Page 1565 - Small Animal Internal Medicine, 6th Edition

CHAPTER 99 Zoonoses 1537

CESTODES
Dipylidium caninum, Echinococcus granulosa, and Echinococ-
VetBooks.ir cus multilocularis are cestodes of dogs or cats that can infect
humans. Wild carnivores are more common definitive hosts
of Echinococcus spp. and shed infective eggs into the envi-
ronment. Echinococcus granulosa eggs can be transmitted
in feces of dogs after ingestion of infected sheep or rabbit
tissues; Echinococcus multilocularis can be transmitted in
feces of dogs or cats after ingestion of an infected rodent.
Transmission to humans occurs after ingestion of the inter-
mediate host (flea, Dipylidium) or eggs (Echinococcus spp.).
Infection of dogs and cats with cestodes is generally subclini-
cal. Dipylidium infection is most common in children after
accidental ingestion of fleas and can lead to diarrhea and
pruritus ani. In humans, after ingestion of eggs, which are FIG 99.1
Cryptosporidium parvum and Toxoplasma gondii oocysts on
immediately infectious, Echinococcus enters the portal cir- a fecal flotation. The C. parvum oocysts are approximately
culation and spreads throughout the liver and other tissues. 4 × 5 µm, and the T. gondii oocysts are approximately 10
Echinococcus multilocularis is most common in the northern × 12 µm.
and central parts of North America but seems to be spread-
ing with the fox population (most common definitive host).
Prevention or control of cestodes is based on sanitation et al., 2010). In one study, cat or dog ownership was not
procedures and use of taeniacides. Praziquantel is labeled statistically associated with cryptosporidiosis in human
for the treatment of Echinococcus spp. Restricting hunting immunodeficiency virus (HIV)-infected people (Glaser
behavior of dogs and cats and feeding only processed or et al., 1998).
cooked foods should lessen potential exposure to Echino- Infection of dogs and cats by Cryptosporidium spp. is
coccus spp. Monthly administration of praziquantel should usually subclinical, but small-bowel diarrhea occurs in some
be considered in dogs and cats allowed to hunt in endemic cases. Immunosuppression may potentiate disease; several
areas. Flea control should be maintained to lessen risk of D. dogs and cats had concurrent FeLV infection, canine distem-
caninum infection. per virus infection, or intestinal lymphoma. Clinical crypto-
sporidiosis is characterized by small-bowel diarrhea and is
COCCIDIANS generally self-limiting in immunocompetent people, but
Cryptosporidium spp. inhabit the respiratory and intestinal fatal infection is common in those with AIDS.
epithelium of many vertebrates, including birds, mammals, The small size (approximately 4-6 µm in diameter) of
reptiles, and fish. Once thought to be a commensal, Crypto- Cryptosporidium spp. oocysts leads to difficulty in diagnosis.
sporidium spp. are now known to cause gastrointestinal tract Routine salt solution flotation and microscopic examina-
disease in several mammalian species, including rodents, tion at ×100 magnification commonly lead to false-negative
dogs, cats, calves, and humans. The organisms have an results. The combination of concentration techniques with
enteric life cycle similar to that of other coccidians that cul- fluorescent antibody staining or acid-fast staining appears
minates in the production of thin-walled, autoinfective to be more sensitive. Multiple enzyme-linked immunosor-
oocysts and thick-walled, environmentally resistant oocysts bent assays for the detection of C. parvum antigen in feces
that are passed in feces (Fig. 99.1). Oocysts (4-6 µm in diam- are commercially available but do not accurately detect C.
eter) are passed sporulated and are immediately infectious felis or C. canis. PCR assay is the most sensitive test to date
to other hosts. Multiple species of Cryptosporidium spp. and is now routinely available but not standardized among
exist, including Cryptosporidium parvum, Cryptosporidium laboratories.
hominis, Cryptosporidium felis, and Cryptosporidium canis. No drug has been shown to eliminate Cryptosporidium
Although some Cryptosporidium infect multiple animal spp. from the gastrointestinal tract. However, clinical signs
species, others have a limited host range. However, strains usually resolve when azithromycin is administered orally at
that infect both pets and people cannot be differentiated by 10 mg/kg q24h, tylosin is administered orally at 10 to 15 mg/
light microscopy from those that infect only pets, so all Cryp- kg q8h, or nitazoxanide is administered orally at 37.5-75 mg/
tosporidium spp. should be considered potentially zoonotic. kg twice, 14 days between doses. Optimal duration of therapy
Cryptosporidium parvum infection of humans after exposure is unknown; some cases have required administration of
to infected calves has been recognized for years, and it is now azithromycin for several weeks before clinical signs resolve.
known that most human infections are from C. hominus. The Avoiding exposure is the most effective prevention. Routine
most common Cryptosporidium spp. isolated from dogs and disinfectants require extremely long contact with the organ-
cats are the host-adapted C. canis and C. felis, respectively, ism to be effective. Drying, freeze thawing, and steam clean-
and so most consider pets with Cryptosporidium spp. infec- ing can inactivate the organism. Surface water collected in
tions to not be significant risks for humans (Lucio-Forster the field for drinking should be boiled or filtered. If the