Some people love mice and rats and keep them as pets. The white ones with the pink eyes are nice, very soft and furry, and many people love them as much as they love their dogs.
And they may be man’s best friend in another sense as well: they are by far the most common experimental animal in medical research. There are hundreds of specially bred lines of mice and rats used for research in various fields of medicine, biology, physiology, psychology, and so on.
There are mice that easily get skin and lung tumors but are resistant to breast tumors, rats that commonly have vaginal prolapse, rats that are aggressive, rats that are passive, mice that have a high incidence of testicular birth defects, mice that have abnormal kidney function, skinny rats, fat rats, mice that have a high incidence of congenital cleft palate when their mothers are injected with cortisone, mice that tend to chew each other’s whiskers off, mice that are immune to tetanus, rats that like to run in wheels and rats that don’t, rats that get asthma, rats that are highly susceptible to malignant tumors of the central nervous system, mice that are resistant to Herpes simplex, mice that are born deaf, and hundreds and hundreds of other genetic variations.
It almost seems that if there’s a behavior, a disorder, or a disease you are interested in, there’s a rat or mouse strain genetically bred to manifest it.
Mice are often used in vaccine development, and until the middle of the twentieth century, they were important in the production of vaccines as well. Today, however, vaccines are produced by tissue culture without the involvement of live laboratory animals. Bacterial vaccines still require serum or blood for culture, and mice are often used as part of the production process for these medicines. Animals, and particularly mice and rats, are widely used in testing for toxicity, tumori-genicity (potential to cause tumors), and potency of many drugs, and for testing the purity and safety of vaccine batches.
And then there are those mice and rats that carry diseases without human intervention, pests that have tormented humankind throughout history, and probably even before history was recorded. These are the rats that were a vector of the Black Death in medieval Europe. These are the mice that carry hantavirus in the American West. These are the animals that figure in the life cycle of the bacterium that causes Lyme disease.
And these are the rodents that the New York City Health Department vividly characterizes, along with cockroaches and flies, as “vermin” when they turn up in a restaurant. Various species of rats and mice live only in close proximity with humans, and in most parts of the world, where humans live, these rodents live with them.
If you’ve been reading, you know that mice and rats don’t carry rabies (although, like many other mammals, they theoretically could). But there are plenty of diseases they do carry, and a number of them can be transmitted to humans under the right circumstances.
In fact, mice are carriers of some of the most deadly viruses known, the various viruses that cause viral hemorrhagic fever, or VHF. While these viruses are not common in the United States, they do occasionally appear, and when they do, it’s not pretty. Their symptoms include bleeding from the mouth, ears, eyes, and internal organs; kidney failure; coma; delirium; seizures; and death.
There is only one place in the United State where four states have a common border: the place where Utah, New Mexico, Arizona, and Colorado all come together, sometimes called the Four Corners. In 1993, a doctor working for the Indian Health Service in Gallup, New Mexico, noted a cluster of five deaths in the Four Corners area and told the health department about it.
No one knew what the problem was, so they organized a conference of doctors and health officials to try to figure it out. Even as they were organizing the conference, more reports of death were coming in. People were coming down with fever, chills, and muscle pain, followed by cough, thrombocytopenia, and elevated serum levels of lactate dehydrogenase.
Patients had a diffuse infiltrate in their lungs visible on X ray. All these symptoms are common in acute respiratory diseases, but the cause of this one was still a mystery. The mortality rate was high, nearly 80 percent, so this was no ordinary disease. Eventually the virus was amplified in the lab by polymerase chain reaction and was found to be a previously undescribed hantavirus. The disease now had a cause, the virus, and a name: hantavirus pulmonary syndrome, or HPS. Now, where was this new virus coming from?
Researchers trapped rodents at the homes of HPS patients in the Four Corners and soon discovered that the deer mouse (Peromyscus maniculatus) is the main host of the virus in that part of the country.
By mid-January of 1995, 102 people in 21 different states had been diagnosed with the disease, and subsequent examinations of rodent populations revealed four other kinds of hantavirus carried by various species of mice. The disease infects human by inhalation of the virus, which is carried into the air when rodent droppings or urine are disturbed.
In North America, the disease does not spread from person to person. There isn’t any drug that treats the disease, but with the recognition of the illness at earlier stages and careful fluid replacement in patients with advanced disease, the mortality rate decreased to 40 percent, a significant improvement over the earlier figures, but hardly cause for celebration. There are now trials going on to see if intravenous ribavirin will be helpful in treatment.
Leptospirosis, a bacterial disease carried by rats and other animals, can easily be transmitted to humans through contact with the animal’s urine, often present in rat-infested alleys and parks in the inner city and very likely in suburban areas as well. If the bacterium enters your body through an open cut, your eyes, your mouth, or your nose, it can cause a nasty flulike illness that in about 10 percent of cases becomes systemic. This form is called Weil’s disease and can cause fever, jaundice, meningitis, kidney failure, and internal bleeding. On rare occasions, it kills people.
You get the symptoms of leptospirosis about two to four weeks after contact with the bacteria. Since lots of other animals besides rodents can be infected, you can get the disease in many different places. It’s an occupational hazard for people who work with animals, and a recreational risk for people who hike, camp, swim, or go boating or whitewater rafting in or near contaminated water. The illness can be successfully treated with doxycycline or penicillin, provided you get it early enough in the course of the disease. In severe cases, intravenous antibiotics are used.
There are about 200 reported cases a year of leptospirosis, which can only be diagnosed with a polymerase chain reaction test of blood or spinal fluid to determine the presence of the bacterium. About half of these cases occur in Hawaii. But the symptoms in most cases are difficult to distinguish from a flu, and few labs perform the complex test for it. So there are probably many more unreported cases of the disease than reported. There was a large confirmed outbreak in 1998 in which more than 100 people participating in triathlon competitions in Wisconsin and Illinois became ill, probably from the polluted water of Lake Springfield.
Ingesting food or water containing rat feces or urine can also give you rat-bite fever. This is a bacterial illness caused by Streptobacillus moniliformis, and it can also be contracted by a bite or scratch of an infected rat. The illness usually begins about two to four days after contact with the infected substance.
The symptoms can get nasty: relapsing fever, aching bones, and a rash on the extremities, palms, and soles. Headache, meningitis, nausea, and vomiting have also been reported. Although most people get better by themselves, intravenous penicillin for a week, followed by another week of oral penicillin, is a good idea because 13 percent of untreated cases are fatal. Diagnosis is not easy, there’s no serologic test, and you can only establish the diagnosis by culturing the bacterium, which is not easy for most labs to do. No one knows if it’s effective to give antibiotics prophylactically after an exposure. Fortunately, rat-bite fever is rare.
The species of rat that infests American farms and cities is called Rattus norvegicus, or the Norway rat, and this is the one you may see hovering around the garbage cans in the back of the restaurant. It’s sometimes called the common brown rat, and it’s very ugly, rat-faced indeed, with coarse brown and black fur and a naked tail.
They weigh about a pound, and including the six-to-nine-inch tail, they can be a foot and a half long. They have keen senses of hearing, smell, taste, and touch, and they’ll eat anything, grain, meat, insects, plant matter. If there’s no other food around, they’ll eat each other. They need about an ounce of water a day to wash it all down. They can chew through wood, wallboard, plaster, and concrete if they’re hungry and there’s food available. They can climb, swim, and fit through any opening larger than a half-inch across.
They’re nocturnal and normally quite shy of humans and other animals (including, usually, each other), but every once in a while they’ll bite a sleeping baby, usually on the face where they smell food. A rat lives about 18 months and starts having babies at three months. Six to 12 young are born after about three weeks of gestation, and the average female gives birth about five times a year. So if a female has about 45 babies a year, that means a lot of rats. All they need is enough food to eat and they’ll be all over the place.
There is no evidence that the house mouse (Mus musculus) you see running across the restaurant floor while you’re eating dinner is likely to give you a disease like hantavirus, but rodent droppings and urine are nevertheless something you’ll want to keep away from the food you eat. There are some diseases that they transmit, mostly minor, like salmonellosis.
And of course some species can be the vector of more serious illness, like Lyme disease. But house mice and humans have lived together, if not always happily, for thousands of years, and continue to do so today. The two species are quite well adapted to each other.
House mice weigh about a half-ounce, and to some eyes they’re kind of cute: furry brown things with big ears and small eyes. If a wall is the slightest bit rough, they can grip on and run right up it, and they can jump more than a foot off the floor. A hole about one-quarter inch in diameter is big enough for them to squeeze through. They like cereal grains, but they’ll eat almost anything their human companions will eat. They don’t live long, but they keep busy: the gestation period is about three weeks, and a female can have as many as 10 litters of five or six young in her one year of life.
There is the occasional, more or less rare disease carried by some species of mouse, and they are the vectors of certain diseases like Lyme, but house mice are in general not a reservoir of serious human illness.
They do a certain amount of economic damage, on farms, for example, they can result in the spoilage of large amounts of food meant for humans, and any restaurant that allows them to frolic in the presence of diners is likely to be out of business soon, but, like cockroaches, they and their excreta are probably more disgusting than dangerous.
