African tick bite fever (ATBF) refers to bacterial infection spread by the bite of a tick.
The tick, Rickettsia africae is a gram-negative, obligate intracellular, pleomorphic bacterium.
It belongs to the genus Rickettsia, that includes many species that are transmitted to humans by arthropods.
African tick bite fever is a type of spotted fever.
Symptoms of ATBF include: fever, headache, muscle pain, and a rash.
African tick bite fever is often asymptomatic or mild in clinical presentation and complications are rare.
Within 7-10 days of inoculation patients develop one or more eschars with regional lymphadenopathy, fever, headache, myalgias, or cutaneous rash.
The site of the bite there is typically a red skin sore with a dark center.
The onset of ATBF symptoms usually occurs 4–10 days after the bite.
It is the second most common cause of a febrile illness in returning travelers from sub-Saharan Africa.
Ricketsia africae is the causative agent of ATBF.
Ricketsia africae is transmitted by Amblyomma ticks across sub-Saharan Africa.
The disease occurs in sub-Saharan Africa, the West Indies, and Oceania.
Inoculation results in a local necrotizing vasculitis manifesting as a single lesion covered with a black scab.
The lesion has affinity for the endothelial cells.
Cattle herds and wild animals are important reservoirs.
Game hunting, travel during the rainy season, and safaris are all associated with ATBF.
Diagnosis is typically based on symptoms, but can be confirmed by culture, PCR, or immunofluorescence.
Serologic diagnosis of rickettsia cannot determine the exact species and serologic testing with acute face serum is usually negative, awaiting seroconversion a month later with a convalescent sample.
Immunohistochemical staining of a escort biopsy can confirm the infection.
Standard treatment for this rickettsial infection is doxycycline 100 mg twice daily for 10 days.
The disease is generally uncomplicated with resolution of symptoms with treatment to be quick.
In the absence of treatment symptom resolution may take weeks.
Prevalence is increasing, and is now second only to malaria as the leading cause a febrile illness in a traveler returning from sub Saharan Africa with an incidence of 4-5.3% in this group.
Preventive measures include avoiding infection by wearing long sleeve clothing that covers both arms and legs, wearing boots, using permethrin on clothing, and frequent application of insect repellent that includes date and frequent tick checks.
Complications with ATBF are rare, as some do not develop symptoms.
Joint inflammation may occur.
The bacterium is spread by ticks of the Amblyomma type ticks spread the rickettsia.
Amblyomma type ticks generally live in tall grass or bush rather than in cities.
There is no vaccine available.
Prevention is by avoiding tick bites by covering the skin, using DEET, or using permethrin treated clothing.
The disease tends to resolve without treatment, but the antibiotic doxycycline appears useful.
Complications are rare and are not life-threatening: Prolonged fever > 3 weeks in duration, arthritis, and moderate to severe headache.
No deaths associated with the disease have been reported.
Chloramphenicol or azithromycin may also be used.
Eschars may or may not be seen, but multiple eschars may be seen and are considered pathognomonic
The Amblyomma ticks can attack cattle or humans and can bite more than once.
Two species of hard ticks, Amblyoma variegatum and Amblyomma hebraeum are the most common vectors of R. africae.
Amblyomma hebraeum transmits the bacteria in South Africa.
Amblyoma variegatum carries R. africae throughout West, Central and East Africa and through the French West Indies.
Amblyomma ticks are most active from November to April.
These tick species frequently feed on cattle other livestock, and found feeding on wild animals in areas where farm animals are not found.
Unlike other hard tick species, which 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.
After the rickettsia bacteria infects humans through a tick bite, it invades endothelial cells in the circulatory system.
The body then releases chemicals that cause inflammation with symptoms of headache and fever.
The process results in a lymphohistiocytic vasculitis that involves immune cell deposition into the endothelial cells that make up vessels.
The inflammation Involves signals to immune cells, T cells and macrophages, to come to the site of the infection.
Rickettsia bacteria replicate around the area of the initial tick bite, causing necrosis and lymph node inflammation, resulting in the
While most patients have mild symptoms, it can cause more serious symptoms in travelers who have never been exposed to the Rickettsia africae bacterium before and are not immune.
malaria, dengue fever, tuberculosis, acute HIV and respiratory infections.
Diagnosis of ATBF is mostly based on symptoms, as many laboratory tests are not specific for ATBF.
Common laboratory test signs of ATBF are lymphopenia and thrombocytopenia, a high C-reactive protein, and mildly high liver function tests.
Biopsies or cultures of a tick eschar are used to diagnose ATBF.
It requires special culture media, or
specialized laboratory tests that use quantitative polymerase chain reactions (qPCR), but can only be done by laboratories with special equipment.
Prevention: of ATBF: wearing long pants shirts, and using insecticides on the skin.
November – April, is when Amblyomma ticks are more active, and Inspection of the body, clothing, gear, and any pets after time outdoors can help to identify and remove ticks early.
ATBF is usually mild, and most patients and treatment with antibiotics for their illness.
The best antibiotic choice are not well known:
Cases have more frequently reported in among international travelers.
In Zimbabwe, where R. africae is endemic, there is an estimated yearly incidence of 60-80 cases per 10,000 patients.