There are multiple infectious agents of the gastrointestinal tract that can be shared between cats and humans. Prevalence rates recently reported in two studies in cats of the United States were 13.1% (Hill et al, 2000) and 40.7% (Spain et al, 2001), respectively. Prevalence rates for most infectious agents were generally greater in cats with diarrhea. These findings emphasize that diagnostic workups for enteric infections are indicated due to potential human health risks. The AAFP recommends that the minimal diagnostic plan to assess for enteric zoonoses in cats with diarrhea include a fecal flotation, Cryptosporidium spp. screening procedure, fecal wet mount, and rectal cytology. Fecal culture should be considered if Salmonella spp. or Campylobacter spp. are on the list of differential diagnoses.

Of the most common enteric zoonoses, Toxocara cati (visceral larva migrans), and Ancylostoma tubaeforme, Uncinaria stenocephala (cutaneous larva migrans) require a period of time outside the host and so are not common direct zoonoses. Most people are infected by contact with the organisms in the environment. Areas such as children's sandboxes may be commonly contaminated with these organisms because of the defecation habits of cats. Prevention of hookworm and roundworm infection is achieved by control of animal excrement in human environments. All kittens should have a fecal flotation performed and should be routinely treated with an anthelmintic such as pyrantel pamoate at least three times. Some administer the drugs at 21 days apart during their initial vaccination period. In heavily infected kittens, deworming with pyrantel can begin at 3 weeks of age and repeated at 2 week intervals (www.cdc.gov/ncidod/dpd/parasites/ascaris/prevention.htm). The queen should be dewormed as well as patent infections may exist. Feces of adult cats should be periodically screened for roundworms and hookworms and anthelmintics should be administered periodically. Administration of heartworm preventatives that also control or eliminate hookworms and roundworms are indicated year-round in areas with a high prevalence.

Cats are the only known definitive host of Toxoplasma gondii that complete the enteroepithelial cycle (sexual phase) that results in the passage of environmentally resistant unsporulated oocysts in feces once for about 7-10 days of their lives. Oocyst sporulation occurs in 1-5 days in the presence of oxygen; sporulated oocysts are infectious to most warm-blooded vertebrates. Infection by T gondii occurs after ingesting sporulated oocysts, after ingesting tissue cysts, or transplacentally. Transplacental infection of humans and cats usually only occurs if the mother is infected for the first time during gestation. Because of the short shedding period and the oocyst sporulation period, T. gondii is not a common direct zoonosis. There was no association between cat ownership and T gondii seroprevalence in a group of HIV-infected humans (Wallace et al, 1993). Veterinary health care providers do not have increased incidence of toxoplasmosis when compared to the general population. Thus, cats do not need to be removed from households with immunodeficient or pregnant humans due to the risk of acquiring toxoplasmosis. Toxoplasma gondii infection can be avoided by avoiding the ingestion of sporulated oocysts in old feline feces and avoiding ingestion of tissue cysts in undercooked meats.

Dipylidium caninum, Echinococcus granulosa, and Echinococcus multilocularis are cestodes that can infect humans. Wild carnivores are more common definitive hosts of Echinococcus spp. and shed infective eggs into the environment. Echinococcus multilocularis can be transmitted in feces of dogs or cats after ingestion of infected vole. Transmission to humans occurs following ingestion of the intermediate host (flea, Dipylidium) or by the ingestion of eggs (Echinococcus spp.).

Cryptosporidium spp. inhabit the respiratory and intestinal epithelium of many vertebrates including birds, mammals, reptiles, and fish. Once thought to be commensal agents, Cryptosporidium spp. are now known to cause gastrointestinal tract disease in a number of mammalian species including rodents, dogs, cats, calves, and humans. It is now apparent that there are multiple strains of Cryptosporidium spp. including C. parvum, C. hominis, C. felis and C. canis. While some isolates infect multiple species, others have a limited host range. However, strains that infect both pets and people cannot be differentiated from those that only infect pets by light microscopy and so all Cryptosporidium spp. should be considered potentially zoonotic. The prevalence of Cryptosporidium spp. oocysts or antigens in dog and cat feces approximates that of Giardia leading to the recommendation that all cats with diarrhea be assessed for this infection. Person-to-person contact with oocysts by fecal-oral contamination or by ingesting contaminated water are the most likely routes of exposure. In one study, cat ownership was not statistically associated with cryptosporidiosis in HIV-infected people (Glaser et al, 1999).

Giardia spp. (flagellate) and Entamoeba histolytica (amoeba) are enteric protozoans that can be transmitted to humans by contact with feces; the cysts do not require an incubation period to become infectious. Giardia spp. infection of cats is common and the organism can be detected in feces of normal cats or those with small bowel diarrhea (and occasionally mixed bowel diarrhea in cats). Because the organism is immediately infectious when passed as cysts in stool, there is potential for direct zoonotic transfer. Based on genetic studies, it is now known that there are multiple Giardia spp. (Thompson et al, 2000). Assemblage A has been found in infected humans and many other mammals including dogs and cats. Assemblage B has been found in infected humans and dogs, but not cats. It also appears that there is a specific genotype of Giardia that is specific to cats. However, as for Cryptosporidium, since it is impossible to determine zoonotic strains of Giardia spp. by microscopic examination, it seems prudent to assume feces from all dogs and cats infected with Giardia spp. are a potential human health risk. Giardia vaccines for SQ administration are now available for cats. Vaccination against Giardia could be considered in cats with recurrent infection and has been evaluated as a therapeutic agent with variable results (Olson et al, 2000; Stein et al, 2003). It is unknown whether treated cats are cured and it is likely that if a treated cat is exposed again it will be reinfected.

Salmonella spp., Campylobacter spp., E. coli, Yersinia enterocolitica, and Helicobacter spp. each infect cats and can cause disease in humans. While Helicobacter pylori was isolated from a colony of cats, it is unlikely cats are a common source of Helicobacter infection for people (Simpson et al, 2000). In 2 recent enteric zoonoses prevalence studies, Salmonella spp. and Campylobacter spp. infections were < 2% in cats. Prevalence of Salmonella and Campylobacter infections is greater in young animals housed in unsanitary or crowded environments. Fecal culture should be considered in cats with fever, neutrophils on rectal cytology, or at high risk of exposure to the organisms.

Approximately 300,000 emergency room visits per year are made by people bitten by animals in the United States (Talan et al, 1999). Approximately 28% to 80% of cat bites become infected and severe sequelae including meningitis, endocarditis, septic arthritis, osteoarthritis, and septic shock can occur. Immunodeficient humans or humans exposed to Pasteurella spp., Capnocytophaga canimorsus (DF-2), or Capnocytophaga cynodegmi more consistently develop systemic clinical illness. Splenectomized humans are at increased risk of developing bacteremia. Mycoplasma infections have also rarely occurred from contact with cats.

Bartonella henselae is the most common cause of cat scratch disease as well as bacillary angiomatosis, and bacillary peliosis, common disorders in humans with AIDS (Table 6). Cats can also be infected with B clarridgeiae, B. koehlerae, and B. weissii (Breitschwerdt and Kordick, 2000). Bartonella henselae has been isolated from the blood of subclinically ill, seropositive cats and also from some cats with a variety of clinical manifestations like fever, lethargy, lymphadenopathy, uveitis, gingivitis, and neurologic diseases. Seroprevalence in cats varies by region but as many as 54.6%-81% of cats in some geographical areas of the United States are Bartonella spp. seropositive (Jameson et al, 1995). The organism is transmitted between cats by fleas and so prevalence is greatest in cats from states where fleas are common (Chomel et al, 1996; Foley et al, 1998). Transmission to humans commonly occurs after cat bites or scratches; the disease appears to be transmitted most commonly from kittens. Blood culture, blood PCR, and serologic testing can be used to determine risk of individual cats but each test has false-positive and false-negative results. Thus, testing healthy cats for Bartonella spp. infection is not currently recommended. Testing should be reserved for cats with suspected clinical bartonellosis. Administration of doxycycline, tetracycline, erythromycin, amoxicillin-clavulanate, azithromycin, or enrofloxacin can limit bacteremia but does not cure infection in all cats and has not been shown to lessen the risk of cat scratch disease. Thus, antibiotic treatment of healthy bacteremic cats is controversial. Treatment should be reserved for cats with suspected clinical bartonellosis. Bartonella spp. infection is more common in flea-infested cats from catteries (Foley et al, 2000). Bartonella henselae replicates in fleas and can survive in flea feces for days (Finkelstein et al, 2002; Higgins et al, 1996). Thus, it is possible that flea feces contaminates our wounds, resulting in infection. Strict flea control should be maintained. Kittens should be avoided by immunodeficient people. Cat claws should be kept clipped and cats should never be teased. Cat-induced wounds should immediately be cleansed and medical advice sought.

Yersinia pestis, a gram-negative coccobacillus found in some areas of the world. Humans are most commonly infected by rodent flea bites, but there have been many documented cases of transmission by exposure to wild animals and infected domestic cats. In the United States from 1977 to 1998, 23 cases of human plague (7.7% of the total cases) resulted from contact with infected cats (Gage et al, 2000). Francisella tularensis is found is some areas and is transmitted by Dermacentor variabilis (American dog tick), D. andersoni (American wood tick), and Amblyomma americanum (Lone Star tick) are known vectors. Human tularemia occurs most commonly following exposure to ticks and less commonly from contact with infected animals. There have been at least 51 cases of human tularemia resulting from contact with infected cats (Capelin and Fong, 1993). Cats are infected most frequently by tick bites or by ingesting infected rabbits or rodents. Most cases of feline tularemia have been documented in the mid-Western states, particularly Oklahoma.

Of the many fungal agents that infect both humans and animal, only Sporothrix schenckii and the dermatophytes have been shown to infect humans upon direct exposure. Histoplasma, Blastomyces, Coccidioides, Aspergillus, and Cryptococcus infections of humans and animals can occur in the same household, but infection of humans generally results from a common environmental exposure rather than by direct contact with an infected animal. Sporothrix is cosmopolitan in distribution and the soil is thought to be the natural reservoir. Infection of cats and humans usually occurs after the organism contaminates broken skin. Cats are thought to be infected by scratches from contaminated claws of other cats; infection is most common in outdoor males. Humans can be infected by contaminating cutaneous wounds with exudates from infected cats (Dunston et al, 1986).