Definition of zoonosis: an infectious disease transmitted from animals to humans or from humans to other animals (reverse zoonosis).
The health and well-being of the veterinarian and staff members must be considered when treating non-mammalian species such as birds, reptiles, and fish. Zoonotic diseases in these taxa exist and the practitioner that works with non-traditional pet species should be acquainted with such zoonoses. Appropriate protective measures such as surgical masks, eye protection, gloves, and disinfectants should be considered when zoonoses are suspected. Additionally, veterinarians must be ready to educate pet owners about the risks associated with exotic pet ownership.
Salmonellosis is the most recognized reptilian zoonosis, though can also be transmitted by birds and fish. Salmonella spp. are gram-negative rods, usually flagellated, and facultative anaerobes of which dozens of serotypes have been implicated in cases of reptile zoonoses. Salmonella can be part of the normal intestinal flora of humans and many animals with no clinical signs present. Many people do not develop disease when exposed to Salmonella; however, others may develop diarrhea, abdominal cramps, and fever. Immunocompromised individuals such as infants, the elderly, and those with immunosuppressive disorders are at a higher risk. The bacteria must be ingested; simply touching an infected animal will not result in disease. Many Salmonella infections in humans are self-resolving within a week; however, the disease can sometimes progress to sepsis and possibly death.
Most infected reptiles are asymptomatic and shedding of Salmonella organisms can be intermittent. As such, all reptiles should be presumed positive and managed accordingly regardless of any Salmonella testing results. Treatment to eliminate the organism from a Salmonella-positive reptile is difficult as antibiotics may suppress excretion of detectable organisms without their complete elimination. Development of antibiotic resistance is likely with such treatment failures.
In 1975, following a rash of Salmonella cases in children infected by pet turtles in the USA, the FDA ruled it illegal to sell turtles less than 4 inches in carapace length.
In one study regarding fish and aquaria, multidrug-resistant Salmonella enterica serovar Paratyphoid B dT+ isolates that were found in patients with gastroenteritis were identical with isolates from their home aquariums. Ornamental fish tanks and their fish inhabitants are also reservoirs for Salmonella.
Birds are also potential carriers of Salmonella. Host susceptibility and development of carrier states varies widely among avian species.
Aeromoniasis is primarily a disease that affects fish, amphibians, and reptiles. The disease is rare in wild or domestic mammals and birds. It is a gram-negative, straight bacillus, with a flagellum and are facultatively anaerobic. The principal reservoir is in rivers, ponds, and lakes as well as areas where salt water meets fresh water. The disease may occur individually, but is often seen in epidemic proportions particularly in fish-farming pools. Freshwater fish are typically affected with a hemorrhagic septicemia. In the acute form of the disease, death may occur without clinical signs. In other cases, there may be scale loss with hemorrhage in the gills, mouth, and base of the fins, cutaneous ulcers, exophthalmia, and ascites. Intensive fish-farming or overcrowded aquaria with poor water quality, inadequate feeding, and temperature stress can create conditions that favor infection.
In colonies of reptiles and amphibians, Aeromonas hydrophila can also cause significant illness. A disease syndrome called "red leg" is widely recognized in amphibians that causes petechial hemorrhages of the skin, cutaneous ulceration, septicemia, and death. In lizards and snakes, Aeromonas infection is often associated with ulcerative stomatitis, hemorrhage and ecchymoses of the integument, pneumonia, and septicemia. Poor hygiene, infrequent water changes, overcrowding, environmental changes, trauma, and other causes of stress are factors that unleash disease.
In man, the disease occurs in two forms: enteric and extra-enteric. The enteric form results in diarrhea, fever, and abdominal pain. Extra-enteric infections may occur from contact with contaminated water in open wounds or from bites or scratches inflicted by reptiles living in water environments. Cellulitis results.
Campylobacter spp. are enteric bacteria commonly implicated in causing diarrhea and enteritis in man. Mammals and birds, both domestic and wild, are important reservoirs for C. jejuni. In developed countries, the incidence rate of human enteritis cases is similar to that of Salmonella. A high rate of infection is found in clinically healthy animals, including birds. Water sources can be contaminated via the fecal route, and is then transmitted orally. Infection in man is often caused by cross contamination in the kitchen of meats with C. jejuni and other foods which do not require cooking. Infection can be acquired directly from animals with diarrhea, especially children who play with these animals and come in contact with the animal's feces.
Mycobacterium spp. are acid-fast bacilli with several species known to cause disease in humans and animals. Exotic pets including birds, reptiles, fish, and mammals have all been associated with Mycobacterium zoonoses. Transmission can occur through direct contact with infected animals or environments, through aerosolization and inhalation, or ingestion.
Mycobacteriosis is a common disease of fish and is usually caused by M. marinum, M. fortuitum, or M. chelonei. Fish can be long-term asymptomatic carriers. When they are symptomatic, clinical signs may include skin ulcerations, loss of scales, poor body condition, distention of the coelomic cavity, and exophthalmia. Granulomas can also be found in the liver, kidneys, and spleen. Infected fish shed the organism into the environment through feces, infected tissues, and skin ulcerations. Aquarists are often exposed when placing forelimbs into fish tanks, and the bacteria can enter through small cuts or abrasions in the skin. To prevent zoonotic mycobacteriosis from fish, aquarists should wear gloves that extend to the elbows to reduce potential exposure while cleaning fish tanks. Additionally, water should not be disposed of where cutaneous inoculation or ingestion of organisms could occur, such as in sinks or bathtubs.
Birds and reptiles are also affected by Mycobacterium spp. that are known to cause infection in humans, including but not limited to M. avium, M. genavense, M. marinum, and M. tuberculosis. In birds and reptiles, disease is often chronic with a wide variety of pathologic lesions including granulomatous and non-granulomatous disease involving the lungs, liver, gastrointestinal tract, spleen, skin, subcutaneous tissue, oral mucosa, gonads, bone, and central nervous system. Potential routes of transmission include direct contact through defects, scratches, or bites in the skin or with inhalation and contact with oral or respiratory mucosa.
Chlamydophila psittaci can cause disease in birds and humans. Also known as ornithosis or parrot fever, it is caused by an intracellular bacteria. In humans, the disease manifests as flu-like symptoms, pneumonia, and possibly death. Birds can be asymptomatic carriers but may exhibit respiratory illness, lethargy, diarrhea, and biliverdinuria. The organism is shed in respiratory secretions and feces. While often associated with parrots, C. psittaci has been isolated from many other bird families. Various tests are available to detect the organism in pet birds. Precautions should be taken by both bird owners and the veterinarian to reduce risk of exposure and identify the pathogen in suspect outbreaks.
"Bird flu" has been a hot topic in the media over the past two decades as it has been linked to hundreds of human deaths primarily in Asia and Africa. Until recently, the species barrier for influenza viruses between humans and birds was thought to be quite strong. However, several pieces of evidence point to the fact that avian influenza viruses can, under certain circumstances, be directly infectious for people. In birds, the nature of influenza virus infections is dependent upon 2 major factors: the species of bird and the pathogenicity of the virus strain involved. Clinical disease often develops following infection in chickens and turkeys, while waterfowl and other wild avian species are often subclinical. Typical respiratory disease (lethargy, inappetence, nasal discharge, sneezing) and reductions in egg production can develop in poultry infected with influenza viruses of relatively low pathogenicity. In contrast, highly fatal, systemic disease ("fowl plague") characterized by edema, hemorrhages, cyanosis of combs and wattles and infection of the CNS can occur in poultry infected with so-called "highly pathogenic" influenza viruses. These viruses are always of the H5 or H7 subtypes, but not all H5 and H7 viruses are highly pathogenic. The various subtypes are named for the 16 hemagglutinin (H) and 9 neuraminidase (N) proteins on the viral surface.
West Nile Virus
West Nile Virus is an insect-borne flavivirus commonly found in Africa, western Asia and the Middle East, and, since 1999, in the Western Hemisphere. In the USA, it initiated an epizootic in local birds, followed by a human epidemic in the New York City area that spread to the remainder of the continent. Migrating birds are thought to be the one of the major contributors to the rapid dissemination of this mosquito-borne virus. In humans, presentation can range from asymptomatic infection to fatal encephalitis. Symptoms may include fever, rash, muscle weakness, flaccid paralysis, seizures, and nausea and vomiting.
Histoplasma capsulatum is a dimorphic fungal agent that has been associated with the droppings of birds. It can grow as a mycelium in soil or material contaminated with bird droppings and upon animal or human infection, converts to a yeast phase. Transmission occurs by inhalation of spores from infected material. It is not contagious and cannot be transmitted from an infected person or animal to another. The acute phase of histoplasmosis is characterized by non-specific respiratory symptoms, often cough-like or flu-like. Disseminated histoplasmosis can affect multiple organ systems and can be fatal if left untreated.