UK DIAGNOSTIC LAB NEWS (CONT.)
Interpretation Due to the histologic changes within the brain, in conjunction with the identification of Neospora DNA within the brain and positive maternal serol- ogy, abortion was attributed to Neospora caninum. Due to the presence of low titers to multiple sero- vars, the positive serological results for Leptospira most likely reflect vaccination or exposure to carrier animals.
Further Information Serum from four additional cows from the same herd was submitted. All tested positive for Neospora caninum ranging from 56.6% to 74.6% inhibition (strongly positive). Additionally, the referring veter- inarian indicated that a bitch had recently whelped in the same barn where the milking herd was kept.
Discussion Neospora caninum is apicomplexan parasite with a lifecycle similar to that of Toxoplasma gondii with canids, rather than felids, as definitive hosts and ru- minants as the most common intermediate hosts.4 In cattle, transplacental infection, so-called endoge- nous infection, is the most important route of infec- tion for calves born on the farm.5 In newly infected adult cattle, disease caused by acute infection is typ- ically subclinical; however, experimentally infected cows develop mild fever and weight loss.1 Te exact pathogenesis of abortion due to Neospora caninum is poorly elucidated; however, the timing of infection is critical to determine the fate of the fetus. Infection during the first and second terms of gestation, pri- or to development of a competent fetal immune re- sponse, is more likely to result in fetal death.1 Calves that are not aborted are likely to be infected, and or- ganisms can be detected in the lung, liver, brain, and spinal cord.1 Typically, aborted fetuses are expelled in a state of moderate autolysis; however, delivery of fresh or mummified fetuses are also possible. Necro- suppurative placentitis is nearly a constant feature regardless of abortion status of the fetus.1
Fetuses are typically devoid of gross changes; howev- er, gross changes within the central nervous system, such as hydrocephalus and cerebellar hypoplasia are occasionally present present.5 Although the organ- ism is present and can be identified in multiple tis- sues via PCR or immunohistochemistry, microscop- ic lesions are often only seen in the central nervous system and consist of multifocal neuropil necrosis with variable meningoencephalitis. In one study of naturally infected animals, myocarditis was also commonly identified.3
10 KVMA News
Projects to develop safe and effective vaccines and treatments for Neospora caninum are ongoing; how- ever, to date, none have been successful in demon- strating efficacy. Animals that are infected are con- sidered infected for life and, at least in one study, seropositive cattle were five-fold more likely to have recurring abortion than seronegative animals.8
Although difficult, control of the parasite and pre- vention of new infections is of paramount impor- tance. Te most effective means to prevent endoge- nous infections is through test and cull protocols or by retaining only those heifers born to seronegative dams; however, due to the difficulty, many producers learn to deal with the decreased productivity rather than developing eradication programs.9 Initial work with embryo transfer protocols showed promise to prevent transplacental infection of calves born
to
cows with high genetic value; however, recent work has demonstrated that Neospora caninum shows tropism for ovarian follicular fluid. Terefore, these advanced reproductive techniques may not be as ef- fective as initially hoped.10
Te presence of dogs on the farm was identified as an important risk factor for seropositivity in cattle.6 Ad- ditionally, puppies have been demonstrated to shed more oocysts than adult dogs.7 For these reasons, controlling interactions between cattle and dogs may be an effective means of preventing new maternal in- fections. Te most important parts of these control measures might include managing pastures, facili- ties, and feedstuffs to minimize contamination with dog feces, preventing puppies from interacting with cattle, and discouraging wild canids from entering cattle facilities. Additionally, restricting dogs from consuming uncooked meat or placentas from cattle may decrease the number of new infections in dogs and thus the number of oocysts shed.9
General Diagnostic Considerations Diagnosis of abortions is notoriously difficult even with use of the most advanced techniques, a causative agent is identified in less than half of cases submit- ted.2 Tis difficulty stems from the lack of pathogno- monic lesions associated with many causes of abor- tion and the fact that lesions, when present, are often obscured by autolysis, especially in cases where the fetus has been dead in utero for prolonged periods of time. Nevertheless, because of the profound losses frequently associated with fetal loss and the potential for zoonotic infection, samples should be submitted to diagnostic laboratories for evaluation.
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