Rapid communicationConfirmation that the dog is a definitive host for Neospora caninum
Introduction
Neospora caninum is an important cause of abortion in dairy cattle (Dubey and Lindsay, 1996). It had been postulated ever since its discovery that N. caninum had a coccidial life cycle that was similar to T. gondii (Dubey et al., 1988a, Dubey et al., 1988b, Dubey and Lindsay, 1996), but only recently have dogs been shown to excrete oocysts and be a definitive host (McAllister et al., 1998). The earliest that oocysts were seen in dog feces was 8 days after infection and they were excreted for 7–19 days. Oocysts extracted from the feces caused fatal infections in γ-interferon knockout (γ-INF-KO) mice, and transmission electron microscopy and immunohistochemistry demonstrated that infection with N. caninum had been established.
The effect of dog age, breed, and immune status on oocyst excretion is not presently known. Similarly, although the role of N. caninum oocysts in the epidemiology of neosporosis is unknown, it is likely that the oocyst plays an important role in the transmission of the parasite to cattle. Because of this, we conducted the present study to confirm and extend the findings of McAllister et al. (1998). Additionally, we report new findings on the biology of N. caninum oocysts.
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Inoculation and examination of dogs
Two mixed-breed littermate dogs (Dog 1, female; Dog 2, male) about 14 weeks of age were used. Before use, both dogs were negative (<1 : 25 titer) for antibodies to N. caninum as determined by the indirect fluorescent antibody test (IFAT) (Cole et al., 1995). Both dogs were vaccinated against the common pathogens of dogs (distemper, adenovirus type-2, parainfluenza, parvovirus, and Leptospira) and were treated with fenbendazole at 50 mg/kg for 3 days to remove nematodes. Tapeworm segments of
Infections in dogs
Dog 1 began excreting N. caninum oocysts 5 days PF and Dog 2 dog began excreting oocysts 6 days PF. Oocysts were detected in the feces of Dog 1 on Days 5 through 10 and on Day 17 PF. Feces from this dog were collected and processed to sporulate and isolate oocysts. Dog 2 had detectable oocysts only on Day 6 and Day 9 PF. Its feces was not used for oocyst collection because of the few oocysts present. Neither dog had diarrhea. Both dogs had normal appetites and were active throughout the period
Discussion
Our study confirms that the dog is a definitive host for N. caninum and extends our knowledge about the behavior of this parasite in dogs fed tissue cysts. Our study demonstrated that dogs excrete oocysts following a prepatent period of 5 days. This is 3 days shorter than the previous report (McAllister et al., 1998). The lack of seroconversion in Dog 2 following excretion of oocysts is interesting and has been demonstrated previously with this isolate of N. caninum (McAllister et al., 1998).
Acknowledgements
The authors thank Susan King, Catherine Walsh, and Kay Carlson for technical assistance.
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