| African tick bite fever | |
|---|---|
 | |
| Leg lesion from aRickettsia africae infection | |
| Specialty | Infectious disease |
| Symptoms | Fever, headache, muscles pains, rash[1] |
| Complications | Rare (joint inflammation)[2][3] |
| Usual onset | 4–10 days after the bite[4] |
| Causes | Rickettsia africae spread by ticks[2] |
| Diagnostic method | Based on symptoms, confirmed byculture,PCR, orimmunofluorescence[3][2] |
| Prevention | Avoiding tick bites[1] |
| Medication | Doxycycline,chloramphenicol,azithromycin[2][3] |
| Prognosis | Good[2] |
| Frequency | Relatively common among travelers tosub-Saharan Africa[2] |
| Deaths | None reported[3] |
African tick bite fever (ATBF) is abacterial infection spread by the bite of atick.[1] Symptoms may include fever, headache, muscle pain, and a rash.[1] At the site of the bite there is typically a red skin sore with a dark center.[1] The onset of symptoms usually occurs 4–10 days after the bite.[4] Complications are rare but may includejoint inflammation.[2][3] Some people do not develop symptoms.[4]
Tick bite fever is caused by the bacteriumRickettsia africae.[2] The bacterium is spread byticks of theAmblyomma type.[2] These generally live in tall grass or bush rather than in cities.[2] The diagnosis is typically based on symptoms.[3] It can be confirmed byculture,PCR, orimmunofluorescence.[2]
There is novaccine.[1] Prevention is by avoiding tick bites by covering the skin, usingDEET, or usingpermethrin treated clothing.[1] Evidence regarding treatment, however, is limited.[2] Theantibioticdoxycycline appears useful.[2]Chloramphenicol orazithromycin may also be used.[2][3] The disease will also tend to resolve without treatment.[3]
The disease occurs insub-Saharan Africa, theWest Indies, andOceania.[1][5] It is relatively common among travelers tosub-Saharan Africa.[2] Most infections occur between November and April.[1] Outbreaks of the disease may occur.[3] The earliest descriptions of the condition are believed to be from 1911.[2] African tick bite fever is a type ofspotted fever.[5] It has previously been confused withMediterranean spotted fever.[2]
African tick bite fever is often asymptomatic or mild in clinical presentation and complications are rare.[6] The onset of illness is typically 5–7 days after the tick bite, although in some cases it may take up to 10 days for symptoms to occur.[7] Symptoms can persist for several days to up to three weeks.[6] Common presenting symptoms include:
Complications are rare and are not life-threatening.[7] No deaths due to African tick bite fever have been reported.[4] Reported complications include:
Rickettsia africae is a gram-negative, obligate intracellular,pleomorphic bacterium.[7] It belongs to the genusRickettsia, which includes many bacterial species that are transmitted to humans by arthropods.[9]
Two species of hard ticks,Amblyomma variegatum andAmblyomma hebraeum are the most common vectors ofR. africae.[10] Typically,A. hebraeum transmits the bacteria in South Africa whileA. variegatum carriesR. africae throughout West, Central and East Africa and through the French West Indies.[10] Other species ofAmblyomma in sub-Saharan Africa can also transmitR. africae and it may be that up to 100% ofAmblyomma ticks in sub-Saharan Africa carryR. africae.[8]Amblyomma ticks are most active from November to April.[1] 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.[7] Unlike other hard tick species, which passively seek hosts by clinging to plants and waiting for a potential host to brush by in passing, the Amblyomma hard ticks actively seek out hosts.[11]
Groups of tourists visiting Africa have returned to their own countries and were diagnosed there as having been infected.[12][13]
Up until 1998, it was thought that only ticks in sub-Saharan Africa carriedR. africae. However, a case of locally transmitted African tick bite fever in the French West Indies led to the discovery ofR. africae carried byAmblyomma varigatum ticks introduced through cattle shipped fromSenegal toGaudeluope more than a century ago.[10]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.[8]R. africae has also been found inAmblyomma loculosum ticks in Oceania.[4]
After the rickettsia bacteria infects humans through a tick bite, it invadesendothelial cells in the circulatory system (veins, arteries, capillaries).[14] The body then releases chemicals that causeinflammation, resulting in the characteristic symptoms like headache and fever. The hallmark of all rickettsial diseases is ahistology (cellular) finding called lymphohistiocytic vasculitis[15] that involves immune cell deposition into theendothelial cells that make up vessels.[7] This occurs secondary to the chemicals mentioned above, as well as damage from the infection, and involves signals to immune cells (T cells and macrophages) to come to the site of the infection.[16]
Rickettsia bacteria species likeR. africae replicate around the area of the initial tick bite, causingnecrosis (cell death) and lymph node inflammation.[14] This is the cause of the characteristiceschar.[14]
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.[6] This disease can, however, cause more serious symptoms in travelers who have never been exposed to theRickettsia africae bacterium before and are notimmune.[17] Travelers who present to a doctor after a trip to affected areas can be hard to diagnose, as manytropical diseases cause a fever similar to that of ATBF.[18] Other diseases that may look similar are malaria, dengue fever, tuberculosis, acute HIV and respiratory infections.[18] In addition to questions about symptoms, doctors will ask patients for an accurate travel history and whether he/she was near animals or ticks.[18] Microbiological tests are available for doctors, but are expensive and often must be done by special laboratories.[19]
The antibiotic treatment available for rickettsiae infections has very few side effects, so if a doctor has a high suspicion of the disease, he or she may simply treat without doing more laboratory tests.[3]
Diagnosis of ATBF is mostly based on symptoms, as many laboratory tests are not specific for ATBF. Common laboratory test signs of ATBF are a low white blood cell count (lymphopenia) and low platelet count (thrombocytopenia), a highC-reactive protein, and mildly high liver function tests.[19]
Biopsies or cultures of a person's tick wound (eschar) are used to diagnose ATBF. However, this requires specialculture media and can only be done by a laboratory withbiohazard protection.[19] There are more specialized laboratory tests available that use quantitativepolymerase chain reactions (qPCR), but can only be done by laboratories with special equipment.[19]Immunofluorescence assays can also be used, but are hard to interpret because of cross-reactions with other rickettsiae bacteria.[8]
Prevention of ATBF centers around protecting oneself from tick bites by wearing long pants and shirt, and using insecticides likeDEET on the skin.[7] Travelers to rural areas in Africa and the West Indies should be aware that they may come in contact with ATBF tick vectors.[7] Infection is more likely to occur in people who are traveling to rural areas or plan to spend time participating in outdoor activities. Extra caution should be taken in November - April, whenAmblyomma ticks are more active.[1] Inspection of the body, clothing, gear, and any pets after time outdoors can help to identify and remove ticks early.[1]
African tick bite fever is usually mild, and most patients do not need more than at-home treatment with antibiotics for their illness.[6] However, because so few patients with this infection visit a doctor, the best antibiotic choice, dose and length of treatment are not well known.[8] 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 asRocky Mountain Spotted Fever.[8]For mild cases, people are usually treated with one of the following:
If a person has more severe symptoms, like a high fever or serious headache, the infection can be treated with doxycycline for a longer amount of time.[7] Pregnant women should not use doxycycline or ciprofloxacin as both antibiotics can cause problems in fetuses.[20]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.[7]
Cases of African tick bite fever have been more frequently reported in the literature among international travelers.[7] Data examining rates in local populations are limited.[7] 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.[21] In Zimbabwe, whereR. africae is endemic, one study reported an estimated yearly incidence of 60-80 cases per 10,000 patients.[7][21]
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.[7]