Infectious Agents

Chapter 57
Infectious Agents: Leptospirosis


Daniel L. Grooms


Department of Large Animal Clinical Sciences, College of Veterinary Medicine, Michigan State University, East Lansing, Michigan, USA


Introduction


Bovine leptospirosis is recognized as a cause of significant reproductive losses including infertility, early embryonic death, abortions, stillbirths, and birth of weak calves. In addition, several other nonreproductive clinical manifestations of leptospirosis have been described.1 Leptospirosis occurs worldwide and is caused by infection with the spirochete bacteria belonging to the genus Leptospira. Leptospira can infect all mammalian species including humans and is a significant zoonotic pathogen.2


Leptospira can be classified on several different levels. The most definitive classification is based on genomospecies which is established by genetic sequence analysis. Leptospira are also classified into serovars based on antigenic relatedness. Over 200 different serovars of pathogenic Leptospira have been identified.3 Serovars can further be defined by genotypic analysis, for example there are two genotypes of serovar Hardjo that are important in cattle, serovar Hardjo type hardjoprajitno and type hardjo-bovis.


Epidemiology of leptospirosis


Prevalence


Leptospira are found worldwide, though different serovars may only be endemic in certain geographic regions. The most common serovars in the United States are Hardjo, Grippotyphosa, and Pomona; however, other serovars can also be found and have been associated with reproductive losses.


Serologic surveys have repeatedly shown that the most common cause of leptospirosis in cattle is infection with serovar Hardjo.4–7 As mentioned earlier, two genetically distinct types of serovar Hardjo have been identified: serovar Hardjo type hardjo-bovis and serovar Hardjo type hardjoprajitno. Serovar Hardjo type hardjo-bovis is common in cattle populations throughout the world, including North America,5 while type hardjoprajitno has been found primarily in the United Kingdom.


Limited studies in the United States estimate herd prevalence of serovar Hardjo to be near 60% in dairies6 and 40% in beef herds.7 In Ontario, Canada, 8% of dairy herds and 44% of beef herds surveyed that were not vaccinated against leptospirosis were found to have cows serologically positive for serovar Hardjo,8 whereas in western Canada seroprevalence appears to be much less in the beef herds surveyed there.9 Herd prevalence may vary regionally with higher rates more likely in temperate climates.7,10 In a large survey of cull cows sampled at slaughter in 49 states, the percentage of cattle serologically positive for a Leptospira serovar was much higher in southern tier and west coast states than other areas of the United States.4 In studies of individual adult cows, seroprevalence to serovar Hardjo ranges from 10 to 30% in both the United States and Canada. Higher seroprevalence rates can be expected in individual herds that are endemically infected.


Accurate data for the frequency of abortions attributable to leptospirosis are lacking in North America. In Northern Ireland, leptospirosis was recognized as being responsible for over half of all bovine abortions, with serovar Hardjo being identified as the causative agent in 97% of those in one study.11 In the previously cited study performed in Ontario, Canada, serovar Hardjo was associated with 6% of abortions while Pomona abortions were not recognized.8 In the United States, multiple diagnostic laboratory surveys of cases in which there was a definitive diagnosis found that Leptospira was identified in less than 10% of aborted fetuses.12 Because of the overall difficulty in making a definitive diagnosis in bovine abortions cases in general, it is likely these surveys underestimate the true prevalence of Leptospira-related abortions.


Maintenance versus incidental hosts


Animals can be divided into maintenance hosts and incidental hosts of Leptospira. Different Leptospira serovars are associated with one or more maintenance host(s), which serve as transmission reservoirs. Animals may be maintenance hosts of some serovars but incidental or accidental hosts of others. In a maintenance host, the disease is maintained by chronic infection of the renal tubules. Transmission of the infection among maintenance hosts is efficient and the incidence of infection is relatively high at a population (or herd) level. Incidental hosts may become infected by direct or indirect contact with the maintenance host. Incidental hosts are not important reservoirs of infection and the rate of transmission is low. Transmission of the infection from one incidental host to another is relatively uncommon. Examples of important maintenance hosts for different Leptospira serovars that commonly cause infection in cattle in North America are listed in Table 57.1.


Table 57.1 Important maintenance hosts for different Leptospira serovars that commonly cause infection in cattle in North America.

























Leptospira serovar Maintenance host
Bratislava Pigs, mice, horses
Canicola Dogs
Grippotyphosa Raccoons, opossums, skunks
Hardjo Cattle, deer
Icterohaemorrhagiae Rats
Pomona Pigs, raccoons, opossums, skunks

Transmission


Transmission of Leptospira can occur either directly or indirectly. Transmission among maintenance hosts, such as serovar Hardjo among cattle, can be both direct and indirect and involves contact with infected urine, placental fluids, or milk. In addition, the infection can be transmitted venereally or transplacentally. Infection of incidental hosts, such as cattle becoming infected with serovar Grippotyphosa originating from raccoons, occurs more commonly by indirect contact with environments contaminated with urine of maintenance hosts.


Leptospira can survive in the environment for days to weeks outside of the animal under warm moist conditions. The organism survives only briefly in hot dry conditions or at freezing temperatures. Because of these characteristics, leptospirosis occurs more commonly during the spring and fall and in temperate climates.6


Pathogenesis


Leptospira gains entrance to the body through mucous membranes and water-softened or abraded skin. After a 3–20 day incubation period, leptospiremia occurs and the organism is disseminated to many organs including the renal system and genital tract. During this period, acute clinical disease may be seen. Antibodies can be detected in serum soon after leptospiremia begins and coincides with the clearance of Leptospira from blood and most organs. However, organisms can persist in the kidneys for several weeks in an incidental host, or 6 months or more in a maintenance host.5,6 Leptospira can also persist in the genital tract of a maintenance host for similar periods of time.6 Leptospira excretion in urine is variable and may last weeks in incidental hosts and months in maintenance hosts. In cattle, serovar Pomona shedding has been observed for up to 15 weeks under experimental conditions,13 while serovar Hardjo shedding typically is for 6–8 months14 but has been reported to be as long as 18 months.15


Clinical disease


Clinical outcomes of Leptospira infection in cattle will depend on the infecting serovar, level of immunity, and physiological state. A summary of clinical reproductive differences between infections with Hardjo and non-Hardjo serovars can be found in Table 57.2.


Table 57.2 Summary of clinical differences between infections with Hardjo and non-Hardjo serovars.










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Aug 24, 2017 | Posted by in GENERAL | Comments Off on Infectious Agents
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Serovar Hardjo Non-Hardjo serovars (e.g., Pomona and Grippotyphosa)
Presentation in adult cows