Borreliosis (Lyme Disease)

Chapter 18 Borreliosis (Lyme Disease)



Robert G. Sherding


Lyme borreliosis is a polysystemic, tickborne disease caused by the spirochete Borrelia burgdorferi. The disease has been associated with polyarthritis in dogs, cattle, horses, and humans. Infection is widespread in the United States, and is endemic in the northeastern coastal states and the upper Midwest. Infection also occurs worldwide and is prevalent in areas of Europe. Borreliosis is transmitted to animals and people by deer ticks.



ETIOLOGY


Lyme borreliosis in dogs and humans is caused by at least 6 subspecies of the Borrelia burgdorferi sensu lato complex. The strain that primarily causes borreliosis in North America is Borrelia burgdorferi senso stricto. These eubacteria are spiral-shaped, flagellated, gram-negative spirochetes that are related to Leptospira, except they do not survive as free-living organisms outside of the host.


B. burgdorferi is easy to isolate from vector ticks but not from clinically affected patients. The organisms are microaerophilic and have special growth requirements. The outer membrane of the spirochete can undergo antigenic variation during the course of infection, which enables the organism to evade the host’s immune response and cause persistent infection. Antigenic variation may limit the effectiveness of vaccines.


Dogs in endemic areas are commonly co-infected with other tickborne agents, especially Anaplasma phagocytophila (see Chapter 17), and mixed infections may contribute to the pathogenesis and severity of disease in some cases.




Prevalence




Key Point


The risk of Borrelia burgdorferi infection correlates with the prevalence of infected ticks in an area and the likelihood of being bitten by a vector tick.



• Borreliosis commonly affects dogs that have recreational exposure to tick-infested vegetation in endemic areas, especially outdoor sporting and hunting dogs.

• In the United States, over 90% of clinically affected dogs are found in the northeastern coastal states and Wisconsin and Minnesota. Within endemic regions, there are focal areas of high and low prevalence. The seroprevalence in dogs in highly endemic areas commonly reaches 50% to 90%, and 60% to 80% of the ticks in these areas may harbor Borrelia spirochetes. The seroprevalence in Southern and Western states is less than 4%.

• 95% of seropositive dogs are asymptomatic.


Transmission


Dogs are infected through the bite of an infected tick. The primary vectors of Lyme borreliosis are Ixodes spp. ticks, also called black-legged or deer ticks. These hard ticks are small (2-3 mm) 3-host ticks that have a 2-year life cycle with larval, nymph, and adult stages.


• In the Northeast, Southeast, and Midwest regions of the United States, Borrelia burgdorferi is carried by Ixodes scapularis (formerly designated I. dammini). In California and other western states, I. pacificus is the carrier. Various other Ixodes ticks are involved in Europe and Asia. Other species of ticks and biting insects such as flies and mosquitoes can carry Borrelia spirochetes, but these are not a significant source of transmission.

• Deer support the adult stage of I. scapularis but are not usually infected by the spirochete. Mice and other small rodents are the main reservoirs of borreliosis because they maintain the larval and nymphal stages of I. scapularis and can become infected with the spirochete. Migratory birds may be important reservoirs because they have the ability to transmit ticks and spirochetes over long distances. Dogs are incidental hosts.

• Western I. pacificus ticks are less efficient as vectors because they prefer to feed on lizards that contain borreliacidal factors.

• Direct transmission between dogs is unlikely. Uninfected dogs housed with infected carrier dogs for 1 year did not become infected. In utero transmission is not likely. Iatrogenic transmission by blood transfusion is a possibility.



Key Point


Transmission of Borrelia infection requires at least 48 hours of attachment and feeding by a vector tick.



• Once inside the animal host, Borrelia spirochetes live intercellularly in the skin and spread through connective tissue migration rather than hematogenously. A small area of erythematous skin around the tick bite may be seen in the first week.


CLINICAL SIGNS




Key Point


In endemic areas, most dogs in the population have serologic evidence of exposure to Borrelia, but never develop clinical signs, indicating that subclinical infection is common.


Most dogs develop persistent subclinical infection that may lifelong. Seroconversion occurs within 3 to 6 weeks after exposure. Clinical disease develops in only 5% of infected dogs, depending on age, immune status, strain of Borrelia, and dose of exposure (number of ticks). In experimental infections, clinical signs develop 2 to 5 months after tick exposure.




Polyarthropathy


Acute, chronic, or intermittent shifting-leg lameness due to polyarthritis is the most common clinical sign.


• Systemic signs can include fever, anorexia, lethargy, lymphadenopathy, and weight loss; however, many animals show no clinical signs other than lameness.

• Swelling and pain in one or more joints may be evident. Arthritis usually occurs first in the joint closest to the tick bite. The carpi are the most frequently affected joints.

• Chronic, progressive, non-erosive arthritis may develop in persistently infected dogs.

• Radiographically, affected joints may have joint effusion, but usually show little or no evidence of degenerative joint disease.

• Synovial fluid analysis reveals neutrophilic (suppurative) inflammation with leukocyte counts ranging from 2,000 to 100,000 nucleated cells per μl, similar to findings in idiopathic immune-mediated polyarthritis.


Renal Disease


A unique syndrome of protein-losing glomerulonephropathy and acute progressive renal failure (“Lyme nephropathy”) has been associated with canine borreliosis, especially in Labrador retrievers and golden retrievers. Borrelia is not the proven cause, and mixed infections or an unrecognized co-infection by another tickborne agent could play a role.


• Renal lesions include immune-mediated glomerulonephritis, renal tubular necrosis, and lymphocytic-plasmacytic interstitial nephritis.

• Clinical findings can include acute vomiting, hypertension, edema, ascites, azotemia, hypoalbuminemia, hypercholesterolemia, proteinuria, and tubular casts in the urine. Most cases progress rapidly over 1 to 2 weeks and die despite therapy.


Other Manifestations


Meningitis, encephalitis, uveitis, and myocarditis with cardiac arrhythmias are seen rarely in association with canine borreliosis, but the relationship of these to Borrelia infection is uncertain.



Feline Borreliosis


Naturally-occurring clinical disease is not well documented in cats; however, many healthy cats are seropositive for Borrelia infection and experimentally infected cats develop polyarthritis, meningitis, pneumonitis, and lymphadenopathy.



DIAGNOSIS


Consider borreliosis in dogs from endemic areas with typical clinical signs, especially if there is a history of exposure to a wooded environment or tick-infested area. The diagnosis depends on finding the organism or antibodies directed against the organism. Serologic testing provides a presumptive diagnosis; however, in endemic areas many healthy dogs have been exposed and are seropositive, leading to a tendency to over-diagnose the disease. Definitive diagnosis requires identification of the Borrelia spirochetes by culture or polymerase chain reaction (PCR) assay, but these are not routinely available to the clinician.


In endemic areas, the differential diagnosis is challenging because other doxycycline-responsive tickborne infectious diseases are often prevalent as well, many of which cause clinical signs that overlap with borreliosis; such as infections caused by Anaplasma, Ehrlichia, Rickettsia, and Bartonella (see Chapters 17 and 19). Lyme borreliosis also is difficult to distinguish from immune-mediated polyarthritis (see Chapter 124).




Key Point


A positive Borrelia antibody titer is a marker for exposure to ticks and the infectious agents they transmit. Consider the potential for other tickborne diseases in any seropositive dog.

Related posts:

  1. Feline Infectious Peritonitis (Feline Coronavirus)
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  3. Keratinization Defects
  4. Canine and Feline Demodicosis

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Aug 27, 2016 | Posted by in SMALL ANIMAL | Comments Off on Borreliosis (Lyme Disease)

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