Important public health considerations, zoonotic diseases are those that can be passed from animals to humans. The CDC reports that 3 of 5 new human sicknesses are transmitted by animals or bugs. For the purposes of our discussion, we will narrow our focus to those diseases which involve cats or dogs. Transmission can occur via direct contact with the pet, contact with their waste, or environmental contamination from their waste. We will also concentrate on diseases that have a relevant clinical incidence; many zoonoses are so rare as to be an insignificant risk. The information contained in this handout is by no means all-inclusive and additional material can be found in the reference section on the last page.
Anyone can be infected with a zoonotic disease, but there are subgroups of people that are disposed toward such infection. First, children, due to their less evolved grooming and washing habits, are more likely to acquire some illnesses and parasites (roundworms and hookworms, for example). As such, it is vital to stress the importance of simple control measures such as washing hands after handling a pet and before eating, and proper handling of any pet waste. Equally important is the washing of any fruits and vegetables before consumption. Additionally, outdoor sandboxes should be covered when not in use as they can serve as a source of infection for children. The elderly and unborn are at increased risk. Immunocompromised individuals should also take extra precautions to avoid exposure; this includes those on chemotherapy or radiation treatment for cancer, on immunosuppressive drugs, infected with HIV or other immunosuppressive diseases, transplant recipients, and women who may be pregnant. Any such person should consult with their healthcare provider for specific information and instructions regarding risk.
There is also a subgroup of pets who are more likely to acquire zoonotic contagions in the first place, thus increasing exposure for their human family. Animals that exhibit predatory behavior (hunting) will have a higher chance of acquiring zoonotic disease. The same holds true for pets that do not receive regular deworming, heartworm prevention, flea/tick prevention and vaccinations. Finally, pets that are fed a raw diet have a higher incidence of not only acquiring zoonotic illness, but passing it on to people as well.
Before discussing the diseases pertinent to our discussion, the following page mentions those that did NOT make the list…...
- Tick-borne bacterial diseases such as Lyme disease, Ehrlichiosis, Anaplasmosis and Rocky Mountain Spotted Fever can be spread to humans, but not using your pet as a vector. A tick bite (or, in the case of RMSF, simply touching an infected tick) is required for transmission.
- Bordetella (the bacteria responsible for “kennel cough”) is transmitted to humans so rarely it is not a clinically significant risk.
- Cryptosporidium, a coccidial parasite, is transmitted to humans so rarely it is not a clinically significant health risk.
- Giardia is a very common protozoal parasite that affects the gastrointestinal tract of dogs, but most human cases are transmitted from other humans. Also, dogs and humans are usually infected with different subtypes or “Assemblages.”
- Cryptococcus is the most common opportunistic fungal infection to affect HIV patients, but reports of animal-to-human transmission involve birds, not dogs and cats.
- Leishmaniasis, a protozoal disease, is spread via the sand fly.
- Pinworms (Enterobius vermicularis), a common gastrointestinal parasite of children, is not carried by dogs or cats.
- Parvo virus can infect humans, but human/canine strains are species-specific and do not cross over.
- Lice are species-specific parasites and dogs and cats are neither reservoirs nor vectors for the human form.
- Whipworms, like roundworms and hookworms (both discussed later), frequently affect pets, but are not transmissible to humans.
- Taenia solium is a tapeworm that can affect humans, but it is acquired by eating inadequately cooked pork. There is a form of tapeworm (Echinococcus multilocularis ) that can be transmitted to humans via pet waste and will be discussed below.
- Helicobacter bacteria represent a significant health concern in humans with gastric ulcers. Dogs and cats may occasionally suffer from H. pylori as humans do, but likely do NOT play a significant role in it's transmission to humans. Transfer of H. canis and H. felis to humans from their pets is VERY rare.
- Avian Influenza is a viral disease that can be transmitted to cats that prey on affected birds, but it's predominant strains (H5N1 and H5N2) cannot travel from cat to human. There are strains (H1N1) that can be passed from human to cat (anthroponosis).
- Anthrax (Bacillus anthracis) is a bacterial zoonosis that is seen in this country (Mississippi delta), but usually usually spread by inhalation, ingestion or cutaneous transmission (usually an open wound that has contact with meat, hide or wool from an infected animal). Carnivores in general seem to be more resistant than herbivores and recovery is not uncommon. Transmission from a dog or cat to human is even more rare.
- Sporotrichosis is a fungal disease caused by Sporothrix schenkii. It is most often acquired from soil, but it has been found on the teeth and claws of cats from infected environments, and within the exudate of cutaneous nodules common to the disease.
- Cheyletiella mites (Cheyletiella yasguri & C. blakei) are an uncommon skin parasite of cats, though passage to humans has been documented.
Rabies is possibly the best-known zoonotic disease. All mammals are susceptible to this virus that affects the nervous system. The primary mode of transmission is exposure to the saliva or brain/nervous system tissue of infected animals, typically in the form of a bite wound. Primary reservoirs in this country include raccoons, skunks, foxes, coyotes and bats. Small mammals/rodents (squirrels, rats, mice, chipmunks) are rarely implicated in rabies transmission, but the exception is the woodchuck. From 1985-1994, woodchucks accounted for 86% of all cases of rabies among rodents reported to the Center for Disease Control. There have been reports of humans contracting rabies via mucous membranes and organ or corneal transplant. Initial clinical signs are compatible with flu-like symptoms, but within days progress to delirium, agitation, hallucinations and insomnia. No single test can diagnose rabies before death, multiple tests are required. The disease is nearly always fatal; to date, just 10 confirmed cases of rabies survival have been documented, with only two not having had any pre- or postexposure prophylaxis. If bitten by a confirmed or suspected animal, a combination of immunoglobulin and vaccinations are started immediately. Initial washing of the wound with soap and water can also significantly reduce the risk of infection, and should be done with any bite wound regardless of rabies risk. When animals with low probability of having rabies bite a human, they can be quarantined for 10 days to monitor for signs of rabies. If the animal passes away, is euthanized for ANY reason, or shows signs compatible with rabies, it must be tested. Testing is performed on brain tissue and obviously necessitates euthanasia. Unvaccinated animals exposed to a rabid animal should be euthanized or placed in 6 month confinement with rabies vaccination given one month prior to release. If, at any time, signs consistent with rabies are seen, the animal is euthanized and tested. Vaccinated dogs and cats so exposed are kept under observation for 45 days. (Note: any bite to a human, by any type of mammal, should prompt a phone call to his/her healthcare provider!)
Ringworm is not really a worm at all, it is a fungal infection caused by Microsporum canis, Trichophyton mentagrophytes and Microsporum gypseum. The first two live predominantly on animals, while M. gypseum is commonly found in soil as well. Pets can pass this dermatophyte to people, and vice versa. Fomite transmission is also common and the spores that live on infected hair shed from an animal can remain infective for years. Dogs and cats may have alopecia, scaling, crusting, or be asymptomatic carriers still capable of transmitting the fungus to people. Diagnosis can be achieved via Wood's lamp or culture. Infections may take weeks to resolve naturally. Because of this and it's zoonotic potential, ringworm is usually treated with either topical and/or oral antifungals, depending on the severity and distribution of the lesions. Humans are often tested by having nail clippings cultured for dermatophytes, and they are treated in a fashion similar to pets. Environmental decontamination is essential; your veterinarian can help you develop such a plan. There are also human species of ringworm that can infect dogs.
Rodents are commonly implicated in human cases of Leptospirosis, though dogs can be the source of infection. Cats are infected VERY rarely. Like pets, humans will contract this bacterial disease by exposure of mucosa or abraded skin to soil and water contaminated with the body fluids (especially urine) of affected animals. Ingestion is another route of infection. Affected animals can shed periodically as long as they are infected with leptospirosis, and the bacteria can last for months in environment. Canine symptoms include vomiting, decreased appetite, abdominal and muscle pain. Affected humans may appreciate these signs as well as headaches. Severe canine cases may proceed to liver or kidney failure. There are vaccinations available that will provide protection against this disease, though no vaccine is 100% effective and there are many strains, not all of which the vaccine will provide protection against. Both dogs and humans are treated with antibiotics.
Brucellosis is, predominantly, a venereal disease affecting multiple species and among the most common zoonoses worldwide. It is the most commonly reported laboratory-acquired infection and is a reportable disease in many states. In some states, if an animal is found to be positive, sterilization or euthanasia is mandatory. Brucella canis is the most common form of the bacteria to infect dogs, but dogs can also be infected with B. abortus (cattle), B. melitensis (goats), and B. suis (pigs) if they are allowed to drink affected milk or eat affected meat or leftover birth tissues. Cats are rarely affected. All dogs to be used for breeding should be tested for Brucellosis, and if positive, culled from the breeding stock. Diagnostics may include blood tests, semen evaluation, or culture of the semen, placenta, fetus or testicles. Clinical signs in the dog include infertility, joint pain and inflammation of the eye, vertebrae and kidneys. Males may exhibit inflammation of testicles, epididymis and scrotum. Infection also commonly leads to abortion late in the canine pregnancy, though pups can be born live and infected (most will die within days). Affected dogs can be treated with antibiotics, but the organism can evade treatment and should never be truly considered eradicated. It is possible for an animal to recover without therapy, but it can take years. Dogs that recover naturally cannot be reinfected, but dogs treated with antibiotics can be reinfected. Sterilized dogs may harbor the bacteria in other body tissues, but sterilization will decrease shedding of bacteria. Most humans acquire the disease by drinking infected milk, inhalation (meat-packing or slaughterhouse employees), and transmission through skin wounds or mucous membranes (meat-packing/slaughterhouse employees, hunters, veterinary staff). Transmission from the family dog is possible, though humans seem to be more resistant to B. canis than other Brucella species. Urine and saliva can be infectious, but birth membranes (placenta) and urogenital secretions are the biggest risk. Fomite transmission has also been recognized. (Note: human-to-human transmission, while rare, can occur. Transmammary, sexual, and post-transfusion/transplantation acquisition have all been reported.) In humans, tests look for presence of bacteria in blood, bone marrow or body fluids, as well as antibodies to the bacteria. Human infections are treated with antibiotics.
Cat Scratch Fever is a bacterial infection caused by Bartonella henselae. Fleas transmit B. henselae from cat to cat by leaving their excretia on the cat's skin. When the cat scratches his/her skin, the organism is transferred to the claws. Fleas do not transmit the disease directly to people. The bacteria can also develop in the mouth, urinary system, eyes and heart. Subsequent transmission to people is usually by cat scratch, but can also be accomplished by the cat biting or licking a person's skin. People with healthy immune systems may have no symptoms or localized rash, fever and swelling of the closest lymph node. The immunocompromised may develop more severe forms of the disease such as vascular lesions in the liver and infection of the heart valves. Antibiotics such as doxycycline and amoxicillin have been shown to clear the infection in cats. Although 40% of cats carry B. henselae at some point in their lives, neither diagnostics or treatment are routine; most cats are asymptomatic and pose very little risk to owners that are healthy. Humans showing appropriate symptoms may be treated empirically with antibiotics or have lesion(s) cultured to positively identify B. henselae.
Roundworm (Toxocara canis, T. cati, T. leonina) is the most commonly reported zoonosis associated with pets in the United States. Dogs and cats can acquire the parasite from their mother's milk, by ingesting another vertebrate host that is infected, or from contaminated soil. Dogs can also acquire the infection in the womb. If the larvated egg is ingested, the parasite will develop in the small intestine. If the larva is acquired in a manner other than swallowing, it will migrate to the lungs where they will be coughed up, swallowed, and then proceed to the small intestine; this is why coughing pups are often checked for worms or empirically dewormed, despite the fact that coughing does not seem like a symptom caused by gastrointestinal parasites. Once an animal acquires the infection, it is 2-4 weeks until adult worms inhabit the small intestine and shed eggs. Identification of Toxocara eggs in the stool is diagnostic, as is the obvious presence of the adult worm in stool or vomitus. Eggs that are shed in to the environment are not immediately infective; it takes 1-3 weeks to become so. This means that the contaminated soil, not the stool itself, is the source of infection. Unfortunately, eggs are very hardy and can survive in the environment for years. Considering the fact that a single adult roundworm can produce as many as 85,000 eggs per day, it is easy to see why infections often persist. There are medications that are labeled for the treatment of roundworms, and most monthly heartworm preventions also control Toxocara infections (one reason why we recommend using heartworm prevention year-round). Humans are not the usual host for roundworms, and when ingested by a human (most often children) the larvae will follow an abnormal migration path that will cause disease in various organs (visceral larval migrans). One of the most common organs affected is the eye (ocular migrans), causing permanent blindness. Testing in humans relies on a combination of clinical signs and evidence of larvae found on blood testing. Treatment for visceral forms usually includes albendazole or mebendazole. The ocular form is much more difficult to treat.
Similarly, hookworms (Ancylostoma caninum, A. tubaeforme) present a health concern not only for dogs and cats, but the people that care for them. This worm is typically acquired via ingestion of larvae in the tissue of other host (usually rodents) or penetration through the surface of the skin. Transmammary transmission is possible in dogs, but not cats. Once in the dog or cat, the larva will find their way to the intestines where they will mature to adults, mate and start producing eggs within 2-3 weeks. These worms feed on your pet's blood and can cause severe anemia, especially in the young or debilitated. On the contrary, pets that are well-nourished and immuno-competent may not show any signs at all. Hookworms can live for up to 2 years in the intestine. Like roundworms, eggs shed in the environment will develop in to infective larva in 2-9 days. Unlike the roundworm, the hookworm can only survive in soil for a few months. There are medications that are labeled for the treatment of hookworms, and most monthly heartworm preventions also control Ancylostoma infections (another reason why we recommend using heartworm prevention year-round). Besides accidental ingestion of larvae via contaminated soil, humans can also acquire this parasite via penetration through the skin (cutaneous larval migrans). Accordingly, people who have regular contact with soil are at highest risk (sunbathers on contaminated sand, children in contaminated sandboxes, gardeners, farmers, exterminators/electricians/plumbers who crawl under raised buildings, etc.). A person suspected of having a hookworm infection would have the stool checked for presence of eggs and, if infection is confirmed, be treated with albendazole of mebendazole.
Toxoplasmosis (Toxoplasma gondii) is a protozoal infection that can affect dogs and, more commonly, cats. Again, animals lacking a healthy immune system are over-represented. Though transplacental or trasmammary transmission is possible in dogs and cats, the most common route of infection is ingestion of mammalian or avian tissue that contains Toxoplasma bradyzoites (tissue stage), or ingestion of stool, water or soil that contains oocysts (environmental stage). Signs can be as vague as fever, weight loss, and lethargy. Infected cats only shed oocysts in their stool for 1-2 weeks, so fecal screening for the oocysts is a poor means of evaluating infections status. Most cats that are shedding oocysts in their stool will be negative on blood tests for Toxoplasmosis. Most cats that are (+) on blood test have finished their shedding phase and are thus very little risk to people.
In people, Toxoplasmosis is a common disease that rarely causes clinical signs. Of the estimated 60 million infected people in the United States, few will show symptoms. But for the pregnant or immunocompromised, effects could be serious. It is important to note that most people who become infected with Toxoplasma gondii will NOT get it from their cat's waste. Rather, they will ingest it by consuming undercooked meat (especially lamb, pork, venison), not washing their hands well enough after handling undercooked meat, or drinking contaminated water. Vertical transmission (mother-to-child) is possible. Acquisition via blood transfusion or organ transplant is rare. Because most cases are asymptomatic, the majority of people will have no idea when/if they become infected. General “flu-like” symptoms may occur. Damage to brain, eye or other organs can occur in more severe or recurrent infections. Infants infected during pregnancy may be stillborn or have lesions related of the brain or eyes. Others may have no clinical signs at birth, but could develop symptoms later in life.
Simple steps to reduce your chance of developing Toxoplasmosis:
1) Freeze meat for several days before cooking, or if intended for raw or undercooked consumption.
2) Cook meat to safe temperatures.
a) Whole cuts of meat (excluding poultry): cook to 145 F°, 3-minute “rest” period
b) Ground meat (excluding poultry): cook to 160 F°
c) Poultry (whole cuts and ground): cook to 165 F°, 3-minute “rest” period
d) Thoroughly wash knives, cutting boards
3) Immunocompetent, nonpregnant persons should clean litter box(es).
4) Change litter daily. (Oocysts require minimum 24 hours to become infectious.)
5) Do not feed your pets a raw diet.
6) Keep cat indoors as this decreases predation.
7) Keep outdoor sandboxes covered when not in use.
8) Wear gloves when gardening.
9) Avoid drinking untreated water, especially in less developed countries.
10) Wash and/or peel all fruits and vegetables prior to consumption.
Testing in people can include blood testing or identification of parasites in tissue and body fluids (specifically CSF). Amniotic fluid can be tested if fetal involvement is suspected. Healthy and nonpregnant persons are sometimes not treated. If treatment is initiated, sulfadiazine, folinic acid, or combinations of the three may employed. For the immunocompromised, treatment may be life-long. Additional treatment may be necessary for ocular involvement.
Women who are pregnant or may become pregnant can request that their PCP perform a test to see if they have already been infected with Toxoplasmosis. This will help determine if any treatment is necessary.
The Sarcoptic mite (Sarcoptes scabiei), also called “scabies,” is a parasite that burrows under the skin and causes an extremely pruritic (itchy) dermatitis; dogs, cats, and people are susceptible to it. In animals, ears, face, elbows and hocks are the most commonly affected regions of the body. Hair loss, scabs and self-mutilation are hallmarks of the infestation. As the mites burrow through the epidermis, they attach their eggs to the tunnel walls. The larva that emerge from the eggs will take approximately 3 weeks to become adults themselves. The easiest means of diagnosis is a simple “skin scrape” test that can be performed by your veterinarian, but because the mite is somewhat elusive (and because treatment is safe, cheap and effective), suspected cases are treated even in light of “negative” diagnostics. This mite cannot live for long off of a host so transmission is dependent on either direct contact or quick transmission via a fomite (non-living object) such as clipper blades. There are approved medications for the treatment of Sarcoptic mange and there is a monthly heartworm prevention (Revolution®) that not only treats existing cases, but prevents new infections as well. Many cases of human scabies (especially institutional outbreaks) are transmitted from other people (Sarcoptes scabiei var. hominis) and it is considered a STD, though non-sexual close contact can also facilitate transmission. Humans often have small red papules at the site of infection, and the dermatitis and pruritus may worsen when the body is warm, such as when under the covers in bed. Like animals, the cornerstone of diagnosis in people is the microscopic identification of the mite (or it's feces). Prescription scabicides are available in topical form. There are no OTC scabicides. The topical medication is applied to the entire body. Clothing, bedding and towels are dry cleaned, sealed in plastic bags for 72 hours, or washed in hot water and dried on high heat.
Campylobacter is an intestinal bacterial infection that man can acquire from dogs and cats (as well as hamsters, ferrets, rabbits and many farm animals) via contact with their feces. However, the biggest risk is the consumption of raw/undercooked meat (especially chicken) or unpasteurized milk. Proper cooking of meat, drinking only pasteurized milk, and appropriate hygiene after handling waste can help decrease risk of transmission. Antibiotics will resolve infection of companion animals. Feces can be evaluated to determine the presence of Campylobacter. Most infections will resolve without medication, but antibiotics are used for those with severe illness or inadequate immune systems.
E. coli bacteria is a major source of enteric (intestinal) infections in animals and people. There are some strains of E. coli that are normal inhabitants of the g.i. tract and do not cause disease (nonpathogenic). However, there are some other strains that cause diarrhea and illness related to the intestines and other parts of the body- these are often called diarrheagenic or enterotoxigenic. Most cases are transmitted through contaminated soil, or ingesting contaminated or improperly prepared food. The CDC reports that the highest-risk foods include: raw fruits and vegetables, raw seafood, undercooked meat or poultry, unpasteurized dairy products, untreated water/ice, and food from street vendors. If your pet ingests E. coli, his/her feces may be a source of infection. Like Campylobacter, diagnosis is based on detection of the organism in the stool. Though rehydration is essential, E. coli infections are often not treated with antibiotics or antidiarrheals as they can actually worsen the disease.
Salmonella is a bacterial infection that is similar to both Campylobacter and E. coli in that the most common means of infections is ingestion of poorly prepared or raw meat, milk and eggs. And, similarly, a pet that has been fed these items and has become infected will eliminate waste that can pass the infection to their human caretakers. Turtles and reptiles are much more likely to be the source in instances of human Salmonellosis caused by an animal. Human blood or stool can be tested for infection and infections are treated with antibiotics. The CDC website has excellent guidelines for preventing Salmonellosis, specifically in children.
Clostridium difficile is a bacteria that produces a toxin (“enterotoxin”) which can make both humans and pets very sick. Like MRSA, it is often a hospital-borne, or nosocomial, infection in people. Diarrhea is the chief clinical sign. This bacteria forms a spore which allows it to last for years in the environment (especially soil). The spore will release the bacteria when ingested. Again, wearing gloves in the garden, and washing fruits and vegetables helps reduce risk. For both humans and animals, having been on antibiotics increases risk of clinical disease as the antibiotics will often kill the normal flora (“good” bacteria) in the intestine, allowing Costridium overgrowth. Ironically, antibiotics (metronidazole, vancomycin) are sometimes used in the treatment of this disease. Transmission from pets to humans has not been definitively proven, but since pets and humans suffer from the same strains of C. difficile, it is safest to assume that affected dogs and cats act as a nidus of infection. Humans stool can be screened for the presence of the bacteria, or the toxin which it produces.
Methicillin-resistant Staphyloccous aureus (MRSA) is a type of bacterial skin infection that does not respond to conventional antibiotic therapy. Traditionally a malady of humans (often acquired in a hospital), it is now recognized in companion animals. This organism can be transmitted back and forth between humans and pets via direct contact, but information found for this paper indicates that most infections start with the human, not the pet. The most common manner in which a human would develop this infection is in any activity (athletics) or venues (gyms, schools and daycare facilities, hospitals, military barracks) that involve crowds and/or shared supplies. People should not try to drain skin lesions themselves, but rather allow their healthcare provider to address the lesion and prescribe appropriate antibiotics.
Echinococcus multilocularis is a parasite that occurs as a tissue cyst in sheep, rodents and rabbits (intermediate hosts). If a dog (the definitive host) or cat eats an intermediate host, Echinococcus will manifest itself not as a cyst, but as an intestinal worm. The feces from that pet then becomes a source of potential infection to humans. The eggs are immediately infective. Humans can also contract the disease by ingesting food or water contaminated with the eggs. Humans that ingest eggs develop cysts in their lungs and liver (called hydatid disease). These cysts can become very large, though they grow slowly. This disease is more common in developing countries where sheep and dogs are more commonly housed together, causes significant morbidity and loss of human productivity and can be fatal. Surgical removal is the cure. Other treatment modalities include chemotherapy, cyst puncture and the PAIR (Percutaneous Aspiration, Injection of chemicals, and Re-aspiration) method. In dogs, infection is diagnosed by fecal evaluation for eggs. For humans, a combination of clinical signs, blood tests and advanced imaging are utilized.
Veterinary Information Network
Companion Animal Parasite Council
Center for Disease Control
World Health Organization