Back To CourseBiology 103: Microbiology
16 chapters | 156 lessons | 12 flashcard sets
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Angela has taught college Microbiology and has a doctoral degree in Microbiology.
If you live in Arizona, particularly northern Arizona, you might have noticed an awful lot of dogs running around. Feral dogs seem to be on the rise not just in Arizona but in many states and countries. What makes the dogs in Arizona noteworthy is not their presence but what they have been bringing with them: ticks. I'm sure it's not a surprise to learn that feral dogs have ticks. What makes the ticks on the feral dogs in Arizona noteworthy is not their presence but what they have been bringing with them: the bacteria Rickettsia rickettsii. This parasite is capable of causing a deadly disease called Rocky Mountain spotted fever. As the dog population has increased, the tick population has increased, and with that has come an increase in the number of Arizonians stricken with Rocky Mountain spotted fever.
Before we dive into Rocky Mountain spotted fever, let's take a quick look at spotted fevers in general. Spotted fever is the name given to a wide range of similar human infections caused by bacteria in the genus Rickettsia, acquired through the bite of an infected tick. Each specific spotted fever is caused by a specific species of Rickettsia and transmitted by a specific tick species, but all cause similar symptoms. Which spotted fever you get depends on where you're bitten, geographically. Nearly every country or continent has a different dominant Rickettsial species. Africa has Rickettsiosis, caused by R. aeschlimannii. Australia has Flinders Island spotted fever caused by R. honei. Ticks in the United States carry Rickettsia rickettsii, which causes Rocky Mountain spotted fever. For the rest of this lesson, we will focus on this specific, and most deadly, spotted fever.
Rickettsia rickettsii is an obligate, intracellular bacterial parasite that causes Rocky Mountain spotted fever. An obligate, intracellular parasite is one that must invade a host cell to survive and reproduce. In the case of R. rickettsii, the bacteria invade the cells that line the blood vessels throughout the host's body. This invasion causes the symptoms of the disease, but we'll get to those in a minute.
In addition to living in the cells lining the blood vessels, R. rickettsii can also be found in the salivary glands and gut of several species of North American ticks, specifically the American dog tick (Dermacentor variabilis), Rocky Mountain wood tick (Dermacentor andersoni), and brown dog tick (Rhipicephalus sanguineus). When an infected tick bites a human host, it injects a cocktail of chemicals to reduce pain, prevent blood clotting, and increase blood flow. But it can also inject any number of parasitic organisms, like Rickettsia. Once in the blood stream, the cellular invasion can begin!
It usually takes between 2 and 14 days after the tick bite for symptoms of Rocky Mountain spotted fever to develop. During this incubation time, the bacteria are invading and damaging the cells that line the blood vessels. This invasion causes inflammation of the vessels, bleeding, blood clots, and a massive immune response. All of these internal changes cause a very sudden onset of fever and severe headache. Many patients also complain of abdominal pain, vomiting, muscle pain, eye redness, and a characteristic red rash. These symptoms can be very vague and variable between patients.
Let's spend a few minutes discussing the characteristic rash commonly associated with Rocky Mountain spotted fever. Around 90% of patients develop small, flat, pink spots on the extremities several days after the tick bite. Sometimes these spots spread to the entire body. The rash develops as the bacteria damage and rupture the capillaries close to the skin. This rash can be a very important tool in diagnosing the disease and assessing severity. About six days after the initial red rash develops, it can darken to a purple color, signifying a worsening of the disease. It is at this point that Rocky Mountain spotted fever transitions from an easily treatable infection to a potentially deadly disease. As the damage to the vessels and capillaries spreads and increases in severity, more and more bleeding and blood clots occur. Clots that form in the extremities can completely block vessels, leading to amputations. Clots in the heart or brain can lead to life-threatening heart attacks and strokes. Without treatment, these complications will kill about 20% of those infected, many within 8 days of the onset of disease.
Luckily, the fever and headaches appear so quickly and so severely that most people will see their doctor before too many days have passed. In most cases, diagnosis of Rocky Mountain spotted fever is based on these symptoms combined with a recent tick exposure and the presence of the rash. The downside is that many tick-borne diseases have similar symptoms, all of which can be vague, highly variable, and flu-like. Misdiagnosis is a common problem but not a serious one. Fortunately, the same treatment is used for most tick-borne diseases. Doctors usually begin treatment without a positive diagnosis in order to head off the disease before it progresses to the more life-threatening stage.
This should not be the end of the diagnosis, though. More in-depth lab tests like blood vessel biopsies and antibody detection blood tests need to be performed to ensure the cause of the symptoms is, in fact, Rickettsia rickettsii. Failure to get a definitive diagnosis can lead to incorrect treatment and long-term health impacts.
With or without a positive diagnosis, the treatment of choice for all patients is the antibiotic doxycycline. Treatment is most effective if started within the first five days of symptoms, and most patients recover fully with no lingering effects.
Rocky Mountain spotted fever was first identified in the western United States in the early 1900s. The location, combined with the spotty rash, helped name the disease. But names are not always appropriate. The vast majority of cases of Rocky Mountain spotted fever occur in the southeastern United States, not the Rocky Mountain region. Today there is an average of 2,000 cases every year, with a 1% fatality rate. This fatality rate has been steadily dropping over the last few decades from a peak of 30%. Increased doctor awareness, better diagnostic tools, and better treatments are responsible, helping to save countless lives.
It's time to review.
Spotted fever is the name given to a wide range of similar human infections caused by bacteria in the genus Rickettsia acquired through the bite of an infected tick. In the United States, the most common, and deadliest, spotted fever is Rocky Mountain spotted fever. People catch the disease through the bite of an infected tick. Tick bites transmit the bacteria Rickettsia rickettsii into the blood stream, where the bacteria invade the cells that line the vessels. This invasion causes inflammation, bleeding, blood clots, and a massive host immune response. What results is the sudden onset of a fever and severe headache. Other symptoms can occur, but they are usually vague, variable, and flu-like.
Ninety percent of patients develop a characteristic spotty red rash on their extremities. This rash can be used to help diagnose the disease. The headache, fever and rash, combined with a recent tick exposure, are all that is needed to make an initial diagnosis and begin treatment. The antibiotic doxycycline is the treatment of choice and is most effective if administered within the first five days. If the disease is allowed to progress untreated for more than 8 days it can be fatal. The unchecked vessel damage can lead to blood clots blocking vessels in the extremities, heart and brain.
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Back To CourseBiology 103: Microbiology
16 chapters | 156 lessons | 12 flashcard sets