Sheet n°245 - June/July 2006
orreliosis, or tick-borne
relapsing fever, is transmitted to humans by the tick
Ornithodoros sonrai. It causes
recurrent fevers in several
areas of Africa. IRD researchers traced the evolution of
the disease in West Africa
from 1990 to 2005 and made
the first determinations of its
incidence in a rural African
population. Persistent drought
has enabled the tick to spread
beyond the Sahelian zone and
has colonized rodent burrows
in most villages in Senegal,
Mali and Mauritania. The incidence of the disease is particularly high: depending on the
year, from 4 to 25% of the
population surveyed manifested borreliosis. Average
annual incidence over a period
of 14 years was 11%, the
highest level for a bacterial
disease observed in Africa.
The same subject can have
the disease repeatedly many
times owing to the bacteria’s
ability to overcome the human
immune system. Tick-borne
relapsing fever has remained
poorly understood, in spite of
its high prevalence, because it
is systematically confused
with malaria which induces
the same clinical signs and
which rages among the same
people.
©IRD/Jean-François Trape
Record morbidity in Africa
from a little-understood emerging disease
B
Ornithodoros sonrai, human vector of Borreliosis.
Borreliosis, or relapsing fever, is a disease provoked by bacteria of the genus
Borrelia. Two of these are encountered in
tropical Africa: Borrelia crocidurae in the
Sahara and Sahel, Borrelia duttoni in East
Africa. This infection causes recurrent
fevers over a long period that can lead to
severe meningoencephalitis and sometimes death. The human vector of Borrelia
crocidurae in West Africa is the tick
Ornithodoros sonrai, which lives in burrows of small wild rodents and is therefore
in close contact with them. This disease
was considered rare up to the end of the
1980s when a team of IRD researchers
showed that in a rural area of the Dakar
region tick-borne relapsing fever was the
second most frequent reason for patients’
consultation at the local dispensary.
Since 1990, the IRD launched a comprehensive research programme on this disease, first in Senegal, then in the whole of
West Africa. In Senegal, the investigation
showed that the tick had colonized savannah and that the disease’s expansion was
closely linked with the decrease in average rainfall since the long drought began in
1970. The advance of borreliosis prompted the research team to extend their epidemiological research and study the longterm trends, by 14 years of measurement
of the infection’s incidence in a rural
Senegalese community which was the
subject of continuous demographic and
health monitoring programme run jointly
by the IRD, the Pasteur Institute and the
University of Dakar.
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Sheet n°245- June/July 2006
The study was conducted from 1990 to
2003 on people in Dielmo, a village in the
savannah in the Sine-Saloum region of
Senegal, in order to determine the frequency and describe the clinical manifestations of malaria, tick-borne relapsing
fever and other fevers not linked to these
two diseases. The research team lived
permanently in the village so that they
could visit each inhabitant daily. Medical
examinations and biological tests were
systematically carried where someone
had a fever or other symptoms that might
signal the disease. The presence of
Borrelia crocidurae was also tested in
people without symptoms at least once a
year. All rodent burrows near houses and
inner yards of concession were counted
and recorded. They were searched for
ticks and the latter’s infection rate by the
bacterium determined. Rodents and
insectivores were captured in order to
study the bacterial reservoir.
For futher information
CONTACTS :
JEAN-FRANÇOIS TRAPE,
IRD 77 Paludologie
Afrotropicale, Senegal,
+221 849 35 82 ;
trape@ird.sn
PRESS OFFICE:
01 48 03 75 19 ;
presse@paris.ird.fr
INDIGO BASE, IRD PICTURE LIBRARY
01 48 03 78 99 ;
indigo@paris.ird.fr
REFERENCES:
LAURENCE VIAL, GEORGES
DIATTA, ADAMA TALL, EL HADJ
BA, HILAIRE BOUGANALI,
PATRICK DURAND, CHEIKH
SOKHNA, CHRISTOPHE ROGIER,
FRANÇOIS RENAUD, JEANFRANÇOIS TRAPE. “Incidence
of tick-borne relapsing fever
in west Africa: longitudinal
study”, 2006. The Lancet,
368 :37-43.
Over the period of study as a whole, an
average 11 % of the population suffered
from borreliosis each year, an exceptional
incidence rate for a disease, whatever its
cause. Only malaria, among the parasitic
diseases and in any case to a lesser degree, and influenza, among viral diseases,
are known to produce a comparable incidence level over such a long period. Tickborne relapsing fever is the second cause
of illness after malaria. All age groups
were affected by it in the population studied.
The IRD team then conducted systematic
surveys in Senegal, Mali and Mauritania in
order to define the geographical distribution of the tick, determine the infection rate
over the whole of its area of distribution
and establish the proportion of villages
affected by the disease. The results showed that the vector is present in massive
numbers in these three countries in any
area where average rainfall is below 750
mm. Out of 30 villages studied, 26 (87 %)
were colonized by the tick which was present on average in 31 % of the burrows in
the villages with a Borrelia crocidurae
infection rate of 21 %. Two-thirds of the
villages studied showed an exposure rate
to tick-borne relapsing fever among inhabitants even higher than that of the people
in Dielmo. It is in most of the rural areas of
Senegal and Mali and in all parts of
Mauritania that the disease is a major
public health problem.
Paradoxically, this emerging disease is
still completely misunderstood by health
care personnel, although it has become
one of the most frequent bacterial infections. Laboratory testing for determining
the cause of a disease is rarely possible in
tropical Africa, particularly in the rural
areas. Borrelia crocidurae can be detected in blood samples only during peaks of
fever. Its density is usually very low and
diagnosis requires the skills of an experienced microscopist. The symptoms of
the disease are exactly similar to those of
malaria which is very frequent in the same
populations. The disease is therefore systematically confused with malaria, and failure of treatment is attributed to antimalarial drugs. Furthermore, the existence of
an animal reservoir and the omnipresence
of rodents in rural areas makes prevention
of the disease hardly feasible. However, a
cheap effective treatment is possible
which involves the use of antibiotics of the
tetracycline family which are available in
most clinics in the bush.
Aude Sonneville, IRD
Translation : Nicholas Flay
KEY WORDS
AFRICA, BORRELIOSIS, TICKS,
EMERGING DISEASES
Marie Guillaume - Signoret, coordinatrice
Délégation à l’information et à la communication
Tél. : +33(0)1 48 03 76 07 - fax : +33(0)1 40 36 24 55 - fichesactu@paris.ird.fr