We’ve covered a lot of diseases over the past few years, but what we haven’t covered much of are the vectors that carry them. A few months ago, we looked at bats. Now, we bring you… rats!
It is hard to find anything good to say about rats. For one thing, they eat garbage. You find them in stinky sewers, musty pipes, attic spaces, and crawling around underneath the third rail of the subway. Rats are often thought of as disgusting, dirty, and creepy; and yet, they are commensal. This means they live right alongside us, above us, and around us. They eat our food, gnaw on our walls, and leave droppings and urine markings in our basements. The Allen County Department of Health in Indiana says, "The word 'commensal' means these rodents live off humans without returning anything of worth", and "we all agree that commensal rodents are good for nothing" (3). Harsh words.
Though it may seem like rats take but never give, sadly this isn’t the case. Rats do give us something. The CDC says that worldwide, rats and mice spread over 35 diseases. These include leptospirosis, salmonellosis, rat-bite fever, murine typhus, and the dreaded plague (2). Rats spread diseases directly and indirectly. An example of direct transmission is the rat’s habit of “urine marking.” Rats do this for two main reasons: sexual advertisement and habitat labeling. They like whatever smells familiar, so they mark everything, including each other, the pathway back to their nests, and their food, with urine. This leaves tiny tell-tale droplets all over surfaces. Bacteria, such as the spirochete Leptospira, accumulate in the kidneys and are shed in urine. When bacteria enter the human body through a cut or mucous membrane, infections can occur. Of the eight diseases that the CDC lists as directly transmitted by rats, five are spread by contact with contaminated urine: Hantavirus Pulmonary Syndrome, Hemorrhagic Fever with Renal Syndrome, Lassa Fever, leptospirosis, and South American Arenaviruses (2).
Rats can also directly spread disease through bites. Rat-bite fever (RBF), caused by the bacteria Streptobacillus moniliformis in North America and Spirilium minus in parts of Asia, is one of the diseases carried and transmitted by rats. The number of rats carrying RBF-causing bacteria varies greatly: recent reports indicate that between 50 and 100 percent of wild rats carry it in some countries and between 10 and 100 percent of lab rats carry the bacteria (6). Humans are infected mostly through bites, but also from contact with contaminated urine. This disease is more common in Asia, and in Japan is known as Sodoku. According to The Humane Society of the United States, more than 15,000 rat bites are reported each year (7). The total number of bites may be under-reported, but they are not as frequent as one might think. A study by Hirshborn and Hodge (4) found that the incidence of rat bites in Philadelphia was a mere 2.12 bites per 100,000 people per year from 1974 through 1984, and only 1.39 per 100,000 people per year from 1985 through 1996. Because wild rats live in such close proximity to humans, bites may occur at night when fingers and toes are exposed. Most rat bite victims are children. Though the idea of being bitten by a rat is scary, there are relatively few cases of rat bite fever reported in the literature. For example, as of 2004, only 200 cases had been documented in the United States. (6).
Rats also spread diseases indirectly, by acting as hosts for parasitic vectors like mites, ticks, and fleas. Plague and murine typhus are both transmitted through the Oriental rat flea, Xenopsylla cheopis. Rats do not carry the plague; when they are bitten by fleas carrying the Yersinia pestis bacteria they die just like we do. The fleas, however, escape the dying rat, and are forced to find other hosts, which might include humans who previously (and possibly unwittingly) provided food and shelter for the rats. This close association provides means for transmission of plague, which has erupted many times in human history: Europe in the 1300s, London in the 1600s, France in the 1700s, and Central Asia in the 1800s. Modern outbreaks still occur, most recently in Madagascar, where more than 30 lives have already been lost (5).
Murine typhus, caused by Rickettsia typhi bacteria, is another illness that is transmitted by the Oriental rat flea. The word "murine" means caused or transmitted by a rodent of the family Muridae or subfamily Murinae, including rats and mice. Murine typhus is also called endemic typhus, and is different than epidemic typhus (which is the disease we usually mean when we refer to typhus). Murine typhus tends to be less severe than epidemic typhus, and occurs rarely in the United States. According to the California Department of Public Health, "in the United States there are about 200 cases of murine typhus reported every year. Most of the cases occur in Texas, California, and Hawaii" (1).
To sum it up in the words of the CDC: "Rodents destroy property, spread disease, compete for human food sources, and are aesthetically displeasing" (2). They are clearly not one of the most beloved animals on Earth. But they are one of the most ubiquitous, at least in places where people also dwell. As long as we have garbage to get rid of, we will not soon get rid of them. Rats consume untold tons of our refuse every year, leaving us with less waste to haul away and toss into landfills. Some estimate that there are as many as one rat for every human in big cities like New York. That may be too many rats to think about, but let's say there are roughly 18 million roaming the New York City area. What follows may be a conservative estimate, since rats also hoard food, perhaps as much as four times the amount they actually eat. But if rats can eat one-third of their body weight in a day, and each rat weighs a pound---well, that is a lot of garbage. Maybe that is the best thing we can say about rats.
1) California Deprtment of Public Health. "Murine Typhus (Endemic Typhus)." Dec 2012. [Retreived December 22, 2013]. From http://www.cdph.ca.gov/HealthInfo/discond/Documents/MurineTyphus.pdf
2) Centers for Disease Control and Prevention. (n.d.). [Retrieved 15 Dec. 2013]. From http://www.cdc.gov/rodents/index.html
3) Fort Wayne/Allen Country Department of Health. (n.d.). [Retrieved 15 Dec. 2013]. From http://www.allencountyhealth.com/divisions/vector/?ID=articles1228767614
4) Hirschborn, R.B. and R.R. Hodge. 1999. Identification of Risk Factors in Rat Bite Incidents Involving Humans. PEDIATRICS Vol. 104 No. 3, p. e35
5) Rabary, Lovasoa. “Bubonic plague claims 32 lives in Madagascar.” (20 Dec. 2013). [Retrieved 20 Dec. 2013]. From http://abcnews.go.com/International/wireStory/bubonic-plague-claims-32-l...
6) Spickler, Anna Rovid. "Rat-Bite Fever Factsheet." (Last Updated August 2013.) [Retrieved 6 Jan. 2013]. From http://www.cfsph.iastate.edu/DiseaseInfo/factsheets.php
7) The Humane Society of the United States. 4 Oct. 2009. [Retrieved 24 Dec. 2013]. From http://www.humanesociety.org/animals/rats/tips/solving_problems_rats.html