Zoonoses are a major area of concern in veterinary practice and public health. Zoonoses are diseases that are transmitted among animals and humans. However, the term usually refers specifically to those diseases which can be transmitted from animals to people. Over 200 zoonoses have been reported but the number of zoonotic diseases that are common in pet animals is much smaller. These include a variety of diseases caused by viral, bacterial, fungal, and parasitic pathogens. Approximately three-fourths of the new diseases that have affected humans over the past decade have been caused by pathogens originating from an animal or from products of animal origin.
The veterinary health care team has a responsibility to monitor potential risks to clients related to zoonotic diseases. In addition, the Occupational Health and Safety Administration mandates that employers minimize potential hazards to employees. A thorough knowledge of the characteristics of infectious agent as well as the mode of disease transmission is essential to avoid potential harm to practice staff and clients. Certain groups of people are more susceptible and suffer more serious effects from zoonotic diseases than others. Children and the elderly are more susceptible because their immune systems function at a lower level than those of normal healthy adults. AIDS, lupus, and chemotherapeutic medications suppress normal functions of the immune system and make an individual more susceptible to disease. Children are also more likely to put contaminated soil or materials in their mouth.
Mode of Transmission
Disease can be transmitted directly (i.e., by direct contact, inhalation) or indirectly. Indirect transmission may involve contact with inanimate objects (fomites) or with disease vectors. Common vectors of zoonotic diseases include fleas, flies, ticks, and mosquitoes. The infectious agents may undergo part of its development in the vectors or the vector may simply transmit the agent. Although diseases transmitted to humans by some arthropod vectors do not meet the strictest definition of a zoonotic disease, they are often treated as such since dogs, cats, and other animals can serve as reservoirs for the disease. These diseases are classified as cyclozoonosis, metazoonoses, and saprozoonoses. A cyclozoonosis requires several components of the disease to occur in several different vertebrate species. Hydatid cyst disease caused by Echinococcus is an example of a cyclozoonosis. A metazoonosis is maintained by both invertebrate and vertebrate species. For example, the viral agent that causes encephalitis is transmitted among mammals and birds by a mosquito. Saprozoonotic diseases depend on an inanimate reservoir, such as soil or water, to maintain the cycle of infection. An example of a saprozoonotic disease is visceral larva migrans. The causative agent for this disease, Toxacara canis, is maintained in soil.
Animal diseases that can be transmitted to humans by arthropod vectors include Babesiosis, Anaplasmosis (Ehrlichiosis), Lyme disease, Rocky Mountain Spotted Fever, West Nile Virus infection, and Equine encephalitis. Diseases that can be acquired from dogs or cats via bites and scratches include pasteurellosis, cat scratch disease, and rabies. Although there are a large number of animal diseases that can be transmitted to people via inhalation or ingestion, only a few of these occur commonly in dogs and cats. These include Giardiasis, Toxoplasmosis, and various diseases caused by nematode and cestode infection. Diseases transmitted by direct contact include Dermatophyosis (dermatomycosis) and Leptospirosis.
Prevention and Control of Zoonotic Diseases
Prevention programs include vaccination of animals in the reservoir population, potential hosts, and people, if vaccines against that disease are available. Vaccines are available to prevent some zoonotic diseases, such as rabies. In geographic areas where the human pre-exposure vaccine is widely available and where routine rabies vaccination of pets occurs, the incidence of human rabies infection has been greatly reduced. Prevention of human infection may also involve treatment of infected animals that may transmit the disease to people. For example, treating puppies and kittens for roundworms and hookworms can prevent contamination of soil by feces containing infective eggs. Other control measures are designed to reduce or eliminate the presence of the reservoir of disease or the vectors that transmit the disease. Such control measures include spraying for mosquitoes, use of tick repellent, treatment of infected animals, and decreased contact with infected animals. Information on some of the most common zoonotic diseases is below.
Animal bites can be a significant source of zoonotic disease. Bacteria transmitted via bite wounds can be from the animal's mouth or on the skin of the person bitten. The majority of dog and cat bite infections are caused by Pasteurella. Mixed infections are also possible and include infections with Staphylococcus aureus, Staphylococcus epidermidis, Streptococcus spp., Bacteroides spp., Fusobacterium spp., and other gram-negative bacteria. These can all cause fever, septicemia, meningitis, endocarditis, and septic arthritis. Cat bites are 10 times more likely to become infected than dog bites. Control measures should be aimed at client education, especially educating children. Approximately 80% of dog bites are inflicted on children. Cat bites occur more commonly in women.
Cat Scratch Disease
Cat scratch diseases, also known as cat scratch fever, is caused by the gram-negative bacillus, Bartonella henselae, present in fecal material of fleas. The bacterium is primarily found in kittens and rarely causes disease in cats. It is transmitted when the kitten bites, licks, or scratches a human. In humans, the disease is usually self-limiting. Blister-like lesions at the site of infection are common and lymphadenopathy ensues. Fever, lethargy, and inappetence may also occur. The disease can be life-threatening complications in immunocompromised individuals. The organisms can be detected using immunologic testing and infected cats can be treated with antibiotics. Individuals in frequent contact with possibly infected cats should wear gloves when handling cats. Clients should be advised to keep the cat's nails cut short and to not allow kittens to lick open wounds. Flea control should also be stressed.
Visceral Larval Migrans
Soil or vegetation contaminated with roundworm eggs, especially Toxacara canis, can result in visceral larva migrans. The disease occurs most often in children that eat soil or handle items that have been in contact with soil that is contaminated with feces that contain Toxocara eggs. The eggs hatch in the individual's gastrointestinal system and the larvae migrate through the organs. Tissues most often affected are the liver, lungs, brain, and eyes. Migrating larvae cause permanent tissue damage. The severity of the disease is related to the numbers of larvae present as well as their location in the body. Eventually, the migrating larvae die and abscesses or granulomas form around them in the tissues. Mild infections are characterized by eosinophilia, fever, hepatomegaly, and pulmonary signs. If the larvae migrate to the eye, there may be loss of vision or the eye. (ocular larval migrans). Larvae that enter the brain and spinal cord can cause seizures and paralysis (neural larval migrans). Control of infection involves treatment of puppies as well as breeding animals. Client education on the dangers of this disease is essential.
Cutaneous Larval Migrans
Larvae of hookworm species are present in soil and are capable of penetrating intact skin. Humans, especially children, can also become infected by ingesting the larvae. Individuals with increased exposure to soil, such as gardeners, utility workers, and even sunbathers, may be at greater risk of infection. When larvae penetrate human skin, their normal migration patterns are disrupted and the larvae remain within the epidermis. The larvae migrate randomly throughout the epidermis causing a condition commonly known as 'creeping eruption'. The lesions are pruritic and appear inflamed.
Infection with the protozoan parasite Giardia can occur with ingestion of the cyst or trophozoite stage of the parasite that is located in soil, contaminated water, or any surface contaminated with Giardia cysts. The parasite is easily transmitted from person to person, animal to animal, and between animals and people. Giardiasis is characterized by severe diarrhea, nausea, abdominal pain, and fatigue. Prevention of infection relies on good sanitation practices, such as regular hand washing, thorough washing of uncooked vegetables, etc.
The incidence of tick-borne diseases in humans has increased exponentially over the past two decades. This is generally thought to be due to reforestation projects that have resulted in increased density of deer population as well as expansion of the range of the habitat of the tick vectors. People are infected when bitten by ticks during outdoor activities in tick-infested areas or from contact with ticks on pets. Control of tick-borne diseases involves wearing tick repellents in areas infested with ticks and keeping pets free of ticks. Common tick-borne diseases include Lyme disease, Rocky Mountain Spotted Fever, Human Granulocytic Anaplasmosis (also called Human Granulocytic Ehrlichiosis), and Babesiosis
Rocky Mountain Spotted Fever is the most severe and most frequently reported rickettsial illness in the United States. Rocky Mountain Spotted Fever causes high fever, headache, chills, severe muscle pain, and malaise. Mortality is high if the disease is not treated. Rocky Mountain spotted fever is transmitted by the dog tick Dermacentor or the Lone Star tick Ambylomma. Lyme disease (borreliosis) is the most commonly reported tick-borne disease and is caused by the spirochete Borrelia burgdorferi. It is transmitted to humans by the deer tick, Ixodes scapularis. Symptoms include rash, fever, headache, and muscle or joint pain. The deer tick may also transmit the causative agent of Human Granulocytic Anaplasmosis. Several species of Anaplasma are implicated in this disease. Symptoms include fever, severe headache, muscle aches, nausea, vomiting, diarrhea, cough, joint pains, and confusion. Babesiosis is caused by several hemoprotozoans in the genus Babesia and is most common in the northeast and midwestern states. The deer tick has also been implicated in transmission of this pathogen.
Leptospirosis is caused by bacteria of the genus Leptospira. Symptoms include high fever, severe headache, chills, muscle aches, and vomiting, jaundice, abdominal pain, diarrhea, or a rash. The bacterium has been found in cattle, pigs, horses, dogs, rodents, and wild animals. Humans become infected through contact with water, food, or soil contaminated with urine from infected animals.
Commonly known as ringworm, dermatophyosis is caused by various species of fungi, such as Microsporum spp. and Trichophyton spp. The disease is transmitted through direct contact with infected animals, their hair, or scales from the lesions, or from fomites such as grooming tools and leashes. Lesions are scaly, circular, with reddened borders, and accompanied by alopecia. Control measures include wearing gloves while handling suspected infected animals. Good hygiene practices and careful attention to proper husbandry may help reduce the development of the disease in animals.
This disease is caused by the protozoal parasite Toxoplasma gondii is common in cats but rarely causes clinical disease in cats. Human infection most often occurs through ingestion of undercooked meats containing infective stages of the parasite. Cats obtain the parasite from ingestion of mice or if fed undercooked meat containing infective stages. Cats shed oocysts in the feces for only a few weeks after infection. Infection requires ingestion of the infective stage. Human infections are usually asymptomatic or flu-like symptoms may occur. Congenital transmission is possible when a woman is first infected during early pregnancy and can produce serious disease in the fetus. Pregnant women should not handle cat litter or should wear gloves. Keep cats indoors. Changing the litter box daily may also reduce the potential for infection as the oocysts require some time in the environment before they are infective.
Lice, Mites, Ticks, Fleas
Many species of external parasites can infest humans. Plague is the best-known zoonotic disease involving the flea as a vector and still occurs in the western United States. The primary reservoir for plague is wild rodents but rabbits and domestic cats may serve as a source of infection for people. If untreated, the mortality rate can be 50%.
Other Zoonotic Diseases
Rabies, Schistosomiasis, Brucellosis, Campylobacteriosis, Q fever, Anthrax, Cryptosporidiosis, and numerous other zoonotic diseases can be transmitted from dogs and cats to people. However, most of these are quite rare in pets.
Control of Zoonotic Diseases
Educating veterinary staff and clients about the dangers of zoonotic diseases is an essential first step in the prevention of infection. Always practice good hygiene when handling animals; assume all animals could be harboring a zoonotic disease. Wash your hands after examining patients and wear gloves when handling anything soiled with animal urine or feces. Speak with your physician about vaccinations for rabies and tetanus.
Mosquito and tick repellants should be used when outdoors and all pets should be regularly checked for ticks. Prevention of disease in pets also requires attention to exposure risks, such as frequency of contact with other animals. Dogs and cats that regularly encounter other animals, (i.e., pet shows, pet parks) should be examined at least twice a year as part of a thorough wellness program.
Veterinary professionals are at greater risk of contracting zoonotic diseases than the general public. Knowledge of how zoonotic diseases are transmitted and maintained in a population is important for preventing spread of diseases and preventing infection in veterinarian staff and clients. Careful attention to stringent husbandry procedures is also vital to preventing these diseases in both pets and people.
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