Etiology

Leptospirosis, a zoonotic disease of worldwide significance that affects many animal species, is caused by infection with leptospiral spirochetes of the species Leptospira interrogans sensu lato. Leptospires are thin, flexible, filamentous (0.1 to 0.2 µm wide and 6 to 12 µm long) bacteria made up of fine spirals with hook-shaped ends (Fig. 42-1). They are composed of a cytoplasmic or protoplasmic cylinder surrounded by an inner membrane that is wound around a straight central axial filament composed of two nonoverlapping longitudinally oriented periplasmic flagella that exit the spirochete in subterminal locations. The inner spiral cylinder and overlapping axial filament are surrounded by a layered outer envelope or membrane. Leptospires are motile, making writhing and flexing movements while rotating along their long axis. As with gram-positive bacteria, the peptidoglycan cell wall is closely associated with the inner membrane, as are some lipoproteins (LipL31) and transmembrane proteins such as signal peptidase and ImpL63. The outer membrane is composed of lipopolysaccharide (LPS), multiple antigenic lipoproteins (LipL21, LipL32, LipL36, LipL41), integral transmembrane proteins (e.g., porin OmpL1), and secretins (e.g., GspD).

Leptospiral nomenclature and taxonomy is complicated by initial classification by cultural isolation and immunologic reactivity before 1989, with subsequent reclassification using genetic technology. Classically, the genus was divided into two species, L. interrogans sensu lato, containing all pathogenic strains, and Leptospira biflexa sensu lato, with all saprophytic strains from the environment. Within these two groupings, leptospires were identified as different serovars by distinct antibody reactivity to differing carbohydrate moieties in their outer-membrane LPS. Serovars are antigenically, and often epidemiologically, distinct organisms within a given species. More than 250 serovars of L. interrogans have been described and further classified into antigenically related serogroups,⁵⁹ of which at least 10 are important for dogs or cats. The term serogroup refers to serovars that share common antigens that often lead to cross-reactions with antibody detection methods. Although serogroups currently have no taxonomic basis, they have been historically useful for epidemiologic tracking and understanding of the disease. In cases where diagnosis is made by serologic means and the organism cannot be cultured or genetically identified, serogroup reactivity is technically the highest level of incrimination that can be determined. Furthermore, attempts to correlate specific clinical features of leptospirosis with a given serogroup or serovar can be misleading because serovars from different geographic areas that are immunogenically within one serogroup can have widely differing genetic composition, pathogenicity, and maintenance hosts. Furthermore, serovars within the same serogroup can belong to multiple different leptospiral genomospecies (see next paragraph). For example, serovars within the serogroup Grippotyphosa can be found in both L. interrogans and Leptospira kirschneri.⁸ Serovars are maintained in nature by numerous subclinically infected wild and domestic mammalian reservoir or maintenance hosts that serve as a potential source of infection and illness for humans and other incidental animal hosts (Table 42-1). Infected incidental hosts develop more severe clinical illness and shed organisms for shorter periods than reservoir hosts. Epidemiologic studies have shown that host preferences can change with time and geographic region of the world.

With the advent of genetic methods, organisms in the genus Leptospira have been classified into 20 genomospecies on the basis of genetic relatedness (Table 42-2).^{46,59,172,176} Both pathogenic and nonpathogenic serovars can occur within the same genomospecies in this new genetic classification. The complete genome sequences are available for six leptospiral strains, including two serovars of L. interrogans (Lai and Copenhageni), two strains of L. borgpetersenii serovar Hardjo, and two strains of L. biflexa serovar Patoc. The genome sizes of these organisms range from 3.9 to 4.6 Mbp.

Epidemiology

Leptospires are transmitted between hosts by direct, or more commonly indirect, contact. Direct transmission occurs through contact with infected urine, venereal and placental transfer, bite wounds, or ingestion of infected tissues. Crowding of animals, as may occur in a kennel situation, might theoretically be thought of as an environmental risk of infection; however, such transfer has not been well documented. Recovered or subclinically infected dogs have been shown to excrete viable organisms in urine intermittently for days to months after their initial infection with a variety of serovar isolates included in groups Canicola and Pomona.51,62,141,207

Indirect transmission, which is relatively common, occurs through exposure of susceptible animals to contaminated water sources, soil, and food. Once outside the host, leptospires do not replicate. The spirochete may remain viable for several months in moist soil that has been saturated with urine. Although evidence exists that spirochetes survive in insects and other invertebrate hosts, the significance of this finding with regard to disease transmission is unknown. The indirect transmission of leptospires may increase when environmental factors favoring the survival of leptospires are optimal. Stagnant or slow-moving warm water, although not essential, provides a suitable habitat for spirochetes. Within aquatic environs, leptospires form biofilms on organic and inorganic surfaces that contain colony clusters of organisms.²⁸⁴ Optimal survival in soil is favored by a neutral or slightly alkaline pH. Spirochetes survive only transiently in undiluted acidic urine (pH 5.0 to 5.5), whereas the opposite conditions provide more suitable habitats. Ambient temperatures between 0° C and 25° C favor the survival of leptospires, whereas freezing, dehydration, and exposure to ultraviolet radiation markedly decrease survival.

Pathogenesis

Leptospires penetrate intact mucous membranes of the mouth, nose, or eyes or abraded, scratched, or water-softened skin. Once entering the warmer mammalian body environment, transcriptional changes occur that enhance the spirochete’s pathogenicity.186,202,202 Organisms multiply rapidly after entering the blood vascular space (Fig. 42-2). They then spread and further replicate in many tissues, including the kidney, liver, spleen, central nervous system, eyes, and genital tract. In some experimental studies, the incubation period until development of clinical signs was 7 days, but this may vary according to infecting dose, conditions of natural exposure, infecting strain, and host immunity. With appropriate increases in serum antibodies thereafter, the host clears the spirochetes from most organs, but organisms may persist in the kidney and be shed in urine for days to months (see the earlier discussion of Epidemiology). The extent of damage to internal organs is variable depending on the host immunity, the virulence of the organism, and host ability to contain the infection.^114,115,115 Many animals develop more than one of these clinical manifestations, and disease expression can vary among outbreaks and geographic areas. Tissue edema and vasculitis may occur in rapid and severe infections that result in acute endothelial injury and hemorrhagic manifestations. These manifestations are typical of the systemic inflammatory response syndrome (SIRS) associated with sepsis (see Chapter 36). Leptospiral LPS stimulates neutrophil adherence and platelet activation,¹⁴³ which may contribute to the inflammatory and coagulatory abnormalities that occur. Tissue factor-mediated coagulation activation and impaired fibrinolysis or platelet activation may be results of inflammatory cytokine responses. The exact mechanisms involved are controversial and remain to be substantiated.^344,361 LipL32 is the major outer surface lipoprotein of pathogenic leptospires. Much research has been conducted to show its relationship with clinical illness. However, pathogenic lesions with LipL32 mutant were indistinguishable from those produced with wild-type infections.²³³

Classically, infections of dogs with various serogroups Canicola, Bratislava, and Grippotyphosa have been associated with predominantly renal or hepatic involvement, whereas serogroups Icterohaemorrhagiae and Pomona produce more hepatic disease. In other studies, high titers to serogroup Pomona were associated with severe renal disease.¹⁰⁶ Studies attempting to link specific disease manifestations with infecting serogroup have generally been based on serologic diagnosis, which is not consistently reliable for accurate identification of the infecting isolate (see Etiology). Further genetic analysis of isolates from natural disease outbreaks will be needed to determine if clinical signs vary as a result of differences in genomic composition and resultant pathogenicity of serologically related strains.

Should recovery occur, it is associated with increased specific antibody in the circulation as early as 7 to 8 days after experimental infection. With complete recovery and elimination of infection, the organisms will eventually be eliminated from the body. However, pathologic changes will persist in the severely affected kidney tissue despite clinical improvement. Recovered dogs that are not given antimicrobial therapy may be more likely to be persistent renal carriers and shed Leptospira in their urine; the duration of shedding has not been determined in all instances and likely varies by the infecting strain (see Epidemiology).

Renal colonization occurs in most infected animals because the organism replicates and persists in renal tubular epithelial cells, even in the presence of serum neutralizing antibodies (see Fig. 42-2).^61,225 Differential expression of surface antigens of the spirochete may be responsible for its ability to evade host immune responses during the chronic shedding period.²²⁴ The organism penetrates the renal capillaries and enters the interstitium, and by 2 weeks after infection, leptospires may be seen within the proximal tubular cells and tubular lumen, which coincides with the onset of shedding. Anti-leptospiral IgG in the renal tubules may also be unable to kill spirochetes because of the absence of complement in the hypertonic interstitium.²²⁵

The invasive capacity of leptospires in tissues may be correlated with their pathogenicity because nonpathogenic leptospires do not penetrate cells as readily as do pathogenic leptospires.²⁶ Acute impairment of renal function may result from decreased glomerular filtration caused by kidney swelling that impairs renal blood perfusion. Damage to the endothelium of small blood vessels may result in ischemic damage to the renal parenchyma. In addition, several leptospiral factors have a toxic effect on cells located within the renal parenchyma. Significant renal damage probably results from the leptospiral LPS and other outer membrane protein components. Leptospiral LPS is a potent activator of macrophages, and stimulates secretion of interleukin-1 and interferon by these cells, in addition to augmenting their killing capacity.¹⁴⁴ Incubation of peripheral blood mononuclear cells with leptospiral LPS results in the dose-dependent release of tissue necrosis factor-α and interleukin-10. The expression of these cytokines, as well as transforming growth factor-β and the T-cell recruitment cytokine interferon inducible protein-10, has also been detected in hamster models of infection. Tissue necrosis factor-α and inducible protein-10 are proinflammatory cytokines and may play a role in the inflammatory response to leptospires.¹⁹⁰ Leptospiral outer membrane proteins, such as LipL32, induce an inflammatory response in renal proximal tubule cells via Toll-like receptors or other ligands.^225,343,343

Unsaturated fatty acids in a glycolipid fraction from leptospires specifically inhibit sodium-potassium adenosine triphosphatase (Na⁺,K⁺-ATPase), which might account for the urinary potassium wasting observed in many patients with leptospirosis.183,236,236 Endotoxin fractions from L. interrogans have been shown to interfere with renal concentrating mechanisms.⁵²

Several hemolysins have been identified in pathogenic leptospires. The first (sphingomyelinase C) was purified from Leptospira borgpetersenii serovar Hardjo.³³ Sphingomyelinase H causes pore formation in target cell membranes and is highly conserved only in pathogenic leptospires.¹⁷⁰ Hemoglobinemia, hemoglobinuria, and icterus that develop in cattle with leptospirosis result from a serovar-specific hemolytic toxin produced by isolates from serogroup Pomona. However, hemolysis in other hosts is uncommon despite the presence of other known hemolytic surface moieties. In contrast, decreased osmotic fragility has been detected in canine leptospirosis,²⁹² making hemolysis less likely.

A number of fibronectin binding proteins have been identified on the surface of pathogenic leptospires including LipL32, as have surface exposed Lig proteins (LigA and LigB).²⁰³ Virulent strains are able to bind complement-4 binding protein, thus interfering with serum neutralization of the spirochete.²³ Some of these virulence factors may explain how pathogenic Leptospira evade host defense mechanisms better than attenuated vaccine strains or cultivated isolates. The extent to which these factors cause tissue damage in leptospirosis remains to be investigated.

The liver is another major parenchymatous organ damaged during leptospiremia. Profound hepatic dysfunction may occur without major histologic changes because of subcellular damage produced by leptospiral toxins. Because there is no hemolysis, the degree of icterus in both canine and human leptospirosis usually corresponds to the severity of hepatic necrosis. Chronic active hepatitis has been a documented sequela to serovar Grippotyphosa infection in dogs.³⁶ Dogs in a breeding colony of beagles in France had similar disease associated with serogroup Australis infection.² Presumably, initial hepatocellular injury and persistence of the organism in the liver result in altered hepatic circulation, fibrosis, and immunologic disturbances that perpetuate the chronic inflammatory response. Extensive hepatic fibrosis and failure may result from this process. Leptospirosis should be considered in dogs with hepatic inflammation or fibrosis.

Acute lung injury occurs as a result of the effects of the toxins of the organism on pulmonary tissue. Fluid exudation within the lung may result from vasculitis, and rarely, acute severe pulmonary hemorrhage may occur.35,323 The degree of pulmonary impairment is often reflective of the prognosis for recovery.^102,196 Geographic variation in the prevalence of acute, severe pulmonary hemorrhage may exist among dogs with leptospirosis. Outbreaks of pulmonary hemorrhage associated with leptospirosis have also been reported in humans and also appear to occur in dogs.

Other body systems are damaged during or subsequent to infection that have not been frequently documented in dogs. A benign meningitis, documented in humans, is produced if some leptospires invade the nervous system or cerebrospinal fluid (CSF) during the acute phase of illness. After onset of the antibody response and clearance of tissue organisms, persisting organisms in the central nervous systems of some patients, especially children, may result in development of immune-complex–mediated meningitis.⁷⁶ Uveitis has also been documented in humans, dogs, and horses, likely through a similar pathogenic mechanism.

Box 42-1 summarizes all the clinical features of leptospirosis. Clinical signs in canine leptospirosis depend on age and immunity of the host, environmental factors affecting the organisms, and virulence of the infecting serovar. A comparison of the published historical and physical findings is in Web Table 42-1. Young animals are usually more severely affected than are adults. Large breed (more than 15 kg), outdoor, adult dogs are the most commonly affected.²¹⁶ In a survey of reported cases from veterinary teaching hospitals in the United States, herding dogs, hounds, and working dogs had a higher risk of infection compared with other breeds.³⁴⁸ However, affected breeds in published reports may vary depending on geographic area, practice setting, or referral base. Small breeds were more frequently affected in cases observed at the teaching hospital at the University of Minnesota (40%) as compared to those at the University of California (16%).³²⁸ Peracute leptospiral infections can be exhibited by massive leptospiremia and death with few premonitory signs. In acute infections, pyrexia (39.5° C to 40° C [103° F to 104° F]), shivering, and generalized muscle tenderness are the first clinical signs. Subsequently, vomiting, rapid dehydration, and peripheral vascular collapse occur. Tachypnea, rapid and irregular pulse, and poor capillary perfusion have been noted. Coagulation defects and vascular injury are apparent with hematemesis, hematochezia, melena, epistaxis, and widespread petechiae. Terminally ill dogs become depressed and hypothermic, and renal and hepatic failure does not have time to develop.

Subacute infections are characterized by anorexia, vomiting, dehydration, and increased thirst. In many dogs with subacute leptospirosis, rectal temperature is within reference ranges. Reluctance to move and paraspinal hyperesthesia in dogs may result from muscular, meningeal, or renal inflammation. Weight loss may occur, usually associated with reduced food intake resulting from hyperesthesia associated with inflammation or from uremia. Abdominal palpation may indicate hyperesthesia and abdominal discomfort from visceral inflammation. The kidneys may be within reference size limits or slightly enlarged and may be painful on palpation. Mucous membranes may appear congested. Coughing and dyspnea may accompany conjunctivitis, rhinitis, and tonsillitis. Progressive deterioration in renal function is exhibited by oliguria or anuria. Renal function in some dogs surviving subacute infections may return to normal within 2 to 3 weeks, or chronic compensated polyuric renal failure may develop. These dogs with chronic renal dysfunction may have polydipsia and polyuria as their primary manifestations for long periods of time until azotemia progresses and eventual uremia ensues.

Icterus or more mild increases in serum bilirubin concentrations may occur in dogs affected with the acute form of the disease (Fig. 42-3). Intrahepatic cholestasis from hepatic inflammation may be so complete that fecal color changes from brown to gray. Dogs with chronic active hepatitis or chronic hepatic fibrosis as a sequela to leptospirosis may eventually demonstrate overt signs of liver failure, including chronic inappetence, weight loss, ascites, icterus, or hepatoencephalopathy. Chronic progressive hepatic disease can occur in the clinical absence of other organ system involvement.²

Intestinal intussusceptions occur with some frequency in dogs with acute infections presumably associated with gastrointestinal (GI) inflammation. Careful abdominal palpation should be performed in dogs that develop persistent vomiting and diarrhea. Feces become scanty in such cases, and hematochezia or melena will be apparent. Development of pancreatitis is a documented sequela to Leptospira infection in humans and may explain dogs having acute abdominal pain as well as persistent vomiting and anorexia in dogs in which azotemia has resolved after hospitalization.

Pulmonary manifestations include labored respiration and coughing.²⁸⁰ Both interstitial pneumonia and pulmonary hemorrhage with alveolar consolidation have been documented as the cause in humans^32,139 and in dogs (Fig. 42-4).^30,35,35

Cardiac manifestations may occur in severely affected dogs.¹⁹⁹ Rhythm disturbances that can be found on physical examination and electrocardiographic abnormalities have been ventricular tachyarrhythmias. Myocardial damage is suspected based on laboratory findings (see Clinical Laboratory Findings, later) and from similar findings in human leptospirosis.³⁰⁶

Uveitis is commonly recognized in naturally and experimentally occurring leptospirosis in humans and certain animals such as the horse. Panuveitis has been reported in association with leptospirosis in a dog³³³ and has been found in several dogs examined by the authors of this chapter.

Abortion is a clinical problem in mares, cows, and sows; however, there are few reports in dogs. A leptospiral serovar Buenos Aires (serogroup Djasiman) was isolated from an aborted fetus of a seropositive bitch.²⁸⁶ Abortion and infertility was associated with a leptospire serogroup Batavia infection in a dog.⁸⁵

Based on serologic surveys of clinically healthy dogs, many leptospiral infections in dogs are chronic or subclinical (see the earlier discussion of Etiology).²⁹⁷ The clinical focus for serotesting in dogs has been on those with renal or hepatic dysfunction; however, serologic and microbiologic evaluation for leptospirosis might be performed on dogs with fever of unknown origin, anterior uveitis, or meningitis and on healthy dogs in kennels, multidog households and neighborhoods, or other environs where infection in other members has been documented.

Hematologic findings in typical cases of canine leptospirosis include leukocytosis and thrombocytopenia. Leukocyte counts fluctuate depending on the stage and severity of infection. Leukopenia, common in the leptospiremic phase, develops into leukocytosis with a left shift. In later stages, white blood cell counts usually range from 16,500 to 45,000 cells/µL. Thrombocytopenia is often a prognostic manifestation of severe cases of leptospirosis, and the degree correlates with the severity of respiratory distress and radiographic pulmonary changes.²⁷¹ Anemia of mild severity is often observed in dogs with leptospirosis. It is usually nonregenerative, which may reflect: the presence of acute oliguric renal failure with overhydration; decreased erythrocyte production from chronic renal failure or chronic inflammatory disease; or acute (less than 4-day duration) blood loss from tissue thrombosis and/or acute pulmonary or GI hemorrhage.

The majority of dogs with leptospirosis are azotemic on initial examination. Increases in serum urea nitrogen and creatinine concentrations are found in dogs with varying severity of kidney disease, although a testing bias may be present because in most retrospective case reports only azotemic dogs were tested (see Web Table 42-2). Hypoalbuminemia can be observed,* (likely from proteinuria or SIRS), and the serum albumin : globulin ratio is usually reduced. The serum globulin concentration may be increased in some dogs.¹¹¹ The disproportionate hyperglobulinemia may relate to chronic antigenic stimulation from protracted infection, or superimposed dehydration from renal failure, or both. However, concurrent persistent infections associated with hyperglobulinemia should be investigated. Electrolyte alterations usually parallel the degree of renal and GI dysfunction except for marked hypokalemia in many cases. Hyponatremia, hypochloremia, hypokalemia, and hyperphosphatemia are present in most cases, whereas hyperkalemia develops only in dogs with terminal oliguric renal failure. In humans, hypokalemia in the presence of acute renal failure has also been thought to be likely associated with leptospirosis.²³⁶ Hypomagnesemia has also been reported in the acute stages of the severe human illness.⁶⁷ Similar associations have not been documented for the infection in dogs. Mild hypocalcemia is related to hypoalbuminemia and decreased concentration of the protein-bound calcium fraction. Blood pH and serum bicarbonate concentration are reduced in severely affected animals, reflecting metabolic acidosis. Hypoglycemia is occasionally present with severe hepatic failure. Hepatic dysfunction may be apparent in some dogs, but it usually is less dramatic than renal failure or is associated with concurrent renal failure. Liver damage is demonstrated by increased serum alanine aminotransferase, aspartate aminotransferase, lactate dehydrogenase, and alkaline phosphatase activities and bilirubin concentration. The increase in serum alkaline phosphatase activity is often proportionally greater than that of alanine aminotransferase activity. Bilirubin concentration may be increased in serum and urine, and the magnitude is usually proportional to the degree of liver impairment and is a very common finding. Marked bilirubinuria usually precedes hyperbilirubinemia. Serum bilirubin peaks by days 6 to 8 after onset of disease. Increased sulfobromophthalein retention (more than 5%) can be found in acute leptospirosis before the onset of icterus and in dogs that later develop chronic active hepatitis. Increased serum amylase and lipase activities may result from their release from inflamed hepatic and small intestinal tissues and from decreased renal excretion. Increased serum amylase and lipase activities and increased pancreatic lipase immunoreactivity are found in some dogs with apparent secondary pancreatitis. Serum cardiac troponin concentrations have been increased in some dogs, suggesting myocardial damage.¹⁹⁹ Serum creatine kinase activity is increased when skeletal muscle inflammation occurs.¹⁹⁹ Urinalysis is very often characterized by glucosuria, tubular or glomerular proteinuria, and bilirubinuria and, in some cases, by increased numbers of granular casts, leukocytes, and erythrocytes in the sediment. Some dogs have an increased ratio of protein to creatinine in their urine in the presence of a low-cellularity urinary sediment, which indicates the kidney as the source of the urine protein. The concurrent presence of glucosuria or casts with proteinuria can be explained by acute tubular injury. Findings of predominantly low- and sometimes high-molecular-weight urine proteins are compatible with tubular and glomerular sources, respectively,^199,365 although glomerular pathology is not a primary feature of leptospirosis, which is predominantly a tubulointerstitial nephritis. Leptospires will not be observed in the urine without special staining or dark-field microscopy.

In experimentally and naturally occurring leptospirosis in dogs, varying degrees of thrombocytopenia, reduced fibrinogen concentration, prolonged activated partial thromboplastin and prothrombin time values, reduced antithrombin activity, increased D-dimer concentration, and increased fibrinogen degradation products have been found, presumably a result of excessive intravascular coagulation.161a,199,238 In many dogs, clotting parameters are within reference limits, or variable thrombocytopenia may be the only abnormality, suggesting compensated hemostatic mechanisms. The degree of thrombocytopenia in dogs with leptospirosis has been correlated with the severity of pulmonary manifestations and higher mortality.^161a,271 Although meningitis is not well documented in dogs, increased protein concentration with a predominance of neutrophils would be detected by CSF analysis.

In humans, assessment of clinical laboratory findings has allowed prediction of severity and mortality in affected patients.82,196 Dyspnea and severe alveolar infiltrates on thoracic radiography, elevated blood leukocyte counts, thrombocytopenia, oliguria, and repolarization abnormalities on electrocardiograms are all poorer prognostic findings. Similar associations for pulmonary disease and thrombocytopenia²⁷¹ and for serum albumin, cardiac tropinin I, C-reactive protein/haptoglobin ratio, urinary albumin, and urinary total protein/creatinine ratio¹⁹⁹ have been made, suggesting a poorer prognosis for affected dogs.

Radiographic and Ultrasonographic Findings

Although coughing and dyspnea are not consistent features of canine leptospirosis, interstitial to nodular alveolar densities, likely associated with pulmonary hemorrhage from endothelial damage and vasculitis, are found in thoracic radiographs of many dogs.30,161a,271 Lesions are most consistently detected in the caudodorsal lung fields. A complete clearance of the lesions occurs in recovered dogs. In decreasing order of prevalence, ultrasonographic abnormalities of the urinary system have included renomegaly, pyelectasia, increased cortical echogenicity, mild perirenal fluid accumulation, and a medullary band of increased echogenicity (Fig. 42-5).⁹³ The medullary echogenic banding is not specific for leptospirosis in dogs and corresponds to a region of renal edema, necrosis, and hemorrhage seen grossly and histologically. Changes suggestive of pancreatic inflammation may also be detected using abdominal ultrasonography.

Only gold members can continue reading. Log In or Register to continue

Share this:

• Click to share on Twitter (Opens in new window)
• Click to share on Facebook (Opens in new window)

Related

Related posts:

1. Poxvirus Infections
2. Laboratory Diagnosis of Bacterial Infections
3. Chlamydial Infections
4. Surgical and Traumatic Wound Infections

Stay updated, free articles. Join our Telegram channel

Join