Volume 4, Number 1—March 1998
The Name Ixodes dammini Epidemiologically Justified
To the Editor: Although a large body of evidence has been interpreted as supporting conspecificity of the deer tick (Ixodes dammini) and the blacklegged tick (Ixodes scapularis), according to Chapter VI, Article 23 L of the International Code of Zoological Nomenclature (1), "A name that has been treated as a junior synonym may be used as the valid name of a taxon by an author who considers the synonymy to be erroneous...."
Current use of I. scapularis to refer to the vector of Lyme disease obscures important epidemiologic issues. One of the reasons for "sinking" I. dammini was to make it easier to diagnose Lyme disease in areas where the disease was thought to be nonendemic: "The belief that I. dammini does not occur south of Maryland and that I. scapularis is a separate and distinct species yet unproven as a natural vector of Lyme disease has caused delays in Lyme disease surveillance in the South. The general attitude among physicians and veterinarians has been that Lyme disease is not a problem in that area, although patients present clinical symptoms of it" (2). Recognizing and reporting Lyme disease in southern and southcentral states should not, however, depend on whether the two ticks are conspecific. Only peer-reviewed descriptions of human cases of Lyme disease, with appropriate documentation of the diagnoses, should be accepted as evidence. Few such reports exist, and the evidence does not convincingly support a conclusion that Lyme disease exists as an epidemic zoonosis in southern states (3). This is not to say that residents outside the well-established eastern United States zoonotic sites (the Northeast and upper Midwest) do not have symptoms that fit one or more aspects of the current Centers for Disease Control and Prevention/Council of State and Territorial Epidemiologists/Association of State and Territorial Public Health Laboratory Directors surveillance definition for Lyme disease. Lyme disease-like infections, mainly manifesting as erythema migrans and strongly associated with Lone Star tick (Amblyomma americanum) bites are commonly seen in southern and southcentral states, but Borrelia burgdorferi does not seem to be the etiologic agent (4).
Enzootic transmission of Lyme disease spirochetes among rodents and ticks had been documented in southern and southcentral states by the late 1980s (5-7). The question, however, is whether there is frequent zoonotic transmission. There are widespread southern U.S. enzootic cycles of Trypanosoma cruzi, but few autochthonous human Chagas disease cases seem to occur because the vectors (such as Triatoma sanguisuga) have behavioral traits that reduce their capacity to serve as zoonotic vectors (8). Nymphal I. scapularis apparently do not frequently bite humans (7,9), although adult ticks do. The major feature of Lyme disease epidemiology in the Northeast and in the upper Midwest, however, is transmission by nymphal I. dammini (10).
Whether the predilection of nymphal I. dammini to feed on humans is environmentally determined or is a heritable trait with undescribed genetic markers remains unexplored. Particular mitochondrial DNA haplotypes seem to be more characteristic of I. dammini (11,12), and the use of such typing methods may enhance future analyses of the vectorial capacity of these ticks. For example, one might test the hypothesis that nymphal ticks removed from residents of sites in coastal North Carolina through Georgia, where both kinds of ticks have been collected, represent only I. dammini. But, if it is "widely accepted" that no differences exist between the two ticks, such studies may never be done. Similarly, many may wrongly assume that Lyme disease, human babesiosis, and human granulocytic ehrlichiosis are, or will become, epidemic throughout virtually all of the eastern United States. An equally likely scenario is that these zoonoses may never become public health problems for more southerly states. For the moment, then, distinguishing tick populations that frequently bite humans from those that rarely do seems to be a rational use of nomenclature, particularly for public health officials.
Dr. Sanders is correct in pointing out that all Ixodes spp. are "three-host" ticks, although my intent in using the term "one-host" was to indicate that all stages of I. cookei tend to feed on the same kind of animal (sometimes a single animal, within burrows), usually woodchucks, skunks, or raccoons. I regret the confusion from my use of the acarologic term in a descriptive context.
- International code of zoological nomenclature. 3rd ed. London: International Trust for Zoological Nomenclature; 1985.
- Oliver JH Jr, Owsley MR, Hutcheson HJ, James AM, Chen C, Irby WS, Conspecificity of the ticks Ixodes scapularis and I. dammini (Acari: Ixodidae). J Med Entomol. 1993;30:54–63.
- Barbour AG. Does Lyme disease occur in the South? A survey of emerging tickborne infections in the region. Am J Med Sci. 1996;311:34–40.
- Campbell GL, Paul WS, Schriefer ME, Craven RB, Robbins KE, Dennis DT, Epidemiologic and diagnostic studies of patients with suspected early Lyme disease, Missouri, 1990-1993. J Infect Dis. 1995;172:470–80.
- Levine JF, Apperson CS, Nicholson WL. The occurrence of spirochetes in ixodid ticks in North Carolina. J Entomol Sci. 1989;24:594–602.
- Kocan AA, Mukolwe SW, Murphy GL, Barker RW, Kocan KM. Isolation of Borrelia burgdorferi (Spirochaetales: Spirochaetaceae) from Ixodes scapularis and Dermacentor albopictus ticks (Acari: Ixodidae) in Oklahoma. J Med Entomol. 1992;29:630–3.
- Luckhart S, Mullen GR, Wright JC. Etiologic agent of Lyme disease, Borrelia burgdorferi, detected in ticks (Acari: Ixodidae) collected at a focus in Alabama. J Med Entomol. 1991;28:652–7.
- Pung OJ, Banks CW, Jones DN, Krissinger MW. Trypanosoma cruzi in wild raccoons, opossums, and triatomine bugs in southeast Georgia, USA. J Parasitol. 1995;81:324–6.
- Lavender DR, Oliver JH Jr. Ticks (Acari:Ixodidae) in Bulloch County, Georgia. J Med Entomol. 1996;33:224–31.
- Spielman A, Wilson ML, Levine JF, Piesman J. Ecology of Ixodes dammini borne human babesiosis and Lyme disease. Annu Rev Entomol. 1985;30:439–60.
- Rich SM, Caporale DA, Telford SR III, Kocher TD, Hartl DL, Spielman A, Distribution of the Ixodes ricinus-like ticks of eastern North America. Proc Natl Acad Sci U S A. 1995;92:6284–8.
- Norris DE, Klompen JSH, Keirans JE, Black WC IV. Population genetics of Ixodes scapularis (Acari:Ixodidae) based on mitochondrial 16S and 12S genes. J Med Entomol 1996;33:78-89. 996;33:78-89.