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Serosurvey of shelter dogs in Virginia for antibodies to Borrelia burgdorferi
Preventive Veterinary Medicine, 10 (1990) 41-46
41
Elsevier Science Publishers B.V., Amsterdam
Serosurvey of shelter dogs in Virginia for antibodies to Borrelia burgdorferi Mary E. Torrence ~, Suzanne R. Jenkins 2, Jay F. Levine 3, William L. Nicholson 3 and Kevin D. Pelzer I 1Virginia-Maryland Regional College of Veterinary Medicine, Virginia Tech, Blacksburg, VA 24061 (U.S.A.) 20ffice of Epidemiology, Virginia Department of Health, Richmond, VA 23219 (U.S.A.) ~North Carolina State University, School of Veterinary Medicine, Raleigh, NC 27606 (U.S.A.) (Accepted for publication 12 July 1990)
ABSTRACT Torrence, M.E., Jenkins, S.R., Levine, J.F., Nicholson, W.L. and Pelzer, K.D., 1990. Serosurvey of shelter dogs in Virginia for antibodies to Borrelia burgdorferi. Prey. Vet. Med., 10:41-46. Increased interest and concern about Lyme disease in the last several years, along with the close proximity of Maryland's endemic foci, has raised questions about the incidence of Lyme disease in Virginia. A canine seroprevalence survey was undertaken in an attempt to estimate the prevalence of borreliosis in Virginia. An ELISA test was used to detect antibodies against Borrelia burgdorferi in sera from 221 shelter dogs from the three different physiographic regions in Virginia. From those animals sampled, only one dog was seropositive, This finding was consistent with other studies which have found limited evidence of Lyme disease in Virginia.
INTRODUCTION
Lyme disease in humans first was recognized in 1975 (Steere et al., 1977 ). Its etiologic agent, Borrelia burgdorferi, was isolated in 1982. Since then, cases have been reported in at least 32 states, including Virginia; Lyme disease now is recognized as the most commonly reported human arthropod-borne disease in the United States (Anderson and Magnarelli, 1984). Eight cases of human Lyme disease were reported in Virginia with onset in 1986, and 20 cases with onset in 1987. Of these 28 cases in Virginia, seven were from a peninsula on the Atlantic side of Chesapeake Bay, known as the eastern shore. In areas endemic for Lyme disease, B. burgdorferi infects a wide range of vertebrate hosts, including rodents, lagomorphs, insectivores, carnivores, ungulates, and several species of birds (Eng et al., 1988; Greene et al., 1988b). Wild rodents, other small mammals and possibly birds serve as reservoirs for 0167-5877/90/$03.50
© 1990 - - Elsevier Science Publishers B.V.
42
M.E. TORRENCE ET AL
the spirochetes (Kornblatt et al., 1985 ). Dogs with clinical Lyme disease may exhibit malaise, fever, lymphadenopathy, and arthritis. Twenty to 50% of asymptomatic dogs from endemic areas have serologic evidence of previous infection (Magnarelli et al., 1984a,b, 1986). Numerous foci of Lyme disease have been identified in several geographic regions by the following methods: ( 1 ) clinical and serologic studies of humans; (2) determining the prevalence of ticks infected with B. burgdorferi, (3) serologic studies of wild and domestic animals. In 1987, B. burgdorferi was identified in ticks collected from raccoons and birds on the eastern shore of Virginia, but not in other localities in Virginia or North Carolina (Sonenshine and Levine, 1988 ). Because dogs develop high levels of antibodies to B. burgdorferi, they are easier to sample, and appear to be at greater risk of exposure to infection than people, canine seroprevalence surveys have been used to evaluate the distribution ofB. burgdorferi (Steere et al., 1977 ). In the summer of 1988, the Virginia Department of Health, in cooperation with the Virginia-Maryland Regional College of Veterinary Medicine at Virginia Tech and the College of Veterinary Medicine at North Carolina State University, conducted a serological survey of dogs in Virginia animal shelters to determine the prevalence of Lyme disease. MATERIALS AND METHODS
Study design A survey was designed to select a random sample of animal shelters from three distinct physiographic regions of Virginia; mountain, piedmont plateau, and coastal plain. Each animal shelter in Virginia was organized by physiographic region and listed alphabetically within that region. Any shelter located in a county on the region's dividing line was eliminated. This left 21, 29, and 14 possible shelters in each region, respectively. A random numbers table system was used to select the first shelter from each region, then every fifth shelter was selected. A 5% prevalence rate was assumed, based on results obtained in other nonendemic areas by the College of Veterinary Medicine in North Carolina. A sample size of 60 dogs per region was estimated using a confidence level of 95% and a tolerance level of _ 5%. An average of 10 dogs per shelter was assumed; therefore, six shelters were selected per region to achieve the necessary sample size. Each shelter was sent an introductory letter explaining the purpose of the study, and a follow-up telephone call was made to arrange appointments for blood collection. In the piedmont region, there was one shelter with no dogs. In the coastal region, there were three shelters with no dogs, and in the mountain region, three shelters refused participation because it was against their policy. In an attempt to maintain the randomness of our study, a second selection using
SEROSURVEYOF DOGS FOR ANTIBODIESTO B. BURGDORFERI
43
TABLEI Shelter description and sample size for each region Shelter
No. of dogs
Mountain region A (rural) B (rural) C (rural) D (suburban)
24 22 29 15
Piedmont plateau A (rural) B (rural) C (suburban) D (rural) E (suburban) F (rural)
6 4 20 7 7 25
Coastal A (rural) B (rural) C (rural) D (rural) E (suburban)
4 13" 10 10 25
*Positive case.
the random numbers table was made to select one alternative shelter for the piedmont and mountain region. Because most of the shelters in the coastal region were extremely small, two non-random larger shelters were selected to achieve the proposed sample size. The distribution of the shelters and sample numbers are shown in Table 1.
Study population The dog population in this study consisted of captured stray dogs and those given up by the owner (designated as pets) at animal control facilities within the region. Blood samples were collected during July and August. Blood was drawn from the cephalic vein using a 10-ml evacuated tube. The goal was to obtain a blood sample from each tractable dog housed in the shelter. Dogs that were quarantined, aggressive, or fractious during venipuncture were not included. The age (by dentition ), sex, breed, and history of each dog (pet vs. stray) was recorded at the time of sampling. The blood samples were centrifuged, and the serum was recovered and frozen. This serum was sent on dry ice to the College of Veterinary Medicine, North Carolina University, for testing by indirect enzyme-linked immunosorbent assay (ELISA) for antibodies to B. burgdorferi.
44
M.E. TORRENCE ET AL.
ELISA procedure The washed whole-cell antigen of B. burgdorferi was prepared and standardized for the ELISA procedure as previously described (Greene, 1988a) with the following modifications. The sonicated suspension was centrifuged at 6 4 0 0 x g for 15 min instead of 10 0 0 0 × g for 30 min. In addition, 50 #1 of the working dilution ( 1:500) instead of a 1 : 1000 dilution of peroxidase-conjugated goat anti-dog IgG heavy-chain specific was added. The absorbance (A) was measured at 410 nm by an automated microplate reader. ELISA values were obtained that related the absorbance of the test sera to those of the positive and negative controls, as shown by the formula: E value= [ (A t e s t - A neg) / (A p o s - A neg) ] X 100. Interpretation of the values were as follows: 0-39, negative; 40-49, suspicious; 50-100, positive. These ranges correspond to reciprocal IFA titers of 64, 64-128, and 128, respectively. RESULTS
The sampled dog population of 221 animals consisted of 42.5% females and 57.5% males. Adult dogs ( > 1 year of age) comprised 80. 1% of the sample population, while 12.2% were 6 months to 1 year of age, and 7.7% were under 6 months of age. The proportions were similar for each of the physiographic regions. Of the 164 dogs whose histories were recorded, 27.3% were considered pets. Samples for the physiographic regions included: 90 samples from four shelters in the mountainous region; 69 samples from six shelters in the piedmont region; 62 samples from five shelters in the coastal region. Borrelia burgdorferi-specific IgG antibodies were detected in one dog (0.45%) out of 221 samples. The ELISA value for this animal was 76.82. The positive dog was an adult female stray of mixed breed from an eastern shore animal shelter (coastal region). DISCUSSION
The results of this study indicate that canine exposure to B. burgdorferi is minimal in Virginia, and limited to the coastal region. This finding is consistent with the low numbers of reported human cases in Virginia, and with studies indicating a limited distribution of the tick, Ixodes dammini, in Virginia (Sonenshine and Levine, 1988 ). The apparent increase in reported cases of human Lyme disease may be due to the increase in recognition or reporting of clinical cases rather than to an actual increase in the prevalence of the disease. The fact that the focus of Lyme disease is limited to the eastern shore region which is sparsely populated, may account for the low numbers of cases reported in humans and dogs in Virginia. In addition, only 10 samples were
SEROSURVEY OF DOGS FOR ANTIBODIES TO B. BURGDORFERI
45
obtained from the eastern shore region where the one positive case was located. More samples of the area may have produced different results. A serosurvey of shelter dogs was selected as the most convenient and efficient m e t h o d for this study as opposed to sampling household pets or veterinary hospital patients. Shelter dogs are more representative of the population within a large geographic area because they consist of a mixture of stray dogs as well as household pets and can be from a large range of locations. The stray dogs would be most likely to have higher exposure to the tick population because of their increased time outdoors and lack of parasite control. A study involving household pets and veterinary hospital patients would increase the a m o u n t of time, money, and personnel needed for the survey. In addition, this population would be a more biased group because they would be more likely to be treated with insecticides and have a narrower range of locality within the geographic region. However, the availability of travel history for the hospital patients would be an advantage over shelter animals. One problem that developed from using the shelters in Virginia was that many of the rural shelters had only zero to three dogs at the time we sampled; therefore, several of the shelters selected were located in more suburban areas. This may have limited the exposure of the dogs to the areas where Ixodes are prominent. However, residential and suburban neighborhoods in Virginia are known to harbor large tick populations. The distribution of dogs from suburban shelters was similar for all three physiographic regions. In addition, because of the small numbers, two non-random shelters were selected. We do not feel that the results were biased as these two shelters were similar to other suburban shelters in the study. Because the coastal region was the most likely region for dogs with borreliosis we felt that an adequate sample size was essential for this region. The selection of these two large shelters was in order to maintain an adequate sample size. The humoral response of dogs naturally infected with B. burgdorferi is not well defined. There is serologic evidence of increased IgM levels (Magnarelli et al., 1984a). Later as IgM levels decrease, an increase in IgG levels occurs. However, the exact interval and duration of the titer response has not been determined. Consequently, a cross-sectional study examining the IgG levels may potentially fail to detect dogs with recent exposure, especially as it takes approximately 2 weeks for IgG levels to increase. However, following exposure, these levels can last up to 8 months (Greene et al., 1988b). While the peak of h u m a n infections appears to be in June and July, 80% of infections occur from May through September. The immature stages, especially the nymph, is thought to be the most c o m m o n stage of infection for dogs and cats. This stage usually feeds on animals and is most active in May and June decreasing by July (Schulze et al., 1985 ). Thus, sampling in July and August does not seem to decrease the potential to discover seropositive animals. In
46
M.E. TORRENCE ET AL.
a d d i t i o n , 82% o f o u r s a m p l e d p o p u l a t i o n w e r e a d u l t s t h a t h a d a h i g h e r likel i h o o d o f h a v i n g b e e n e x p o s e d d u r i n g a p r e v i o u s season. This study confirms our suspicions that the transmission of Lyme disease is p r e s e n t l y m i n i m a l in V i r g i n i a a n d p o s s i b l y is l i m i t e d to t h e c o a s t a l region w h e r e t h e v e c t o r L d a m m i n i h a s b e e n l o c a t e d . T h i s is to b e e x p e c t e d a l o n g t h e n o r t h e a s t e r n c o a s t a l r e g i o n (Burgess, 1986 ). I n a d d i t i o n , this s t u d y s h o w s t h a t c a n i n e serologic s u r v e y s m a y b e o n e w a y to d e t e r m i n e foci o f b o r r e l i o s i s ( M a g n a r e l l i et al., 1 9 8 4 a ) , a n d t h e r e c o g n i t i o n o f a s e r o p o s i t i v e d o g f r o m a s m a l l s a m p l e o f p o u n d d o g s o n t h e e a s t e r n s h o r e suggests t h a t c o n t i n u e d m o n i t o r i n g s h o u l d b e c o n s i d e r e d in this region.
REFERENCES Anderson, J.F. and Magnarelli, L.A., 1984. Avian and mammalian hosts for spirochete-infected ticks and insects in a Lyme disease focus in Connecticut. Yale J. Biol. Med., 57: 627-644. Burgess, E.C., 1986. Natural exposure of Wisconsin dogs to the Lyme disease spirochetes (Borrelia burgdorferi). Lab. Anim. Sci., 36: 288-290. Eng, T.R., Wilson, M.L., Spielman, A. and Lastavica, C.C., 1988. Greater risk of Borrelia burgdorferi infection in dogs than in people. J. Infect. Dis., 156:1410-1411. Greene, R.T., Levine, J.F., Breitschwerdt, E.B., Walker, R.L., Berkhoff, H.A., Cullen, J. and Nicholson, W.L., 1988a. Clinical and serologic evaluations of induced Borrelia burgdorferi infections in dogs. Am. J. Vet. Res., 49: 752-757. Greene, R.T., Levine, J.F., Breitschwerdt, E.B.and Berkhoff, H.A., 1988b. Antibodies to Borrelia burgdorfen in dogs in North Carolina. Am. J. Vet. Res., 49(4): 473-476. Kornblatt, A.N., Urband, P.H. and Steere, A.C., 1985. Arthritis caused by Borrelia burgdorferi in dogs. J. Vet. Med. Assoc., 186: 960-964. Magnarelli, L.A., Anderson, J.F., Burgdorfer, W. and Chappell, W.A., 1984a. Parasitism by lxodes dammini (Acari:Ixodidae) and antibodies to spirochete in mammals at Lyme disease foci in Connecticut, U.S.A.J. Med. Entomol., 21: 52-57. Magnarelli, L.A., Anderson, J.F., Apperson, C.S., Fish, D., Johnson, R.C. and Chappell, W.A., 1984b. Antibodies to spirochete in white-tailed deer and prevalence of infected ticks from foci of Lyme disease in Connecticut. J. Wildl. Dis., 20:21-26. Magnarelli, L.A., Anderson, J.F., Apperson, C.S., et al., 1986. Spirochetes in ticks and antibodies to Borrelia burgdorferi in white-tailed deer from Connecticut, New York State, and North Carolina. J. Wildl. Dis., 22:178-188. Schulze, T.L., Bowen, G.S., Lakat, M.F., Parkin, W.E. and Shisler, J.L., 1985. The role of adult Ixodes dammini (Acari: Ixodidae) in the transmission of Lyme disease in New Jersey, U.S.A. J. Med. Entomol., 22: 88-93. Sonenshine, D.E. and Levine, J.F., 1988. Lyme disease in Virginia and North Carolina. The results of a study performed for the Virginia Department of Health and the North Carolina Department of Health in collaboration with the Virginia Commission of Game and Inland Fisheries. The Old Dominian University Research Foundation, Norfolk, VA, 40 pp. Steere, A.C., Malawista, S.E., Snydman, D.R., Shope, R.E., Andiman, W.A., Ross, M.R. and Steele, F.M., 1977. An epidemic of oligoarticular arthritis in children and adults in three Connecticut communities. Arthritis Rheumatol., 20: 7-17. Update: Lyme disease and cases during pregnancy - United States, 1985. Morb. Mortal. Wkly. Reader, 34: 376-384.
41
Elsevier Science Publishers B.V., Amsterdam
Serosurvey of shelter dogs in Virginia for antibodies to Borrelia burgdorferi Mary E. Torrence ~, Suzanne R. Jenkins 2, Jay F. Levine 3, William L. Nicholson 3 and Kevin D. Pelzer I 1Virginia-Maryland Regional College of Veterinary Medicine, Virginia Tech, Blacksburg, VA 24061 (U.S.A.) 20ffice of Epidemiology, Virginia Department of Health, Richmond, VA 23219 (U.S.A.) ~North Carolina State University, School of Veterinary Medicine, Raleigh, NC 27606 (U.S.A.) (Accepted for publication 12 July 1990)
ABSTRACT Torrence, M.E., Jenkins, S.R., Levine, J.F., Nicholson, W.L. and Pelzer, K.D., 1990. Serosurvey of shelter dogs in Virginia for antibodies to Borrelia burgdorferi. Prey. Vet. Med., 10:41-46. Increased interest and concern about Lyme disease in the last several years, along with the close proximity of Maryland's endemic foci, has raised questions about the incidence of Lyme disease in Virginia. A canine seroprevalence survey was undertaken in an attempt to estimate the prevalence of borreliosis in Virginia. An ELISA test was used to detect antibodies against Borrelia burgdorferi in sera from 221 shelter dogs from the three different physiographic regions in Virginia. From those animals sampled, only one dog was seropositive, This finding was consistent with other studies which have found limited evidence of Lyme disease in Virginia.
INTRODUCTION
Lyme disease in humans first was recognized in 1975 (Steere et al., 1977 ). Its etiologic agent, Borrelia burgdorferi, was isolated in 1982. Since then, cases have been reported in at least 32 states, including Virginia; Lyme disease now is recognized as the most commonly reported human arthropod-borne disease in the United States (Anderson and Magnarelli, 1984). Eight cases of human Lyme disease were reported in Virginia with onset in 1986, and 20 cases with onset in 1987. Of these 28 cases in Virginia, seven were from a peninsula on the Atlantic side of Chesapeake Bay, known as the eastern shore. In areas endemic for Lyme disease, B. burgdorferi infects a wide range of vertebrate hosts, including rodents, lagomorphs, insectivores, carnivores, ungulates, and several species of birds (Eng et al., 1988; Greene et al., 1988b). Wild rodents, other small mammals and possibly birds serve as reservoirs for 0167-5877/90/$03.50
© 1990 - - Elsevier Science Publishers B.V.
42
M.E. TORRENCE ET AL
the spirochetes (Kornblatt et al., 1985 ). Dogs with clinical Lyme disease may exhibit malaise, fever, lymphadenopathy, and arthritis. Twenty to 50% of asymptomatic dogs from endemic areas have serologic evidence of previous infection (Magnarelli et al., 1984a,b, 1986). Numerous foci of Lyme disease have been identified in several geographic regions by the following methods: ( 1 ) clinical and serologic studies of humans; (2) determining the prevalence of ticks infected with B. burgdorferi, (3) serologic studies of wild and domestic animals. In 1987, B. burgdorferi was identified in ticks collected from raccoons and birds on the eastern shore of Virginia, but not in other localities in Virginia or North Carolina (Sonenshine and Levine, 1988 ). Because dogs develop high levels of antibodies to B. burgdorferi, they are easier to sample, and appear to be at greater risk of exposure to infection than people, canine seroprevalence surveys have been used to evaluate the distribution ofB. burgdorferi (Steere et al., 1977 ). In the summer of 1988, the Virginia Department of Health, in cooperation with the Virginia-Maryland Regional College of Veterinary Medicine at Virginia Tech and the College of Veterinary Medicine at North Carolina State University, conducted a serological survey of dogs in Virginia animal shelters to determine the prevalence of Lyme disease. MATERIALS AND METHODS
Study design A survey was designed to select a random sample of animal shelters from three distinct physiographic regions of Virginia; mountain, piedmont plateau, and coastal plain. Each animal shelter in Virginia was organized by physiographic region and listed alphabetically within that region. Any shelter located in a county on the region's dividing line was eliminated. This left 21, 29, and 14 possible shelters in each region, respectively. A random numbers table system was used to select the first shelter from each region, then every fifth shelter was selected. A 5% prevalence rate was assumed, based on results obtained in other nonendemic areas by the College of Veterinary Medicine in North Carolina. A sample size of 60 dogs per region was estimated using a confidence level of 95% and a tolerance level of _ 5%. An average of 10 dogs per shelter was assumed; therefore, six shelters were selected per region to achieve the necessary sample size. Each shelter was sent an introductory letter explaining the purpose of the study, and a follow-up telephone call was made to arrange appointments for blood collection. In the piedmont region, there was one shelter with no dogs. In the coastal region, there were three shelters with no dogs, and in the mountain region, three shelters refused participation because it was against their policy. In an attempt to maintain the randomness of our study, a second selection using
SEROSURVEYOF DOGS FOR ANTIBODIESTO B. BURGDORFERI
43
TABLEI Shelter description and sample size for each region Shelter
No. of dogs
Mountain region A (rural) B (rural) C (rural) D (suburban)
24 22 29 15
Piedmont plateau A (rural) B (rural) C (suburban) D (rural) E (suburban) F (rural)
6 4 20 7 7 25
Coastal A (rural) B (rural) C (rural) D (rural) E (suburban)
4 13" 10 10 25
*Positive case.
the random numbers table was made to select one alternative shelter for the piedmont and mountain region. Because most of the shelters in the coastal region were extremely small, two non-random larger shelters were selected to achieve the proposed sample size. The distribution of the shelters and sample numbers are shown in Table 1.
Study population The dog population in this study consisted of captured stray dogs and those given up by the owner (designated as pets) at animal control facilities within the region. Blood samples were collected during July and August. Blood was drawn from the cephalic vein using a 10-ml evacuated tube. The goal was to obtain a blood sample from each tractable dog housed in the shelter. Dogs that were quarantined, aggressive, or fractious during venipuncture were not included. The age (by dentition ), sex, breed, and history of each dog (pet vs. stray) was recorded at the time of sampling. The blood samples were centrifuged, and the serum was recovered and frozen. This serum was sent on dry ice to the College of Veterinary Medicine, North Carolina University, for testing by indirect enzyme-linked immunosorbent assay (ELISA) for antibodies to B. burgdorferi.
44
M.E. TORRENCE ET AL.
ELISA procedure The washed whole-cell antigen of B. burgdorferi was prepared and standardized for the ELISA procedure as previously described (Greene, 1988a) with the following modifications. The sonicated suspension was centrifuged at 6 4 0 0 x g for 15 min instead of 10 0 0 0 × g for 30 min. In addition, 50 #1 of the working dilution ( 1:500) instead of a 1 : 1000 dilution of peroxidase-conjugated goat anti-dog IgG heavy-chain specific was added. The absorbance (A) was measured at 410 nm by an automated microplate reader. ELISA values were obtained that related the absorbance of the test sera to those of the positive and negative controls, as shown by the formula: E value= [ (A t e s t - A neg) / (A p o s - A neg) ] X 100. Interpretation of the values were as follows: 0-39, negative; 40-49, suspicious; 50-100, positive. These ranges correspond to reciprocal IFA titers of 64, 64-128, and 128, respectively. RESULTS
The sampled dog population of 221 animals consisted of 42.5% females and 57.5% males. Adult dogs ( > 1 year of age) comprised 80. 1% of the sample population, while 12.2% were 6 months to 1 year of age, and 7.7% were under 6 months of age. The proportions were similar for each of the physiographic regions. Of the 164 dogs whose histories were recorded, 27.3% were considered pets. Samples for the physiographic regions included: 90 samples from four shelters in the mountainous region; 69 samples from six shelters in the piedmont region; 62 samples from five shelters in the coastal region. Borrelia burgdorferi-specific IgG antibodies were detected in one dog (0.45%) out of 221 samples. The ELISA value for this animal was 76.82. The positive dog was an adult female stray of mixed breed from an eastern shore animal shelter (coastal region). DISCUSSION
The results of this study indicate that canine exposure to B. burgdorferi is minimal in Virginia, and limited to the coastal region. This finding is consistent with the low numbers of reported human cases in Virginia, and with studies indicating a limited distribution of the tick, Ixodes dammini, in Virginia (Sonenshine and Levine, 1988 ). The apparent increase in reported cases of human Lyme disease may be due to the increase in recognition or reporting of clinical cases rather than to an actual increase in the prevalence of the disease. The fact that the focus of Lyme disease is limited to the eastern shore region which is sparsely populated, may account for the low numbers of cases reported in humans and dogs in Virginia. In addition, only 10 samples were
SEROSURVEY OF DOGS FOR ANTIBODIES TO B. BURGDORFERI
45
obtained from the eastern shore region where the one positive case was located. More samples of the area may have produced different results. A serosurvey of shelter dogs was selected as the most convenient and efficient m e t h o d for this study as opposed to sampling household pets or veterinary hospital patients. Shelter dogs are more representative of the population within a large geographic area because they consist of a mixture of stray dogs as well as household pets and can be from a large range of locations. The stray dogs would be most likely to have higher exposure to the tick population because of their increased time outdoors and lack of parasite control. A study involving household pets and veterinary hospital patients would increase the a m o u n t of time, money, and personnel needed for the survey. In addition, this population would be a more biased group because they would be more likely to be treated with insecticides and have a narrower range of locality within the geographic region. However, the availability of travel history for the hospital patients would be an advantage over shelter animals. One problem that developed from using the shelters in Virginia was that many of the rural shelters had only zero to three dogs at the time we sampled; therefore, several of the shelters selected were located in more suburban areas. This may have limited the exposure of the dogs to the areas where Ixodes are prominent. However, residential and suburban neighborhoods in Virginia are known to harbor large tick populations. The distribution of dogs from suburban shelters was similar for all three physiographic regions. In addition, because of the small numbers, two non-random shelters were selected. We do not feel that the results were biased as these two shelters were similar to other suburban shelters in the study. Because the coastal region was the most likely region for dogs with borreliosis we felt that an adequate sample size was essential for this region. The selection of these two large shelters was in order to maintain an adequate sample size. The humoral response of dogs naturally infected with B. burgdorferi is not well defined. There is serologic evidence of increased IgM levels (Magnarelli et al., 1984a). Later as IgM levels decrease, an increase in IgG levels occurs. However, the exact interval and duration of the titer response has not been determined. Consequently, a cross-sectional study examining the IgG levels may potentially fail to detect dogs with recent exposure, especially as it takes approximately 2 weeks for IgG levels to increase. However, following exposure, these levels can last up to 8 months (Greene et al., 1988b). While the peak of h u m a n infections appears to be in June and July, 80% of infections occur from May through September. The immature stages, especially the nymph, is thought to be the most c o m m o n stage of infection for dogs and cats. This stage usually feeds on animals and is most active in May and June decreasing by July (Schulze et al., 1985 ). Thus, sampling in July and August does not seem to decrease the potential to discover seropositive animals. In
46
M.E. TORRENCE ET AL.
a d d i t i o n , 82% o f o u r s a m p l e d p o p u l a t i o n w e r e a d u l t s t h a t h a d a h i g h e r likel i h o o d o f h a v i n g b e e n e x p o s e d d u r i n g a p r e v i o u s season. This study confirms our suspicions that the transmission of Lyme disease is p r e s e n t l y m i n i m a l in V i r g i n i a a n d p o s s i b l y is l i m i t e d to t h e c o a s t a l region w h e r e t h e v e c t o r L d a m m i n i h a s b e e n l o c a t e d . T h i s is to b e e x p e c t e d a l o n g t h e n o r t h e a s t e r n c o a s t a l r e g i o n (Burgess, 1986 ). I n a d d i t i o n , this s t u d y s h o w s t h a t c a n i n e serologic s u r v e y s m a y b e o n e w a y to d e t e r m i n e foci o f b o r r e l i o s i s ( M a g n a r e l l i et al., 1 9 8 4 a ) , a n d t h e r e c o g n i t i o n o f a s e r o p o s i t i v e d o g f r o m a s m a l l s a m p l e o f p o u n d d o g s o n t h e e a s t e r n s h o r e suggests t h a t c o n t i n u e d m o n i t o r i n g s h o u l d b e c o n s i d e r e d in this region.
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