Transfusion-transmitted tick-borne infections: A cornucopia of threats

Transfusion-Transmitted Tick-Borne Infections: A Cornucopia of Threats David A. Leiby and Jennifer E. Gill Over the past several decades, the frequency of contact between humans and ticks has increased dramatically. Concomitantly, several newly recognized tick-borne pathogens have emerged joining those already known to be transmitted by ticks. Together these factors have led to an enhanced public health awareness of ticks, tick-borne agents, and their associated diseases. Reports that several of these agents are transmitted by blood transfusion have raised concerns about blood safety. The primary agents of interest are members of the genus Babesia, but Anaplasma phagocytophilum, Rickettsia rickettsii, Colorado tick fever virus, and tickborne encephalitis virus also have been transmitted by transfusion. In many cases, these agents and their diseases share common features including vectors, symptoms, and diagnosis. Unfortunately, they also share the common problem of insufficient epidemiologic and

transmissibility data necessary for making informed decisions regarding potential blood safety interventions. Although further surveillance and epidemiologic studies of tick-borne agents are clearly needed, at present only the Babesia warrant consideration for active intervention; because donor management strategies based on risk-factor questions are inadequate, leukoreduction not effective for agents found in red cells and pathogen inactivation remains problematic for red cell products. Despite the present unavailability of screening assays, some form of serologic and nucleic acid testing may be justified for the Babesia. Given that interactions between humans and ticks are likely to increase in the future, vigilance is required as new and extant tickborne agents pose potential threats to transfusion safety. © 2004 Elsevier Inc. All rights reserved.

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agents of human ehrlichiosis were described in the United States. Ehrlichia chaffeensis, the agent of human monocytic ehrlichiosis (HME) was first recognized in 1986,6-8 and Anaplasma phagocytophilum (formerly Ehrlichia sp, E phagocytophila), the agent of human granulocytic ehrlichiosis (HGE), was first described in 1994.9,10 Similarly, 2 new Babesia-like organisms were described in 1993 and 1996, WA-1 from Washington and MO-1 from Missouri, respectively.11,12 These series of events clearly suggest that new tick-borne agents continue to be recognized and additional ones will likely be identified in the future. Reports that several tick-borne agents (eg, B microti, A phagocytophilum, Rickettsia riskettsii) have been transmitted by blood transfusion have raised concerns about blood safety. These concerns were brought to the forefront in 1997 when 12 members of the Iowa National Guard developed Rocky Mountain Spotted Fever (RMSF) and/or ehrlichiosis after field training (Ft. Chaffee, AR)

ICKS AND THE varied pathogenic microorganisms that they transmit to humans have taken on enhanced public health significance during the last several decades. It is now recognized that in the United States alone, ticks transmit over a dozen pathogens, including those representative of bacteria, rickettsia, protozoa, and viruses.1,2 In part, ticks became more prominent vectors of human disease as interactions between humans and ticks increased during the past 20 years. Greater contact between ticks and humans has been fostered by the movement of people to rural areas and more frequent outdoor recreational activities, both of which place humans and ticks in closer proximity. Additionally, the reforestation of agricultural land and suburban neighborhoods has provided suitable habitat for the proliferation of rodents, deer, and other animals that play critical roles in perpetuating the life cycles of ticks and tick-borne agents. It is also clear that during this same time period newly recognized etiologic agents of disease transmitted by ticks have emerged. In the mid-1970s, an epidemic of arthritis with an unknown etiology was described in residents living near Old Lyme, CT. Several years later, a newly described spirochete, Borrelia burgdorferi, was identified as the causative agent of the malady now known as Lyme disease.3-5 Subsequent studies revealed that B burgdorferi is widely distributed in the United States and other parts of the world. More than a decade later, 2 newly recognized

From the Department of Transmissible Diseases, American Red Cross Holland Laboratory, Rockville, MD. Address reprint requests to David A. Leiby, PhD, Department of Transmissible Diseases, American Red Cross, 15601 Crabbs Branch Way, Rockville, MD 20855. E-mail: leibyd@usa. redcross.org © 2004 Elsevier Inc. All rights reserved. 0887-7963/04/1804-0005$30.00/0 doi:10.1016/j.tmrv.2004.07.001

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during which they experienced extensive tick exposure.13 Several of the ill guard members reportedly donated blood 24 to 72 hours before the onset of symptoms. Because of concerns regarding potential blood transmission, a voluntary recall of blood components collected at the military base during 3 separate blood drives was issued and a multistate investigation was initiated, but no evidence of transmission was identified in 10 recipients. Ultimately, these and other events led the Centers for Disease Control and Prevention, the Food and Drug Administration, the National Institutes of Health, and the Department of Defense to jointly convene a workshop in early 1999 entitled, “The Potential for Transfusion Transmission of Tick-Borne Agents.” The primary goals of the workshop were 4-fold: (1) to review the epidemiology and biology of tick-borne agents transmissible by blood transfusion; (2) to identify the risks posed by these agents to the US blood supply; (3) to identify research needs, particularly those related to diagnostic test development; and (4) to discuss whether interventions are needed to reduce the risk of transfusiontransmitted tick-borne infections.14 A primary conclusion of this workshop was that based on biologic and epidemiologic evidence, the Babesia appeared to present the greatest risk of transmission by transfusion. However, it was implicitly implied that other tick-borne agents could not be ignored and that rational approaches are needed to address all tick-borne agents including increased surveillance, development of more specific and sensitive diagnostic tests, and continued investigation of new product management techniques (eg, pathogen inactivation). Although the Babesia continue to be the agents of greatest concern, an expanding group of tickborne agents also warrant our attention and further examination. However, despite an increased awareness regarding potential and actual cases of transfusion-transmitted tick-borne agents, to date no interventions have been implemented to protect the blood supply from any of these agents. In large part, insufficient clinical and epidemiologic data are available for most agents to make informed decisions regarding the need for blood screening or other measures to protect the blood supply. Additionally, test manufacturers have been reluctant to develop specific tests for these emerging agents because of perceived market limitations.

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The present review is designed to examine those tick-borne agents that have been shown to be transmitted by blood transfusion and those for which transmission is likely to occur. Several recent articles on this subject have provided excellent reviews of single or multiple tick-borne agents, providing separate discussions of each agent.15,16 Rather than duplicate these existing reviews, we have attempted herein to provide an integrated review designed to examine the common features of these agents (eg, vectors, symptoms, diagnosis, and so on) and to provide an expanded discussion of potential donor management strategies. ETIOLOGIC AGENTS, VECTORS, AND GEOGRAPHIC DISTRIBUTION

The tick-borne diseases discussed in this review can generally be considered zoonoses in which humans are accidental, dead-end hosts. The exception, however, is when the causative tick-borne agent is transmitted by blood transfusion. By definition, ticks are the critical vectors for transmitting these agents to blood donors who unwittingly pass them along to blood recipients. For several agents discussed herein, ticks of the genus Ixodes are the principal vector and have been described as maintaining a guild of pathogens, comprised of spirochetes, piroplasms, ehrlichiae, and arboviruses17 (Table 1). Perhaps the most widely recognized tick-borne agent transmitted by Ixodes ticks is B burgdorferi, which causes Lyme disease. However, despite causing thousands of cases of Lyme disease in North America, Europe, and other parts of the world, this agent has not been shown to be transmitted by blood transfusion. Several explanations for the apparent lack of transfusion cases have been proposed. As will be discussed later, Lyme disease like many other tick-borne diseases is characterized by nonspecific symptoms that often go unrecognized. Introduction of the spirochetes intravenously may in fact prevent the formation of the erythema migran lesion often associated with B burgdorferi infections. Finally, the spirochetemic phase of disease may be very short, severely limiting the likelihood of transmission by transfusion. Because of its apparent lack of transmissibility, B burgdorferi will not be discussed further in this review. Piroplasms of the genus Babesia cause human disease (ie, babesiosis) in many areas of the world

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Table 1. Selected Tick-Borne Agents Posing Blood Safety Risks

Agent

Babesia spp Babesia microti WA-1 B divergens Ehrlichia chaffeensis Anaplasma phagocytophilum Rickettsia rickettsii Colorado tick fever virus Tick-borne encephalitis virus

Geographical Distribution

Vector(s)

United States (northeast, upper Midwest) United States (Pacific Coast) Europe United States

Ixodes scapularis I pacificus I ricinus Amblyomma americanum

United States, Europe North America Western United States, Canada Eastern and Central Europe, Russia, Asia

I scapularis, I pacificus, I ricinus Dermacentor variabilis, D andersoni D andersoni I persulcatus, I ricinus

including the United States, Latin America, Europe, Africa, and Southeast Asia.18 With few exceptions, however, human babesiosis has been concentrated in the United States and Europe. In the United States, hundreds of cases of babesiosis have been attributed to infections with the rodent babesial parasite B microti.14,19 Sporadic cases of human babesiosis have also been caused by the Babesia-like organisms MO-1, CA-1, and WA-1 in Missouri, California, and Washington, respectively.11,12,20 More recently, the first case of human babesiosis caused by the bovine parasite B divergens was reported in the United States.21 In contrast, most European cases of human babesiosis are caused by B divergens,22 but other human cases have been attributed to infections with B bovis, B canis, B microti, and the newly described Babesialike organism EU-1.23,24 In the United States, B microti is transmitted to humans by the black-legged tick I scapularis. Both adult and nymphal ticks infected with the parasite are capable of transmitting the infection to humans. Also crucial to the enzootic life cycle of B microti is the white-footed mouse, Peromyscus leucopus, which serves as a reservoir host for the parasite. White-tailed deer, although not a competent host for B microti, play an important role as a maintenance and transport host for adults of I scapularis. The WA-1 parasite is thought to have a similar enzootic life cycle, but the tick vector has not been clearly identified, although it is thought to be I pacificus. In Europe, the sheep tick, I ricinus, has been implicated as the probable vector in human cases of babesiosis, particularly those attributable to B divergens.23 The tick vector for a recently described Japanese isolate of Babesia is less clear

Number of Transfusion Cases Reported

⬎50 2 0 0 1 1 1 2

but by analogy is thought to be I persulcatus, the main vector of Lyme borreliosis in Japan.25 Worldwide there are 5 rickettsial agents that cause human cases of ehrlichiosis: E sennetsu, E canis, E ewingii, E chaffeensis, and A phagocytophilum.7-10,26,29 With the exception of the latter 2 species, human infections are rare and at this time pose a minimal threat to transfusion medicine. As mentioned previously, A phagocytophilum is the etiologic agent of HGE, first described in 1994. The agent has been reported in the United States, parts of Europe, and a recent study suggested its presence in Korea.30,31 The geographic distribution of A phagocytophilum in the United States is concentrated in the Northeast, upper Midwest, and Pacific Northwest but continues to expand. In the Northeast and upper Midwest, the principal vector is I scapularis, whereas the vector on the Pacific Coast is thought to be I pacificus. The European vector is likely to be I ricinus, whereas a Korean vector has not been identified. The etiologic agent of HME, E chaffeensis, occurs primarily in the southeastern and south-central United States, but sporadic cases have been reported from other locations. The primary US vector of E chaffeensis is the Lone Star tick, Amblyomma americanum, and the reservoir host is the white-tailed deer. As suggested by the disease name, this agent infects human monocytic blood cells; however, despite this intracellular location, no transfusion cases have been recognized or reported to date. Serologic evidence of E chaffeensis infections have been reported from patients in Asia, Europe, and Africa, but definitive evidence for the agent in these locations remains controversial.31-34

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Another rickettsial agent transmitted by blood transfusion is Rickettsia rickettsii, the agent of RMSF, the most common fatal tick-borne disease in the United States. Worldwide, a variety of rickettsial agents belonging to the spotted fever group are reported to cause similar disease.35 In humans, R rickettsii is an obligate intracellular pathogen that infects and multiplies within the nucleus or cytosol of endothelial cells lining blood vessels. Geographically, R rickettsii is limited to the Western Hemisphere, causing RMSF in North America and the virtually indistinguishable Brazilian SF in South America. RMSF occurs throughout much of the continental United States, but the highest number of reported cases are from the southeast and south-central states.36,37 In North America, R rickettsii is transmitted by the American dog tick (Dermacentor variabilis) and the Rocky Mountain wood tick (Dermacentor andersoni). In Latin America, R rickettsii is transmitted primarily by Rhipicephalus sanguineus and the Cayenne tick, Amblyomma cajennen.38 There are 2 main groups of viruses that pose threats to blood safety. The first, Colorado tick fever (CTF) virus, is a member of the genus Coltivirus, family Reoviridae. CTF virus infections are concentrated in the Rocky Mountain regions of the United States and Canada and are transmitted by adult D andersoni ticks. As for many of the other agents already discussed, CTF is a zoonotic disease maintained between immature stages of ticks and several host animals, including various mice, porcupines, squirrels, and chipmunks.39 In humans, the virus infects hemopoietic cells, particularly erythrocytes, thus enhancing the likelihood of its transmission by blood transfusion. The other viral agents of interest are the tick-borne encephalitis viruses (TBEV). These agents, members of the genus Flavivirus within the family Flaviviridae, are broadly classified into 3 main subtypes: European, Siberian, and Far Eastern.40 The European subtype is limited to Central and Eastern Europe where I ricinus serves as the primary vector.41 The Siberian and Far Eastern subtypes are primarily found in Eastern Europe and Asia, particularly Russia, where they are transmitted by I persulcatus. A closely related TBEV, Powassan virus, occurs in Canada and the northeastern United States. Human infections result from exposure to ticks, probably I cookei, which are often found on woodchucks and skunks.42

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CLINICAL FEATURES

Symptoms Just as many of the tick-borne agents share common vectors, the disease symptoms elicited by their infections are often similar and nonspecific in nature. In most instances, these agents produce self-limiting infections that are characterized by the following flu-like symptoms: fever, headache, myalgia, and malaise. However, immunocompromised patients show an increased susceptibility to infection, often leading to severe medical complications, even death. This is particularly relevant for blood recipients since by definition many are immunocompromised. The flu-like symptoms associated with human babesiosis appear 1 to 4 weeks postinfection, usually resolving within a few weeks. However, more serious disease complications can occur in immunocompromised patients, particularly the elderly, including hemolytic anemia, thrombocytopenia, and renal failure. Babesial infections are especially problematic for asplenic persons who can develop extremely high, life-threatening parasitemias (up to 85%).22 The mortality rate for clinically apparent B microti infections in the United States approaches 5%.43 Compared with B microti, human infections with WA-1 and B divergens are more virulent, causing fulminant cases of disease with rapid onset.11,23 Most cases of ehrlichiosis are also subclinical or mild, characterized by fever, headache, and general malaise that appear 1 to 3 weeks postinfection. A rash is observed in approximately one third of HME patients but is less frequently observed in patients with HGE.30,44 Severe cases of ehrlichiosis can be characterized by acute renal failure, gastrointestinal bleeding, respiratory distress syndrome, and secondary opportunistic infections; fatality rates of up to 5% for HGE and 10% for HME have been reported.32 The early clinical presentations of RMSF are similarly nonspecific and difficult to distinguish from other infections already discussed. Symptoms appear 2 to 14 days after exposure and include fever, nausea, vomiting, headache, myalgia, and malaise. What differentiates RMSF from other infections discussed herein is the appearance of the characteristic spotted fever rash 2 to 5 days after onset of fever. The rash usually appears on the palms, wrists, forearms, ankles, and soles, but in a

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small number (10%-15%) of patients a rash never develops. Manifestations of more serious disease include meningoencephalitis, myocarditis, adult respiratory distress syndrome, and renal failure.37 Infections with the CTF virus are generally mild with symptoms appearing several days to 2 weeks postinfection. Onset is sudden and characterized by chills, biphasic fever, headache, malaise, and myalgia that last for up to several weeks.39 Mild anemia can occur, but significant leukopenia is more common. Severe cases of CTF may include encephalitis, meningitis, myocarditis, and hemorrhagic manifestations, particularly in children, with persistent viremia in some cases of up to 120 days because of survival of the virus within maturing erythrocytes.45 TBEV symptoms appear 2 to 28 days postinfection with the European subtype producing a relatively mild, less severe disease compared with the Siberian and Far Eastern subtypes. Although the subtypes share the common symptoms of headache, nausea, vomiting, and myalgia, the European subtype is distinguished by the appearance of a biphasic febrile illness, in contrast to the monophasic febrile illness shown by the Eastern subtypes.46 During the initial phase of disease, the TBEV multiplies at the inoculation site, thereby initiating a febrile reaction. Later, as the viremia increases, TBEV spreads through the lymphatics to the central nervous system where it may elicit febrile headaches, aseptic meningitis, meningoencephaliltis, and meningoenchephalomyeliltis.41 More severe forms of disease can lead to paresis and other neurologic complications that eventually cause death. Although symptoms resolve in some cases, many TBE patients develop chronic symptoms commonly describes as postencephalitic syndrome. Diagnosis The differential diagnosis of tick-borne diseases is complicated by similar clinical manifestations, overlapping geographic distributions of the agents, and shared vectors. Further hampering diagnosis is the inability of most people infected with a tickborne agent to recall an associated tick bite.19,47,48 Thus, accurate diagnosis of infection will depend on a combination of the following: clinical signs and symptoms (previously discussed), direct detection of the organism in blood, serologic measure-

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ment of antibodies, and antigen-based tests (eg, polymerase chain reaction [PCR]). Direct detection of Babesia infection can be made by examination of stained peripheral blood smears for infected red blood cells. However, the sensitivity of this technique is directly proportional to host parasitemia levels and care must be taken to differentiate this agent from the malaria parasite. Smear sensitivity can be enhanced by inoculating susceptible hamsters with patient blood. Babesia readily replicate in hamsters, producing infections that can be identified in smears of hamster blood several weeks later. Perhaps the most effective diagnostic tools are serologic assays, particularly the indirect immunofluorescent assay (IFA), which detect Babesia antibodies.49 During the acute phase, when antibody production is limited, PCR assays have been developed that amplify highly conserved sequences of Babesia spp.50-52 As a caveat, both PCR and hamster inoculation are limited by sample and inoculum size, respectively. Thus, despite their sensitivity, a negative result does not exclude the possibility of infection, particularly one in which parasites circulate intermittently in the blood or remain lodged in sequestered locations. HME and HGE can be diagnosed by examination of stained peripheral blood smears for infected leukocytes containing characteristic intracytoplasmic inclusions called morulae. E chaffeensis and A phagocytophilum infections are further differentiated by the phenotype of the infected cell, monocytes, and neutrophils, respectively. Sensitive PCR assays for the detection of amplified rickettsial DNA are also available.53,54 However, after the first week of infection, the bacteremic phase of infection rapidly wanes, thereby limiting the effectiveness of direct detection by smear and PCR.30 Thereafter, serologic detection by IFA is the most frequently used assay for clinical diagnosis.55 Diagnosis of RMSF is almost exclusively based on the presentation of clinical signs and symptoms because diagnostic tests outside of IFA have limited value. Indeed, although direct identification of the agent in tissue can be accomplished by immunostaining, particularly when a skin biopsy is taken from the rash of an infected person, this technique offers only 70% sensitivity. Similarly, PCR of rickettsial DNA is reportedly even less sensitive.2 Again an IFA test, in this case designed to detect immunoglobulin (Ig) M and IgG antibodies 7-10

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days after the onset of illness, is the most widely used diagnostic test.56 A variety of techniques have been reported for diagnosis of CTF virus infection with mixed results. Infected erythrocytes can be identified by immunofluorescent staining, but this technique lacks sensitivity.57 Serologic assays (eg, complement fixation, enzyme-linked immunosorbent assay, Western blot) have been described for detection of IgM and IgG antibodies, but the late appearance of antibodies during the acute phase makes detection of early infections problematic.58-60 However, early infections can be detected using a multiplex reverse-transcription PCR system designed to detect the viral genomic segments designated M6, S1, and S2.58 Cases of TBE are usually diagnosed by viral isolation and serologic testing.46 Viral isolation is accomplished by a variety of techniques including animal inoculation, cell culture and plaque assays. Serologic testing generally uses enzyme-linked immunosorbent assay to target IgM antibodies because detection of IgG antibodies is complicated by cross-reactivity with other flavivirus infections.41,61 Detection of viral RNA by reverse-transcription PCR has been described in cerebral spinal fluid, but its use in a clinical environment has been limited.62 Treatment For most of the agents discussed herein, treatment primarily consists of antibiotic therapy. Babesiosis is generally treated with a combination of quinine and clindamycin; however, adverse drug reactions do cause patients to cease or avoid this treatment. The recent introduction of atovaquone and azithromycin for treatment of babesiosis has proven to be fortuitous because this drug combination is equally efficacious in the absence of adverse reactions.63 In cases of severe babesiosis or those in which antibiotic therapy is unsuccessful, exchange transfusion has proven to be an effective alternative.15,64,65 Ehrlichiosis is also treatable by antibiotic therapy, in this case using doxycycline or tetracycline hydrochloride.30,32 Similarly, RMSF is treated with tetracycline unless central nervous system manifestations require use of chloramphenicol.16 Treatments for CTF and TBE are primarily supportive because patients usually recover without complications, although postexposure prophylaxis with anti-TBE immunoglob-

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ulin has been recommended for some cases of TBE.16,66 Concurrent Infections Further complicating the diagnosis and treatment of tick-borne diseases is the potential for a host to be infected concurrently with 2 or more tick-borne pathogens. Concurrent infections occur as the result of ticks infected with 2 or more agents or perhaps when the endemic range of 2 or more agents transmitted by different tick vectors overlap. In Connecticut, persons who lived in or who spent time in tick-infested areas were observed in some cases to have antibodies for up to 3 tickborne agents, all transmitted by I scapularis: B burgdorferi, B microti, and E equi (ie, A phagocytophilum).67 Similar observations were made for Lyme patients from Wisconsin and Minnesota.68 Accurate and complete diagnosis of all tick-borne infections is critical for successful treatment. Indeed, although Lyme disease, babesiosis, and HGE are all treatable with antibiotics, a different antibiotic is required in each case. Some have also suggested that concurrent infections may cause 1 agent to produce a more severe disease of longer duration than normal, thereby masking the presence of the other infecting agent(s).69 This hypothesis, however, remains controversial and awaits further investigation.70 SEROPREVALENCE

Our understanding of the prevalence of tickborne pathogens in human populations is extremely limited. Although there have been a few systematic seroprevalence studies reported for B microti and A phagocytophilum, published studies are virtually nonexistent for other agents discussed in this review. For this latter group of agents, the primary source of information regarding their distribution and relative frequency largely comes from reported cases. Babesia spp seroprevalence estimates have been reported primarily from the United States and Europe. US seroprevalence studies have focused on B microti in the Northeast where seroprevalence rates range from 0.3% in Connecticut to 9.5% in Lyme disease patients.14,71,72 Several reports of B microti in blood donors have been published with rates as high as 4.3% on Shelter Island, NY73 (Table 2). Preliminary estimates of WA-1 seroprevalence in residents of northern California and

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Table 2. Seroprevalence of Tick-Borne Agents in Blood Donors Agent

B microti

B divergens WA-1 A phagocyotphilum

Location

Cape Cod Shelter Island, NY Wisconsin Connecticut Germany Germany Sacramento, CA Westchester County, NY Wisconsin Connecticut

Washington ranged from 0.9% (1 of 115) to 17.8% (39 of 219).11,20,74 The only study of WA-1 in blood donors identified 25 of 124 (20.8%) Sacramento, CA, donors with evidence of antibodies to WA-1 (Table 2). However, the observed high seroprevalence rates for WA-1 in blood donors from areas in which the parasite is not endemic suggests that the available WA-1 serologic tests may lack specificity.22 Babesia studies in Europe have been limited, perhaps because of the underlying misconception that Babesia infections rarely occur there. Among Swedish Lyme borreliosis patients, the seroprevalence rate for B divergens was reported to be 13%.75 A study of 100 healthy German blood donors identified 8 (8%) with antibodies to B microti.76 A more recent German study reported that the seroprevalence rates for B microti and B divergens were 5.4% (25 of 467) and 3.6% (17 of 467), respectively.77 Contained within this latter German study were 120 healthy blood donors, 2 (1.7%) with antibodies to B microti and 1 (0.8%) with antibodies to B divergens (Table 2). Relatively few studies have also been published for A phagocytophilum, 0.4% of northern California, 3.4% of New York, 14.9% of northwestern Wisconsin residents and 1.8% of Korean patients with febrile illness had antibodies to A phagocytophilum.31,74,78,79 Two recent studies reported seroprevalence rates for A phagocytophilum in blood donors (Table 2). The first reported that 11.3% of donors from Westchester County, NY, were found to have A phagocytophilum antibodies.80 In another study, 0.5% of Wisconsin donors and 3.5% of Connecticut donors had antibodies to A phagocytophilum.71 A phagocytophilum rates in Connect-

Number Tested

Number Positive (%)

779 115 999 1007 1,848 120 100 120 25 159 992 992

29 (3.7) 5 (4.3) 3 (0.3) 6 (0.6) 6 (0.3) 2 (1.7) 8 (8.0) 1 (0.8) 124 (20.8) 18 (11.3) 5 (0.5) 35 (3.5)

icut blood donors were also found to be similar in 2001 (4.1%) and 2002 (3.2%).81 In the latest US summary of notifiable diseases, 261 cases of HGE and 142 cases of HME were reported nationwide during 2001.82 These figures, however, must be viewed as conservative estimates because HGE and HME only recently became notifiable diseases and reporting from all states remains incomplete. RMSF has been a notifiable disease for many years and during 2001, 695 cases were reported in the United States.82 Accurate estimates for the CTF and TBE viruses are unavailable. TRANSFUSION TRANSMISSION

Cases The intracellular niche of many of the agents discussed in this review provides an ideal mechanism for transmission by blood transfusion. However, with the exception of Babesia, only one or two cases of transfusion-transmitted infection have been reported for most tick-borne agents.(Table 1) Transfusion cases involving these agents generally occur when an infected donor gives blood during the asymptomatic phase of disease. Implicated donors are then identified later by lookback, after recognition of symptomatic disease in the blood recipient, or after the donor becomes symptomatic and reports the infection to the blood center. To date, at least 50 cases of transfusion-transmitted B. microti have occurred, with most cases identified during the last 20 years.83 While several new transfusion cases are identified yearly, they are rarely published because the case reports are not considered novel. All cases of transfusion-

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transmitted Babesia reported to date have occurred in the United States, with the exception of an indigenous case in Japan and a Canadian case attributed to a donor who acquired the parasite in the United States.84,85 The WA-1 parasite has also been implicated in two U.S. transfusion cases, despite being first described in 1993.52,86 As mentioned previously, many other transfusion cases have likely occurred that were misdiagnosed or not recognized because of the absence of clinical symptoms. Reported Babesia transfusion cases have involved blood recipients who ranged in age from neonates to 79 years of age. Most recipients were immunocompromised, often multiply transfused, and in at least four cases involved asplenic individuals.15 As one might predict, most transfusion cases involved red cell units; however, at least four cases have been attributed to platelets, most likely contaminated with Babesia-infected red cells. The reported incubation period for transfusion-transmitted babesiosis is 1-9 weeks. A 1991-1992 study in Connecticut calculated the risk of acquiring babesiosis from a unit of packed red blood cells as 1 in 601 or 0.17% (95% CI, 0.004%-0.9%) and for platelets as 0 in 371 or 0% (95% CI, 0-0.8%).87 A more recent Connecticut study used different methodology to calculate the risk of transfusiontransmitted babesiosis as 1 in 1,800 for transfused red cell units.88 For the other tick-borne agents discussed in this review, reported cases of transmission by blood transfusion remain rare. There has been one reported case for A. phagocytophilum, R. rickettsii, and the CTF virus, while two cases associated with the TBEV have been reported. A case of A. phagocytophilum transmission was reported in Minnesota involving a patient suffering from rheumatoid arthritis, who was transfused with two red cell units.89 Infection with A. phagocytophilum was confirmed in this recipient less than 2 weeks later by blood smear, serology (1:512) and PCR. The recipient was treated with doxycycline and the symptoms resolved rapidly. The implicated donor was found through lookback and reportedly was asymptomatic. However, this donor who had a history of Lyme disease and extensive deer tick bites 2 months prior to donation, was seropositive for A. phagocytophilum at 1:2048. The lone case of transfusion-transmitted R. rickettsii occurred after blood, obtained from an

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asymptomatic donor 3 days before the onset of RMSF, was transfused to a patient experiencing severe anemia.90 The donor died six days after the onset of RMSF symptoms. The implicated blood unit was stored at 4oC for 9 days before transfusion. The recipient exhibited fever and headache of unspecified origin 6 days post-transfusion. Shortly thereafter, the hospital was notified that the recipient had received blood from a donor diagnosed with RMSF. Treatment for RMSF was initiated immediately and the recipient recovered fully within a few weeks. Infection with R. rickettsii was confirmed in recipient blood samples by serology, animal inoculation and cell culture. As for R. rickettsii, the lone case of transfusiontransmitted CTF virus involved a donor who was asymptomatic, but apparently donated during the agent’s infectious incubation period.91 A Montana resident reported the onset of an acute febrile illness four days after removing a tick and 18 hr after donating blood. Six days later the donor was confirmed to have CTF by animal inoculation and IFA. Approximately11 days post-donation, the regional blood collection center was notified of the donor’s diagnosis. Tubing from the donor’s most recent donation, which had been stored for 2 weeks at 4oC, contained CTF virus in the serum. Approximately a week earlier, blood from this unit had been transfused to an 82-year-old patient undergoing exploratory laparotomy. The recipient experienced a prolonged febrile illness after surgery and 23 days post-transfusion CTF virus was detected in the cellular fraction of the patient’s blood. The recipient experienced an uneventful recovery. Although specific details are limited, two cases or transfusion-transmitted TBEV (Kumlinge disease) have been reported from a single donor who had been to Kumlinge Island, Finland and later became ill hours after donating blood.92 In both cases the recipients became febrile shortly after transfusion, were treated with non-steroidal antiinflammatory drugs, and subsequently recovered. Survival in Blood Components A key factor in transmissibility of tick-borne agents is their relative capacity for survival in stored blood components. For several of the tickborne agents discussed herein, their intracellular location provides a suitable environment for their survival in stored blood components. Measurements of agent survival time in blood products

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have been derived from two primary data sources. The first are laboratory experiments in which whole blood units, components, or tubes of blood are spiked with pathogens or pathogen infected cells and then the viability of the agent is measured at regular intervals. Alternatively, for those agents with documented transfusion cases, survival data can be extrapolated from the age of the product that transmitted the infection. (Table 3) B microti, which infects red cells, survived in blood tubes maintained at 4°C for 21 days.93 This estimate, however, should be viewed as conservative because the blood tubes provided less than ideal conditions for survival of the blood cells. A transfusion case suggests that the parasite survives and can transmit the infection for at least 35 days.94 Because red cell units are stored for a maximum of 42 days, it appears that B microti remains viable for virtually the entire shelf life of the red cell product. Less information is available for WA-1, but, based on a recent transfusion case, the parasite can survive in red cells for a minimum of 6 days.86 Similarly, studies on the survival of A phagocytophilum in refrigerated blood suggests that it can survive for at least 18 days, thereby enhancing the likelihood of transfusion transmitted cases of HGE.95 The lone transmission case of A phagocytphilum occurred after the transfusion of a 30-dayold packed red blood cell unit, again indicating that longer survival in blood products should perhaps be anticipated.89 Another rickettsial agent, R rickettsii, apparently survives for at least 9 days in refrigerated whole blood as described in the only published transfusion case involving RMSF.90 Studies on the survival of viral agents transmitted by blood transfusion are very limited. The transfusion case implicating CTF virus transmission indicated that the virus remains viable in blood for at least 8 days,91 but laboratory studies

Table 3. Survival of Selected Tick-Borne Agents in Blood Agent

Experimental Studies

Transfusion Cases

B microti WA-1 A phagocytophilum E chaffeensis R rickettsii CTF virsus

21 days NA 18 days 11 days NA 18 months*

35 days 6 days 30 days NA 9 days 8 days

Abbreviations: NA, not available; CTF, Colorado tick fever. *Refrigerated blood clots

have shown that the virus can be re-isolated from clots of infected blood cells maintained at 4°C for 18 months.96 Information on the survival of the viral agents responsible for TBE has not been reported to our knowledge. PREVENTION OF TRANSMISSION

The observation that a number of tick-borne agents have been transmitted by blood transfusion raises the question of whether intervention strategies are needed to prevent their transmission. For some tick-borne agents, perhaps no response is warranted because of the self-limiting nature of infection, ease of treatment, and/or the relatively few cases of transmission (often only one) that have been reported to date. However, as discussed previously, the general lack of research focused on these agents makes informed decisions extremely difficult. Indeed, fundamental epidemiologic information is just now becoming available for several of these agents. In those cases in which interventions may be required, strategies implemented for other agents transmitted by blood transfusion (eg, human immunodeficiency virus, Plasmodium spp) have been considered for the control of tick-borne agents. Among these approaches are donor management strategies that focus on identifying and deferring at-risk donors by risk factor questions. Other options include either testing the blood of at-risk donors or to test all collected blood for any or all tick-borne agents. Alternatively, leukoreduction and pathogen inactivation techniques have been developed recently that are reported to significantly reduce or eliminate the presence of pathogens in blood products. One or several of these approaches may prove useful in preventing transfusion transmission of tick-borne agents. Risk Factor–Based Donor Management The fundamental risk factor for acquiring any tick-borne infection is contact with a tick vector. Thus, exposure to ticks has been suggested as a potential risk factor on which to base eligibility for blood donation.14 A recently published study investigated the frequency of self-reported tick bites in random blood donors from 6 geographically distinct blood collection regions over a 6-month period.71 A total of 103 of 2,482 (4.1%) donors reported a tick bite, with rates varying by region from 0.7% to 9%. However, the relatively high

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overall rate coupled with local rates at times approaching 10% suggests that outright deferral of donors based solely on a reported tick bite would not be an acceptable donor management strategy. As an alternative strategy, exposure to a tick bite could be used to identify at-risk donors for serologic testing, leading to deferral of only confirmed seropositive donors who presumably would be limited in number. However, a comparison of B microti and A phagocytophilum seroprevalence rates in random Connecticut blood donors with those donors reporting tick bites showed no significant differences.71 The relative inability to link selfreported tick bites with increased levels of donor seropositivity is likely related to interactions between the host and tick vector. First, as previously discussed, many people infected with a tick-borne disease do not recall an associated tick bite.19,47,48 Second, the absence of tick-borne diseases in some donors may be attributable to “tick avoidance” behavior characterized by preventive measures (eg, insect repellent, protective clothing) and/or routine examination for and prompt removal of attached ticks.71 The latter approach would greatly reduce the likelihood of pathogen transmission because ticks usually must feed on the host for 48 to 72 hours to successfully transmit most tick-borne agents, with the exception of A phagocytophilum and R rickettsii, which are transmissible after only 24 hours.1,97-100 Taken together, donors reporting tick bites may actually be less likely to be infected with tick-borne diseases than those who do not report tick exposure.71 A related approach is to suspend blood collections in highly endemic areas, particularly during periods of peak tick activity (ie, late spring and summer). Unfortunately, this approach may actually be counterproductive, resulting in the unnecessary deferral of large numbers of healthy donors.15 The potential for blood donors to be chronic carriers of disease (eg, babesiosis) and capable of transmitting pathogens year round further adds to the ineffectiveness of seasonal deferrals.101,102 Likewise, donors who become infected on vacation or during travel to an endemic area would not be precluded from donating blood upon returning home to a nonendemic area. Despite these arguments, deferral of donors from well-defined and isolated communities of high incidence, such as

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islands located off the coast of New England, may warrant consideration.15 Blood Screening Control strategies dependent on blood testing algorithms remain problematic because of the unavailability of a Food and Drug Administration– approved and –licensed blood screening test. However, a recent report describing the development of a peptide enzyme immunoassay designed to detect B microti antibodies suggests that suitable tests may be forthcoming.103 Similarly, another report cited the potential for diagnostic assays designed to detect secreted B microti antigens in serum.104 When, and if, a licensed test becomes available there are several potential blood screening strategies that could be implemented. For practical purposes, the following discussion will be limited to B microti, the only tick-borne agent for which blood screening is presently justifiable. The distribution of B microti in the United States is primarily restricted to the Northeast and Upper Midwest. Thus, one could screen only in these endemic regions, but this approach would require clearly defined areas of endemicity, a task that is becoming increasingly difficult. As discussed earlier, regional testing also would not preclude donors, who become infected in an endemic area, from donating in a nonendemic area; a scenario that has already led to at least 2 transfusion-transmitted cases of babesiosis.85,105 Given these caveats and the continued geographic spread of B microti, universal blood screening of all US blood donors may prove to be easier logistically and more cost-effective. An alternative approach would be to provide those recipients at greatest risk for acquiring a serious babesial infection (eg, elderly, immunocompromised, asplenic) with blood products that have been tested for B microti and found to be negative. This approach assumes that blood recipients who are not at risk for acquiring a serious infection would only experience a relatively mild, self-limiting infection if transfused with an infected blood component. A similar approach is presently used for to prevent transfusion-associated cytomegalovirus transmission in the United States.

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A final issue for consideration are the early acute or “window period” infections common to most tick-borne agents. During this phase of infection, serologic tests have low sensitivity because of limited host antibody production. To overcome this deficiency, a nucleic acid– based test (eg, PCR) is required to identify blood donors with window period infections. Thus, blood screening strategies designed to address B microti (and perhaps other tick-borne agents) will likely require both nucleic acid and serologic tests to ensure blood safety. Product Manipulation Product manipulation methodologies designed to reduce, inactivate, or eliminate pathogenic agents continue to evolve. These methodologies fall into 2 primary categories: leukoreduction and pathogen inactivation. Leukocyte reduction via specialized filters has been implemented to eliminate or decrease the transmission of viral pathogens, to minimize febrile nonhemolytic reactions, and to prevent adverse transfusion responses attributable to HLA alloimmunization. Although not implicitly designed to prevent the transmission of tick-borne agents, leukoreduction may prevent or at the very least reduce transmission of several of these agents (eg, A phagocytophilum, E chaffeensis) because of their location within leukocytes. A recent study using the ricketsial agent of scrub typhus, Orientia tsutsugamushi, showed that filtration reduced the number of infectious rickettsiae by up to 105.106 However, a study of E chaffeensis survival in leukoreduced red cells stored at 4° to 6°C reported that the agent could be reisolated from the supernatant fraction after filtration.107 Therefore, leukoreduction may remove rickettsia contained within cells but not those in extracellular locations. The Babesia and CTF virus, however, are unlikely to be affected by leukoreduction because of their location within RBCs. An alternative approach to leukoreduction is pathogen inactivation, which has shown the potential to reduce or eliminate tick-borne pathogens from platelet and red cell units. Pathogen inactivation usually involves the photo excitation of an inactivating compound added to a blood component. The actual inactivation of the pathogen is accomplished by the formation of covalent adducts to the agent’s nucleic acids or by the generation of reactive oxygen intermediates that serve as the effecter mechanism for inactivation. Psoralen pho-

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tochemical inactivation, using 4⬘-(aminomethyl)4,5⬘,8-trimethylpsoralen hyrdochloride, has been shown to be feasible for eliminating rickettsia of O tsutsugamushi from infected mononuclear cells previously added to platelet concentrates.108 Indeed, mice injected with 4⬘-(aminomethyl)-4,5⬘,8trimethylpsoralen hydrochloride–treated platelets showed no signs of infection with the scrub typhus agent as opposed to control mice that became ill or died. Another photosensitizing agent, N-(4-butanol) pheophorbamide, effectively eradicated erythrocytes infected with B divergens from whole blood when illuminated for 10 minutes.109 However, when this compound was illuminated for 10 to 30 minutes, hemolysis ranging from 0.1% to 0.5% was observed after 15 days of storage at 4°C and a further 48-hour incubation at 37°C. Based on these studies, pathogen inactivation may prove beneficial for elimination of Babesia and rickettsiae in blood products, but recent clinical trial setbacks may delay its possible implementation. SUMMARY

For the foreseeable future, tick-borne agents will continue to be of concern to blood safety. Of particular concern is B microti, which is responsible for an ever-increasing number of transfusion cases. Based on the number of transmission cases and the potential severity of disease in immunocompromised recipients, it is perhaps time that sound interventions to prevent its transmission be considered for implementation. Although relatively few transfusion cases have been reported for the other agents discussed in this review, these agents should continue to be monitored via active surveillance for changes in their transmission status. Further epidemiologic studies of many of the agents should be encouraged to better understand their prevalence, transmissibility, and management. New tick-borne agents continue to emerge and need to be evaluated as well. Indeed, Ixodes ticks have recently been identified to also carry Bartonella henselae, the agent of cat-scratch disease.110,111 The threat posed by this and other as-yet-undescribed agents remains equivocal. Thus, the challenges posed by tick-borne agents are varied and complex, but through vigilance and active, targeted research efforts we can minimize the threat these agents pose to blood safety.

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