Results
TB was first recorded in the late 1990s in this population and spread within ~ 8 years amongst all social groups and exposing most individuals to the disease. Between 1993 and 2020, 71% of all individuals were exposed to TB at some point during their life (n = 2,427/3,420). The proportion of individuals exposed varied markedly between study years (mean ± SD: 54.8 ± 38.4%), with no TB exposure recorded prior to 1997. Exposure levels proceeded to rise rapidly until 70 – 95% of individuals were exposed in any given year after 2006 and remained at comparably high levels thereon after. In contrast, clinical signs of the disease increased in frequency less steeply. Between 0 to 29.4% of the study population showed clinical signs (mean ± SD: 8.6 ± 9.0%, with 0 to around 6% of individuals dying with confirmed TB (mean ± SD: 4.2 ± 4.6%) in any given year (Figure 1, Supplementary Figure 1).
Individuals were most often exposed at birth (75.3%, n = 1827), or upon development of TB signs in their current resident group (19.8%, n = 472). The remaining exposed individuals immigrated into a group already experiencing TB (5.4%, n = 128). Only 22.9% of exposed individuals (n = 555), corresponding to 16.1% of the study population, developed clinical signs of TB within 1.5 years (mean ± sd: 520 ± 421 days). The majority of individuals reported with clinical signs were confirmed to have died with or from TB (n = 398), leading to a TB-related mortality rate of 11.6% in the population. This does not account for individuals infected, but not showing overt signs of TB, so it is likely this figure underestimates the real extent of TB related mortality.
We observed marked inter-individual variation in TB progression and outcomes, with time between exposure and first clinical signs ranging from 0 (for index cases in a group) to 2,971 days (Figure 2, Figure 3). The mean duration between exposure and signs was ~1.4 years (mean ± SD: 520 ± 421 days, median: 440 days). Individuals survived the onset of clinical signs for up to 2,756 days, although some individuals were recorded with signs only at the time of their death. On average, individuals survived the onset of clinical TB for ~6.6 months (mean ± SD: 200 ± 328, median: 70 days), indicative of the rapid progression of TB after development of clinical signs (Figure 2, Figure 3, Supplementary Figure 2). Comparing exposed individuals progressing to clinical TB with exposed individuals never developing TB signs, we found high variation in survival in both groups: Individuals developing TB signs survive between 5 and 3,541 days (mean ± SD: 720 ± 529, median = 603 days) past exposure, asymptomatic individuals survived up to 4,123 days (mean ± SD 432 ± 485, median = 269 days) past exposure. Asymptomatic individuals thus seemed to die significantly earlier (MWU-test: U = 723844, p < 0.001).
On the group level, we focused on 91 meerkat groups lasting between 196 and 9,235 days (mean ± SD: 1,513 ± 1,935 days, median: 660 days). Individuals with clinical TB were present in 62 (68.1%) groups, only eight of which persisted until the end of the study. Additional four groups present at the end of the study had not been in contact with TB. TB prevalence among groups varied between 0% before TB detection in the study and 96% in 2013 and 2016, with an average of 59.5 ± 33.9% of groups exposed to TB (Figure 1). TB was present in most groups that did not persist (n = 54, 68.4% of extinct groups).
After the first detection of clinical TB, groups persistence varied between only 46 and 6356 days (mean ± SD: 1,200 ± 1,352 days, median: 569 days) and notably the two longest surviving groups in our dataset persisted despite being exposed to TB since ~ 17 and 21 years, respectively. Marked variation in group survival past TB exposure is retained when only considering groups that disintegrated after detection of TB, after which these groups survived up to 8,243 days (mean ± SD: 1,591 ± 1,858 days, median: 573 days).