Tick-borne disease diagnosis: moving from 3Dx to 4Dx (Proceedings)

In just the past 3 to 5 years, advanced diagnostic capabilities have enhanced our ability to detect infectious pathogens in the dog and have given credence to the term "emerging" infections. However, "emerging infectious diseases" recently described, in fact, not be emerging at all...as it appears; many of these infections have, quite likely, existed in dogs for several years. It's the emerging technology that has enabled our ability to detect these infections.

AND...it's MUCH more than Blue Dots! As we move from the 3Dx test to the 4Dx testing platform, veterinarians will soon have the ability to test for Anaplasma phagocytophilum antibody, in addition to Borrelia burgdorferi C6 antibody, Ehrlichia canis antibody, and canine heartworm antigen. This advancement in testing capability comes with a new learning curve for clinicians since most, in fact, are not immediately familiar with the clinical presentation of dogs infected with A. phagocytophilum (formerly, Ehrlichia equi).

Specifically, the introduction of a 4Dx Snap Test emphasizes the importance of understanding not only the indications for performing the test, but, most importantly, the implications of a POSITIVE vs. NEGATIVE test result in the sick, as well as the 'healthy' patient.

For any dog presented with fever, decreased appetite, lethargy, and petechia over the oral mucous membranes, testing for the presence of tick-borne disease is clearly indicated...it's become the standard of care in veterinary medicine. However, indications for testing patients that do not have overt clinical signs can be equally important. It's called Surveillance Testing...and is part of a health or "wellness" testing profile for dogs with any risk of tick exposure.

Surveillance Testing for antibody associated with tick-exposure may, in fact, be the best, earliest test of infection or impending illness. Rather than wait for clinical signs to become so obvious that the owner is compelled to have the dog examined...Surveillance Testing provides immediate insight on tick-borne disease exposure in an individual patient...at a reasonable cost to the client.

The implications of a POSITIVE test in a patient with clinical signs consistent with tick-borne disease are obvious...the test drives treatment decisions as well the need to perform follow-up examination of the patient. But the POSITIVE test in a seemingly healthy patient, what some will call 'confusing', actually isn't. In fact, the POSITIVE test in a healthy patient drives the need for further patient assessment and justifies, at the least, a thorough physical examination and a laboratory profile that includes hematology and biochemistry. Antibody POSITIVE patients without physical signs can have significant laboratory abnormalities, such as hypoalbuminemia with hyperglobulinemia, leukocytosis, thrombocytopenia, and anemia, that not only support infection but also warrant immediate therapeutic intervention.

Also...the patient with a POSITIVE test result that has a normal laboratory profile reveals whether or not the patient has been adequately or appropriately treated with a tick preventative. While it is well known that infection with a tick-borne pathogen may never cause clinical illness in some dogs, the ability to define 'exposure' through antibody detection is an important opportunity to educate the client on the use of tick preventatives...over and above what they might purchase at a pet store!

Knowledge that the patient is NEGATIVE, and is therefore unlikely to have had tick exposure, at least in part, denotes adequate tick prevention and reduced risk of exposure. The NEGATIVE test in the sick patient has a high degree of "negative" predictive value...ie, it is highly unlikely that the patient with a NEGATIVE test has the infection it was tested for. The NEGATIVE test result in a sick patient, therefore, drives additional diagnostic testing...but with the knowledge that, exposure to the agents tested is unlikely to be an issue for that patient.

I'm speculating now, but it's reasonable to assume that in the future we will see a 5Dx or even a 6Dx Snap Test. Regardless of the testing platform, technology will certainly allow veterinarians to perform in-hospital detection of antibody to other pathogens in the near future.

Infectious diseases such as Canine Ehrlichiosis, Anaplasmosis, and Lyme Borreliosis will continue to play an important role in companion animal practice. But it stands to reason, there are more pathogenic organisms, and more clinical manifestations associated with infection, and co-infection, occurring in dogs and cats that will remain undiagnosed until technological advances improve our diagnostic sensitivity. As new infectious disease diagnostic tests enter the marketplace, veterinarians will not only gain important insight on the amount of exposure dogs actually have to vector-borne organisms, but also the remarkable spectrum of physical and laboratory abnormalities that can occur in the individual patient infected with one...or more...pathogenic organisms.

The 10 Most Common Questions concerning Canine Ehrlichiosis

1. There are many ehrlichiae capable of infecting dogs...but we currently only test for one (E. canis)...which infections are most significant for dogs?

E. canis (canine monocytotropic ehrlichiosis) is probably the most important. It's certainly the most common infection in dogs in the US. However, infections with E. chaffeensis, E. ewingi (canine granulocytotropic ehrlichiosis), and E. equi are also known to infect dogs living in the US.

What compounds conventional diagnostic strategies is the fact commercial tests are simply not available (yet) for many of these infections. Furthermore, it's becoming more apparent that individual dogs can be (and are) infected with multiple tick-borne pathogens simultaneously...such as Anaplasma spp and Neorickettsia.

2. What is the spectrum of clinical signs associated with ehrlichiosis?

The standard answer goes like this:Incubation is 8 to 20 days.

ACUTE (2-4 weeks): signs may be mild to absent (at least to the owner) or may include lethargy, anorexia and possibly epistaxis. IF...the owner seeks medical attention, fever may be detected evidence of spontaneous bleeding. A laboratory profile may reveal thrombocytopenia, low albumin with elevated globulin (total protein may be elevated). THEN...they get better...whether or not they receive treatment.

SUB-CLINICAL (months to years [life?]): any clinical signs resolve...and any treatment given appears to have worked well...THEN...they may get worse...

CHRONIC (months): This is where infections become complicated. Serious clinical signs may develop months to years following exposure and infection. Signs are highly variable. Usually, thrombocytopenia with petechiation or ecchymoses, and epistaxis develop. Pancytopenia seems less common today. Hyperglobulinemia/hypoalbuminemia may be present. Granular lymphocytosis (counts between 5000 and 17,000 lymphocytes/μL). Less common are a variety of multisystemic signs, including peripheral limb edema, vasculitis (generalized), neurological signs ranging from head tilt (vestibular) to disorientation to meningitis and seizures. Hyperviscosity syndrome, retinal detachment, and a host of immune-complex disorders affecting joints and kidney may develop. Death is possible...despite aggressive treatment.

BUT...the variation in physical signs and laboratory values is significant. For example...the absence of thrombocytopenia does NOT exclude a diagnosis of ehrlichiosis. As such, it is not feasible to rely exclusively on clinical or laboratory findings to establish a diagnosis. Testing is important. The message here is not to limit testing to only those patients that have obvious physical signs that are distinctively associated with ehrlichiosis..."test outside the box"!

3. Is co-infection with other tick-borne agents significant with ehrlichiosis?

With increasing significance...YES. The more we look, the more we discover oc-infected dogs...and this is true in humans who are infected with tick-borne disease. AND...it's the co-infected patient that makes it difficult to establish a diagnosis on the basis of clinical signs.

What's still needed in veterinary medicine is a testing platform that will allow screening of individual patients for multiple tick-borne pathogens, simultaneously! And work is currently underway to achieve that.

4. Is there any breed, sex, or age predilection for development of ehrlichiosis?

There is no age or sex predilection for canine ehrlichiosis. However, there are reports suggesting German shepherds may be more susceptible to infection than other breeds. When clinical signs do develop, the clinical course seems to be more severe and prognosis poor. This is suggested to be associated with a breed-specific compromise in cell-mediated immunity.

5. Where is the geographic risk for infection (exposure) the greatest?

This map denotes exposures in dogs (2005 data) based on ELISA serology for E. canis antibody. BUT...today, such maps have limited relevance considering that pets frequently travel with ownersa fact clearly leads to exposure risk.

6. How long does a tick have to feed in order to transmit the infection?

Interestingly...that is not known! It's an important fact since newly infected larval and nymph ticks can transmit the infection for 155 days, reservoir ticks can "overwinter" with their ehrlichia...becoming an early spring threat. Since most topic tick preventatives can take from 2 to 3 days to effective kill ticks, the time required to effectively transmit the organism is important!

7. How should the Snap Test for E. canis be interpreted in a sick patient vs. a healthy patient?

The SICK dog with a POSITIVE E. canis antibody test (blue-dot on the Snap 3Dx) should be treated (see below). Following treatment, clinical and laboratory assessments need to made as necessary to document resolution of the clinical illness. Follow-up testing is NOT recommended...(see question #9)

The HEALTHY "appearing" dog with a POSITIVE E. canis antibody test (Surveillance Testing) should be evaluated further. Not is a thorough physical examination indicated (ie, look carefully for petechia), but a laboratory profile that includes at least a CBC and a biochemistry panel is VERY important. Dogs with no abnormalities...have been exposed to Ehrlichia and have sustaining levels of antibody...and may never become ill. Treatment is not indicated.

Dogs with a NEGATIVE E. canis antibody test have not been exposed to the organism. There is no cross reaction on the test... a NEGATIVE dog, with clinical or laboratory signs, does not have E. canis exposure. However, it could be infected with a different type of tick-borne pathogen. But it could be a completely different diagnosis.

The dog with a NEGATIVE test result, in the absence of clinical or laboratory changes...is not infected.

8. Treatment recommendations for a dog with a POSTIVE test result and clinical signs consistent with ehrlichiosis include:

Several options:

• Doxycycline, 10 mg/kg, orally (rarely IV), q12-24h, for 28 days.

• Minocycline, 10 mg/kg, orally (rarely IV), q12h, for 28 days.

• Tetracycline, 22 mg/kg, orally, q8h, for 28 days.

• Chloramphenicol, 15-25 mg/kg, orally (or IV or SC), q8h, for 28 days.

• Imidocarb dipropionate, 5 mg/kg, IM, once, then repeat in 2-3 weeks.

• Amicarbalide, 5-6 mg/kg, IM, once, then repeat in 2-3 weeks.

Short term prednisolone (2-7 days) at 2 mg/kg, daily, may be indicated in severe or life-threatening cases.

It is NOT believed that combining parenteral with oral therapy provides any therapeutic advantage.

9. Will the E. canis antibody test (3Dx Snap Test) become negative following treatment?

This is important...NO...not for several weeks or even months. Unlike post-treatment testing patients for the Lyme C6 antibody, the titer of E. canis antibody does not fall following treatment. THEREFORE...the 3Dx test result can remain positive for extended periods following treatment for ehrlichiosis.

10. What's the prognosis for a dog with ehrlichiosis following treatment?

Physical signs, if present, may appear to resolve within 1-2 days. Resolution of thrombocytopenia is the most rapid laboratory response (7-10 days following treatment). It is recommended to recheck platelet counts monthly for up to 3 months following treatment to monitor possible relapse. Changes in serum proteins may require 6 to 12 months to resolve.

However, relapse is possible...as is re-infection. Infections may become chronic and resistant to treatment.

It has been suggested that resistant infections may be more common than previously realized...leading to recrudescence of clinical and laboratory abnormalities..

The 10 Most Common Questions concerning Canine Lyme Disease

1. Recent studies show a dramatic increase in human Lyme disease...has the geographic distribution of Canine Lyme disease changed?)

Typically, canine Lyme disease occurs with the highest prevalence in the same locations that human Lyme disease is reported...the Northeastern States and the upper Midwest. Increased numbers of positive tests in dogs living in Texas suggest that risk of exposure may be significant, particularly around major metropolitan areas. Significant numbers of cases are reported also in Southern California. BUT...it is reported in virtually all States...the reason is not related to the presence of vector ticks...it's related to the fact that dogs traveling with their owners can and do travel into high prevalence areas, where they have increased risk of exposure. Practicing in a non-endemic area (e.g. Denver CO) is no assurance that a dog presented to the practice will not be infected.

2. What clinical signs are characteristically associated with an active infection?

Experimentally, the time between infection and development of clinical signs ranges from 2 to 6 months.

Systemic signs include fever, shifting leg lameness, joint swelling, enlarged lymph nodes, anorexia and lethargy....all of which are rapidly responsive to antimicrobial therapy.

Arthritis. Polyarthritis is the MOST common reported clinical signs.

Lyme Nephritis. This poorly described, but acute and frequently fatal, complication associated with B. burgdorferi infection is being reported with increasing frequency in dogs from Lyme endemic states...particularly the Northeastern US. (see #3 below). Infections are uncommon and are most often reported in Labradors (Black?) and golden retrievers.

Meningitis. Neuroborreliosis has been reported in humans; to date, there are no reports in dogs.

Other. The erythema migrans (EM) 'bullseye' lesion at the site of tick attachment has been reported in about 80% of humans exposed to Lyme disease. Some dogs are known to manifest a faint EM-type lesion..but only transiently. The hair is a problem is identifying it. Additionally, rare reports of myocarditis (and death) and rheumatoid arthritis are cited.

3. Lyme Nephritis...how does this manifest clinically, and how common is it?

Although uncommon, reports of Lyme nephritis are characteristically that of an acute onset renal failure associated with both glomerular and tubular disease. Most often reported in large breed dogs (especially Labradors and golden retrievers) residing in the Northeastern US. Sporadic cases from Minnesota and Wisconsin have been noted. The prognosis is poor, despite aggressive and early treatment. Most dogs die shortly after clinical signs of acute renal failure become apparent. Recent studies at Cornell University (Dr. Richard Goldstein) suggest that the underlying lesion is likely associated with immune complex deposition in the kidneys of affected dogs. Studies also suggest NO correlation between prior vaccination and risk of developing Lyme nephritis.

4. Is there any breed, sex, or age predilection for infection?

No. However, it is cited that the severity of signs and propensity to develop signs is greatest in younger dogs vs. older dogs. Also, the onset of clinical signs is typically associated with a rise in antibody titer, suggesting a significant immune-mediated component to the clinical disease.

5. How long does a tick have to feed in order to transmit the infection?

Infection requires at least 50 hours of continuous tick attachment. This is important...reason...the topical tick preventatives can require from 2 to 3 days to effectively kill attached ticks. While tick death may not be particularly rapid, it is rapid enough to prevent transmission. That's been documented in independent studies. IT'S ALWAYS BETTER TO PREVENT LYME DISEASE THAN TREAT IT!

6. How should the Snap Test for C6 antibody (canine Lyme disease) be interpreted in a sick patient vs. a healthy patient?

The C6 antibody test is an important and significant technological advancement in Lyme diagnosis in dogs...and, recently, in humans. Because there is a strong correlation between infection and POSITIVE antibody test result, a positive test generally denotes infection...it does NOT predict impending clinical disease.

A POSITIVE test result in a dog with compatible clinical signs obviously justifies immediate treatment (see below). NONE of the available Lyme disease vaccines will cause a FALSE POSITIVE test on the 3Dx Snap testing platform.

A POSITIVE test result in a healthy appearing dog should be interpreted as an exposed and infected dog...a positive test result is NOT predictive of impending clinical disease. Despite confirmed infection, some dogs will never develop clinical signs. That said, discussions with veterinarians from various locations throughout the US suggest that most clinicians recommended treating a POSITIVE, healthy-appearing dog when that dog is known to reside in a Lyme endemic area of the US. In non-endemic parts of the US, most veterinarians will elect NOT to treat, but simply advise the client that evidence of exposure (and infection) is present and that should lameness or myalgia develop, treatment should be initiated. NOTE: there are also comments from practitioners who indicate that treating the "healthy" POSITIVE dog quickly resulted in a "healthier" dog... There are NO reliable laboratory changes supporting a diagnosis of Lyme disease in the healthy appearing dog with a POSITIVE test result.

A NEGATIVE test result indicates the patient is not infected at that time.

7. What's a "Quantitative C6 Antibody" test and what are the indications for testing a patient?

The Quantitative C6 Antibody test is a 'send-out' test available only through IDEXX Laboratories. Because of the sensitivity of the C6 antibody test and it's correlation with active infection, it has been suggested that determination of the C6 antibody concentration may be predictive of impending clinical illness...AND...can offer the clinician an opportunity to monitor a decline in antibody response subsequent to treatment.

Although it's a bit early to confirm the findings, there is reasonably good correlation between antibody titer and clinical illness when the titer (concentration) of C6 exceeds 30 units of antibody. Most dogs with clinical signs show good resolution of clinical signs with treatment...and this correlates well with a decline of the C6 antibody titer (usually over 2 to 6 months). In some (BUT NOT ALL) cases, treatment results in a decline in antibody concentration such that the Snap Test will become Negative. Don't count on this happening.

Veterinarians who wish to monitor the results of treatment or who desire to assess the risk of a healthy patient with a POSITIVE test may elect to submit serum to IDEXX Laboratories for a Quantitative C6 Antibody Assay.

8. Treatment recommendations for a dog with a POSTIVE test result and clinical signs consistent with ehrlichiosis include:

No surprises here...

• Doxycycline, 10 mg/kg, PO, q12h, for 30 days.

• Amoxicillin, 20 mg/kg, PO, q8h, for 30 days.

• Azithromycin, 25 mg/kg, PO, once daily, for 10-20 days.

• Ceftriaxone, 25 mg/kg, IV or SC, once daily, for 14 to 30 days.

• Chloramphenicol, 15-25 mg/kg, PO or SC, q8h, 14 to 30 days.

Drugs such as Chloramphenicol and Ceftriaxone are reserved for patients with neurologic or cardiac (arrhythmia) manifestations.

Vaccination...IS NOT PART OF THE TREATMENT REGIMEN! There is NO indication for vaccination as part of the therapy for canine Lyme disease.

Treatment will NOT clear B. burgdorferi. Development of a persistent carrier state is likely.

9. Will the Lyme C6 antibody test (3Dx Snap Test) become negative following treatment?

(NOTE: this is quite different that E. canis antibody titers following treatment).

It might...clinical signs do tend to correlate with antibody titer. With treatment, the infected dog will experience a decline in the number of spirochetes...which, in turn results in a decline in antibody (C6)...which, in turn is associated with resolution of physical signs. While most dogs will experience a significant decrease in antibody concentration with treatment (may take up to 6 months), the occasional patient will see a decline in the antibody concentration such that the Snap Test becomes NEGATIVE...that's not known to be a 'better' response compared to one that has a significant decline in antibody but remains POSITIVE on the Snap Test.

10. What's the prognosis for a dog with Lyme disease?

The earlier treatment is started following the onset of clinical signs, the better. However, that's not always possible. There is generally good resolution of clinical signs with 2-3 days following treatment. But, although it is customary to continue oral treatment for a 30 day period, relapse does occur (even in dogs isolated in tick-free environments) 8 to 12 months later. In clinical practice, distinguishing relapse from re-infection may be impossible.

Clinical Insights

Anaplasma phagocytophilum infection (or...more properly: canine granulocytotropic anaplasmosis, CGA)

What is it?

It's a tick-borne disease...recently recognized in the Northeast and upper Midwestern US in dogs that presented with clinical signs consistent with E. canis...but were consistently NEGATIVE for E. canis antibody. Turns out...it was what we then called...Ehrlichia equi.

Quite recently, E. equi was reclassified an anaplasma and is now referred to a Anaplasma phagocytophilum.

Why the emphasis on this organism and the infection it causes?

Two reasons: first, it ranks among the 3 most common tick-borne infections in dogs in the US...with E. canis and Lyme disease.

Second...the 3Dx is expected to become a 4Dx in 2006...with a spot for A. phagocytophilum antibody added. This will be helpful.

How is the infection characterized?

Very much like E. canis...although there is no known sex predilection, a breed predisposition has been suggested (large breeds [Labradors and golden retrievers] may dogs have greater risk. Physical signs include fever, lethargy, and anorexia. Muscle pain is described in over half of the affected dogs. Muscle pain and weakness are described. Joint pain is uncommon. Although mild to moderate thrombocytopenia occurs in 80% of affected dogs, spontaneous bleeding disorders are not described. Laboratory changes look like ehrlichiosis. Thrombocytopenia and lymphopenia predominate (80% of cases). Hypoalbuminemia (and hyperglobulinemia) are relatively common. Dogs may have a mild anemia.

Geographic Distribution?

In addition to Lyme endemic areas (it's transmitted via the Ixodes tick family), infections seem to be occurring in several locations throughout the US. Occurrence maps will be presented during the presentation.

Treatment?

Doxycycline is the recommended treatment (10 mg/kg, orally, q12h, for 28 days). Also, rifampin and levofloxacin are reported to be effective. Chloramphenicol can be used in puppies to avoid the risk of doxycycline 'labeling' of enamel.I did find any reference to the use of imidocarb dipropionate to treat canine anaplasmosis.

Experimental studies suggest that treatment can be curative...therefore, having an antibody test (when it become available) does add significantly to ability to screen dogs for the most common tick-born infections.