Wild rats are widely recognized the world over as carriers of disease. Most famously, the black rat has long been condemned as being responsible for the catastrophic outbreak of bubonic plague in the 14th Century. It is true that the black rat did host a species of rat flea that harbored the Yersinia pestis bacteria that causes plague.
There were, however other factors involved in the “perfect storm” that was the plague pandemic which helped make it so deadly, a set of circumstances that is unlikely to be seen again. As frightening as bubonic plague is, in our modern age of antibiotics it is no longer the certain death sentence it once was.
The Center for Disease Control (CDC) of the United States has identified more than ten illnesses and diseases that affect humans and which are spread by rodents. The rodent’s taxonomic family is vast, and not all of these diseases are primarily associated only with the two true rat species rattus rattus (black rat) and rattus norvegicus (brown rat).
Various serious diseases can be spread via wild mice, rabbits and even domesticated hamsters as well as many other members of the rodent family. Some of these are commonly referred to as rats, but are in fact different species altogether. Whether with cause or not, the rat has been singled out as the primary culprit and the two major species have long been condemned as dangerous pest animals.
Rat Diseases in Humans
Most of these diseases are also transmissible by other members of the rodent family up to and including bubonic plague, which can also be found in squirrels, prairie dogs, and chipmunks. Additionally, there are several sicknesses that are primarily spread by other rodent species but which can infect and sicken the two rat species. A rat sick with one of these diseases can pass the bacteria or virus on to humans, via direct contact, bites or exposure to urine and feces contaminated dust.The CDC identifies the following as the illnesses most often associated with the two true rat species. Included are some of the major rat disease symptoms that present in humans.
• Leptospirosis: also known as Weil’s disease, this illness is most often contracted by exposure to water or food that has been contaminated by rat/rodent urine and feces. It can alternately be spread via contact through the skin or mucous membranes (such as inside the nose) with water or soil that is contaminated with the urine from infected animals. Gold medal winning British sculler, Andy Holmes died of the disease in 2010 after being exposed to rat urine contaminated water while rowing. Deaths from Weil’s disease occur when the infection causes kidney and liver failure. This is rare; most people affected suffer only flu-like symptoms and make full recoveries. Some people are infected and show no symptoms at all.
Symptoms of Leptospirosis: High fever, headache, chills, muscle aches, vomiting, jaundice (yellowing of skin and eyes), red eyes, abdominal pain, diarrhea and rash. The infected person may recover, then fall ill again with more severe symptoms. Kidney and liver failure can occur, as can meningitis. Duration of illness may be from a few days to three weeks. Without treatment recovery can take several months.
• Plague: This infamous killer has three clinical forms, bubonic (marked by buboes, tender and swollen lymph nodes) pneumonic (attacks the lungs and respiratory tract, the most serious); and septicemic (blood poisoning). The disease is contracted via the bite of a rat flea or direct contact with an infected black rat or other infected animal. The Yersinia pestis bacterium enters the body through the flea bite, attacking the nearest lymph nodes first. Both bubonic and septicemic plague require the bite of a flea or handling the dead rodent; pneumonic plague can be passed from human to human via infected droplets of saliva suspended in the air as the patient coughs. The plague still exists in many parts of the world including the American west and parts of South America, Africa and Asia where a few cases are reported each year. Without treatment, plague can still be fatal; however modern antibiotics are extremely effective against the disease if caught in the early stages.
Symptoms of Plague: Bubonic plague presents with a sudden onset of fever, headache, chills, and weakness and one or more swollen, tender and painful lymph nodes (called buboes). Pneumonic plague sufferers experience fever, headache, weakness, and a rapidly developing pneumonia with shortness of breath, chest pain, cough, and sometimes bloody or watery mucous. Septicemic plague presents with fever, chills, extreme weakness, abdominal pain and shock. Also sometimes seen is bleeding into the skin and other organs. Extremities such as fingers, toes and nose can turn black and die. Septicemic plague can occur as the first symptom of plague, or may develop from untreated bubonic plague.
• Rat-Bite Fever: As the name implies this illness is most frequently caused by the bite or scratch of an infected rat, although mice have been implicated in some cases. It can also be transmitted via handling of the dead rodent, and eating or drinking food or water that has been contaminated by rat feces. The infection exists worldwide although there are two different strains of bacteria that cause the illness. In the US and Europe it is Streptobacillus moniliformis (streptobaccilliary RBF) in Africa and Asia Spirillum minue (spinillary RBF). The disease is sometimes known as Haverhill Fever when contracted via contaminated food or water.
Symptoms of Rate Bite Fever: The symptoms of streptobaccilliary RBF include: fever, vomiting, headache, muscle pain, joint pain, rash. Spinillary RBF presents with fever ( which may be remittent, i.e. occurring over and over; development of an ulcer at the site of a bite wound when that is the form of transmission;, swelling near the site of the rat bite; swollen lymph nodes and rash ( following partial healing of the original bite wound). Haverhill fever (contaminated food/water) has somewhat different symptoms including more severe vomiting than the other two forms and a sore throat.
Streptobacillary RBF usually presents within 3-10 days after contact with the infected rodent, but can take up to three weeks show symptoms, long after the initial bite or scratch has healed. Spinillary RBF has a somewhat longer incubation period, from 7-21 days. All of these forms of Rat Bite Fever can be serious and sometimes fatal without treatment.
• Salmonellosis is an infection carried by both rats and mice. It is caused by the salmonella bacteria and occurs worldwide. It is transmitted via the consumption of food and water contaminated by the droppings of the carrier rodents.
Salmonellosis symptoms include: diarrhea, fever, and abdominal cramps 12 to 72 hours post initial infection. Duration of the illness is usually 4 to 7 days, and most persons who contract the infection recover without treatment. Sometimes however the diarrhea may be so severe that hospitalization may be required, as the Salmonella infection may spread from the intestines to the blood stream, and then to other body sites. Death occurs in these cases if the illness is not treated promptly with antibiotics. The elderly, infants, and those with compromised immune systems are more susceptible to the severe form of the illness.
• Hantavirus Pulmonary Syndrome: spread primarily by deer mice and other mice species, rats can be directly infected by this disease. Humans usually fall ill with the disease after being exposed to and inhaling dust that is contaminated by urine and feces. Direct contact with an infected animal is another means of exposure; rarely the disease is passed on through a bite from an affected animal.
Hantavirus Symptoms include: fatigue, fever and muscle aches, especially in the large muscle groups—thighs, hips, back, and sometimes shoulders; these are universal and occur early in the disease’s progression. Secondary symptoms which often present include: headaches, dizziness, chills, and abdominal problems, such as nausea, vomiting, diarrhea, and abdominal pain. About half of all HPS patients experience these symptoms. Late symptoms which present 10 days after exposure include fluid in the lungs, coughing and inability to draw sufficient breath. HPS has a 38% fatality rate.
• Hemorrhagic Fever with Renal Syndrome (HFRS): is spread by brown rat, field mice and bank voles. It is caused by the viruses of the Bunyaviridae family of Hantaviruses. Similarly to the American strain of Hanta virus it is most often contracted through inhalation of dust contaminated by the urine and feces of the animals, direct handling of a diseased animal ,and rarely through bites. This disease can also be transmitted from one person to another, although this is a rare occurrence. It is not a disease generally known in the new world countries; it is most frequently reported in eastern Asia, Russia, Korea, Scandinavia, western Europe, and the Balkans.
HRFS symptoms include: sudden onset of intense headaches, back and abdominal pain, fever, chills, nausea, and blurred vision. Flushing of the face, inflammation or redness of the eyes, or a rash may also occur. Late occurring symptoms are low blood pressure, acute shock, vascular leakage, and acute kidney failure, which can cause severe fluid overload in the body. Symptoms of HFRS usually develop within 1 to 2 weeks after exposure to infectious material, but in rare cases, they may take up to 8 weeks to develop. The disease’s severity depends on the particular virus within the Bunyaviridae family that is causing the infection. Severe symptoms are usually caused by the Hantaan and Dobrava viruses, more moderate symptoms are common in infections originating with the Seoul, Saaremaa, and Puumala virus. Complete recovery can take weeks or months.
Rat disease in pets
Obviously pet rodents are capable of transmitting and being infected by the same diseases as their wild counterparts; reputable breeders and pet stores will have their breeding stocks tested against the most common rat diseases. But what about cats and dogs? Can they be sickened by some of these same rat borne pathogens? Or are there others?While the CDC details rat diseases in humans, their causes, symptoms and treatments, what about pets?
Two diseases transmissible to cats are tapeworms and Leptospirosis.
Cats that are allowed outside will invariably hunt small animals and birds. When a cat eats a wild rodent, it is most at risk from tapeworms (Taenia, Diplopylidium, Echinococcus, Hymenolepis and Spirometra). Most cats carry these parasites and exhibit no symptoms of illness from them; however high worm populations in a cat’s gut can result in weight loss, distension of the abdomen and discomfort. Some of the tapeworm species are transmittable to humans, and can be passed along from cat to owner.
Cats have a natural resistance to the Leptospirosis bacteria which is endemic in rats and other rodents. Ingestion of an infected animal is not likely to sicken the cat, but again, the bacteria can be carried by the cat and transmitted to humans. Dogs are susceptible to Leptospirosis as well and just like humans, canines at great risk of kidney or liver disease and failure as a result of exposure to the bacteria.
Toxoplasmosis is another parasite that does not materially sicken the cats that carry the organism, but which presents risk to humans, especially pregnant women who are more likely to miscarry if exposed.
Toxoplasmas are only able to reproduce in the digestive tract of cats and are excreted in feline feces. Here the organism contaminates soil and rodents become infected. Recent studies have shown that the Toxoplasma damage the brains of rats and mice, making them attracted to cat urine—which they normally fear and avoid. This in turn results in mice and rats that have lowered fear of their natural enemy, and slower responses, making them easier prey. The organism thus completes its life cycle when a cat easily catches and eats the brain addled rodents. This effect is unique for being cat urine specific, affected rats do not lose their general ability to fear other dangers.
Cats are resistant to most rat and rodent borne diseases, evolution having selected for this resistance in the eons that cats have been hunting and eating these animals as prey. Owners of pet cats should have them regularly tested for worms and given worm treatments if they are allowed outside to hunt.
Cats kept indoors are usually not exposed to rodent borne parasites and diseases; especially if the house they live in has been rat proofed or rodent populations are kept under control. Any cat which is allowed to catch prey should be taken to a vet if any signs of illness present, for the safety of the cat as well as the humans and dogs in the household.
As already noted, dogs are at particular risk from Leptospirosis. Dogs that hunt and kill rats may be directly affected by the bacteria, or may contract the disease via a cat carrier in the same household. Dogs can also pick up the bacteria by merely biting a rat, through rat urine found in puddles and from the ground. In dogs Leptospirosis presents with high fever, severe thirst, bloody diarrhea, vomiting and abdominal discomfort. Death can occur from infection induced kidney and liver damage. Humans can contract the bacteria from their pet dogs, resulting in Weil’s diseases.
There are at least five other rat borne diseases that dogs are susceptible to and which they can, in turn infect their human caretakers with. These are:
• Rat Bite Fever (streptobacillus moniliformis form). Dogs can be infected by the bacteria by biting or eating infected rats. Dogs do not themselves get sick, but serve as carriers for the infection, passing it along to humans.
• Tularemia: caused by the Francisella tularensis bacterium. Rabbits, hares and rodents, including rats, are particularly susceptible to the organism, and often die in large numbers during an outbreak. If a dog eats a rat (or other animal) infected by this organism or drinks water contaminated by infected rodent waste, the dog can become ill as well as passing the disease on to humans. Symptoms in dogs include refusal to eat, fever, cough, vomiting, and diarrhea. If not caught and treated, it can lead to death. In humans the symptoms are specific to the mode of infection (deer fly, ticks, contaminated water, handling dead infected animals, etc.) but can include: chills, eye irritation, fever, headache, joint stiffness, muscle pain, ulcers on skin, shortness of breath, sweating and weight loss.
• Bubonic plague: dogs can become carriers of the Yersinia pestis bacterium if they are bitten by the flea of an infected rat. Dogs do not tend to be sickened by bubonic plague but can pass the disease on to human caretakers.
• Salmonellosis can be transmitted to a dog that encounters the fecal material of rats and other rodents. Symptoms in canines include: fever, diarrhea, dehydration and shock. Death can occur if untreated, and the dog can spread the infection to human through its feces.
• Rabies: the rabies virus is considered the type species of the Lyssavirus genus. All warm blooded animals can be infected by and become carriers of the rabies virus, rats are not excluded. Rabies is usually contracted by dogs that are bitten by rats. The symptoms of rabies in dogs include paralysis of legs, abrupt and radical changes in behavior, avoidance of water, light and noise. Death occurs within ten days. Dogs can transmit rabies to humans in their saliva.
While there are many rat diseases that pose a risk to humans and their household pets, a few common sense measures can greatly reduce the risk of exposure to both. Rat proofing a home, keeping areas clear of debris, not allowing cats to free range outdoors, vaccinating household pets against rabies, careful handling of cat and dog waste, frequent washing of hands, wearing filtered masks when cleaning out dusty attics and sheds are but a few of the preventative measures that can be undertaken.
Most of the diseases described herein are not easily caught by humans if sanitary measures are in place and caution is observed; so panic is not warranted. Being forewarned of the diseases and the risks of their transmission is the best armor against their danger.