From 1985 through 2009, a total of 10,257 bats were submitted to the Massachusetts Department of Public Health for rabies testing: 8,850 big brown bats (Eptesicus fuscus, 86.3%), 1,074 little brown bats (Myotis lucifugus, 10.5%), 94 Keen long-eared bats (Myotis keenii, 0.9%), 48 red bats (Lasiurus blossevillii, 0.5%), 17 hoary bats (Lasiurus cinereus, 0.2%), 17 silver-haired bats (Lasionycteris noctivagans, 0.2%), 1 zoo-submitted Seychelles fruit bat (Pteropus seychellensis), and 156 (1.5%) unspeciated bats. The proportion of rabies-positive bats by species was 5.0% (443/8,850) big brown bats, 3.6% (39/1,074) little brown bats, 23.5% (4/17) hoary bats, 8.3% (4/48) red bats, 5.9% (1/17) silver-haired bats, and 3.2% (3/94) Keen long-eared bats; 751 (7.3%) of 10,257 bats were not suitable for rabies testing. Among all rabies-positive bats, 89.3% were big brown bats, 8.0% were little brown bats, and 2.7% were of less frequently submitted species. During 2005–2008 (), submissions of little brown bat sharply increased then decreased, although the proportion that was rabies positive (2%–4%) remained stable.
Bats submitted for rabies testing in Massachusetts, USA, 1985–2009. Black line indicates Eptesicus fuscus, red line indicates Myotis lucifugus, and blue line indicates other pooled bats.
The average annual number of bat submissions increased significantly from 103 during 1985–1991 to 302 during 1992–1998 and to 675 during 1999–2009 (p<0.0001) (). The average annual number of confirmed rabid bats increased from 7 to 19 to 28 for those periods, and the proportion of bats positive for rabies decreased from 6.9% (50/720) to 6.4% (135/2,113) to 4.2% (311/7,424). The proportion of rabid bats was significantly lower during 1999–2009 (p<0.05).
Rabies in bats in Massachusetts, USA, 1985–2009. Black line indicates number of bats submitted and red line indicates percentage of bats positive for rabies.
Among 961 testable bats submitted before RRV introduction, 76 (7.9%) were positive for rabies compared with 420 (4.9%) of 8,545 bats submitted after RRV introduction. No positive association was identified between RRV introduction and proportion of bats positive for rabies, even when adjusting for potential confounders such as bat species (big brown and little brown vs. other pooled species), reason for bat submission (human and pet exposure, human exposure only, and pet exposure only vs. undefined), and time of submission (1985–1991 and 1992–1998 vs. 1999–2009). Limited RRV strain typing results performed on 52 rabies-positive bats showed that all bats were infected with non-RRV (X. Wang et al., unpub. data).
Before publication of the 1999 ACIP recommendations, the most common reason for testing bats was pet exposure only, which accounted for 50.3% of submissions during 1985–1991 and 43.5% during 1992–1998 (). After 1999, the most common reason for bat testing was human exposure, accounting for 72.0% of submissions during 1999–2009. Although the number of bat submissions because of human exposure increased with time, the rabies-positive proportion of these bats decreased from 10.3% (17/165) to 5.8% (51/885) to 3.8% (204/5,343) during the 3 periods. No significant differences in rabies positivity among bats submitted for different reasons were found during 1999–2009 ().
Reasons for bat submissions for rabies testing, Massachusetts, USA, 1985–2009
Characteristics of bats submitted for rabies testing, Massachusetts, USA, 1985–2009*
Clinical signs were reported for 2,291 (24%) of 9,537 bats submitted since 1992 (). The most common signs were death (1,206, 52.6%), disorientation (514, 22.4%), lethargy (358, 15.6%), or aggressiveness (275, 12.0%). Bats described as having aggression, ataxia, disorientation, or lethargy were significantly more likely to have rabies than were bats with no reported signs (p<0.05). Bats found dead were no more likely to have rabies than were bats reported alive before submission.