia, lacrymation (runny eyes), production of porphyrin around the eyes and nose which may also appear on the inside of the forelegs as they attempt to wash themselves. Older rats usually develop sneezing and sometimes swelling of the salivary glands at the angle of the jaw (parotid) and up the sides (submaxillary). Swelling of the Harderian and lacrymal glands around the eyes may also occur. These early symptoms develop 4-6 days post contact. However, as the first rats infected often show only mild symptoms, it may be 2 weeks or more before you realise your rats ar infected.

The eyes may then become severly inflamed, leading to keratoconjunctivitis. This is not caused by clinical infection of the eye, but by the disrupted production of tears from the infected lacrymal and Harderian glands. If this does not get better quickly then the cornea may become opaque and ulcerated. The eye itself may become greatly enlarged (megaloglobus). Dark red encrustations may be present around the eyes and nose due to the porphyrin released from the damaged Harderian glands. Other affects include weight loss, because the rat often stops eating or drinking, reduced fertility and infection of the lower respiratory tract (trachea and lungs). Like many respiratory viruses, RCV causes flattening and loss of the cilliated cells lining the passageways of the lungs etc. This leads to mucus buildup and subsequent activation of any bacterial respiratory pathogens who may be living there, including Mycoplasma and CAR bacillus. Pneumonia, accompanied by fluid build up in the lungs, is not uncommon in severe cases and is the most likely cause of death when it occurs.

On the whole, this virus is usually non fatal, although occasional outbreaks, including the latest UK one, can produce a significant number of deaths. Rats on steroids do not shed the virus for significantly longer than other rats. There is evidence that some strains of rats are more susceptible to serious illness than others.

Convalescence

Rats mostly cease producing the virus when they start producing antibodies, that is 10-14 days post infection. however the symptoms may take longer than that to resolve. Usually it takes 4-6 weeks post infection for all of the symptoms to completely subside and some scarring in the respiratory tract may be left, leading to occasional snuffles. The odd rat may go on to develop full blown Mycoplasmal respiratory disease. Eye problems, including clouding and ulceration of the cornea and megaloglobus, can also resolve in 4-6 weeks. However, some rats damage their eyes while they are swollen and this can lead to loss of the eye due to secondary bacterial infection. A few may also be left with glaucoma.

Following infection, the rat appears to produce antibodies for around six months. After this, while the rat does not produce symptoms if infected again with RCV, it can still shed that virus for another 7 days.

Diagnosis

The swelling of the salivary and lacrymal glands is considered diagnostic as it does not occur in any other known rat infection. However a blood test can be performed looking for antibodies around ten days after exposure. Both ELISA and IFA tests are available.

Spread and Control

RCV spreads rapidly among rats housed in open cages and presumably, at gatherings of rat owners and their animals. The infection rate is normally 100% of rats held in one place. Transmission appears to be primarily by droplets (sneezed out by infected rats) or by direct contact. However, RCV can survive for up to two days dried onto surfaces. Therefore, so called 'fomite' transmission (that is via cages, surfaces, clothes etc) may play a part. Intrauterine infection does not occur. Due to the rapidity of spread, most rats in a room will have had the infection and become immune within 3-5 weeks of the initial contact with the virus, even if some rats still have some symptoms left to resolve.

The virus can be killed by normal household disinfectants and temperatures of 83C. Cool washes are probably not sufficient to kill this virus.

If you get this infection in your rats, then you should avoid contact with other rats or rat owners for 6-8 weeks to be on the safe side, but it is probably impossible to prevent spread within your own rats without elaborate quarantine procedures that are unlikely to achieve anything other than prolonging your outbreak.

There is no treatment for this infection but supportive therapy can help. This includes blanket adminstration of pain killers, such as soluble asprin or Ibruprofen in the drinking water to nursing for the worst affected. Rats who are not eating or drinking, should be dropper fed in order to improve their chances. I have found that diluted Nutrical, sweetened milk with a spot of alcohol and Polyaid can all make a big difference to a rat's chances.

Antibiotics will not touch the virus but the use of chloramphenicol eye drops does help prevent secondary eye infections. Likewise rats with the worst respiratory symptoms may be helped by a course of an antimycoplasmal antibiotic such as Baytril or Doxycycline.

This is a common virus and most outbreaks pass off without deaths or serious illness. However, when a more virulent strain does appear, it is important that committees act quickly to prevent the spread of the infection. This means stopping shows and 'gatherings' until the outbreak has subsided. However, this will not work unless individual members behave in a responsible manner and keep away from more 'informal' meetings. Likewise care must be taken when bringing in rats from unknown or dubious outside sources. These rats can often be the source of new disease strains even if they appear well. Therefore you should quarantine these rats for 2-3 weeks. Alternatively, if you do mix these rats in with your own, you will need to watch yours carefully for a few weeks. During this time, you should keep away from other rats.