Overview of Canine Zoonotic Diseases
Whether you own a dog or a cat, a bird or a reptile, a rabbit or fish, you should be aware that your pet can have an effect on your health by infecting you with certain diseases. These are called zoonotic diseases, which are animal diseases that can be transmitted to humans.
You may already know about some of the more common zoonotic diseases: Lyme disease is a bacterial disease transmitted by tick bites; malaria is transmitted by the Anopheles mosquito, and bubonic plague is transmitted by rats, or rather by fleas that become infected by biting the rats. However, you should also be aware of several common zoonotic diseases that can be transmitted by your pet. Most common are:
Cat scratch disease – a bacterial disease caused by bacteria carried in cat saliva. The bacteria can be passed from a cat to a human through biting or scratching.
Psittacosis – a bacterial disease you can get by inhaling dust from dried bird droppings.
Rabies – a viral infection caused by a virus found in the saliva of infected animals and is transmitted to pets and humans by bites. Infected bats, raccoons, foxes, skunks, dogs or cats provide the greatest risk to humans.
Toxoplasmosis – a parasitic disease you can acquire from soil or other contaminated surfaces by putting your hands to your mouth after gardening, cleaning a cat’s litter box, or by touching anything that has come into contact with cat feces.
Ringworm – the most common zoonotic disease transferred from animals to humans. It is a contagious fungal infection that can affect the scalp, the body (particularly the groin), the feet and the nails. Despite its name, it has nothing to do with worms. The name comes from the characteristic red ring that can appear on an infected person’s skin.
All animals can acquire zoonotic diseases, but animals at increased risk include: outdoor pets, unvaccinated animals, pets that are immunocompromised (a suppressed immune system), poorly groomed animals and animals that are housed in unsanitary conditions. People with immune disorders, on chemotherapy or immunosuppressive therapy may be at increased risk of infection.
Animals with zoonotic diseases may exhibit a variety of clinical signs depending on the type of disease. The signs can vary from mild to severe. As a pet owner you should know your animal and be aware of any changes in behavior and appearance.
What to Watch For
Signs of zoonotic diseases may include:
Bruising under the skin
Veterinary Care for Zoonotic Diseases in Dogs
Your veterinarian will need a good history, including an accurate travel history, and complete physical examination in order to make an accurate diagnosis. Since there are so many different kinds of zoonotic diseases, your veterinarian will also do various diagnostic tests. Some of these may include blood tests, cultures, x-rays and ultrasounds.
Treatment depends on the specific diagnosis and may include antibiotics, anti-parasitic drugs or anti-fungal drugs; intravenous fluids; symptomatic care for associated conditions (e.g. vomiting or diarrhea); and analgesic (pain) medication.
Not all animals with zoonotic diseases are serious risks to people, but good hygiene practices should always be observed. Proper education, a good understanding of the disease and its method of transmission are a vital part of home and preventative care. Use proper hygiene and sanitation when handling pets and their excretions and maintain a good program of veterinary care.
In-depth Information on Zoonotic Diseases in Dogs
There are a large number of zoonotic diseases that can potentially affect people, caused by a wide variety of bacteria, parasites, viruses and fungal organisms. People may become infected by a number of different routes. Poor sanitary habits may lead to the ingestion of small amounts of animal waste products and transmission of zoonotic disease. Fecal waste is a source of many bacterial and parasitic infections, and even urine contamination can lead to disease (e.g. Leptospirosis). Ingestion of undercooked food products, skin contact with infectious agents (e.g. ringworm, fleas, mites), and bite wound or scratches are all potential modes of zoonotic transmission.
Many zoonotic diseases are not directly transmitted from animal to people, but they require an intermediate host (vector), such as a flea or a tick, for transmission to occur. The dog or cat brings the vector into the household where humans can become exposed.
Even though many zoonotic diseases include some very common aliments in animals, serious disease in people is relatively uncommon. Certain individuals, however, are at increased risk.
Young children are probably at highest risk because they are more likely to be exposed to animal excrements during play. Additionally, wild animals or cats may defecate in sand boxes where children play. Poor hygiene habits practiced by children also make them naturally at increased risk for many zoonotic diseases.
People with suppressed immune function because of chemotherapy, organ transplants, or immunosuppressive drugs are also at increased risk. Individuals with acquired immunodeficiency syndrome (AIDS) are also more susceptible. In addition, some zoonotic diseases (e.g. toxoplasmosis) that cause only mild or self-limiting disease in healthy people may be life threatening in immunocompromised individuals.
Finally, certain culinary practices may lead to an increased risk of contracting a zoonotic disease. Eating raw or undercooked beef is a common cause of transmission of toxoplasmosis. Eating undercooked eggs may lead to salmonellosis. Hikers that drink unfiltered or untreated water have a greater risk of acquiring giardia.
A good knowledge of the most common zoonotic diseases and routine health care with good husbandry and sanitation practices will significantly decrease the likelihood of either you or your pet acquiring a zoonotic disease. Your veterinarian routinely provides yearly exams, preventative internal and external parasite control programs and vaccinations. These services dramatically reduce the zoonotic potential of disease. Additionally, veterinarians usually provide information and consultation on training and behavioral issues. This advice is extremely important, since the most common zoonotic diseases caused by small animals are bite and scratch wounds.
Most Common and Significant Zoonotic Diseases
The following list includes the most common and most significant zoonotic diseases:
Visceral larval migrans is caused by the roundworm Toxocara cati or Toxocara canis. Children become infected after ingesting the eggs, from a contaminated environment.
Cutaneous larval migrans is caused by the hookworm Ancylostoma braziliense or Uncinaria stenocephala. Transmission occurs as the larvae penetrate the skin, which may cause severe itching and skin lesions in people.
Dipylidiasis or tapeworm infection. The most significant tapeworm infection is caused by Echinococcus granulosus and E. multilocularis. It is only rarely reported, but is significant since it can cause internal cystic masses in people and animals that can be fatal. It is most prevalent in sheep ranching areas and dogs usually become infected through the ingestion of sheep entrails or contaminated fecal material.
Toxoplasmosis is a protozoan infection whose main host is the cat, but it can infect many different species. It is one of the most common zoonotic diseases, but people are generally not infected through contact with a cat; ingestion of raw or under cooked meat is the most common method of transmission.
Cryptosporidiosis is also a protozoan organism that can cause diarrhea in domestic animals and people. Infection in people is usually self-limiting, except in the immunocompromised, where it can be fatal.
Giardia is a protozoan in the intestines of most animals. It can cause diarrhea in people and animals. Some animals may harbor the parasite, but display no symptoms. Animals and people usually acquire the disease by drinking stagnant water infected with giardia cysts.
Dirofilariasis, or heartworm disease, is caused by a parasitic worm that invades the animal’s heart by way of the mosquito. Both cats and dogs can be affected, but it is much more common in dogs.
Brucellosis is a significant bacterial disease in breeding dogs. Infection occurs during breeding or through exposure to reproductive secretions or milk. People can become exposed through these secretions, although infection is rare.
Leptospirosis infection is most significant in dogs, but may occur in many mammals. Infection with the bacteria can lead to severe kidney and liver disease, with fever and bleeding tendencies. Exposure to infected urine, tissue or blood can lead to infection in people. Most infections occur due to contact with urine-contaminated water.
Salmonella and Campylobacter are probably the most common gastrointestinal bacterial infections, although, infection in people usually is through contaminated food (especially undercooked eggs) or water rather than pets. Infection in animals and people occurs through contact with infected stool. Both organisms may cause diarrhea, abdominal cramping, fever and dehydration.
Cat-scratch disease (cat-scratch fever, Bartonellosis) is caused by the organism Bartonella. Cats do not show any clinical signs, but carry the organism. People become infected when scratched by an infected cat.
Group A streptococcal infections are important to mention mostly due to misconceptions about the disease. This is the bacteria that causes strep-throat in people, and people are the natural reservoir for the organism. Dogs (and rarely cats) become infected from people, and then are usually are asymptomatic (not ill).
Q fever is caused by the bacteria Coxiella burnetii. Cats become infected through the bite of an infected tick. The disease is usually asymptomatic in cats, but it can cause spontaneous abortions. People may become exposed if the organism becomes airborne during parturition (giving birth) or from fetal and placental tissue.
Rabies has a high fatality rate. Although rare in people in the United States, it can infect any mammal, usually via bite wounds. The main reservoirs of the virus are raccoons, skunks, and bats, not dogs and cats.
Mycotic (Fungal) Causes
Dermatomycosis (ringworm) infection in caused by the fungal organism Microsporum canis. It causes skin lesions and itching in both humans and animals. Transmission is through direct contact.
Sporotrichosis (Rose grower’s disease) is caused by the fungal organism Sporothrix schenckii. People and animals can become infected through traumatic penetrating wounds.
Disease Requiring Intermediate Hosts for Transmission
Lyme disease is caused by the organism Borrelia burgdorferi and requires an infected tick (usually the deer tick) for transmission.
Rocky Mountain spotted fever and Ehrlichiosis are rickettsial diseases caused by Rickettsia rickettsi and Ehrlichia canis respectively. Ticks transmit both, and the diseases cause similar clinical signs.
Plague is caused by the bacterium Yersinia pestis. Rodents are the main reservoir, and fleas transmit the disease. Dogs usually have only mild signs, but cats may have severe signs causing death. The disease is rare in the United States, but is occasionally seen in the southwest.
Tularemia is caused by the bacterium Francisella tularemia. Rabbits and rodents are the main reservoir and the disease is transmitted by blood sucking insects (ticks), or through the ingestion of infected meat.
Leishmaniasis is a protozoal infection that occurs most often in the tropics. It is transmitted by the sand fly, but the mode of transmission in this country is not completely understood.
Diagnosis of Zoonotic Diseases in Dogs
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.
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.
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.
Treatment of Zoonotic Diseases in Dogs
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.
Follow-up Care for Zoonotic Diseases in Dogs
Optimal treatment for your dog requires a combination of home and professional veterinary care. Follow-up can be critical, especially if your dog does not improve rapidly. Administer all medications as directed. Alert your veterinarian if you are experiencing problems treating your dog.
It is very important to have the proper information about the specific disease in order to prevent the potential spread of the disease to people. If a zoonotic disease is suspected, appropriate steps need to be taken to ensure that family members are adequately informed and protected from potential infection. This might involve isolating your pet, or taking greater care in cleaning your pet’s excrement. Conversely, many diseases have zoonotic potential, but are not directly contagious to people (e.g. tick borne diseases), and thus, do not require such precautions.
Follow-up blood tests may be required to ensure your pet is adequately responding to therapy.