Chapter 19 Systemic Bacterial Infectious Diseases
A large number of bacteria can infect dogs and cats. This chapter focuses on leptospirosis, brucellosis, and bartonellosis. Chapter 18 is devoted to Lyme borreliosis. Other infectious diseases caused by bacteria are summarized in Table 19-1, and most are described in the respective organ-system chapters. Bordetella bronchiseptica is associated with the canine infectious tracheobronchitis (see Chapter 12).Mycoplasma haemofelis and Mycoplasma haemominutum are hemotropic infections (see Chapter 22). Mycobacterial infections are associated with chronic skin disease (see Chapter 39). Actinomycosis and nocardiosis are causes of pyothorax (see Chapter 164). Tetanus and botulism cause severe neuromuscular dysfunction (see Chapters 128 and 130, respectively). Salmonellosis, campylobacteriosis, yersiniosis, and Clostridium perfringens and Cl. difficile primarily involve the intestinal tract (see Chapter 69).
LEPTOSPIROSIS
Etiology
Transmission
• Wild animal populations serve as reservoirs of infection that persistently shed Leptospira organisms and contaminate the environment. Suburban “backyard” wildlife such as raccoons, skunks, squirrels, voles, and other rodents may be especially important sources of infection.
• Exposure usually occurs by mucocutaneous contact with Leptospira organisms in the environment, most importantly contaminated surface water, but also food, bedding, soil, vegetation, or fomites. The organisms penetrate mucosa or abraded skin. In addition, transplacental, venereal, and bite-wound transmission can occur.
Pathogenesis
• Leptospira preferentially infect renal tubule epithelium, often causing acute tubular injury and renal failure. Renal colonization and urine shedding are prolonged, continuing for months to years after clinical recovery.
• Leptospira can injure hepatocytes, resulting in hepatocyte dysfunction, cholestasis, acute hepatic necrosis, jaundice, and occasionally chronic hepatitis and hepatic fibrosis (especially L. grippotyphosa).
Clinical Signs
• Systemic signs: This can include acute onset of anorexia, depression, fever, vomiting, dehydration, stiff gait and reluctance to move (generalized myalgia), and congested mucous membranes. Vascular collapse and peracute death are seen in some animals.
• Acute renal failure: Systemic signs combined with renal pain and polyuria-polydipsia (mild cases), oliguria, or anuria (severe cases).
• Acute vasculitis and DIC: Systemic signs combined with widespread tissue edema and petechial and ecchymotic hemorrhages, hematemesis, melena, hematochezia, and epistaxis.
• Chronic hepatitis and hepatic fibrosis: Signs of chronic liver failure develop; e.g., weight loss, ascites, jaundice, and hepatic encephalopathy.
Diagnosis
Diagnostic Imaging
• Kidney size can be normal or enlarged. Ultrasonographically, kidneys show increased cortical echogenicity in 75% of cases and pyelectasia in nearly 50% of cases. Some dogs have a distinctive echoge-nic demarcation of the corticomedullary border (“medullary rim sign”). Perinephric effusion may be seen.
Serology
Microscopic Agglutination Titer
• Although single titers do not definitively diagnose current active infections, MA titers at 1 : 400 or greater are suggestive, and at 1 : 800 or greater they are highly indicative of leptospirosis. However, booster vaccines given in the preceding 2 to 3 months can produce titers that overlap with these values. Most post-vaccinal titers are 1 : 100 to 1 : 400, but rarely titers as high as 1 : 3,200 can occur within 3 months of vaccination. The magnitude of the MA titer can be blunted by early antibiotic therapy.
• MA titers can be negative in the first week of acute illness, requiring a second titer 2 to 4 weeks later for diagnosis. Use paired MA titers during acute illness and recovery (convalescence) to demonstrate a 4-fold rise in titer as a criterion to distinguish current infection from previous infection or vaccination. Because the timing of the titer peak varies, take a convalescent titer 2 to 3 weeks after acute illness, and possibly another 1 to 2 weeks later.
• Ideally, the lab will test for the various serovars listed in the Etiology section, and potentially others known to occur in the geographic region. The serovar with the highest titer is interpreted to be the infecting serovar and other serovars with lower titers are considered to represent nonspecific crossreactivity and/or background titers from prior vaccination.
Identification of Leptospiral Organisms
Microscopic Identification
• Darkfield microscopy is used to detect leptospires on a wet mount of fresh urine. Accuracy is highly operator dependent, and the high rate of false positives and false negatives makes this an unreliable test.
Treatment
• For supportive care: Administer intensive fluid and electrolyte therapy (see Chapter 5) and, depending on clinical findings, initiate appropriate treatment for acute renal failure (see Chapter 77), acute hepatic failure (see Chapter 71), and DIC (see Chapter 23), as recommended in other chapters in this book. Oliguric renal failure and fulminant DIC are life-threatening complications of leptospirosis that often need the most immediate attention and intensive care.
• For leptospiremia: Initially give parenteral antibiotics, such as penicillin G (25,000–40,000 units/kg IV, q12h) or ampicillin (22 mg/kg IV, q6–8h). Once vomiting resolves, switch to oral amoxicillin (22 mg/kg PO, q8–12h) and continue treatment for 2 weeks.
• For leptospiruria: To eliminate the renal carrier state once amoxicillin treatment has been completed, give doxycycline (5 mg/kg PO q12h) for 2 weeks. Other alternatives for treating the carrier state include macrolides (erythromycin, azithromycin), fluoroquinolones, or aminoglycosides (avoid in dogs with renal impairment).
Prevention
• Bivalent Leptospira bacterins (L. canicola and L. icterohaemorrhagiae) have been widely used for routine canine vaccination since the 1970s. Bivalent vaccines have reduced the incidence of infections caused by L. canicola and L. icterohaemorrhagiae, but they do not crossprotect against other serovars that now account for most leptospiral infections in dogs.
• Use multivalent vaccines that contain the serovars L. pomona, L. grippotyphosa, L. canicola and L. icterohaemorrhagiae to give a broader spectrum of protection.
• Vaccinate dogs over 12 weeks of age in high-prevalence areas or high-risk situations with at least 3 doses, 3 to 4 weeks apart, for primary immunization, followed by annual revaccination (see Chapter 7).
• Anaphylactic reactions (facial edema, pruritus, hypotension, or dyspnea) occur with leptospiral bacterins, and the incidence may be increased in small breeds and puppies less than 9 weeks of age. Newer improved vaccines derived from Leptospira antigenic subunits will likely cause fewer allergic reactions.
CANINE BRUCELLOSIS
Etiology
Clinical Signs
Other Uncommon Forms of Brucellosis
• Diskospondylitis: This causes spinal pain progressing to paresis and ataxia with spinal cord compression. The radiographic appearance of diskospondylitis is distinctive, but other bacteria also can cause this.
• Uveitis: This manifests as anterior uveitis (aqueous flare, hyphema, secondary glaucoma, corneal edema) or chorioretinitis (vitreal haze, retinal lesions or detachment).
Diagnosis
• Routine CBC, urinalysis, and serum chemistries are normal except for occasional hyperglobulinemia.
• Semen abnormalities—more than 80% of sperm are morphologically abnormal (immature sperm, deformed acrosomes, swollen midpieces, detached tails, head-to-head agglutination); increased semen leukocytes; and aspermia in advanced cases.
