Photo by Steve Hillebrand, USFWS.
Occurrence of Hemorrhagic Disease in Illinois: Four Decades of Spatial and Temporal Changes
Reports of hemorrhagic disease (HD) affecting ruminants in the U.S. date to the late 1800s, when residents in the South described a condition affecting deer as black tongue. The colloquial term refers to white-tailed deer found dead — usually near water sources — with a discolored and swollen tongue that protruded from the mouth (Figure 1). Since then, two viruses (Family Reoviridae) have been found responsible for two of the major vector-borne viral diseases affecting domestic and wild ruminants in the U.S. The diseases—bluetongue (BT) and epizootic hemorrhagic disease (EHD)—are caused by bluetongue virus (BTV) and epizootic hemorrhagic disease virus (EHDV), respectively. As these viruses similarly affect ruminant species, the illness caused by BTV and/or EHDV is referred to as hemorrhagic disease (HD).
BT and EHD are found worldwide in temperate and tropical regions, where the vector-ruminant hosts cycle is maintained naturally (Figure 2). Culicoides biting midges (aka no-see-ums, pinyon gnats, punkies, five-O’s, moose flies) are the vectors responsible for HD transmission. Culicoides species transmit the viruses that cause HD, although not all Culicoides spp. are competent vectors. In the U.S., C. sonorensis is the primary vector due to its broader distribution, and C. insignis is regarded as a competent vector in the southeastern U.S.
Animals infected with BT/EHD viruses may suffer from subclinical or non-apparent clinical signs or severe illness and mortality. However, some will remain asymptomatic carriers of the virus, posing as sources of infection for vectors and other animals. Note that it is important to take into consideration the host species’ physiology, immunology, and genetics when evaluating HD outbreaks. For instance, sheep, cattle, and goats are primarily affected by BTV, while EHDV notably impacts ruminant wildlife (in specific white-tailed deer). Because several domestic and wild ruminant species are susceptible to infection by EHDV and BTV, it is only through laboratory diagnostic testing that BT and EHD can be distinguished.
The increase of reported HD cases and geographical expansion of HD overtime have been documented in recent decades in North America and worldwide (Stallknecht et al., 2015). Using 38 years of historical data (1982-2019), a recent study by Dorak et al. (2022) evaluated the occurrence of HD cases in Illinois wild white-tailed deer. The study published in Scientific Reports titled “Spatial epidemiology of hemorrhagic disease in Illinois wild white-tailed deer” showed the increase in cases and expansion of the geographic distribution over time (video link). They determined county-level HD cases reported in 24 out of 38 years. The first year with reported cases was 1998, and by 2019, 99/102 counties in Illinois had reported BTV/EHDV cases at least once (Figure 3 A). The only counties with zero reports were Boone, DeKalb, and Kankakee, all located at the northern part of the state. The researchers also provide details of the years with significant outbreaks and high-rate disease clusters (Figure 3 B). For example, 2006, 2007-2009, 2012, and 2013 were important outbreak years because large deer die-offs across multiple counties were reported (Figure 3 C). Furthermore, HD cases clustered in space and time, corroborating the localized virus transmission during the periods of the outbreaks. Interestingly, while the results show that wild white-tailed deer in Illinois have been exposed to both BTV and EHDV, only EHDV was detected during HD outbreaks. Thus, EHDV was the virus driving HD outbreaks in Illinois.
Dorak et al. (2022) demonstrated that a northward expansion was first identified during the 2012-2013 outbreak. High mortalities at northern latitudes could be expected as naïve populations could be exposed to the incursion of HD viruses as suggested by Ruder et al. (2015). Another essential detail identified by Dorak et al. (2022) was that general disease surveillance throughout the year helped identify BTV/EHDV infection outside what is typically considered the regular HD transmission period—which in Illinois is during the summer months (June to September) when Culicoides are active. The emergence and/or re-emergence of vector-borne infectious diseases occur over time when the interplay between ruminant host, vector, and the environment favors disease transmission (Figure 1). Dorak et al. (2022) found cyclical patterns in Illinois with high HD cases in 2007 and again in the 2012-2013 outbreak. Coincidentally, severe drought and high temperatures affected the U.S. during those years and were associated with major HD outbreaks reported in wild white-tailed deer and cattle.
Based on historical regional HD patterns in the U.S.—which have shown patterns of occurrence every 2 to 3 years (enzootic) or every 8 to 10 years (epizootic)—the next outbreak in Illinois could occur in approximately 1 to 2 years. Besides enzootic and epizootic patterns of HD outbreaks, there are patterns in HD mortality in wild ruminants that can vary between zones. These zones are known as endemic, epidemic, and incursive zones. In endemic zones, BT and/or EHD infection is common (high prevalence of seropositive animals, but disease is uncommon). On the other hand, epidemic zones are those where outbreaks appear periodically, and the prevalence of seropositive animals varies annually. Incursive zones are regions where seropositive animals are sparse and infection is rare. Moreover, monitoring HD should remain a priority in captive and wild ruminant populations to identify the state’s enzootic or epizootic HD patterns, zone(s) types, and potential changes in patterns over time.
Furthermore, recognizing and understanding the multiple factors contributing to or interfering with HD’s life cycle is crucial for preventing and controlling infectious diseases. For instance, changes in precipitation, wind velocity, and temperature can affect the distribution and abundance of the arthropod vector and the development of the virus in the hosts. In addition, ruminant host behavior could also be affected by climate change. For example, agglomeration in waterways due to drought conditions and heat may predispose animals to acquire HD, as increased contact with vectors makes ruminant hosts more susceptible.
HD outbreaks impact animal populations—from wildlife to captive cervid herds and the agricultural industry—and have an economic and social effect. As the authors explained, “Only through surveillance can a complete picture of the variables associated with the EHDV/BTV triad (vector-host-environment) be developed and used to assess management programs designed to protect wild and domestic ruminants.” However, there are multiple challenges while conducting surveillance during HD outbreaks that the authors recognized (Figure 4).
The expansion of BT and EHD beyond traditional boundaries in the U.S. and Europe has shown the potential to “increased morbidity when entering susceptible animal populations” (Ruder et al., 2015). The results of Dorak et al. (2022) corroborate the importance of expanding surveillance efforts, collecting precise geographic locations during outbreaks, and the vital role of virus isolation in helping wildlife agencies understand and predict HD outbreaks and better inform the public.
Dr. Nelda Rivera‘s research focuses on the ecology and evolution of new and re-emerging infectious diseases and the epidemiology of infectious diseases, disease surveillance, and reservoir hosts’ determination. She is a member of the Wildlife Veterinary Epidemiology Laboratory and the Novakofski & Mateus Chronic Wasting Disease Collaborative Labs. She earned her M.S. at the University of Illinois at Urbana-Champaign and D.V.M at the University of Panamá, Republic of Panamá.
Dr. Nohra Mateus-Pinilla is a veterinary Epidemiologist working in wildlife diseases, conservation, and zoonoses. She studies Chronic Wasting Disease (CWD) transmission and control strategies to protect the free-ranging deer herd’s health. Dr. Mateus works at the Illinois Natural History Survey- University of Illinois. She earned her M.S. and Ph.D. from the University of Illinois Urbana-Champaign.
Dr. Jan Novakofski studies prion diseases or infectious agents composed entirely of protein in animals such as “mad cow disease” and scrapie. His efforts are contributing to better understanding the genetics and transmission of these types of diseases to protect the health of animals and humans. He earned his B.S., M.S. and PhD from the University of Wisconsin, Madison.