Lyme borreliosis remains a laboratory diagnosis
Author's note: Last month began a two-part series on Lyme disease in dogs. Beginning on page 6S of the April issue, the series addressed cause, transmission, pathogenesis and clinical disease. This month, the discussion continues on diagnosis, treatment and prevention.
Unfortunately, Lyme borreliosis disease in animals has become more of a laboratory diagnosis rather than a clinical diagnosis.
There are no specific hematologic or serum chemistry profile changes pathognomonic of Lyme borreliosis, although CSF, joint fluid and urine may show inflammatory changes. The synovial fluid changes in infected dogs have been best substantiated with increased cell counts of 5,000 to 100,000 cells/ml and neutrophils predominating (>95 percent).
Tickborne transmission results in differences in the outer surface proteins expressed on the Borrelia burgdorferi organisms and differences in the host's serological responses as compared to experimental infections with injected cultured Borrelia burgdorferi.
The current serologic screening procedure available for animals are based on ELISA and some indirect FA assays. Most ELISA and indirect FA assays use whole cells with many cross-reactive proteins present in other bacteria, especially other Borrelia species and Leptospira species. The presence of increased serum antibody titer to Borrelia burgdorferi signifies exposure to the organism but does not prove that the organism causes clinical illness.
In endemic areas, asymptomatic animals are often seropositive. In addition to seropositive results with a validated ELISA or indirect FA assay, the animal should have a history of tick exposure with compatible clinical signs and a rapid response to antibiotic therapy.
The dependency on serologic testing results from the fact that culture and microscopic detection of the Borrelia from specimens of tissues or body fluids are uncommon. Rather than being a test for Lyme borreliosis disease, serology should be viewed more appropriately as seroreactivity to Borrelia burgdorferi.
The time course of serologic testing is also important in determining whether active or past infection is responsible for seropositivity. Early serodiagnostic results are usually negative because the immune response to Borrelia burgdorferi developed gradually. Titer increases almost always precede clinical lameness and fever in infected dogs. High or persistent serologic titers could indicate past exposure or infection and, as such, are difficult to interpret as single samples and absolute values of titers can vary widely.
In general, data from most ELISA and indirect FA assays suggests that titers >1:64 to 1:128 indicate the least significant level of seroreactivity. Titers can be much higher in recently exposed, actually infected animals. High serum antibody titers seem to decline or disappear with antibiotic therapy, but increases in titers that occur after six months following termination of antibiotic therapy are associated with proliferation of surviving bacteria. Vaccinated dogs show seroreactivity for months to years after vaccination, making a diagnosis difficult with whole cell antigen products. Although neutralizing serum antibody titers after vaccination may decrease with time, most ELISA and indirect FA titers remain increased for extended periods, interfering with the standard serologic testing procedures.
Ongoing infections are characterized by high whole Borrelia cell antibody titers. The use of separated or purified protein antigens in ELISA assay or confirmation with western immunoblotting test should improve specificity of Borrelia burgdorferi antibody assays without loss of sensitivity.
The pattern of serum antibody reactivity after field tick infestation also differs from that produced after vaccination. Western immunoblotting test helps determine that clinical significance of whole cell serum antibody titers when reactivity to other bacteria such as Leptospira species occurs.
A number of novel surface protein antigens may also serve as serologic markers of field-exposed infections because in vitro derived Borrelia organisms do not express these surface protein antigens. There is a unique protein called VlsE from Borrelia burgdorferi incorporated in the newly developed canine Snap, 3Dx test (IDEXX Laboratories). This serologic test detects the presence of antibody to a unique outer surface protein of Borrelia burgdorferi in field-infected dogs.
Both Borrelia burgdorferi bacterins and recombinant OspA vaccines are available for dogs and offer protection against infection. In the past, a positive serologic test result, denoted exposure to Borrelia burgdorferi whether or not the dog had previously been vaccinated with any of these commercially licensed Lyme borreliosis vaccines available in the USA. It is also important to note that this new canine Snap, 3Dx test does not detect serum antibody that develops following vaccination.
However, among dogs with clinical signs potentially associated with Lyme borreliosis, a positive canine Snap, 3Dx test may be interpreted as active infection. In the absence of clinical signs, a positive canine Snap, 3Dx test result indicates prior exposure to Borrelia burgdorferi or a dog that has a subclinical infection. In studies completed with several hundred samples from various parts of the country, the test does not detect serum antibodies induced by other pathogenic or nonpathogenic Borrelia species. With canine sera to date, an excellent correlation has been observed between positive canine Snap, 3Dx test results and western immunoblotting procedures. The canine Snap, 3Dx test may serve as a valuable seroprevalence and diagnostic tool in determining levels of true infection of Lyme borreliosis in healthy dogs and may be incorporated into the practice in a routine annual health examination.
Several effective antibiotics are available for the treatment of dogs with clinical signs of Lyme borreliosis: doxycycline, amoxicillin, chloramphenicol, azithromycin and ceftriaxone. The antibiotics most commonly used are doxycycline (10 mg/kg q12h PO for a minimum of 30 days) or amoxicillin (20 mg/kg q8h PO for a minimum of 30 days). Chloramphenicol is especially good for dogs with neurologic manifestations and is administered at 15-25 mg/kg orally q8h for 14-30 days. Antibiotics, such as doxycycline, azithromycin (5 mg/kg q12h for a minimum of 20 days) and ceftriaxone (20 mg/kg q12h for a minimum of 14-30 days), are effective in diminishing organism load in a Borrelia-infected dog, but they do not eradicate the Borrelia.
Young and growing animals should not be treated with doxycycline - because of yellowing of the developing teeth. Improvement should be seen within three days of antibiotic treatment. If improvement is not seen, another diagnosis should be considered.
Lyme borreliosis disease can be prevented by avoiding tick infestation and/or by annual vaccination. Tick engorgement on dogs may be prevented by tick repellents containing DEET or permethrin, amitraz tick collars and/or by grooming dogs daily. Controlling the tick population in the environment would be limited to small areas. Attempts to reduce the deer and/or rodent population has had limited success.