Tuesday, June 07, 2005

Epidemiology of rat bites

Lindsay Beyerstein of Majikthise fame led me to an extremely interesting site awhile back and I have been browsing it ever since. It is the Rat Behavior and Biology page ("Anne's Rat Page") and it is full of interesting information about these animals that are dependent upon and constant companions of humans. They are amazing, resourceful and in many ways mysterious animals. They are also feared, loathed and occasionally dangerous. Since this is a public health blog, it is the health aspects that interest me particularly. Hence this little post on the epidemiology of wild rat bites.

The kind of epidemiology we will discuss briefly here is called descriptive epidemiology, essentially the description of a disease, condition or trait by characteristics of "person, place and time" (who gets what, where, and when). You'd think this would be pretty straightforward, but alas, there is little known about the epidemiology of wild rat bites. If you think about it you can see the information might be difficult to obtain. While reporting animal bites is often legally required, only a tiny fraction are actually reported. We have to rely on special purpose surveys, of which there have been very few. Anne's Rat Page summarizes some of the information here:
Men vs. women
Women were slightly more likely to be bitten than men (51.5% women vs. 48.5% men, Childs et al. 1998; 58% women vs. 42% men Ordog et al. 1985).

The average age of bite patients tended to be relatively young: Ordog et al. (1985) found that the average age of a bite patient was10.8 years, with a range of 5 months to 42 years. The majority (74%) of bite patients were under 15, while 45% percent were under five years old.

Childs et al. (1998) found an older median age of 22 years with a range of less than 1 to 93 years.

Infirmity and weakness
Ninety percent of Ordog's rat-bite patients were either children, or had a physical or mental disability such as diabetes, a psychiatric disorder, intoxication, or a prior wound (Ordog et al. 1985).

Where on the body do rats bite?
Most bites were to extremities. Ordog et al. (1985) found that 70% percent of rat bites were to the upper extremeties: the arm, wrist, hand, or finger. Eighteen percent were to the lower extremities of leg, thigh, or buttocks. The remaining 14% percent of the bites were to the face. Most of these bites were to areas of the body exposed while the patient was asleep.

Childs et al. (1998) found that 59.8% of bites were to the upper extremities of arm, wrist, hand or finger. Twenty-eight percent were to the lower extremities of leg, foot, or toe, while 9.3% were to the head, face or neck. The remaining 2.4% were to the body and trunk.

Bitten person's location and activity
All of Ordog's (1985) rat bites occurred in the patients' own homes. Most people were bitten at night while they are asleep (72%, Ordog et al. 1985; 54.6% Childs et al. 1998). One bite occurred when a patient tried to feed a wild rat by hand (Ordog et al. 1985)

Type of wound
Sixty-one percent of the bites were single or multiple punctures. Fourteen percent were lacerations (all less than < style="font-weight: bold;">Treatment and infection rate

Most rat bites were not severe. Childs et al. (1998) found that most bites were simply washed and 98% of 514 patients were immediately released. Ordog et al. (1985) examined 50 rat bite patients, and only one patient (2%) developed a bacterial infection that required antibiotics.

When bite wounds were swabbed, only 30% of the wounds had a positive bacterial culture. Of these, 43% were Staphylococcus epidermidis. The remainder were Bacillus subtillus, diphtheroids, and alpha hemolytic streptococcus (Ordog et al. 1985).

(Anne's references can be found here)
The most recent paper I could find (and not contained in Anne's reference list) was by Hirschhorn and Hodge, "Identification of risk factors in rat bite incidents involving humans" in the journal Pediatrics, volume 104, 1999. If you are interested, you can download a .pdf for free here.

Using reports filed under the Philadelphia health code between 1976 and 1997, Hirschhorn and Hodge provide a picture of reported cases in an older northeast urban environment. 622 bites were reported and confirmed in this period, with the incidence decreasing in later years:
Thus, the victim profile found in the earlier years (1974–1984) of the Philadelphia study closely follows the patterns found in the New York City study and the Baltimore city studies mentioned previously. Typically, the victim of a rat bite in Philadelphia is most commonly a child of either gender, ve not changed over the 22-year span of this study. Although most rat bite victims are black, when ethnic population is taken into consideration, Hispanics have a four times higher incidence rate than do blacks. Still, one fact that has not changed over the 22-year span is that poor younger children are bitten by rats more often than are adults.
So if you get bitten (or more likely your child gets bitten) the wound is unlikely to get infected (2% in Ordog's series). What about other diseases? One frequently mentioned and feared disease you don't have to worry about is rabies. No documented cases ever in the US and hardly any anywhere. Plague? No problem. Plague kills rats (humans are infected by rat fleas that jump from the dead and dying hosts). But there are a couple of other things, including rat bite fever (infection with Streptobacillus moniliformis), a febrile, sometimes fatal illness. Rare, but it happens. If you are so inclined, you can read about it here.

Rat bites are a disease of poverty. They are fairly common (second most common reported animal bite after dog bites). Maybe as problems go, this isn't at the very top of the list. But it is something that shouldn't happen in a decent society. Make of that what you will.