Diseases that are transmitted from animals to humans. Zoonoses (the singular form is zoonosis) are also termed zoonotic diseases. They are infectious diseases passed from animals (Fig. 1) to humans. Zoonotic agents are found among all major categories of microbes, including bacteria, fungi, parasites, and viruses. Although any individual can acquire a zoonotic infection, young children, older adults, and immunocompromised individuals [for example, human immunodeficiency virus (HIV)–infected individuals or people receiving chemotherapy] are most at risk. See also: Animal virus; Bacteria; Disease; Fungi; Infectious disease; Medical bacteriology; Medical mycology; Medical parasitology; Microbiology; Parasitology; Virus
Animals can also serve as reservoirs for zoonotic agents; that is, they are a source of the microorganism that allows transmission to the human population. Domesticated animals can often serve as a bridge for zoonotic agents that are commonly found in feral animal populations. However, in some cases, animals serve only as indicators that an organism is present in the environment. In these instances, they become infected from contaminated soil, water, or air, as do humans. Additionally, there are diseases, called anthroponoses, that are transmitted from humans to animals. See also: Disease ecology
Nervous system infections
Important zoonotic agents that can invade the nervous system in humans include Toxoplasma gondii and Cryptococcus neoformans.
Many adult humans are seropositive for Toxoplasma gondii. Feline species are the only definitive hosts for the organism. Cats harbor sexual stages of the parasite in their gastrointestinal tract and shed infectious oocysts in their stool. The organism has been found in all areas of the world, in hundreds of species of birds, and in many species of warm-blooded animals, which act as intermediate hosts. Intermediate hosts become infected by ingesting sporulated oocysts from cats. Humans can contract the disease by ingesting inadequately cooked meat of the intermediate hosts with infectious bradyzoites and tachyzoites, or by ingesting oocysts from soil contaminated with cat feces. Humans can be infected by ingesting any source of meat with Toxoplasma, but lamb, mutton, and pork are most commonly infected. See also: Cat; Toxoplasmida
Cryptococcus neoformans (Fig. 2) is an important pathogen in immunosuppressed patients and occurs commonly as a premonitory sign of acquired immune deficiency syndrome (AIDS), where it causes severe life-threatening meningitis. Cryptococcus is taken up by the human host via the respiratory tract. The organism has been found in soil, plants, bird feces, raw milk, and fruit juices, as well as the oral cavity, gastrointestinal tract, and skin of healthy humans. It is commonly found in pigeon roosts and in soil contaminated by pigeon feces, and has been isolated from droppings of other bird species. See also: Acquired immune deficiency syndrome (AIDS); Meningitis; Pathogen
Respiratory tract infections
Bordetella bronchiseptica is a secondary bacterial invader that occurs commonly in the respiratory tracts of dogs, cats, and pigs, and less commonly in horses. It complicates the management of viral respiratory disease in dogs, and has been associated with clinical signs of fever, anorexia, coughing, and nasal discharge. Immunocompromised patients should avoid exposing themselves and their dogs to environments with sources of B. bronchiseptica, including kennels and other settings where dogs are housed closely together. See also: Dogs
Gastrointestinal tract infections
Agents causing primary gastrointestinal diseases that have been documented to be transmitted to humans from an animal source include Salmonella, Campylobacter, Mycobacterium, Cryptosporidium, and Giardia. Agents that have been implicated as zoonotic agents include Entamoeba, Isospora, and various microsporidian parasites.
Salmonella is an important pathogen in humans, and salmonellosis is a major communicable disease problem. Contaminated animals and animal products (particularly poultry and beef) are a major source of Salmonella for humans. The most common source is from contaminated foods. Strict hygiene in food preparation is needed to avoid contaminating cold-served food products (such as salads and coleslaw) with raw meat. It is estimated that the rate of infection of Salmonella in domestic animals is 1–3%. See also: Foodborne disease; Salmonelloses
Salmonellosis is frequently diagnosed in dogs and cats, and these animals can also be a source of infection for humans. Confining dogs and cats in the house as much as possible minimizes their contact with fecal material of other animals. Avoidance of feeding raw meat to pets prevents their contracting the disease from a well-known source of salmonellosis. Salmonella is also carried commonly by cold-blooded animals.
Campylobacter is another common pathogen that can be transmitted from animals to humans. Campylobacter is a significant cause of enteritis and diarrhea in humans all over the world. Transmission of C. jejuni to humans occurs commonly from contaminated water and food (including meat products, especially poultry). Campylobacter jejuni is found generally in water sources, where it can live for several weeks at low temperature. Many animals, including cattle, sheep, swine, fowl, dogs, and cats, excrete Campylobacter species. Immunocompromised individuals should be aware of the risks of owning or coming in contact with puppies and kittens and their excreta. See also: Diarrhea; Waterborne disease
Cryptosporidium is a protozoan parasite that is only 1–6 micrometers (μm) in size. It lacks host specificity and infects the gastrointestinal tract of a wide range of mammals. Cryptosporidium parvum is the most important species in humans and domesticated animals. It has been shown to occur naturally in mice, rabbits, guinea pigs, cats, dogs, squirrels, raccoons, horses, pigs, sheep, goats, cattle, and several species of nondomesticated ruminants. Fecal contaminations of food and water contribute to the high prevalence of cryptosporidial infection in persons residing in areas with poor sanitary conditions. Contaminated food, fomites, flies, and water are sources of infection with Cryptosporidium. See also: Protozoa
Giardia is a flagellate protozoan parasite that infects the gastrointestinal tract of many species of animals, including dogs, cats, horses, pigs, cattle, sheep, goats, and other mammals. It naturally infects humans, and pets are potential sources of Giardia for humans. See also: Giardiasis
Parasitic diseases of animals
Some parasitic diseases of animals have zoonotic potential. The dog heartworm, Dirofilaria immitis, is endemic in many areas of North America and the rest of the world. Humans can be accidentally infected by D. immitis microfilaria-carrying mosquito vectors due to the high prevalence of D. immitis, although the disease is rare. See also: Heartworm; Mosquito
Toxocara canis (Fig. 3) and T. cati are roundworms of the dog and cat. Humans acquire infection with these parasites by ingesting embryonated eggs shed in the feces of animals. Visceral larva migrans occurs when eggs hatch, penetrate the intestine, and migrate through body tissue. Migration can result in multiple abscesses, eosinophilic granulomas, hepatomegaly, and pneumonitis with eosinophilia. Eggs of Toxocara are extremely resistant in soil and remain infectious for years. Routine yearly fecal exams and treatment of positive animals with anthelmintics (anthelminthics) is recommended to decrease the potential for this problem.
Ancylostoma caninum and A. braziliense are hookworms of dogs and cats that rarely cause cutaneous larva migrans in humans. Hookworm eggs are shed in feces into the environment, hatch, and undergo two larval molts to produce infectious, third-stage larvae. Hookworms infect their definitive hosts by penetrating the skin. In the correct host, larvae migrate from the skin to the lungs, ascend the respiratory tree, and are swallowed to resume development in the intestine, where they mature into adult hookworms. Although some human infections may be associated with pneumonitis and eosinophilia when the larvae invade the lung or another organ tissue, most cases are limited to the skin.
Echinococcus granulosus and E. multilocularis are tapeworms that cause hydatid disease or echinococcosis. Echinococcus granulosus is a tapeworm whose life cycle involves dogs, coyotes, wolves, and dingos as the primary hosts and sheep, swine, cattle, moose, caribou, and humans as intermediate hosts. This parasite is endemic in North America, South America, England, Africa, the Middle East, Australia, and New Zealand. Echinococcus multilocularis is a tapeworm that cycles between dogs, cats, and foxes as definitive hosts, and between voles, lemmings, cattle, horses, swine, and humans as intermediate hosts. Echinococcus multilocularis is endemic in north-central Europe, Alaska, Canada, and the central United States. Dogs and cats that ingest wild game and rodents can be infected with E. multilocularis and can be a risk to humans.
Fleas are common, difficult to eliminate, and capable of transmitting several important zoonotic diseases. In many locations, fleas are mostly a nuisance problem, causing pruritis and hypersensitivity in animals and humans. Control of fleas must focus on the environment (indoor and outdoor) of pets. Pets may be treated for fleas weekly, but treatment must coincide with treatment of the environment.
Dipylidium caninum is a common tapeworm of dogs and cats. Cats and dogs are infected during grooming by ingesting the flea intermediate host containing an infective cysticercoid. Cysticercoids mature into adult tapeworms in the intestine of their normal host. Tapeworm eggs that are shed in the feces of an infected dog or cat are eaten by flea larvae, where they hatch and develop to the cysticercoid in the body cavity of the flea. Humans, especially children, are accidentally infected when they swallow a flea while crawling on the floor or while playing with pets.
Animals are exposed to ticks (Fig. 1) when they roam fields and wooded lots. Ticks transmit many zoonotic diseases, including babesiosis, tick fever, rickettsial diseases, Lyme disease, Q fever, relapsing fever, typhus, ehrlichiosis, and tularemia. Humans are exposed to ticks by similar means, but in some cases can acquire ticks from animals. Pets kept indoors are less likely to acquire ticks. Humans can use tick repellants, should check themselves for ticks after a walk in the woods, and should avoid tick-infested areas. See also: Lyme disease; Rickettsioses; Tularemia
Bartonella species, including B. henselae, are members of the rickettsial family and have been implicated as causative agents in cat scratch disease and bacillary angiomatosis (vascular tumors of the skin, internal organs, bone marrow, and lymph nodes). In humans, cat scratch disease is characterized by persistent regional lymphadenopathy in the lymphatic drainage area of the site of a recent cat bite or cat scratch. In immunocompetent hosts, cat scratch disease tends to be a disease of children and is usually self-limited and localized to regional lymph nodes. It rarely causes systemic or widespread infections. Bartonella henselae is widespread in cat populations. Because arthropods are known vectors for the transmission of rickettsias, prevention should be focused on flea control and minimizing direct transmission by cats. See also: Bartonellosis; Cat scratch disease
Pasteurella multocida is an important pathogen in animal bites and a documented life-threatening pathogen in immunocompromised patients. Transmission occurs via traumatic wounds (such as bites and scratches), by the respiratory route via aerosol, and by ingestion. To minimize transmission of P. multocida, humans should avoid being bitten or scratched by pet dogs and cats. See also: Pasteurella; Pasteurellosis
Overall, the risk of contracting zoonotic diseases is likely to be small, but physicians and veterinarians can decrease this risk by educating individuals on the transmission and prevention of these diseases.