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National Wildlife Health Center Newsletter March 2019

Updates on disease surveillance, vaccine development, African Swine Fever, white-nose syndrome, modeling causes of mortality, chronic wasting disease, and coral reefs.

US contributions to international wildlife disease surveillance

View of the Milton Friend Building of the National Wildlife Health Center.  The Milton Friend Building was dedicated on October 17, 2018 in honor of the founding director of the NWHC, Dr. Milton Friend.

The World Organisation for Animal Health (OIE) is an intergovernmental organization made up of 182 Member Countries and is responsible for improving animal health and welfare worldwide. Each Member Country is encouraged to appoint a National Focal Point for Wildlife whose responsibilities include establishing a network of wildlife experts, or integrating with existing networks, within his/her country to support the collection and submission of wildlife disease information to the OIE under the authority of their country’s OIE Delegate.

The national and international reporting of wildlife diseases is important to build situational awareness of wildlife disease and health, build national knowledge capacity, increase coordination among agencies, and integrate wildlife health into other surveillance frameworks. The OIE currently monitors 53 non-listed diseases and unusual morbidity and mortality events affecting wildlife because of their potential to threaten biodiversity or as early warnings for human or agricultural health threats. The USGS National Wildlife Health Center (NWHC) contributes to these international reports from data collected in collaboration with our state, federal, and tribal partners and maintained at NWHC.  However, we recognize that many partners maintain data that could also be useful for these reports and we continually strive to further increase information sharing among the wildlife health community through publicly accessible data platforms such as WHISPers (Wildlife Health Information Sharing Partnership-event reporting system).  For more information or to report a disease of interest please contact US OIE National Focal Point for Wildlife: Dr. Jonathan Sleeman, Center Director, USGS National Wildlife Health Center, (608) 270 2401;


Oral delivery of vaccine to control rabies in vampire bats

Treatment of rabies transmitted by vampire bats (Desmodus rotundus) to cattle or people is an ongoing economic burden to Central and South American countries. As the range of vampire bats continues to expand northward, they are expected to inhabit southern Texas within the next decade thereby increasing rabies transmission risk to livestock in this area. Historically, culling of vampire bats to reduce rabies transmission has been the most commonly applied management practice. However, bats provide many ecosystem services and some studies have even shown culling to be counterproductive to disease control (Streicker et al. 2012, 10.1098/rspb.2012.0538).

USGS National Wildlife Health Center (NWHC) scientists are seeking alternative methods to control this disease including development of an effective and practical oral rabies vaccine that can be applied to the skin of vampire bats. Laboratory trials were initiated at NWHC in the winter of 2018 on vampire bats captured from Mexico to establish the efficacy of a vaccine developed in collaboration with the University of Wisconsin. The intended goal of this project is to find better ways to manage rabies in bats, reducing risks to humans and domestic animals.

Little brown bat with white-nose syndrome. (Credit: Marvin Moriarty, U.S. Fish and Wildlife Service.)


Development of vaccine for white-nose syndrome

USGS National Wildlife Health Center (NWHC) scientists are developing potential topically applied vaccine candidates against the fungus that causes white-nose syndrome. Several vaccine candidates have been tested to determine which provides the best protection to hibernating bats. Scientists are now conducting a second confirmatory animal trial in hibernating, captive little brown bats (Myotis lucifugus), to ensure that the vaccine is safe and effective. Scientists also continue to work closely with the US Department of Agriculture Center for Veterinary Biologics to get the experimental vaccine registered and available for field testing. Once developed, the goal is to confer disease resistance to vulnerable bats and safeguard their populations.


African Swine Fever

Feral pigs at Lower Suwannee National Wildlife Refuge, Florida
If African swine fever is introduced into United States, feral swine could potentially become a reservoir for the virus. Feral pigs at Lower Suwannee National Wildlife Refuge, Florida. (Credit: Steve Hillebrand, U.S. Fish and Wildlife Service. Public domain.)

African Swine Fever (ASF) is a highly contagious and fatal disease of domestic and wild swine. The disease, characterized by pigs developing hemorrhagic lesions on their skin and internal organs, is caused by the African Swine Fever virus (ASFV), a double-stranded DNA arbovirus. High mortality and prolonged environmental survival of the virus make it difficult to eradicate. African swine fever is endemic to Africa and is historically known to have been introduced twice, in 1957 and 2007, into Europe. In January 2018, ASF was detected near the Ukrainian border. In August of last year, the virus was reported in China and subsequently has been detected in over 25 provinces and municipalities. In Europe, dissemination has mainly been through circulation in wild boar, while in China the domestic swine industry has been implicated. An additional mechanism by which ASF virus can spread is through movement of infected pork products, which are suspected in the transmission of ASFV to wild boar in Belgium. Since October 2018 Japan, South Korea, and Taiwan have also detected multiple instances of ASFV in pork products carried in the luggage of passengers arriving from China and Vietnam. The virus has continued to spread in 2019, with 1,235 on-going and 434 new ASF outbreaks reported to OIE in the first two weeks of February.

While ASFV has never been detected in North America, repeated introductions into Europe, more recent introduction and subsequent spread in China and Vietnam, and seizure of over one million pounds of illegally imported Chinese pork products in New Jersey highlight the capacity for intercontinental movement of the virus. The main impact of introduction of ASFV to North America would be to the domestic swine industry. However, given that wild boar in Belgium are thought to have been infected by ingestion of contaminated pork products, and that ASFV-positive food items have been interdicted at border crossings in several other countries, introduction of this virus into the United States, with subsequent exposure of feral swine, is a realistic scenario.

State and federal agencies should remain vigilant and report sightings of sick or dead feral swine to their local USDA Wildlife Services Biologist (1-866-4-USDA-WS). As a Foreign Animal Disease (FAD), all field response to suspect cases of ASF in the United States will be led by USDA. As an Affiliate Member of the USDA National Animal Health Laboratory Network, the USGS National Wildlife Health Center is approved to conduct testing for ASF in feral swine should our assistance with national surveillance for this FAD be requested. 

Information Sources:

Channel News Asia. 10/23/18. Japan finds African swine fever in sausage brought from China. (link)

CNN. 10/2/18. Health officials “very worried” as African swine fever spreads in Europe and Asia. (link)

Dixon, L. K., 2017. Preparing for the worst: African swine fever in China. Vet Record 181:115.

Guinat, C. et al., 2017. Effectiveness and practicality of control strategies for African swine fever: what do we really know? Vet Record 180:97. 3/15/19. Feds seize 1 million lbs. of pork smuggled from China to N.J. port amid African swine fever outbreak. (link)

OIE, 2019. African Swine Fever Situation Report No. 9, Jan 18-31. (link).


White-nose syndrome continues to spread and impact new species

Mexican free-tailed bat
Mexican free-tailed bat (Tadarida brasiliensis). (Credit: Ann Froschauer, US Fish and Wildlife Service. Public domain.)

White-nose syndrome (WNS) is a fungal disease causing severe declines of bat populations in North America. Identifying ways to reduce impacts of this disease is a priority but is inhibited by lack of an experimental animal model that does not require use of wild-caught bat species already in decline due to WNS. Scientists at the USGS National Wildlife Health Center tested whether the Mexican free-tailed bat (Tadarida brasiliensis), one of the most abundant species of bats in the Americas, could serve as a suitable animal model for WNS research. While T. brasiliensis was susceptible to experimental infection with the causative fungus, Pseudogymnoascus destructans, under conditions that induced hibernation, the species exhibited limited pathology from the fungus. These results indicate that T. brasiliensis is not likely a suitable experimental model for WNS research. However, 

understanding the potential for T. brasiliensis to become infected with P. destructans is timely given that this fungal pathogen has recently spread to the range of this long-distance migrating bat species. Recovery of viable WNS-causing fungus from experimentally infected T. brasiliensis indicates potential for this species to contribute to spread of the pathogen where it coexists with other species of bats affected by WNS. 

For more details about this work, see:  Verant, et al. 2018. Experimental infection of Tadarida brasiliensis with Pseudogymnoascus destructans, the fungus that causes white-nose syndrome.  mSphere 3:e00250-18.


When your best guess just isn’t good enough: Improving estimates of cause-specific mortality

A fundamental task for wildlife managers is understanding causes of mortality in a population and their associated impacts on population vital rates. However, in many cases the definitive cause of a mortality is unknown and the biologist assigns the most likely cause based on their expert opinion. Although their opinion may be informed by information such as the carcass condition, evidence at the death site, and field necropsy results, often, it is a best guess. Once the biologists have made their best guess, analyses are then conducted as if there is no ambiguity associated with this assignment, and management decisions are made based on the results. Ignoring the uncertainty around the cause of death parameter can inadvertently impact the validity of management decisions. For example, the evidence may be inconclusive regarding whether a prey species was killed by a predator or died of other causes and was subsequently scavenged by the predator. Without accounting for this uncertainty the use of controversial measures such as predator control may not only be unwarranted, but also ineffective at changing the population’s vital rates. USGS National Wildlife Health Center scientists developed a new statistical approach that allows biologists to incorporate their expert opinion as well as associated uncertainty into models for cause-specific mortality. When evaluated via simulations and applied to a dataset on mortality in a white-tailed deer (Odocoileus virginianus) population in Wisconsin, scientists improved the biologist’s ability to accurately estimate survival and sources of mortality by reducing bias in the estimates and their associated precision. A description of this statistical modeling approach can be downloaded from:


A white-tailed deer fawn taking refuge in vegetation.
A white-tailed deer fawn taking refuge in vegetation. (Credit: Daniel Walsh, USGS. Public domain.)

Expanding chronic wasting disease diagnostic tools for management agencies

Currently the most common tissues used for chronic wasting disease (CWD) surveillance are post-mortem samples (e.g., retropharyngeal lymph nodes). The diagnostic tests most widely used for these samples are immunohistochemistry (IHC) or ELISA.  Because these tests rely on high concentrations of disease associated prion protein they cannot detect CWD at its earliest stages in tissues collected from animals and are ineffective for detecting prions in environmental samples. The main difficulty in developing new diagnostic tools is that the causative agent of CWD, prions, are proteins and lack a genome, precluding the use of powerful genetic tools such as PCR, deep sequencing, and bioinformatics that have revolutionized investigation of other diseases.  However, recent advancements in in vitro prion conversion assays are overcoming the limitations of current CWD diagnostics, and now allow detections of CWD prions with >10,000 fold the sensitivity of traditional assays. Due to its high throughput capability, reliance on recombinant bacteria instead of live animal tissue, and proven repeatability real-time quaking induced conversion (RT-QuIC) development has been progressing rapidly over the last decade. Nevertheless, there are still assay limitations (e.g., it has only been optimized for a limited number of sample types) and availability has been restricted primarily to research laboratories.  The USGS National Wildlife Health Center is partnering with University of Wisconsin and Wisconsin Department of Natural Resources to develop and optimize RT-QuIC technology for use on biological samples as well as environmental samples. The purpose of this work is to make this technology more readily accessible to management agencies thereby opening exciting new avenues of research to help better manage this insidious disease.  


Coral mortality continues along the Florida Reef Tract

The Florida coral reef tract spans 330 nautical miles from Martin County, FL to Dry Tortugas National Park and is co-managed by the Florida Department of Environment Protection, National Oceanic and Atmospheric Administration (NOAA), National Park Service (NPS), and the US Fish and Wildlife Service (USFWS). The Florida reef tract generates $6.3 billion annually for the tourism industry and supports critical fisheries habitat necessary for recreational and commercial fishing. In addition, the reefs provide storm surge and erosion protection for the 6 million people who live along Florida’s Atlantic coast.

The Florida Department of Environment Protection (FL DEP) first noticed unusually high mortality rates in stony corals, the species primarily responsible for the structure of coral reefs, in 2014 near the Port of Miami. Since that initial identification of an unusual event, the disease has spread to the northern limits of the reef tract and south through the lower Florida Keys as of February 2019. Unlike previous coral disease events that tended to be limited in species affected, proportion affected, and geographic scale, the ongoing outbreak affects greater than 20 species of corals, a large proportion of individuals (30-60% depending on the species) and almost the entire reef tract over the past 5 years. Currently, the cause of this event is unknown, but an emerging infectious disease is suspected.

The USGS National Wildlife Health Center continues to provide expert consultation and courtesy review to the Florida Reef Tract Disease Advisory Committee, co-led by FL DEP and NOAA. Ongoing collaborations include review of histopathological slides by one of only three US based coral disease pathologists (Dr. Thierry Work, USGS NWHC Honolulu Field Station) and participation in expert workshops and phone conferences relating to response activities.


Controlling invasive corallimorphs at Palmyra Atoll

Corallimorph infestation on coral reef 2
Invasive corallimorphs at Palmyra Atoll National Wildlife Refuge. (Credit: Thierry Work, USGS. Public domain.)

Beginning in 2007 USGS National Wildlife Health Center Honolulu Field Station (HFS) documented the spread of an invasive corallimorph on the coral reefs at Palmyra Atoll National Wildlife Refuge (PAL). After determining that a wrecked longline vessel on the western shelf of the atoll had contributed to the phase shift from corals to corallimorphs on the atoll, the wreck was removed in 2013 by US Fish and Wildlife Service (USFWS). Surveys performed in 2016 demonstrated marked declines in corallimorphs near the wreck site but also revealed new infestations to the southwest and increasing encroachment towards a particularly diverse patch of corals located to the east of the island. In 2018, HFS and USFWS successfully initiated treatment trials to test biodegradable toxic paste or hot water to control the corallimorphs. Moreover the treatments had limited to no adverse collateral effects on native biota (clams and corals). The next step is to scale up treatment to medium size areas in preparation for a broader atoll-wide control effort. 


More Information from NWHC

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