In Cameroon, the National Tuberculosis Control Program continuously monitors infections caused by M. tuberculosis and the resistance of these strains to anti-tuberculosis drugs. Distinguishing NTM from TB presumptive people is a major challenge, which requires sensitive test. In the present study, 236 (4.5%) people with positive cultures in the BACTEC MGIT 960 instrument who were initially presumed to have TB, were infected with NTM. These findings demonstrated that we need to find NTM strains in TB presumptive people who have a negative TB test. These results are lower than those obtained in northern India, which presented a NTM frequency of 29% between 2013 and 2015 [13]. The low frequency of NTM in our study may be due to the fact that the data were collected only from TB-NRL which is one of two laboratories which perform LPA for NTM identification in Cameroon. In addition, this low frequency could be also due to the lack of reagents, because 128 NTM strains out of 236 were not identified. The similarity of symptoms between TB and NTM infections can lead to overlooking of NTM infections. Microscopy which is the most widely used test in limited resource laboratories, does not distinguish NTM from MBTC. This is why it is necessary to have a good system for transporting samples from remote settings to the TB-NRL where LPA is available. In our study, 47, 88% NTM were isolated among sample with positive microscopy and 52.11% NTM were isolated among samples with negative microscopy although microscopy was repeated using three samples from each patient. These findings demonstrated that NTM-infected patients may be wrongly diagnosed in remote laboratory where only microscopy is used as initial diagnostic test for TB. Consequently, some patients infected with NTM and with negative microscopy would not be treated while some patients infected with NTM and with positive microscopy would be treated as TB patient. This can lead to unfavorable outcomes, increased morbidity and risk of mortality. Previous study conducted in Pakistan also demonstrated the isolation of NTM (6%) among people with negative microscopy [14]. Our findings demonstrated the diagnosis value of culture considered as gold standard, and highlight the usefulness of reverse hybridization-based line probe assay, GenoType Mycobacterium CM and GenoType Mycobacterium AS to diagnose NTM infections among presumptive TB people. However, these tests are expensive, require infrastructure and qualified staff. The development of a point of care test for NTM diagnosis will be helpful in laboratories with limited resources. Some countries like China revealed an increase in the prevalence of NTM strains from 4.3% in 1979 to 22.9% in 2010 through the surveillance system [15]. These findings demonstrates the need to monitor NTM infections in presumptive TB people with a negative TB test and the need to implement a surveillance system with a good transport system for samples from limited resource laboratories to the TB-NRL.
Our study showed that M. fortuitum was the most represented specie with a frequency of 32.41% followed by M. intracellulare with a frequency of 19.44%. The results obtained by Karamat et al. in Pakistan revealed that M. avium complex (55%) and M. abscessus (25%) were the most represented in pulmonary and extrapulmonary samples [14]. These findings showed that epidemiology of NTM infections in Cameroon differs from data obtained in other countries.
The largest proportion of strains was isolated from pulmonary samples with 56% isolated from sputum, followed by 31% isolated from bronchial aspirates (31%). These results are similar to those obtained by Gonzalo et al. in Uruguay [16], which found that pulmonary swabs had the highest proportions of NTM although the proportion of sputum was higher (66.6%) compared to the findings obtained in our study. Studies conducted by Ali et al. [17], in Iran also revealed that the highest frequency of NTM was found in pulmonary sample (27.1%).
The search for comorbidities was carried out only on three participants infected with NTM. This low number of participants is explained by the unavailability of clinical data in laboratories due to the lack of surveillance system of NTM in Cameroon. Among the people infected with NTM one was infected with HIV. The studies conducted by Surendra et al. in India also found that out of 42 patients infected with NTM, one patient was infected with HIV [18]. In our study, other comorbidities such as alveolar pneumonia, type 2 diabete and pulmonary tuberculosis have also been identified in these people. Previous studies have revealed that in most cases, several factors such as the existence of a pulmonary diseases or immunosuppression promote infections by NTM [19]. In this study, chronic bronchitis was the most frequent comorbidity in these patients, in contrast to the studies conducted by Matesanz et al. which revealed that chronic obstructive pulmonary disease was the most frequent comorbidity in patients infected with NTM [20].
This study has several limitations. The low frequency of NTM is also attributed to the collection of data in the TB-NRL only while two laboratories carry out GenoType Mycobacterium CM and GenoType Mycobacterium AS in Cameroon. All the NTM were not identified due to lack of reagents, and the absence of surveillance system of NTM in Cameroon do not allowed us to show the evolution of NTM infections. Although the comorbidities have been identified, the number of patients was insufficient to be able to establish an association with the occurrence of NTM infections.