Tick tick, can make you sick
We have long been familiar with common tick-borne infections that occur in the United States, such as Lyme disease, anaplasmosis, ehrlichiosis, babesiosis, Rocky Mountain spotted fever, Colorado tick fever, southern tick-associated rash illness, and tularemia.
However, there are also six, either newly or more frequently recognized diseases in the U.S. that are known or presumed to be tick-borne. These are currently rare diseases, and in some cases very little is known about them. Three of these are viral — Powassan disease, Heartland virus infection and Bourbon virus infection. The remaining three diseases are bacterial, of which two are rickettsial — Rickettsia parkeri rickettsiosis, and R. phillipi (Rickettsia 364D) rickettsiosis. One is spirochetal — Borrelia miyamotoi infection. All of these tick-borne infections are probably zoonotic, with humans as an accidental host.
While not household names, all of them undoubtedly are much more common than previously recognized. We will certainly be seeing more of these diseases in the future, especially as we look for them. Furthermore, the habitat of infected ticks is spreading for at least three probable reasons: global warming; the fact that ticks can attach to birds and be carried long distances; and growth of suburban neighborhoods into surrounding wooded areas.
What follows is a brief discussion of these six diseases.
There are two lineages of Powassan virus (POWV) in the United States. Lineage 1 appears to be associated with Ixodes cookei or I. marxi, the ticks of woodchucks and squirrels. Lineage 2 is sometimes called deer tick virus, and is associated with I. scapularis, the ticks of white-footed mice. Both lineages have been linked to human disease.
Although discovered in 1958, POWV, a Flavivirus, has been described as an increasing cause of encephalitis in the U.S. since 2006. About 10 cases per year are recognized. The virus is found in small mammals in eastern Canada and the U.S., and is transmitted to man by Ixodes spp. (usually I. scapularis) and Dermacentor ticks. Cases have occurred mainly in the Northeastern states and in Minnesota and Wisconsin. The disease is also found in Eurasia. The virus causes severe encephalitis, which is fatal in 10% of cases, and 50% of those surviving are permanently disabled neurologically. Mild or asymptomatic disease occurs as seropositivity has been found in some healthy individuals. Ticks cannot acquire the virus from feeding on infected humans, who act as a dead-end host. Unlike B. burgdorferi, which usually requires at least 24 hours of tick attachment for transmission of the pathogen, POWV can be transmitted in just 15 minutes.
The Heartland virus, a Phlebovirus, was first recognized in cases that occurred in 2009 in Missouri. There have been 10 cases recognized in the U.S. — eight in Missouri, one in Tennessee and one in Oklahoma (according to a participant at a recent conference, there have actually been about 20 cases). The disease is presumably transmitted by the Lone Star tick (Amblyomma americanum). The clinical illness consists of fever, leukopenia and thrombocytopenia. Two patients died; at least one had comorbidities. A related tick-borne Phlebovirus — accompanied by severe fever with thrombocytopenia syndrome and first described in 2010 — causes a similar illness in China, South Korea and Japan.
Bourbon virus, a Thogotovirus, was first isolated from a patient who died in eastern Kansas in the spring of 2014. Another case was described in May in Oklahoma; he recovered. It is thought to be tick-borne because the illness resembles other tick-borne diseases, and the first patient had multiple tick bites. The illness included high fever, myalgias, abnormal liver chemistries, leukopenia and thrombocytopenia.
R. parkeri rickettsiosis (RPR) was first described in a man with the infection in 2002. It is transmitted by the Gulf Coast tick (A. maculatum). The disease occurs in Southern and East Coast states and resembles a mild case of Rocky Mountain spotted fever (RMSF). Although the number of cases reported has been low (the largest series consists of 12 patients), it is thought to be much more frequent and often misdiagnosed as RMSF. This is because most commercial serologic assays do not distinguish between spotted fever group species. A. maculatum are found in all states bordering the Gulf of Mexico and some other Southern, Middle Atlantic, and central states. R. parkeri has been detected in A. maculatum ticks in many of these states. There is an incubation period of 2 to 10 days after the tick bite during which a sore appears at the bite site. This is followed by symptoms that include fever, fatigue, myalgia, headache and a maculopapular or vesiculopapular rash involving primarily the trunk and extremities and often the palms or soles. The presence of an eschar at the location of the tick bite helps differentiate RPR from RMSF, where eschars are rare.
R. phillipi (formerly known as Rickettsia 364D) was first reported as a cause of spotted fever rickettsiosis in 2008. All of the reported cases (about 12) have occurred in California. R. phillipi is transmitted by Pacific Coast dog ticks (Dermacentor occidentalis).
The incubation period is 3 to 14 days, after which symptoms occur, including fever, rash, headache, myalgia, anemia, thrombocytopenia and/or transaminase elevation. An eschar is found at the site of the tick bite.
B. miyamotoi infection was first described in patients in the U.S. in 2013 in New England and southern New York. Although B. miyamotoi has been found in Lyme disease vectors globally (including I. pacificus in the Western United States, I. ricinus in Europe, and I. persulcatus and I. ricinus in Russia), the only human infections reported have been from Russia in 2011 and now the U.S. Its prevalence in tick vectors for Lyme disease is only about 10% of that for B. burgdorferi, but varies geographically. The ratio of B. burgdorferi to B. miyamotoi presence in I. scapularis nymphs in the Northeastern U.S. has been reported to range from 4:1 to 16:1, and in California, B. miyamotoi presence in nymphal and adult I. pacificus has been reported to approach and even exceed that of B. burgdorferi. One immunocompromised patient in the U.S. had meningoencephalitis, and three patients had fever, chills and muscle aches. A very recent serological study of 11,515 patients presenting with fever in the Northeastern U.S. revealed 97 cases with B. miyamotoi infection. Symptoms were high fever, chills, marked headache and myalgia or arthralgia. It seems likely that B. miyamotoi causes undiagnosed infections in any area where Lyme disease is endemic.
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- For more information:
- Donald Kaye, MD, is a professor of medicine at Drexel University College of Medicine, associate editor of ProMED-mail, section editor of news for Clinical Infectious Diseases and an Infectious Disease News Editorial Board member.
Disclosure: Kaye reports no relevant financial disclosures.