Black Rats, Brown Rats, and the Plague

May 31, 2012

I've been spending a lot of time thinking about rats. Thankfully, it is not because I have a problem in my apartment! Unfortunately, for many people in L.A., rats are a serious pest, and it's not just one type of rat. The most serious rodent offenders in our cities are the brown (aka Norway) rat, Rattus norvegicus, and the black rat, Rattus rattus.  

What species of rat is this? Here on the North Campus we have camera trap images and footage of rats hanging out underneath the bridge. But what type of rat is this? Since Jim Dines, our Mammalogy Collections Manager, wasn't available, I decided to try and figure it out myself. Doing a Wikipedia search for brown rats, I came across a nice diagram that helped me to make an identification. What species do you think it is?    

    Comparison of the physique of a black rat, Rattus rattus,  with a brown rat, Rattus norvegicus, from wikipedia   Using this diagram I looked closely at the ears and the eyes of the rat. Based on the relatively large size of the ears and the eyes, I determined the above image was of a black rat rather than a brown rat. I showed the image to Jim and he confirmed that it was indeed a black rat!        

      Black rat (top) and brown rat (bottom) from the Museum collection. Note the tail to body length ratio.   Regardless of the species, why do people hate rats? As I referenced in the introductory paragraph, rats are sometimes pests in our homes, but what exactly do people think about rats? I did a Google search for, "why do people hate rats," and this is what I found. "Cute or not they're germ-ridden disease carrying vermin who in addition, can cause untold damage. THAT'S WHY." I also found this: "Rats are pests to humanity, but I personally believe that people especially hate rats because they subconsciously identify with them and see them as a reminder of themselves." Finally, someone else wrote: "A lot of rat-hatred goes back as far as plague. Rats were responsible for the disease that killed thousands of people." But, is plague a worry for us today in L.A.? Not really here in the city (I hear a collective YAY)! Firstly, according to the Centers for Disease Control and Prevention the last cases of urban, rat-associated plague occurred in L.A. in the 1920s. However, plague is present in L.A. County, and the principal mode of infection is from infected fleas living on wild rodents in rural areas. These rodents include California ground squirrels (remember the recent blog post?) and chipmunks! According to L.A. public health officials, "the major threat of plague to humans is in the rural, recreational and, wilderness areas of the Angeles National Forest, as well as the Santa Monica and San Gabriel Mountains." But, before you swear off recreating in our lovely parks forever, know that there have only been four cases of the plague in L.A. since 1979, none of which were fatal (another YAY for antibiotics and modern medicine). P.S., Plague is actually caused by a bacteria, Yersinia pestis, that lives in the blood and other bodily fluids of fleas, rodents, and other mammals.  

(Posted by: Lila Higgins)


Nice post. BTW, i love rats. If people knew how smart sweet and sociable they were (talking about fancy rats as opposed to wild rats-and then who knows?-they might think differently. As far as the plague, fleas were the carriers of the disease and spread it to rats. You don't see people jumping on a chair when they see a flea, though....

That would probably be because fleas don't usually carry bacteria our body can't handle. A flea needs to bite an infected animal to ingest the bacteria, and animals today don't usually carry human threatening bacteria. When a wild rat comes into play, we can't be certain what it has come across, and if we avoid it, we also avoid the fleas and whatever they carry. On the other hand, fancy rats rarely have the chance to come in contact with infected rodent.. People should realize that as long as it's not wild, the only danger is Rat Bite.

I shoot every rat I see. 69 was the count in 2011.

While I find this interesting I have a question that although does not relate to humans' fear of rats does refer to their association with the Black Death. My history books claim that it was the black rat that spread the bubonic plague, but my home encyclopedias say it was the brown rat. Wikipedia goes on to say that it was the black rat that spread the disease with its fleas, but was reduced when the larger brown rat drove most black rat populations out of urban areas - taking the fleas with them. Which is true? And why can't the rat fleas infect both if they can also infect squirrels regardless of the specific rodent species?

I would certainly like a clarification of this point also. Black or brown. which is it? And why?

The brown rat didn't spread the disease but stop it. The brown rat is very strong and immune to fleas. They really killed fleas.Brown rats are considered heroes in certain places because of it.

Brown rats don't carry plague. They are an incredibly tough, resourceful creature and are becoming immune to every known poison. They do not tolerate black rats and will kill them. They are also highly intelligent, and their problem solving abilities are truly amazing. This has made them one of the most successful animals on the planet. It's a blessing they do not carry plague, otherwise we'd probably all be dead now as brown rats are everywhere!

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