Zoonoses are defined by the World Health Organization as diseases and infections which are transmitted between vertebrate animals and man. Veterinary practitioners are often the first to address questions on possible transfer of disease from pets to owners who are their clients. The practitioner is expected to be the expert "on site" and provide accurate detailed information and solutions to problems. This challenging duty carries a great deal of responsibility and may also be associated with legal liability. The task of being knowledgeable about zoonoses is even more difficult when considering that the number of organisms known to cause zoonotic infections is greater than 250. Moreover, there is a constant flow of new data and information about these pathogens and newly discovered disease agents. Although human physicians are an important part of the network that should advise pet owners, they are often surprisingly ignorant about zoonoses which they consider to be rare and a relatively minor cause of illness in their human general practice.
The concept of the human-animal bond has a major importance in the modern society where pets are considered a part of the family and affection to them sometimes replaces relationships present in the traditional family. The interaction between humans and pets has a particular significance in special populations such as chronically-ill hospitalized patients, disabled people or prisoners. Contact of animals with people that have special needs could be very meaningful and rewarding to these individuals. But, particular consideration for zoonotic transmission of disease is often needed when relating to humans with immunosuppression or special susceptibility to infection. These individuals include HIV+ patients, cancer patients undergoing chemotherapy or radiation therapy, transplantation recipients undergoing immunosuppression to prevent organ rejection, patients with severe allergic or autoimmune diseases receiving immunosuppressive therapy, children or adults with congenital immunodeficiencies and more. Veterinary advice is often sought under these circumstances and practitioners should be prepared to provide helpful information on the risk of pet ownership to their clients. It should be borne in mind that having pets is of extreme importance to some immunosuppressed patients and therefore the benefits of having an animal should be weighed against the relative risk of acquiring infection from it. Normally, people do not need to be advised to part with their pets but rather to take precautions related to ectoparasite control, personal hygiene, feeding, vaccination and disease testing.
Fortunately, of the roughly 250 zoonotic organisms reported to be naturally transmitted to people, only about 40 are transmitted from dogs and cats. However, more pathogens are transmitted from pet birds, exotic reptiles and mammals, farm animals and wildlife animals.
The spectrum of zoonotic organisms is very wide and includes all classes of pathogens from viruses, through bacteria, algae, fungi, and protozoa to helminths. Furthermore, the transmission routes of these organisms are diverse and include spread by bites and through saliva, scratch or skin contact, aerogenic spread, urogenital dispersal, and vector-borne infections. The latter infections are commonly transmitted by pet ectoparasites such as ticks, fleas, flies, sand flies and mosquitoes.
Toxoplasmosis is a major protozoal zoonosis that threatens humans in particular HIV+ people and pregnant women. Toxoplasma gondii may infect any warm blooded animal with the cat being the definitive host. It can infect most cells types with a predilection to the central nervous system. Shedding of T. gondii oocysts in cat feces is usually restricted to a short period of time (7-10 days) during which large numbers of oocysts can be shed. Oocysts become infective only after sporulation which typically occurs 1-5 days after infected feces is excreted depending on the humidity and temperature conditions. However, infection of humans by meat containing cysts and not by ingestion of fecal oocysts appears to be the major route of infection to people. No association was found between cat ownership and T. gondii seroprevalence in a group of HIV+ people. Toxoplasmosis can be avoided by preventing the ingestion of sporulated oocysts and eating well cooked meat.
Bartonella henselae is a bacterium which is the major cause of cat scratch disease, bacillary angiomatosis, and peliosis hepatis. The two latter disorders are abnormal proliferation of small blood vessels resembling tumors which are common in AIDS patients, whereas cat scratch disease is mainly found in people with an intact immune system. Cats are mostly sub-clinical carriers of infection and fleas are responsible for transmission between cats. The prevalence of infection in cats is related to the extent of cat exposure to fleas and may reach more than 50% of the feline population in warm and humid regions where fleas are highly abundant. Humans appear to be infected by seeding of flea feces containing bacteria into the skin during cat scratch and possibly also during bites. Prevention of this infection in cat owners is especially important for HIV+ owners. It consists of flea control, clipping of cat claws, antiseptic cleaning of scratch and bite sites immediately after the event, screening and avoiding giving cats that are positive for B. henselae by PCR to HIV+ patients.
Rabies is still a very important lethal viral infection of warm blooded animals. Although it is not present or virtually eradicated in some countries, it is still prevalent in most countries in the world and responsible for the death of an estimated 55,000 humans per year worldwide. Vaccination of pets is compulsory in many countries and it is the cornerstone of the control of this disease. Campaigns to reduce and eradicate wild life rabies by distribution of oral vaccination have been successful in some countries. In addition, systems for quarantine of animals suspected of infection, routine post-mortem testing of animals for rabies, and post exposure preventative treatment to humans suspected of exposure to infection, are operated in many countries in which this disease is endemic.
Giardiasis is an enteric disease caused by the protozoa Giardia duodenalis in people. There is a large strain variation in giardiasis and pets can be infected with strains that are infectious to humans or species-specific strains that do not infect people. Giardia assemblages A and B are shared by humans and dogs. Assemblage A is shared by cats and humans and there are more specific assemblages infect animals but not humans such as assemblages C and D which infect dogs and assemblage F that infects cats. Waterborne outbreaks of giardiasis are mostly associated with infection related to Giardia from wild life and livestock. In contrast, infection from pets is usually associated with direct fecal contamination. Prevention consists of avoiding hand to mouth contact while handling animals, litter box hygiene and handling feces with gloves.
Q fever is caused by the gram-negative coccobacillus bacteria Coxiella burnetii. It is mostly a hazard in people who are in contact with domestic ruminants but is also sporadically reported in urban settings from people who have been in contact with infected cats or dogs. Cats can be infected by ticks which carry these bacteria, by ingestion of contaminated tissues, or by aerosol in a contaminated environment. Parturient cats are an important source for human infection and people become infected by aerosols from placental tissues. Pneumonia is the major clinical manifestation of Q fever in people.
Helminth parasites pose a threat to human health and it is the duty of the attending veterinarian to emphasize the potential zoonotic risks of these infections to pet owners and to offer preventative anthelminthic treatment. Early and repeated deworming of puppies and kittens is necessary for preventing and controlling these potentially zoonotic infestations. Ascarid worms can be transmitted transplacentally or transmammarily and puppies or kittens may be born infected or acquire infection very early in their lives. Humans become infected with animal ascarids when they accidentally ingest larvated eggs. Larvae hatch in the intestine and can migrate to visceral organs, the eye or the central nervous system resulting in larval migrans with symptoms depending on the effected tissues. Hookworms whose eggs are shed in animal feces are infectious to humans by accidental ingestion of infective larvae, or more commonly by skin penetration. Infective larvae that penetrate the epidermis cause cutaneous larval migrans. The tapeworms Echinococcus granulosus and E. multilocularis cause hydatidosis with large hydatic cysts in the internal organs of humans which necessitates surgical removal if resectable.
There is a long list of additional pet-related zoonoses that threaten human health. Pets may be directly related to the transmission of some infectious agents or in other cases merely sentinels for the presence of infection. Some additional zoonoses include tularemia, plague, salmonellosis, borreliosis (Lyme disease), leptospirosis, anaplasmosis, tuberculosis, cryptosporidiosis, blastomycosis and sporotrichosis.
To summarize, transmission of infection from pets to humans can be avoided by prevention of animal infection, vaccination, routine deworming, extermination of ectoparasites and strict observation of personal hygiene. The veterinarian has an important role in informing, educating and guiding pet owners to exercise these measures. The risk of disease transmission from pets to AIDS patients and other immunosuppressed individuals can be greatly reduced if guidelines for prevention of disease are followed. Thus, the mental benefit of having a pet in such conditions frequently greatly surpasses the risk of acquiring a contagious disease from it.
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