diff --git a/health_check/mixins.py b/health_check/mixins.py
index 139707c4..d56167e4 100644
--- a/health_check/mixins.py
+++ b/health_check/mixins.py
@@ -2,6 +2,7 @@
 from collections import OrderedDict
 from concurrent.futures import ThreadPoolExecutor
 
+from django.db import connections
 from django.http import Http404
 
 from health_check.conf import HEALTH_CHECK
@@ -63,9 +64,10 @@ def _run(plugin):
             try:
                 return plugin
             finally:
-                from django.db import connections
-
-                connections.close_all()
+                if not HEALTH_CHECK["DISABLE_THREADING"]:
+                    # If this connection was opened by the child thread,
+                    # it must be manually closed.
+                    connections.close_all()
 
         def _collect_errors(plugin):
             if plugin.critical_service: