Bat rabies in Washington State: Temporal-spatial trends and risk factors for zoonotic transmission (2000-2017)

Rabies is a zoonotic viral disease that can affect all mammals. In the United States, the majority of human rabies cases are caused by bats, which are the only known reservoirs for rabies virus (RABV) in Washington State. We sought to characterize bat RABV epidemiology in Washington among bats submitted by the public for RABV testing.

We examined temporal and spatial trends in RABV positivity (% positive) for taxonomically identified bats submitted to diagnostic laboratories during 2006-2017. For a subset of Myotis species, we evaluated sensitivity and predictive value positive (PPV) of morphological identification keys, using mitochondrial markers (cytochrome b) as a reference. For bats tested during 2000-2016, we analyzed RABV positivity by circumstances of encounters with humans, cats, and dogs.

During 2006-2017, RABV positivity for all bat species was 6.0% (176/2,928). Among species with ≥100 submissions, RABV positivity was 2.0%-11.7% and highest among big brown bats (Eptesicus fuscus). An increasing trend in annual positivity was significant only for big brown bats (P = 0.02), and was circumstantially linked to a geographic cluster. Sensitivity and PPV of morphological identification keys was high for M. evotis but varied for M. lucifugus, M. californicus, M. yumanensis, and M. septentrionalis. A positive RABV result was significantly associated with nonsynanthropic species, abnormal behavior, abnormal hiding, injury, biting, found in a body of water, found alive, found outdoors, and caught by a dog.

Monitoring passive RABV surveillance trends enables public health authorities to perform more accurate risk assessments. Differences in temporal and spatial trends in RABV positivity by bat species indicate the importance of collecting taxonomic data, although morphological identification can be unreliable for certain Myotis species. Current public health practices for RABV exposures should be maintained as RABV infection in bats can never be excluded without diagnostic testing.