This article is from the Nov./Dec 1992 AFRMA Rat & Mouse Tales news-magazine.

By Carmen Jane Booth, D.V.M.

Three books that contain medical information on rats and mice are: 1. The Biology and Medicine of Rabbits and Rodents by John E. Harkness and Joseph E. Wagner; 2. Clinical Laboratory Animal Medicine, An Introduction by Donald D. Homes; 3. Laboratory Animal Medicine edited by James G. Fox, Bennett J. Cohen, and Franklin M. Loew.

VIRAL DISEASES OF MICE continued:

Mouse Hepatitis Virus Infection (MHV)

Mice are the only host for MHV, although other rodent species may carry serum antibodies to the virus. This virus is widespread, difficult to detect, highly contagious, and has numerous affects on the immune system of infected mice. There are several strains of mouse hepatitis (inflammation of the liver) virus all of which belong to the collection of viruses known as, Coronaviridae. Corona viruses are made up of single stranded RNA (ribonucleic acid) rather than double stranded DNA (deoxyribonucleic acid) and replicates in the cytoplasm of infected cells.

MHV is transmitted through respiratory aerosols, feces, cannibalism, and experimentally from the mother to the pups through the placenta. Although MHV infection may be asymptomatic, it is always highly contagious. Debilitated mice (example, those infected with the protozoan, Eperythrozoon coccoides; or certain lymphomas (1.)) are more predisposed to active infection. The course of MHV varies with the particular virus strain and host strain. Young mice are more susceptible than older mice and more often demonstrate signs of enteritis (inflammation of the intestine) and encephalitis (inflammation of the brain), where as the old more often develop hepatitis.

The majority of MHV infections are not apparent (subclinical). Suckling mice are susceptible usually between days 7 and 13 from birth and can develop an encephalitis with spasticity and tremors or a severe yellow, sticky diarrhea with high mortality. Older mice lose weight, have decreased breeding efficiency, red-brown urine, jaundice (yellow tinted gums, skin etc.), and lower mortality. Mouse hepatitis must be differentiated from other infectious diseases that cause diarrhea, runting, or death in suckling mice, such as mouse pox, reovirus 3, EDIM (epizootic diarrhea of infant mice), Tyzzer’s disease, and salmonellosis. Mice with neurologic signs must be differentiated from neoplasms in the CNS (central nervous system), musculoskeletal degeneration, and mouse encephalomyelitis, or M. neurolyticum toxicosis.

On necropsy, the distribution and severity of lesions depend on multiple factors including host age, mouse strain, and MHV strain. In susceptible adults and weanlings, yellow-gray foci of hepatic necrosis are seen with varying degrees of frequency. Intestinal hemorrhage, sanguinous peritoneal exudates, or icterus may accompany hepatic lesions. In suckling mice, intestinal lesions are more common. There is often no milk in the stomach, and the intestinal track is distended with gas and a watery to mucoid yellowish fluid. Histopathologically there is villous blunting and vacuolation, desquamation, and syncytial cell formations in the absorptive epithelium.

Clinical history, necropsy, and histopathologic signs are helpful in determining a diagnosis of MHV infection. Complement fixation, ELISA, immunofluorescence or serum neutralization serologic tests may be used to detect and differentiate MHV infections from other murine viral infections.

There is no treatment for mouse hepatitis virus infection, but elimination of ectoparasites, careful husbandry, and control of feral mouse populations will help prevent entry of the virus. Mouse hepatitis virus does not affect humans and poses no public health significance.

1. Lymphoma: any neoplastic (abnormal growth) disorder of the lymphoid tissue (lymph nodes, thymus, spleen, lymphocytes, etc.). For all the new AFRMA members, I would run a reprint of general information from previous newsletters.

GENERAL HUSBANDRY
The most important forms of care that people can give their animals to minimize health problems are proper nutrition, clean water, clean housing, a safe environment, adequate cage space and TLC (Tender Loving Care). The best diet for rats and mice is one that meets all the vitamin, mineral, protein, and caloric requirements for each species during all the phases of their life: growth, lactation, reproduction, etc. The easiest and safest diet to feed is a preformed laboratory block diet, especially if you are raising large numbers of animals. Purina is one of the more well-known manufacturers. They have put a great deal of effort into making high quality diets specific for rats and mice as well as other animals. If this is not available in your area, a high quality dry dog food combined with a mixed grain rodent food should be adequate.

The nutritional requirements of rats and mice are different from each other as well as other species of animals; and as a result, dog food does not have all of the essential nutrients in the correct proportion needed by rats and mice. This is why it is also important to offer them a mixed grain rodent food. The reason that I do not recommend feeding only a mixed grain rodent food is because some ingredients are selectively eaten over the others. For example, sunflower seed and peanuts, which are high in fat, are usually the first things eaten and often the rodents will not eat the other ingredients, which contain beneficial nutrients. Essentially, the rodents can become junk-food junkies. Since currently I only have one rat, she gets a variety of feeds: lab blocks, a mixed grain diet that does not contain peanuts and is low in sunflower seeds, and at times, small pieces of assorted fruits, vegetables, and homemade whole-grain breads.

One additional word of caution about feeding vegetables and fruits: they need to be well washed and free of all dirt, insecticides, and preservatives. With the short life-span of our little friends, along with their propensity for developing cancer and other diseases, anything that we can do to decrease their exposure to harmful substances is beneficial. (Anyone suspecting their animal(s) of having a medical problem should seek the care of their local veterinarian.)

Back to top