Zoonotic Diseases in Dogs
Dr. Douglas Brum
A good history is extremely important in diagnosing a potential zoonotic disease. Since many zoonotic diseases are limited to fairly specific areas of the country, your veterinarian will take into account the pet's geographic location as well as his travel history. The potential exposure to other infectious animals or vectors is also an important part of the history. Animals that roam free, or have significant contact with other animals, are much more likely to contract a zoonotic disease than animals that live in apartments with limited outside access.
Because there are so many possibilities, your veterinarian will base his diagnosis on a history, physical exam and various diagnostic tests as dictated by the symptoms.
The complete blood count (CBC) is a useful test in evaluating a potential zoonotic disease because it evaluates the red and white blood cell lines. Elevations in the white blood cell count may indicate a bacterial infection. If a "left shift" (an increased number of immature white blood cells) is also present, a life threatening infection may be present. Anemia may also be detected due to blood loss or red blood cell destruction.
A biochemical profile is needed to measure many metabolic functions. It is especially useful when a bacterial or tick borne disease is suspected. Many times these diseases affect a variety of organ systems, and the test is a very useful screening test. Liver and kidney values may be evaluated, as well as electrolytes, total protein and globulin levels. The blood sugar is also measured, which may be decreased in some bacterial infections.
The urine needs to be evaluated (urinalysis) if kidney disease is suspected. Infection and proteinuria (abnormal protein in the urine) may also be detected. If infection is suspected, the urine is also cultured.
A fecal examination is a simple test that can diagnose many types of intestinal parasites. With roundworm and hookworm infestations, the eggs of the parasite are commonly found on microscopic exam. Tapeworms are easier to diagnose by direct visual inspection of the stool, since the tapeworm eggs are commonly enclosed in small packets, or segments, and do not break up in the stool. Tapeworm segments are commonly observed as small grains of rice on the stool surface or around the anus. Giardia is difficult to find on a routine fecal exam, so if giardia is suspected, specific tests (ELISA test or zinc sulfate floatation) need to be ordered.
Fecal cultures. The stool can be directly cultured for specific bacterial infections. The gastrointestinal bacterial infections Salmonella and Campylobacter are diagnosed through a fecal culture.
Heartworm test. An occult heartworm test, that tests for the presence of the adult worm, or a filarial filter test, that tests for the adult's microscopic larvae, are accurate and quick tests for detecting heartworm disease.
Fungal cultures, specifically dermatophyte test medium (DTM), are used to diagnose ringworm. Small amounts of infected hair are placed in the culture medium and fungal growth usually occurs within 4 to 7 days.
Blood cultures. If bacteremia (bacterial infection that has spread to the blood) is suspected the blood may be cultured. Often, at least two samples, 1 hour apart, are recommended. If possible, antibiotics should be withheld prior to the cultures, since the antibiotics will inhibit bacterial growth. Tularemia, plague, salmonella and brucellosis are some infections that may cause a bacteremia.
Skin scrapings are needed to diagnose sarcoptic mange. Multiple samples are often required to demonstrate the mites.
Chest and abdominal x-rays are often useful in evaluating for systemic illness. Pneumonia, masses or fluid in the chest may be noted on chest films. Abdominal x-rays are useful in evaluating animals with gastrointestinal symptoms. Animals with brucellosis may also have evidence of diskospondylitis (infection of the intervertebral disc), which can be seen in an x-ray.
An abdominal ultrasound may be used to identify tissue or internal organs that appear abnormal. If needed, a biopsy may be performed to obtain a tissue sample. The biopsy result may suggest a specific zoonotic disease, or may even identify the organism. An ultrasound with or without biopsy may be quite useful when suspecting leptospirosis, toxoplasmosis, echinococcus, tularemia and leishmaniasis.
A coagulation profile is recommended if there is evidence of a bleeding disorder. Clotting tests should include an activated clotting time (ACT), a prothombin time (PT), activated partial thromboplastin time (APTT) and a platelet count. Zoonotic diseases that may affect an animal's ability to clot include: leptospirosis, salmonellosis, Rocky Mountain spotted fever, ehrlichiosis, plague, tularemia and leishmaniasis.
Arthrocentesis (inserting a needle into the joint space and aspirating a sample of synovial (joint) fluid) is a useful diagnostic procedure in diseases that cause a polyarthritis (inflammation in multiple joints). Lyme disease, Rocky Mountain spotted fever and ehrlichiosis are common causes polyarthritis.
A bone marrow sample may be needed if the platelets, red blood cells or white blood cells are decreased (since they are all produced in the bone marrow). Occasionally the causative agent may be identified on the sample (ehrlichiosis, tularemia and leishmaniasis).
Blood (serum) titers for specific diseases may show evidence of exposure to certain disease organisms. Titers must be interpreted with caution, since high titers do not absolutely diagnose the disease. The titers indicate exposure to the agent, and do not have to correlate with recent infection. Occasionally, a second titer might be recommended 2 to 3 weeks later to determine if the titer values are rising, as increasing titers generally indicate active infection. In any case, serum titers should always be evaluated in light of the entire clinical and historical picture.
Proper therapy requires an accurate diagnosis. Many times, treatment is based on the clinical condition and a presumptive diagnosis, while a definitive diagnosis is pending. Since there are so many types of zoonotic diseases, the treatment is based on the specific organ system affected. For example, animals with kidney failure are treated with intravenous fluids. Animals with signs of a bacterial infection are treated with systemic antibiotics. Treatment courses vary from a single dose of anti-parasitic medication to hospitalization and intensive care management. General treatment options include:
Antibiotics - Antibiotics are needed for bacterial causes of zoonotic diseases, including brucellosis, leptospirosis, Q fever, plague and tularemia. Many times, antibiotics are initially given intravenously, to be followed-up orally at home. Treatment for tick borne diseases (Lyme disease, Rocky Mountain spotted fever and ehrlichiosis) also requires antibiotic therapy (usually doxycycline). Treatment of choice for toxoplasmosis is clindamycin.
A variety of anti-parasitic drugs may be given for roundworms, hookworms and tapeworms. These are very safe and effective drugs that are given routinely in many health care programs.
Anti-fungal therapy might include topical or systemic (given orally) treatments. Common drugs for systemic therapy include: griseofulvin, ketoconazole and itraconazole.
Intravenous therapy may be required for supportive care in the critical patient, animals in shock, dehydrated or septic (bacterial blood infection).
Symptomatic therapy for associated conditions - Medications or dietary changes may be needed to support an animal through treatment. If there is severe vomiting, the anti-emetics (drugs that reduce vomiting) metoclopramide (Reglan®) or chlorpromazine (Thorazine®) may be used.