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Emerging Trends in Tick-borne Diseases in the United States

JUN 15, 2016 | NICOLA M. PARRY, BVSC, MRCVS, MSC, DIPACVP

Update on Borrelia miyamotoi 

Alan Barbour, MD, a professor at the School of Medicine, University of California, Irvine, discussed Borrelia miyamotoi, another Borrelia group spirochete that is transmitted by ixodid tick species and may be the cause of another emerging TBD in this country.
 
According to Dr Barbour, in the United States, B. miyamotoi disease (BMD) was first reported in 2013 in the Northeast, but is becoming increasingly common and should be considered in all areas where infections transmitted by deer ticks are endemic. B.miyamotoi is distributed across a similar geographic range as B. burgdorferi in North America, Dr. Barbour said. However, although B. burgdorferi prevalence is about ten times that of B. miyamotoi in most areas, he noted that the prevalence of B. miyamotoi is similar to that of B. burgdorferi—and often higher—in some areas of of California. Another important feature, Dr. Barbour noted, is that most cases of BMD have occurred in July and August, suggesting disease transmission by larval ticks.
 
The symptoms of BMD are nonspecific and include fever, headache, chills, myalgia, and arthralgia. BMD can also be severe and include meningoencephalitis. Symptoms can persist for weeks to months, and relapse of fever can also occur in some cases. Some patients with BMD may also be coinfected with B. burgdorferi. Compared with patients with Lyme disease, those with BMD tend to have a higher fever and are more likely to require hospitalization, in particular because of meningoencephalitis; they also have a low platelet count and abnormal liver enzyme levels. The public health significance of BMD therefore relates to its similar presentation to human granulocytic anaplasmosis (HGA), Dr. Barbour said.
 
Detection of B. miyamotoi in diagnostic specimens by using phase contrast or dark field microscopy can be difficult, and Dr. Barbour recommended examination of a thick blood smear and immunofluorescence staining to identify spirochetes. PCR is also available to detect the organism, as is a serological assay to detect antibodies to the glycerophosphodiester phosphodiesterase (GlpQ) of B. miyamotoi. 
 
Doxycycline is the preferred initial treatment choice for patients with BMD, Dr. Barbour said, in particular because it is also effective against Lyme disease and HGA. Treatment with a beta lactam antibiotic may also be necessary for patients with central nervous system disease, he said.
 

Emerging Tickborne Viral Diseases

According to Nicholas Komar, PhD, from CDC, Fort Collins, Colorado, although tick-borne viruses known to cause disease in the United States are rare, their incidence may be changing. He discussed four such viruses—none of which causes more than a mean of 10 cases annually in the United States.
 
Colorado tick fever virus (CTFV) causes the greatest number of human cases of tick-borne viral diseases in this country, he said. CTFV is transmitted to humans by the bite of an infected Rocky Mountain wood tick (Dermacentor andersoni). The disease is typically mild and manifests as fever, chills, headache, body aches, and fatigue. In most cases, these symptoms resolve without treatment. Some patients have developed symptoms such as skin rash and abdominal pain. And, in rare cases, encephalitis, hemorrhage, and death have been reported.  
 
Powassan virus, a flavivirus, causes the second highest number of human cases of tick-borne viral diseases in the United States, and is transmitted by the bite of an infected tick, including Ixodes ticks as well as tick species that feed on small mammals. Although typical disease manifestations of Powassan virus illness include fever, vomiting, headache, speech difficulties, and seizures, more severe disease—including encephalitis—can also occur. No specific treatments exist for Powassan virus illness, but with good supportive care, 90% of patients with severe disease will survive. However, persistent neurological sequlae are common.
 
Heartland virus (HRTV), a phlebovirus, was first isolated in 2009 from a patient at Heartland Regional Medical Center in Missouri. The virus is transmitted by the bite of an infected lone star tick (Ambylomma americanum), and so far, less than 20 cases have been reported to the CDC, all occurring in Missouri, Oklahoma, and Tennessee, where the tick vector is in prevalent. These cases have all involved men, although it is unknown whether this reflects a biological risk factor, or merely a behavioral risk factor because all affected men were active outdoors in heavily-infested habitats.
 
Symptoms in these patients included fever, fatigue, headache, nausea, and diarrhea. The patients also had low white blood cell counts and most required hospitalization because of their symptoms. Because these features are similar to those in cases of ehrlichiosis, the patients with HRTV disease were treated with antibiotics, but did not respond to therapy. Although no specific treatments exist for HRTV disease, supportive therapies can help improve symptoms. The case fatality rate is approximately 10%.
 
Bourbon virus, a thogotovirus, was first isolated in June 2014 from a man in Bourbon County, Kansas, who died after a period of illness following a tick bite. The patient’s symptoms were similar to those of HRTV disease and included fever, weakness, headache, nausea, diarrhea, anorexia, myalgia, and arthralgia. He also had a low white blood cell count. However, overall, this virus still remains poorly understood, Dr. Komar said.
 

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