What is rickettsia (tick bite fever)?
Rickettsia (tick bite fever) is caused by a bacterial infection transmitted by ticks. This condition occurs in many areas of the world and is often known by a variety of names (see table below). This article will describe some of the general features of tick bite fever.
What causes tick bite fever?
The organism that causes tick bite fever belongs to the Rickettsial family of bacteria. As can be seen from the table, there are a number of different species of Rickettsias.
These organisms are relatively small – about 2/1000ths of a millimetre long – and are only able to survive inside cells. They are found in certain wild and domestic animals, and ticks acquire the organisms when they feed on these animals.
When the tick bites a human, the bacterium is transmitted in the saliva. The bacteria can also be transmitted from ticks to their offspring when still in the egg stage i.e. the Rickettsiae infect the eggs of the tick and thus infect the offspring.
In various parts of the world, different species of tick and Rickettsia are involved in causing tick-bite fever, and these forms of the disease are also given different names (see table).
|Rocky Mountain spotted fever ||R. ricketsii||USA || |
|Boutonneuse ||R. conorii||Africa, Meditteranean, India||Marseilles fever, Mediterranean spotted fever, African tick bite fever.|
|Queensland tick typhus||R.australis||Australia|| |
|North Asian tick typhus ||R. sibirica||Siberia, Mongolia || |
In South Africa, the cause of tick bite fever is either R. conorii (as in the table), or R. africae.
How do you contract tick bite fever?As mentioned above, the organisms are transmitted in the saliva of an infected tick when it bites humans. Alternatively, if the tick is crushed on your skin, the rickettsias may be able to enter through a small abrasion.
Being bitten by ticks usually occurs in rural or wilderness areas i.e. when you are out camping, hiking in long grass etc. The ticks that are able to harbour the organism belong to either the Amblyomma, Dermacentor or the Rhipicephalus family of ticks. The Amblyomma ticks actively seek out humans to feed on, while Rhipicephalus ticks tend to lie in wait on grass, and will bite you if you walk past.
Symptoms and signs of tick bite feverIf you get bitten by an infected tick, the incubation period (the period between being infected and displaying symptoms) is about five to seven days. Symptoms can vary, depending partly on the organism involved. Your age and underlying health may also influence the severity of the infection.
Typical features may include the presence of a black mark where the bite occurred, and fever, severe headache and a rash. The black mark at the site of the tick bite is called an eschar (or a tache noir), and looks like a small ulcer (2-5mm in diameter) with a black centre. It may look something like a spider bite. The eschars can be single or multiple and can sometimes be very difficult to find. The eschar usually appears once the fever appears, as does the headache and malaise (general feeling of ill-health). Lymph nodes near the eschar may be enlarged.
A rash is usually, but not always, a feature of tick bite fever (it is supposedly less likely to occur in someone infected by R. africae), but when it is present, it consists of small red marks on the skin, sometimes raised slightly above the surface. It typically starts on the limbs and spreads to the trunk, and can involve the entire body, including the palms of the hands and soles of the feet.
What is the outcome of tick bite fever?African tick bite fever is usually mild, and death and serious complications are very uncommon. This is in contrast to Rocky Mountain spotted fever, which is usually a more severe illness. Complications include encephalitis (inflammation of the brain), pneumonia and damage to the brain and heart.
How is tick bite fever diagnosed?Your doctor will examine you and question you about your symptoms and whether you could have had recent exposure to ticks. The typical scenario would be that you had been hiking or camping in a rural or wilderness area, and were therefore in contact with ticks. About a week later, you would have experienced a severe headache, fever and generally felt poorly. You may also have a rash and/or an eschar, and enlarged lymph nodes near the eschar.
The presence of the rash and an eschar is a very strong diagnostic sign for tick bite fever. Some other conditions that may be confused with tick bite fever are infection with Neisseria meningitidis (the meningococcus, which causes meningitis and a skin rash), measles and German measles. The pattern of the rash is different in measles and German measles, and the rash in meningococcal infection tends to occur more rapidly, and looks like severe bruises rather than small red marks.
Because the organism lives inside cells, it is very difficult to culture it in a laboratory, and this isn’t routinely offered as a diagnostic test. However, your body makes antibodies (proteins that attack invasive substances or organisms) to the bacteria, and these can be looked for in the laboratory. These serological tests are the main method of confirming a diagnosis of tick bite fever. However, sometimes the tests only become positive after a couple of weeks, so while they may help to confirm a diagnosis, they are not always immediately helpful early on in the infection. If you have typical symptoms of the infection, and a history of possible exposure to ticks, your doctor may decide to treat you before results of the serological tests are known.
How is tick bite fever treated?Some forms of tick bite fever are fairly mild and self-limiting – people may get better on their own without specific treatment. This can take up to two weeks however, and treatment with an antibiotic can shorten the duration of symptoms and reduce the chance of a serious side-effect. In severe cases, antibiotic therapy is more important, and can be life saving. The antibiotic doxycycline is the preferred agent for treating tick bite fever. Some people are not able to take doxcycline, in which case chloramphenicol, or sometimes ciprofloxacin, may be used instead.
Experience with using ciprofloxacin is more limited than with doxycycline or chloramphenicol. In addition, the use of antibiotics like clarithromycin and azithromycin for treating tick bite fever is being studied. These agents may be especially useful for treating pregnant women with tick bite fever.
Can tick bite fever be prevented?
The easiest may to prevent tick bite fever is to avoid being bitten by ticks. Avoiding rural or wilderness areas where ticks are likely to occur is one way to achieve this, but not a great solution if you enjoy hiking and camping. Other measures are generally common-sense, such as wearing insect repellents and long trousers and sleeves. There is no vaccine against tick bite fever, and taking prophylactic antibiotics (as one does for malaria) has never been shown to be effective or necessary.
When to call the doctorThings that should make you suspicious of tick bite fever would be potential exposure to ticks (either when hiking or camping, or you think you’ve been bitten by a tick imported by your favourite pet!) along with fever, headache, a rash and a black ulcer-like lesion (the eschar). In this sort of rather obvious scenario, it is advisable to see your doctor. Unfortunately, the eschar is not always visible, so the absence of an eschar shouldn’t lull you into a false sense of security. Another problem is that the symptoms of fever, headache and a rash are very common (many viral infections, for example, can cause this).
In these less obvious situations, the decision of whether or not to consult a doctor depends to some degree on how ill you feel, and what your underlying state of health is (your age, any other health problems, any regular medication etc). As with most illness, some common-sense is required, but if you are in any doubt and you are feeling unwell, it would be best to consult your doctor.
(Written by Dr Andrew Whitelaw, University of Cape Town and Groote Schuur Hospital)