Leptospirosis is caused by infection with antigenically-distinct serovars of Leptospira interrogans sensu lato. Infected animals shed spirochetes in their urine which subsequently contaminate the environment. Leptospirosis is considered an emerging zoonotic disease, most commonly affecting younger adult, large breed, outdoor dogs. The clinical spectrum of disease includes hepatic and renal failure, and vasculitis. There are at least 200 pathogenic serovars, and at least 10 of these serovars can affect dogs and cats, although reports of feline disease are rare. Each serovar is adapted to one or more mammalian hosts (maintenance hosts), and disease in incidental hosts tends to be more severe. The serovars are grouped into about 25 different serogroups. Different strains that vary slightly antigenically sometimes exist within serovars, and sometimes these strains belong to different leptospiral species, as determined on the basis of their DNA relatedness.
Use of the Microscopic Agglutination Test to Study Leptospiral Epidemiology
Currently, identification of the infecting serovar is usually based on results of serology using the microscopic agglutination test (MAT), which is serogroup specific. Reference serovars grown in liquid media are exposed to serial dilutions of the patient's sera. The end-point is the highest serum dilution that causes 50% of the organisms to agglutinate. Numerous antigens are used to identify the serogroup causing disease. The report to the veterinarian lists the serovars tested and the respective titers. The serogroup with the highest titer is generally interpreted as the infecting one; lower titers represent cross reactions.
Unfortunately, accurate prediction of the infecting serogroup is difficult because of these cross-reactions, 'paradoxical reactions' (where a heterologous reaction is positive while the homologous reaction is negative), and interference by vaccination. In one human study, the predominant serogroups at a titer of > 100 correctly predicted less than 50% of infecting serovars.1 The serovars included in the assay may not include the serogroup infecting the patient. When performed at the CDC, the MAT includes 23 different serovars, but most veterinary diagnostic laboratories include only 6-7 serovars. In addition, there evidence is accumulating that results from different laboratories may vary.2 This is important, because most epidemiologic studies of leptospirosis in dogs have relied upon the results of the MAT. These studies have included reports that:
1. Suggest a correlation between infecting serovar and clinical spectrum of disease in dogs. Most recently, serogroup Pomona was associated with more severe renal disease and a worse outcome compared with other serogroups.3,4
2. Suggest a decreasing prevalence of disease associated with positive test results for Canicola and Icterohemorrhagiae, and increasing reports of disease associated with positive test results for serovars Bratislava, Grippotyphosa, and Pomona. Vaccine pressure, increased testing for a greater range of serovars, and increasing contact between dogs and wildlife, have been suggested as reasons for this change. Because of this phenomenon, currently available vaccines now include serovars Grippotyphosa and Pomona, as well as Canicola and Icterohemorrhagiae.
Current Knowledge of Geographic Variation of Canine Leptospirosis
There is a paucity of literature on geographical variation of leptospiral epidemiology in dogs, within North America and worldwide. Geographic variation in the epidemiology of leptospirosis in humans can reflect the density of pathogenic leptospires in the environment, exposure to excess rainfall and flooding, and the degree of contact between the population and the maintenance host for various serovars. The dog is the maintenance host for Canicola. Rats are the host for Icterohemorrhagiae. Raccoons, skunks, voles, opossums, and possibly squirrels are reportedly maintenance hosts for Grippotyphosa; cattle and pigs for Pomona; pigs for Bratislava and cattle for Hardjo. In 1980, serovar Icterohemorrhagiae was reported to be the most prevalent serovar in dogs living in Detroit, which had a large population of Norway rats, 75% of which were chronically infected with serovar Icterohemorrhagiae.5 Nevertheless, epidemiologic studies suggest that host preference can change over time and with geographic location, and the prevalence of different serovars may change as a result of the introduction of new maintenance hosts, agricultural practice, and recreational activities.
Recently, we sought to compare the clinical presentation, serovar reactivity, laboratory findings, and outcome associated with canine leptospirosis between dogs presenting to the University of Minnesota Veterinary Medical Center, and dogs presenting to the University of California Veterinary Medical Teaching Hospital.4 A search of medical records for dogs diagnosed with leptospirosis from January 2001 to April 2006 was made. Variables recorded included signalment, zip code, living environment, presentation date, vaccination history, presenting signs and laboratory abnormalities, results of leptospira serology, and outcome. Ninety dogs met the inclusion criteria, 55 from CA and 35 from MN. The proportion of small breed dogs was greater in the MN group compared with the CA group (40% vs 16%, P = 0.02). Case numbers peaked in fall in MN and winter in CA. Preliminary data suggest that the two laboratories used agreed on the pattern of seroreactivity, although the magnitude of the titers was not always the same. Titers were highest to serogroup Pomona in 64% of CA but only 14% of MN dogs. Titers were highest to serogroup Grippotyphosa in 34% of MN but only 2% of CA dogs.
The results of this study suggested that the prevalence of different serovars in dogs with leptospirosis may vary widely depending on geographic location. Furthermore, the environmental risk factors may also vary depending on geographic location. Because of this, recommendations for control and prevention of leptospirosis, including vaccination recommendations, may require modification depending on geographic location. Unfortunately, variation in the results of testing with the MAT in different laboratories limits our ability to compare the results of epidemiologic studies between different geographical locations. Prospective studies are required that either 1) use the same laboratory to perform MAT testing on samples from different geographical locations; 2) use alternative methods of leptospira identification, such as direct PCR typing methods or use of isolation and typing methods such as pulsed field gel electrophoresis,6 multilocus sequence typing,7 fluorescent amplified length polymorphism,8 or multiple-locus variable number tandem of repeats analysis (MLVA).9 The difficulties associated with isolating leptospires from clinical material have hampered the latter approach. Nevertheless, this approach has recently been used to study the epidemiology of leptospirosis in the Andaman Islands and allowed correlation between serovars causing outbreaks and those causing sporadic disease, and have emphasized the importance of isolation in the study of the epidemiology of leptospirosis.6
1. Levett P, et al. Clin Infect Dis 2003;36:447.
2. Miller MD, et al. J Vet Intern Med 2007;21:624 (abstr).
3. Goldstein RE, et al. J Vet Intern Med 2006;20:489.
4. Sykes JE, et al. J Vet Intern Med 2007;222:624 (abstr).
5. Thiermann AB. Am J Trop Med Hyg 1977;26:970.
6. Herrman JL, et al. J Clin Microbiol 1992;30:1696.
7. Thaipadungpanit J, et al. PLoS Negl Trop Dis 2007;1:e56.
8. Vijayachari P, et al. J Clin Microbiol 2004;42:3575.
9. Slack A, et al. J Med Microbiol 2006;55:1549.