Page 1566 - Small Animal Internal Medicine, 6th Edition

1538 PART XIV Infectious Diseases

owner of a dog or cat with Cryptosporidium spp. infection minimal associations including one study of HIV-infected
would like to know whether the pet is carrying a pet species individuals (Wallace et al., 1993). In addition, one study of
VetBooks.ir or zoonotic species, genotyping can be performed commer- veterinary staff members who worked frequently with cats
had low seroprevalence rates suggesting exposure to cats
cially in the United States (www.dlab.colostate.edu).
Toxoplasma gondii is a ubiquitous coccidian with world-
(Shuhaiber et al., 2003). Cats infected by T. gondii usually
wide distribution (Lappin et al., 2010). Most seroprevalence did not increase their risk for acquiring T. gondii infection
studies performed in the United States suggest that at least do not shed or shed lower numbers of oocysts on second-
30% of cats and humans have previously been exposed. Cats ary exposures (Zulpo et al., 2018) and did not have repeat
are the only known definitive host of the organism, com- oocyst shedding after being administered cyclosporine in
pleting the enteroepithelial cycle (sexual phase) that results a recent study (Lappin et al., 2015). Cats do not need to
in the passage of environmentally resistant unsporulated be removed from households with immunodeficient people
oocysts in feces. Oocyst sporulation occurs in 1 to 5 days in or pregnant women because of the risk for acquiring toxo-
the presence of oxygen; sporulated oocysts are infectious to plasmosis (www.cdc.gov/parasites/toxoplasmosis/gen_info/
most warm-blooded vertebrates (see Fig. 99.1). After infec- pregnant.html). Prevention of T. gondii infection is sum-
tion by T. gondii, an extraintestinal phase that ultimately marized in Box 98.1.
leads to the formation of tissue cysts containing the organ-
ism develops. Infection by T. gondii occurs after ingestion of FLAGELLATES, AMOEBA, AND CILIATES
sporulated oocysts, after ingestion of tissue cysts, or trans- Giardia spp. (flagellate), Entamoeba histolytica (amoeba),
placentally. Transplacental infection of humans and cats and Balantidium coli (ciliate) are enteric protozoans that can
usually occurs only if the mother is infected for the first be transmitted to humans by contact with feces; the cysts do
time during gestation. not require an incubation period to become infectious. Ent-
In dogs and cats, clinical disease from T. gondii infec- amoeba histolytica infection is extremely rare in dogs and
tion occurs occasionally and is manifested most commonly cats; B. coli infection is rare in dogs and has not been reported
by fever, uveitis, pulmonary disease, hepatic disease, and in cats.
CNS disease (see Chapter 98). Infected immunocompetent Giardia spp. infection of dogs and cats is common and
humans are generally asymptomatic; self-limiting fever, can be detected in feces of normal dogs and cats, and in those
lymphadenopathy, and malaise occur occasionally. Transpla- with small-bowel diarrhea (and occasionally mixed-bowel
cental infection of humans results in clinical manifestations, diarrhea in cats). Clinical signs of disease are generally more
including stillbirth, hydrocephalus, hepatosplenomegaly, severe in immunodeficient individuals. Because the organism
and retinochoroiditis. Presence of T. gondii antibodies has is immediately infectious when passed as cysts in stool, direct
been associated with presence of behavioral abnormalities zoonotic transfer is possible. Genetic studies have detected
in people, but a direct cause and effect has not been estab- multiple Giardia spp., and most dogs and cats are infected
lished (Fuglewicz et al., 2017). Chronic tissue infection in with the host-adapted assemblages C, D, and F (Scorza et al.,
humans can be reactivated by immunosuppression, leading 2012). However, as is the case with Cryptosporidium, because
to dissemination and severe clinical illness; this has been determining zoonotic strains of Giardia spp. by microscopic
commonly associated with drug-induced immunosuppres- examination is not possible, assume that feces from all dogs
sion and AIDS. Approximately 10% of humans with AIDS and cats infected with Giardia spp. are a potential human
develop toxoplasmic encephalitis. health risk. Fecal examination should be performed on all
Toxoplasma gondii oocysts are most effectively demon- dogs and cats at least yearly, and treatment with drugs with
strated in cat feces after sugar or other centrifugation tech- anti-Giardia activity, such as fenbendazole, metronidazole,
niques. Clinical toxoplasmosis is difficult to diagnose in or febantel/praziquantel/pyrantel, should be administered if
humans, dogs, and cats but usually involves the combina- indicated (see Chapter 28). Febantel/praziquantel/pyrantel
tion of clinical signs, serologic test results, organism demon- is now labeled for the treatment of giardiasis in dogs in
stration techniques, and response to anti-Toxoplasma drugs some countries. Centrifugation techniques (zinc sulfate or
(Lappin, 2010; see Chapter 98). sugar) are considered by most parasitologists to be optimal
Although T. gondii is recognized as one of the most for demonstration of cysts. If fresh stool is available from
common zoonoses, humans are usually not infected by dogs or cats with diarrhea, examination of a wet mount
direct contact with cats. The oocyst-shedding period usually to detect the motile trophozoites may improve sensitivity
lasts several days to several weeks (≈7-10 days if the cat was (see also Chapter 91). Monoclonal antibody-based immu-
infected by tissue cyst ingestion). Because oocysts have to nofluorescent antibody tests, fecal antigen tests, and PCR
sporulate to be infectious, contact with fresh feces cannot assays are available but should be used in addition to, not in
cause infection. Cats are quite fastidious and usually do not lieu of, fecal flotation, which can also reveal other parasites.
allow feces to remain on their skin for periods long enough The combination of a fecal centrifugation technique with a
to lead to oocyst sporulation. Oocysts were not isolated from Giardia antigen test has approximately 97% sensitivity. Fecal
the fur of cats 7 days after completion of the oocyst-shedding PCR assay results can be less sensitive due to presence of
period in one study. Most studies evaluating cat owner- fecal PCR inhibitors (Hascall et al., 2016). The Compan-
ship as a risk factor for human toxoplasmosis has showed ion Animal Parasite Council does not recommend treating