African tick bite fever
|African tick bite fever|
Leg lesion after Rickettsia africae infection
|Classification and external resources|
African tick bite fever (ATBF) is a tick-borne disease caused by Rickettsia africae in sub-Saharan Africa and the West Indies. The bacterium is transmitted to humans by ticks in the Amblyomma genus. African tick bite fever is a type of spotted fever. Symptoms, when present, typically occur within 4–10 days of transmission and can include a fever, headache, rash and inoculation eschars at bite sites. The clinical symptoms are typically mild and resolve within 3 weeks. Complications are exceedingly rare.
The diagnosis of African tick bite fever is often based on clinical signs and symptoms. Definitive diagnosis is made by culture, PCR, or immunofluorescence. Data about the appropriate treatment of ATBF is limited. Symptoms tend to be mild and resolve on their own. However, when people seek medical treatment, physicians may treat the infection similarly to other rickettsial infections, such as Rocky Mountain spotted fever.
Signs and symptoms
African tick bite fever is often asymptomatic or mild in clinical presentation and complications are rare. The onset of illness is typically 5–7 days after a tick bite though the range of symptom onset is 4–10 days. Symptoms can persist for several days to up to three weeks. Common presenting symptoms include:
- Muscle aches
- Inoculation eschar(s)
- Swollen lymph nodes near the site of the bite
- Maculopapular and/or vesicular rash
- Prolonged fever > 3 weeks in duration
- Reactive arthritis 
Rickettsia africae is a gram-negative, obligate intracellular, pleomorphic bacterium.
In Sub-Saharan Africa, the most common vectors of R. africae are two species of hard ticks, Amblyomma variegatum and Amblyomma hebraeum. Other species of Amblyomma in sub-Saharan Africa can also transmit R. africae and it may be that up to 100% of Amblyomma ticks in sub-Saharan Africa carry R. africae. Amblyomma ticks are most active from November to April. These tick species frequently feed on cattle and other livestock, but can also be found feeding on wild animals in areas where farm animals are not found. Unlike other hard tick species, which passively seek hosts, the Amblyomma hard ticks actively seek out hosts. These ticks are known to be more aggressive, and several ticks may attack at once, leading to clusters of African tick bite fever cases within groups of persons traveling or working together.
Up until 1998, it was thought that only ticks in sub-Saharan Africa carried R. africae. However, a case of locally transmitted African tick bite fever in the French West Indies led to the discovery of R. africae within Amblyomma variegatum ticks on Guadelupe Island. Now, R. africae has been isolated from ticks on several Caribbean islands, though the only cases in humans in the Caribbean have occurred in the French West Indies. Recently, R. africae has also been found in Amblyomma loculosum ticks in Oceania.
After the rickettsia bacteria infects humans through a tick bite, it invades endothelial cells in the circulatory system (veins, arteries, capillaries). The body then releases chemicals that cause inflammation, resulting in the characteristic symptoms. The hallmark of all rickettsial diseases is a histology (cellular) finding called lymphohistiocytic vasculitis that involves immune cell deposition into the cells that make up vessels. This occurs secondary to the chemicals mentioned above, and damage from the infection, and involves signals to immune cells (T cells and macrophages) to come to the site of the infection.
Rickettsia bacteria species like R. africae replicate around the area of the initial tick bite, causing necrosis (cell death) and lymph node inflammation. This is the cause of the characteristic eschar.
Many patients with ATBF who live in areas with a high number of infections (Africa and the West Indies) do not visit a doctor, as most patients only have mild symptoms. This disease can, however, cause more serious symptoms in travelers who have never been exposed to the Rickettsia africae bacterium before and are not immune. Travelers who present to a doctor after a trip to affected areas can be hard to diagnose, as many tropical diseases cause a fevers similar to that of ATBF. Other diseases that may look similar are malaria, dengue fever, tuberculosis, acute HIV and respiratory infections. In addition to questions about symptoms, doctors will ask patients for an accurate travel history and whether he/she was near animals or ticks. Microbiological tests are available for doctors, but are expensive and often must be done by special laboratories.
The antibiotic treatment available for rickettsiae infections has very few side affects, so if a doctor has a high suspicion of the disease, he or she may simply treat without doing more laboratory tests.
Diagnosis of ATBF is mostly through clinical signs and symptoms, as many laboratory tests are not specific to ATBF. Common laboratory test signs of ATBF are a low white blood cell count (lymphopenia) and low platelet count (thrombocytopenia), a high C-reactive protein, and mildly high liver function tests.
Biopsies or cultures of a patient's tick wound (eschar) are used to diagnose ATFB. However, this requires special culture media and can only be done by a laboratory with biohazard protection. There are more specialized laboratory tests available that use quantitative polymerase chain reactions (qPCR), but can only be done by laboratories with special equipment. Immunoflorescence assays can also be used, but are hard to interpret because of cross-reactions with other rickettsiae bacteria.
Treatment and prevention
African tick bite fever is usually mild, and most patients do not need more than at-home treatment with antibiotics for their illness. However, because so few patients with this infection visit a doctor, the best antibiotic choice, dose and length of treatment are not well known. Typically doctors treat this disease with antibiotics that have been used effectively for the treatment of other diseases caused by bacteria of similar species, such as Rocky Mountain Spotted Fever.
For mild cases, patients are usually treated with one of the following:
If patients have more severe symptoms, like a high fever or bad headache, the infection can be treated with doxycycline for a longer amount of time (100 mg orally 2 times daily for 7 days).Pregnant women should not use doxycycline or ciprofloxacin as both antibiotics can cause problems in fetuses. Josamycin has been used effectively for treatment of pregnant women with other rickettsial diseases, but it is unclear if it has a role in the treatment of ATBF.
Cases of African tick bite fever have been more frequently reported in the literature among international travelers. Data examining rates in local populations are limited. Among locals who live in endemic areas, exposure at a young age and mild symptoms or lack of symptoms, as well as decreased access to diagnostic tools, may lead to decreased diagnosis. In Zimbabwe, where R. africae is endemic, one study reported an estimated yearly incidence of 60-80 cases per 10,000 patients.
Looking at published data over the past 35 years, close to 200 confirmed cases of African tick bite fever in international travelers have been reported. The majority (~80%) of these cases occurred in travelers returning from South Africa.
- Boutonneuse fever
- Rocky Mountain spotted fever
- Flea-borne spotted fever
- List of cutaneous conditions
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