To the Editor: African tick-bite fever (ATBF) is caused by Rickettsia africae and
remains the most common tickborne rickettsiosis in sub-Saharan Africa (
1
,
2
). We describe an outbreak of ATBF in 10 of 34 French tourists on their return from
South Africa in March 2005. Fever, skin rash, and multiple eschars on the legs developed
in the index case-patient (patient 9, Table). After informed consent was obtained,
the tourists completed a questionnaire for epidemiologic and clinical data. Acute-
and convalescent-phase serum samples were collected when possible for serologic analysis
performed at the Unité des Rickettsies. The samples were tested against a panel of
antigens including R. typhi, Francisella tularensis, Coxiella burnetii, Borrelia burgdorferi,
Anaplasma phagocytophylum, R. felis, R. helvetica, R. conorii subsp. conorii strain
Malish, R. africae, R. sibirica mongolotimonae, R. massiliae, and R. slovaca, as previously
described (
3
). A case of symptomatic confirmed ATBF was defined as clinical illness and positive
serologic results against R. africae, whereas a case of probable ATBF was defined
as typical clinical symptoms without definite serologic evidence of R. africae infection.
Table
Epidemiologic, clinical, and serologic information for 10 patients with African tick-bite
fever*
Patient
Sex/age (y)
Tick bite
Delay before onset (d)
Fever
Headache
Myalgia
Eschar (site)
Skin rash
1st serum† IgG/IgM
2nd serum† IgG/IgM
Diagnosis
1
M/62
No
7
Yes
No
No
Multiple (legs)
No
NA
NA
Probable
2
F/58
No
6
Yes
No
Yes
Multiple (legs, arms)
No
64/32
64/128
Confirmed
3
M/58
No
6
No
Yes
No
Single (trunk)
No
64/32
128/16
Confirmed
4
F/51
No
6
No
Yes
Yes
Multiple (legs, trunk)
No
0/64
128/16
Confirmed
5
M/58
No
5
Yes
No
Yes
Multiple (legs)
No
512/0
512/0
Confirmed
6
F/57
No
5
No
No
Yes
Yes (unknown)
Yes
NA
32/16
Confirmed
7
M/65
No
5
Yes
Yes
Yes
Multiple (hands)
No
128/64
512/128
Confirmed
8
F/59
No
10
No
No
No
Multiple (legs, arms, trunk)
No
64/8
128/32
Confirmed
9
M/53
No
3
Yes
Yes
Yes
Multiple (legs)
Yes
0/0
1,024/512
Confirmed
10
M/51
No
8
Yes
No
Yes
No
Yes
32/32
64/64
Confirmed
Total (%)
0
60
40
70
90
30
*NA, not available; Ig, immunoglobulin; male-to-female ratio, 60%; mean age = 57.2
± 4.5 years.
†Identical results obtained with both Rickettsia africae and R. conorii
antigens.
Of the 34 travelers, 30 completed the questionnaire and 20 consented to give at least
1 serum sample. After their return to France, symptoms compatible clinically with
ATBF developed in 10 of the travelers (Table) and 9 had positive serologic results
and/or a seroconversion for spotted fever group-rickettsia, including R. africae (Table).
The median time from illness onset to serum testing was 19 days. Thus, 9 of the travelers
had probable and 1 had possible (no serum was available) ATBF. Including both probable
and possible cases, the illness rate for the whole group was 33.3% (10/30). None of
the travelers reported a history of tick bite. The delay between probable exposure
and onset of symptoms was 3-10 days (mean ± standard deviation 6.1 ± 1.9 days). Multiple
eschars on the legs or arms were seen in 7 (70%) of 10 patients. Eight patients received
doxycycline (200 mg per day) for a mean of 10.8 ± 5.9 days (range 5-20), 1 patient
received pristinamycin for 8 days, and 1 patient received no treatment. All patients
recovered fully without sequela; however, 6 patients reported convalescent-phase asthenia
and 1 reported chronic insomnia, which had not occurred previously, for 2 months after
the illness. Among the 10 remaining travelers, for whom a serum sample was available,
with no clinical evidence of ATBF, 5 were positive for R. africae with only immunoglobulin
M (IgM) at a titer of 1:32 in 4 cases and IgG at 1:128 with IgM at 1:32 in 1 case
(an acute-phase serum from this patient showed IgG at 1:32 and IgM at 1:32). The 5
other travelers had negative serologic results. Results of serologic testing for other
bacteria were negative for all travelers. Twenty-four travelers (80%), including the
10 symptomatic patients, reported using topical insect repellent daily.
Most cases of ATBF are reported in clusters of travelers exposed to ticks during game
hunting or safaris, as described here (
1
,
3
-
5
). The estimated incidence of African tick-bite fever in safari travelers is 4%-5.3%
(
4
) but higher incidence may be reported as emphasized in our study. In our study, epidemiologic
and clinical data for the 10 symptomatic patients were obtained in accordance with
current knowledge of ATBF (
2
).
Skin biopsy samples remain the best tool to isolate or detect R. africae (
2
,
6
). However, specific serologic tests, especially immunofluorescence assays, remain
the most widely used microbiologic test worldwide (
7
). No commercially available test for ATBF exists but due to extensive cross-reactions
between spotted fever group rickettsiosis, commercial kits based on the detection
of R. conorii antibodies can be used for the diagnosis of ATBF. Most tourists reported
using topical insect repellents without any efficacy. Applying repellents to exposed
skin provides little protection against ticks because they can crawl underneath clothing
and bite untreated portions of the body (
8
). Thus, treating clothing with synthetic pyrethroid insecticide is recommended to
complement the topical repellant (
8
).
In conclusion, our study emphasizes the importance of ATBF as a common cause of flulike
illness in travelers returning from South Africa, but with a higher rate than malaria,
typhoid fever, or other tropical fevers. The most important clinical clues are the
presence of clustered cases with multiple inoculation eschars. Healthcare professionals
who are providing advice should inform persons traveling to endemic areas of Africa
of the risk of contracting ATBF and the importance of protecting themselves against
tick bites. Chemoprophylaxis with doxycycline is not recommended, however, this recommendation
may be evaluated in future clinical trials.