**3. Discussion**

The described case concerns the transmission of Beijing type XDR-TB between family members in one household, where the source of infection was the mother, who was profusely positive for mycobacteria. TB in the 10-month-old daughter was confirmed in a microbiological test by isolating the XDR strain of the Beijing 265 genotype from gastric lavage, and it had an identical molecular code to the strain isolated from the mother. Due to the absence of clinical symptoms of TB in the infant, antimycobacterial treatment was not initiated, and the child was followed-up in the outpatient setting.

Cases of tuberculosis in children are reported many times, and a claim has been made that they are able to control the disease progression with no need for clinical intervention. Research from the period before the use of chemotherapy shows that the majority of children recover from tuberculosis without any treatment, and pathological changes in the lungs seen in radiographs often resolve spontaneously. It has also been demonstrated that *Mycobacterium tuberculosis* strains can be cultured from recently infected asymptomatic children [12–14]. Contemporary studies, including the described case, provide information on TB in children with microbiologically confirmed asymptomatic MDR-TB that requires no treatment [15–17].

There may be various reasons for the presence of MTBC in biological specimens collected from the respiratory tract of asymptomatic children. First, it can result from the natural history of infection where children, after a recent primary infection, might temporarily shed viable mycobacteria in the absence of an active form of the disease. This phenomenon was first documented in the 1930s [16]. Second, latent infection covers the spectrum of clinical situations, from inactive (latent) mycobacterial infection to periods of subclinical proliferation of *Mycobacterium tuberculosis*, which, however, is insufficient to cause lung damage [18,19].

How, then, should we classify a totally asymptomatic person with microbiologically confirmed tuberculosis but without clinical or radiological symptoms suggesting an active disease? Is it a case of an active disease?

Tuberculosis in children, especially XDR-TB, is still a serious therapeutic problem. Guidelines on its treatment are similar to those that apply to adults, but there are no antimycobacterial medications dedicated to the younges<sup>t</sup> population of patients [20]. The only dosage regimen relies on dividing a tablet normally prescribed to adults, which is highly controversial. In 2017, paediatric TB physician Dr Jeffery Starke in his speech opening the 48th World Union Conference on Lung Health emphasized that "( ... ) children have the same right as adults to benefit from tuberculosis care and research. It is time that we put these words into action ( . . . )" [21].

Although drugs for MDR-TB in adults are used in older children, currently there are no recommendations on the administration of bedaquiline to children aged <6 years (<15 kg bw) due to the lack of data on its pharmacokinetics and safety [22]. In the treatment of XDR-TB in children aged 3–6 years, bedaquiline can be replaced with delamanide, but it should not be used in children <3 years (<10 kg bw) [23].

The number of XDR-TB cases in young children reported to date is limited. There are fragmentary data on the treatment of XDR-TB in children, ending either with success or fatality [24].

In the described household, apart from the 10-month-old infant, there were also two other infected young children, but they have not developed active tuberculosis so far. It is known that not all infected people have the same risk of developing the active form of TB. In the immunocompetent adult population, the lifetime risk of TB is approximately 5 to 10%; half of these cases develop active TB in the first 2–3 years following infection. On the other hand, in the population of immunocompetent infected infants who received no prophylactic treatment, up to 50% develop active TB within 6–9 months after infection, and the disease might be severe and life threatening [1]. International guidelines recommend monitoring close contacts of patients with MDR-TB for at least 2 years, or for at least 4 years in the case of XDR-TB [25].

It has been proven that BCG vaccination is important in the course of tuberculosis in children. The protective effect of the vaccine is manifested in the form of a lower incidence of tuberculosis in vaccinated children and less frequent progression of the latent form into active disease. Despite the fact that BCG vaccination does not eliminate the risk of infection among children in close contact with smear-positive adults, it significantly influences the course of the disease, protecting against severe TB, especially in the younges<sup>t</sup> children [26].

Particular attention should be paid to the fact that the described case concerns the transmission of MDR-TB caused by the Beijing genotype *Mycobacterium tuberculosis*. In antimycobacterial therapy, the choice of medication is based on drug-susceptibility testing, but in the case of Beijing-TB, the problem appears to be more complex. Some studies have suggested that exposure to Beijing strains is more often associated with progression to the active form of TB than in the case of infection with mycobacteria, representing other genotypes [27]. However, Canadian studies showed no such correlation [28]. Analyses carried out in Vietnam and Iran proved that Beijing-TB is more common in the population of younger adults, and the incidence of this disease decreases with age [29,30].

Undoubtedly, the Beijing genotype is associated with an increased risk of acquired drug resistance and a more difficult clinical course. The increased transmissibility of "Beijing strains" compared to other molecular families of MTBC has also been confirmed. In the present case report, we described the transmission of a strain identified as the Beijing 265 subtype. In contrast to the Beijing 1 subtype, detected in patients with a spectrum of TB forms, from drug sensitive to XDR-TB, the Beijing 265 clone in Poland has been isolated only from patients with MDR, pre-XDR and XDR-TB [31].
