Jane E. Sykes, BVSc(Hons), PhD, DACVIM
School of Veterinary Medicine, University of California-Davis, Davis, CA, USA
Leptospirosis is caused by infection with various serovars of Leptospira interrogans sensu lato. Organisms are transmitted by direct contact with infected urine, bite wounds, or ingestion of infected tissues, or indirectly through contact with infected water, soil, food, or bedding. Survival of leptospires is promoted by stagnant warm water, a neutral or slightly alkaline pH, and temperatures between 0 and 25°C. The seasonality of the disease is variable depending on local climactic conditions, especially rainfall. In areas with year-round rainfall, the disease may occur throughout the year.
There are over 200 pathogenic serovars, which are grouped into antigenically related serogroups. Serovars known to infect and cause disease in dogs include Canicola, Icterohaemorrhagiae, Grippotyphosa, Pomona, Ballum, Bratislava, Autumnalis, Bataviae, Australis, and Hardjo. Classification of leptospires is gradually moving from predominantly serovar-based classification to that based on genetic typing (genotype-based classification). Each serovar (and more accurately, each genotype) is adapted to one or more mammalian host species (maintenance hosts). Other hosts act as incidental hosts. Disease in incidental hosts tends to be more severe and the duration of shedding is generally shorter. Maintenance hosts include dogs (Canicola), rats (Icterohaemorrhagiae), small wildlife mammalian species such as voles and raccoons (Grippotyphosa), cattle and pigs (Pomona), pigs (Bratislava), cattle (Hardjo), and mice (Ballum). The prevalence of infection with a serovar in dogs depends on the degree of contact between the dog population and the maintenance host for that serovar.
The most common serovars thought to infect dogs before the introduction of the Leptospira vaccines were Icterohaemorrhagiae and Canicola. Vaccines containing only serovars Icterohaemorrhagiae and Canicola do not protect against infection by other serovars. Since introduction of the bivalent bacterins containing these two serovars, in North America and Europe, there have been decreasing reports of disease associated with seroconversion to Canicola and Icterohaemorrhagiae, and increasing reports of disease associated with seroconversion to serovars Pomona, Grippotyphosa, Autumnalis and Bratislava (North America) and Sejroe, Australis and Grippotyphosa (Europe). Vaccine pressure, increasing contact between dogs and certain wildlife reservoir hosts and increased testing have been suggested as reasons for this change. In truth, the actual serovars causing disease in dogs worldwide remain uncharacterized because the disease is diagnosed by serology, and serologic test results are not predictive of the infecting serovar.
Pathogenic leptospires penetrate abraded skin or mucus membranes and multiply rapidly in the bloodstream and tissues, causing renal failure, hepatic injury, and vasculitis. The disease is multisystemic and may also involve the pancreas (pancreatitis), gastrointestinal tract (gastroenteritis), eye (uveitis), and lungs (leptospiral pulmonary hemorrhage syndrome, or LPHS). In humans, Leptospira can also cause meningitis, which is most commonly manifest as a severe headache. Clinical manifestations may also depend on the age of the host, the infectious dose, and the strain of Leptospira involved.
Most infections are subclinical. Younger, large breed, outdoor adult dogs are commonly affected, but the disease can occur in any dog breed and at any age; dogs that live in cities may become infected as a result of exposure to rodent reservoir hosts. One recent study showed an increase in the percentage of small breed dogs diagnosed with leptospirosis between 1970 and 2009 (Lee et al. 2014). Younger animals tend to be more severely affected. Male dogs may be predisposed.
Lethargy, anorexia, vomiting, pyrexia, dehydration, abdominal pain, and increased thirst and urination are common signs of acute leptospirosis. Reluctance to move due to myositis, icterus, and uveitis may be noted. Respiratory difficulty may result from pulmonary hemorrhage, which is often associated with the development of moderate anemia.
Leukocytosis, thrombocytopenia, azotemia, hypoalbuminemia, and mild to moderately elevated liver enzyme activities are common. Urinalysis may reveal isosthenuria, proteinuria, glucosuria, and casts. Although it occurs with other causes of renal tubular damage, glucosuria in addition to azotemia can be a "red flag" for a diagnosis of leptospirosis. Proteinuria is typically low-level (urine protein:creatinine ratio < 5). Thoracic radiography may reveal a focal or diffuse interstitial to bronchointerstitial pattern; alveolar patterns may represent pulmonary hemorrhage. Occasionally, mild pleural effusion is evident. Hepatomegaly, splenomegaly, renomegaly and/or peritoneal effusion may be evident from abdominal radiography. Hyperechoic renal cortices and mild renal pelvis dilation are occasionally seen with abdominal ultrasound.
Diagnosis of leptospirosis requires a high clinical suspicion for the disease based on knowledge of the range of clinical presentations that suggest leptospirosis. This is because currently accurate diagnosis is retrospective and generally based on serology using the microscopic agglutination test (MAT). Respective titers are provided for each of several different serovars in order to increase the chance of antibody detection. Studies in humans and dogs have shown that the serovar with the highest titer can vary over time and that paradoxical crossreactivity to multiple serovars occurs after exposure to a single serovar. Thus, the MAT does not accurately predict the infecting serovar, and therefore should not be used for this purpose. Titers may be negative in the first week of illness. Positive titers early in the course of an illness may reflect residual postvaccinal titers or prior subclinical infection, and are not diagnostic for the disease. Demonstration of a fourfold rise in titer is required over a 1–2-week interval. It is the author's opinion that leptospirosis serology should only be performed in a paired fashion or not at all, because of the limited utility of a single positive titer, regardless of its magnitude. Postvaccinal titers against Icterohaemorrhagiae, Canicola, Grippotyphosa, and Pomona occasionally rise as high as 1:6400 for a few months after vaccination, and these can interfere with interpretation. The results can also vary dramatically between laboratories (Miller et al. 2011). Use of a laboratory with a high level of quality control is recommended, or a laboratory that participates in the International Leptospirosis Society's proficiency testing scheme. More information on leptospirosis diagnosis at the author's institution can be found on the UC Davis leptospirosis laboratory testing website (www.vetmed.ucdavis.edu/foley_lab/leptospira/index.cfm).
Recently, an LipL32-based ELISA assay has become available in North America for detection of antibodies to pathogenic leptospires in dogs. In the future, rapid in-clinic kits for leptospirosis serology may also become available. These assays yield qualitative (positive or negative) results and may be most useful for screening dogs for the presence or absence of antibodies. Should these kits yield negative results, then the clinician should consider whether it may be too early for the animal to have developed antibodies (as can occur with the MAT). Another test should be performed one week later to see if the animal seroconverts. Should these kits yield positive results, then the clinician should consider whether previous vaccination has occurred (assuming they cross-react with vaccine-induced antibody, as is the case for the LipL32-based ELISA). Previous exposure without clinical disease should also be considered as a reason for positive results. Thus, in the author's opinion, a positive result using this kit should stimulate reflex testing with acute and convalescent quantitative serology using the MAT.
Darkfield microscopy of the urine is not recommended as sole test for diagnosis because of the large number of false positives and false negatives. Silver staining and fluorescent antibody or immunoperoxidase staining of tissue specimens can also yield false negatives and do not help identify the infecting serovar. Culture is difficult because of the fastidious growth requirements of leptospires and the need for specialized media, but is the only way to truly identify an infecting serovar. Cultures must be incubated for several weeks. Repeated specimen collection may be required due to intermittent shedding. The sensitivity of PCR assays is still not well established, and they do not provide information about the infecting serovar, although they have been used to provide information on genotype. The author's experience is that PCR may be insensitive for diagnosis of canine leptospirosis, but the sensitivity and specificity may vary geographically depending on the serovars present and shedding patterns that occur for those serovars. PCR assays are best performed on blood and urine concurrently because urinary shedding begins 10 days after the onset of infection. UC Davis now offers a multimodality approach to diagnostic testing for leptospirosis that includes serology with or without culture and PCR.
Specific treatment involves initial use of parenteral penicillin derivatives for leptospiremia. In the author's hospital, ampicillin is generally used (20 mg/kg IV q 6–8 h, adjusting dose down if severe azotemia is present) for up to 14 days or as long as the patient is vomiting or appears nauseated. It is recommended that treatment then be changed to doxycycline (5 mg/kg PO q 12 h) for 2 weeks, in order to eliminate the carrier phase. Doxycycline can be used instead of penicillins if vomiting does not occur after administration. Supportive therapy is also indicated for acute renal failure (e.g., IV fluids, H2 blockers, antihypertensives, gastric protectants, antiemetics, phosphate binders, packed red cells, and nutritional support). The use of hemodialysis can improve survival in dogs with severe renal failure. Approximately 50% of the patients with leptospirosis at the author's institution are dialyzed, and the average number of treatments required before polyuria and recovery occurs is 3. Euthanasia or death due to leptospirosis is recorded in 18% of our dogs.
In North America, vaccines are available for serovars Canicola, Icterohaemorrhagiae, Pomona, and Grippotyphosa and in widespread use. The vaccines are generally safe and efficacious and studies suggest they provide a 1-year duration of immunity (Minke et al. 2009; Klaasen et al. 2003). Although it was prevalent when the two-way (Canicola and Icterohaemorrhagiae) vaccines were in widespread use, vaccine failure appears to be extremely rare with the current 4-serovar vaccines (Hennebelle et al. 2013). In Europe, only 2-way vaccines have been available until very recently, and disease has been occurring in vaccinated and unvaccinated dogs. New vaccines are being introduced that contain three (Icterohaemorrhagiae, Canicola, and Grippotyphosa) or four (Icterohaemorrhagiae, Canicola, Grippotyphosa, and Bratislava) serovars. Leptospira bacterins have been associated with occasional acute, severe allergic reactions, but the incidence of these reactions has decreased dramatically in recent years, and reaction rates appear to be approaching those of distemper-hepatitis-parvovirus vaccines, even in small breed dogs. Vaccination against pathogenic leptospires is strongly recommended for dogs living in areas where leptospirosis occurs, and are recommended even for small breed dogs that are confined to urban backyards, because of the possibility of infection as a result of rodent exposure. Minimizing access to rodents, farm animals, and other wild animals also should help prevent infection.
Public Health Risk
Leptospirosis remains an important zoonosis, although most documented human leptospirosis in North America results from recreational activities that involve water, rather than contact with dogs. Because dogs are generally incidental hosts, they may not shed for significant periods of time, although more studies are required to confirm this, and there are anecdotal reports of leptospirosis in staff that work in veterinary hospitals. Human leptospirosis is typically a 'flu-like illness,' but in some cases may be associated with vomiting, diarrhea, shock, jaundice, renal failure, pneumonia, meningitis, or abortion. Any animal with acute renal failure should be treated as a suspect. Warnings should be placed on cages, gloves should be worn while handling these dogs, and bleach or iodine-based disinfectants should be used to clean areas soiled with urine. Owners should be warned that without specific treatment, leptospires may be shed in the urine for months despite clinical recovery. The ACVIM has published consensus guidelines for the diagnosis, treatment, and prevention of leptospirosis in dogs (Sykes et al. 2011).
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