La tuberculose humaine et animale au Tchad : contribution à la mise en évidence et caractérisation des agents causaux et leur implication en santé publique

Chad is a high-incidence country for clinical tuberculosis. Within the last decade, the
annual incidence rate increased from 120 human tuberculosis cases per 100’000 people
to 370/100’000 in 2004. The presence of bovine tuberculosis was suspected through
tuberculination studies and meat inspections at slaughterhouses.
Although more sensitive and specific tuberculosis diagnostic tests exist, before this
thesis, tuberculosis was solely diagnosed in Chad by microscopy of acid-fast bacilli
(AFB) in specimens from suspected cases. Microscopy alone does not allow the
identification of different mycobacteria species nor is it a sensitive method; the
detection threshold is higher than 104 bacilli / ml for a smear to be classified as AFB
positive. Mycobacteria are very heterogeneous with regard to their host tropism, their
habitat and their pathogenity. The identification of the different mycobacterial species is
only possible after strain isolation and subsequent typing. Thus, a laboratory culture
represents a key element to obtain adequate information on the situation of tuberculosis
in a country.
A mycobacteriology laboratory has its specificities in comparison to a classical
bacteriology laboratory and during this study the following relevant steps to identify
mycobacterial species were established:
1) Preparation of culture media and especially the Lowenstein-Jensen medium
2) Decontamination of specimens with N-acetyl-L-cysteine sodium hydroxide
3) Biochemical testing
4) Molecular spoligotying
5) Diagnosis of anti-tuberculosis drug resistance
The specimens treated during the study were mainly sputum and urine samples of
patients coming to a health center for smear examination and specimens of affected
organs from condemned livestock carcasses after meat inspection in N’Djaména. In
total, 639 clinical samples and 201 organ samples from the slaughterhouse were
cultivated and from 26.4% and 65.6% of cultures, respectively, mycobacteria were
isolated. Spoligotyping revealed 35 M. tuberculosis strains - all isolates being from
human specimens - and 55 M. bovis strains from organs of cattle.
We obtained 25 different spoligotypes of M. tuberculosis, of which 7 were clustered and
18 were unique. Ten M. tuberculosis strains corresponded to the “Cameroon family”
and 10 spoligotypes were already described in the international spoligotype database
(SpolDB3.0). Thirty-two percent of our spoligotypes have not yet been described in
literature or entered into a database. Drug resistance testing of M. tuberculosis strains
showed 39% were resistant to at least one drug and 27% were resistant to Izoniazide.
The majority of patients with strains resistant to Izoniazide originated from the saheliansaharian
zone rather than from the more humid Southern zones of Chad. No strain was
resistant to Streptomycin or to Rifampicin.
Out of the 55 M. bovis isolates, 26 derived from cattle of the Arab breed and 29 of the
Mbororo breed. Twelve different spoligotypes were identified and 92.7% of strains
were clustered. Within our typed isolates, most strains lacked the direct repeat (DR) 30
as described for Cameroonian strains. Typical spoligotypes could not be assigned to one
of the two cattle breeds.
Typing of 52 non-tuberculous mycobacteria from clinical specimens and organs from
the slaughterhouse showed that 15% were M. fortuitum. Three strains isolated from
cattle were M. fortuitum 3rd variant which is genetically closely related to M.
farcinogenes - the causal agent of farcy in Central Africa.
The microbiological study of tuberculosis in Chad has confirmed through isolation and
strain typing the existence of tuberculosis among suspected cases in humans and in
livestock. Anti-tuberculosis drug testing has now been established in Chad and has
revealed that the proportion of strains resistant to Izoniazide is high. The higher
incidence of bovine tuberculosis in Mbororo cattle in comparison to Arab cattle was
demonstrated by isolation and characterization of M. bovis strains.
In conclusion, this study has generated the first microbiological results on human and
animal tuberculosis in Chad. These results will inform Chadian public health specialists
and can serve as the basis to define further investigations.