To the Editor: Incidence of pulmonary nontuberculous mycobacterial disease (PNTMD)
is reportedly increasing globally (
1
,
2
). Although such an increase is expected in Japan (
3
,
4
), the epidemiologic situation is unclear. The most recent survey, which used the
1997 American Thoracic Society diagnostic criteria, reported that the incidence rate
for PNTMD in 2007 was 5.7 cases per 100,000 person-years (
5
). To update the data, we performed a nationwide hospital-based survey in Japan.
After a preliminary survey of 20 hospitals, we developed and disseminated questionnaires
to all 884 hospitals in Japan that were certified by the Japanese Respiratory Society.
The surveys asked about the number of newly diagnosed cases, from January through
March 2014, of PNTMD, pulmonary Mycobacterium avium disease, M. intracellulare disease,
or M. avium complex (MAC; the combination of the first 2 species listed); pulmonary
M. kansasii disease; pulmonary M. abscessus disease; and tuberculosis (TB) for inpatients
and outpatients. Hospital respondents returned the completed questionnaires by mail,
fax, or Internet. To avoid potential reporting bias and misclassification, we counted
only cases that met the 2007 American Thoracic Society/Infectious Diseases Society
of America statements (
6
) and excluded cases diagnosed at other hospitals. Because the source population can
be ascertained by using the epidemiologic data for TB as a reportable disease, to
estimate the incidence rate of PNTMD, we used the ratio of TB to PNTMD cases. The
PNTMD incidence rate was calculated as the national incidence rate of TB multiplied
by the ratio of new PNTMD to TB cases reported by the responding hospitals (online
Technical Appendix Figure 1, http://wwwnc.cdc.gov/EID/article/22/6/15-1086-Techapp1.pdf).
To clarify the chronologic changes in incidence, we followed the same method for comparing
TB and PNTMD used in a prior epidemiologic study in Japan (
5
). We established methods for maximizing survey response rates and facilitating ease
of completion by offering extensive support to survey recipients (Technical Appendix
Table 1).
We achieved a high response rate of 62.3% (551 hospitals), and in all regions the
response rate exceeded 50% (Technical Appendix Table 2). The numbers of newly diagnosed
cases were 2,327 for TB and 2,652 for PNTMD. Because the incidence rate for TB was
reported to be 12.9 cases per 100,000 person-years, that of PNTMD was estimated to
be 14.7 cases per 100,000 person-years, which is ≈2.6 times the incidence rate reported
in 2007 (Figure). By using the same method, we found the incidence of pulmonary MAC,
M. kansasii, and M. abscessus disease to be 13.1, 0.6, and 0.5 cases per 100,000 person-years,
respectively (Technical Appendix Table 2). The ratio of pulmonary M. avium disease
to MAC was higher in the northern and eastern parts of Japan, whereas the ratio of
pulmonary M. intracellulare disease to MAC was higher in the southern and western
parts of Japan (Technical Appendix Figure 1).
Figure
Incidence (no. cases/100,000 person-years) of pulmonary nontuberculous mycobacterial
(NTM) disease, culture-positive tuberculosis (TB), and smear-positive TB in Japan
during 1980–2014. The nationwide survey revealed that the incidence rate of pulmonary
NTM disease exceeds that of TB. The epidemiologic survey before 1988 was conducted
annually by the same research group; subsequently, another group performed the epidemiologic
survey only in 2001 and 2007.
From this survey, we observed that the incidence rate of PNTMD may exceed that of
TB and that incidence rates of PNTMD in Japan may be among the highest worldwide (Figure).
This finding implies that the prevalence of PNTMD as a chronic infection is estimated
to be much higher than that of TB.
We assume that the high rates of PNTMD in Japan are consistent with data suggesting
that Asians are particularly susceptible to PNTMD (
1
,
7
,
8
). Other factors contributing to the increase might be the simplified diagnosis according
to the 2007 American Thoracic Society/Infectious Diseases Society of America statements,
increased awareness by medical staff, population aging, and increased frequency of
medical checkups with computed tomography of the chest.
Another finding was the characteristic gradient clustering of the ratios of M. avium
and M. intracellulare (Technical Appendix Figure 2). This finding supports the widely
accepted belief that environmental factors strongly affect the epidemiology of PNTMD;
therefore, the role of factors such as soil, humidity, temperature, and saturated
vapor pressure should be seriously considered (
9
).
We also found dramatic increases in incidence of pulmonary M. abscessus disease and
pulmonary MAC disease, whereas incidence of pulmonary M. kansasii disease was stable.
Although we did not distinguish M. massiliense from M. abscessus, the incidence rate
for pulmonary M. abscessus disease increased from 0.1 cases in 2001 to 0.5 cases per
100,000 person-years in 2014. This epidemiologic tendency should be monitored (
10
).
This study has several limitations. First, differing characteristics between the responding
and nonresponding hospitals could cause bias. Second, we did not collect data outside
of hospitals. Third, incomplete reporting could undermine the accuracy of our estimates
(Technical Appendix Tables 3, 4). Therefore, the epidemiologic data should be verified
by using other approaches (Technical Appendix Table 1).
The dramatic increase in incidence rates for PNTMD warrants its recognition as a major
public health concern. Because the prevalence rates of this currently incurable lifelong
chronic disease are estimated to be high, the effect on the community could be enormous.
Further investigations are needed.
Technical Appendix
Incidence rates for mycobacterial infections in Japan during 1980–2014; response rate,
results, characteristics, and limitations of survey of newly diagnosed pulmonary nontuberculous
mycobacterial disease and mycobacterial disease, January–March 2014, Japan; and comparison
of hospitals that did and did not respond to the survey.