Norval R A, Donachie P L, Meltzer M I, Deem S L, Mahan S M
University of Florida/USAID/SADC Heartwater Research Project, Veterinary Research Laboratory, Causeway, Zimbabwe.
Vet Parasitol. 1995 Jul;58(4):335-52. doi: 10.1016/0304-4017(94)00733-s.
The occurrence of endemic stability for heartwater (Cowdria ruminantium infection) is a controversial issue, because the means by which young cattle and other neonatal ruminants become infected and acquire immunity to the disease have never been adequately explained. We conducted a study in a heartwater-endemic area in southeastern Zimbabwe to investigate the relationships between calf immunity to C. ruminatium, infestations of the tick vector Amblyomma hebraeum and dam, colostral and calf antibody titres to C. ruminantium. Two groups of cows (tick-infested and acaricide-treated) were artificially inseminated and then monitored through pregnancy and lactation by means of tick counts and serum antibody titres. The calves born to the cows in each treatment group were similarly monitored from birth until after weaning, when they were challenged with a heartwater stabilate (Ball-3 vaccine). Colostrum was collected from the cows on the days that the calves were born. Serum and colostrum samples were screened for Cowdria-specific antibodies using an indirect fluorescent antibody test. The cows and calves in the acaricide-treated group remained essentially tick-free for the duration of the study. The cows in the tick-infested group were continuously infested with A. hebraeum, but their calves only became infested between 12 and 31 weeks after birth. Cowdria-specific antibodies were detected in the sera of the cows and calves and in the colostrum of both treatment groups, but the titres were consistently higher in the tick-infested group than in the acaricide-treated group. Antibodies, probably of maternal origin since the precolostral serum titres were negative, were present in the calves of both groups for at least 8 weeks after birth. Between Weeks 24 and 52 postpartum, the antibody titres of the tick-infested group of calves were significantly correlated with the numbers of A. hebraeum present on the animals. The antibody titres of the calves in the acaricide-treated group increased considerably following challenge with heartwater stabilate, 60 weeks after birth. No clinical heartwater was detected in either group of calves following inoculation of stabilate, and it was concluded that the calves in both groups were immune to the disease. In the acaricide-treated group, immunity did not correlate with exposure to tick-transmitted infections. Hence, we concluded that the calves in both groups had probably been infected by vertical transmission around the time of birth. Such early infections are likely to have been controlled by maternal antibodies or by an unidentified immune mechanism.(ABSTRACT TRUNCATED AT 400 WORDS)
心水病（反刍兽考德里氏体感染）地方流行性稳定状态的出现是一个有争议的问题，因为从未充分解释过幼牛和其他新生反刍动物感染并获得该病免疫力的途径。我们在津巴布韦东南部的心水病流行区进行了一项研究，以调查犊牛对反刍兽考德里氏体的免疫力、蜱传播媒介希伯来花蜱的感染情况以及母牛、初乳和犊牛针对反刍兽考德里氏体的抗体滴度之间的关系。两组母牛（感染蜱和用杀螨剂处理过的）进行人工授精，然后在整个怀孕和哺乳期通过蜱计数和血清抗体滴度进行监测。每个处理组的母牛所生的犊牛从出生到断奶后都进行类似的监测，断奶后用一种心水病稳定株（Ball - 3疫苗）进行攻毒。在犊牛出生当天采集母牛的初乳。使用间接荧光抗体试验对血清和初乳样本进行考德里氏体特异性抗体筛查。在研究期间，用杀螨剂处理过的组中的母牛和犊牛基本无蜱。感染蜱的组中的母牛持续感染希伯来花蜱，但其犊牛仅在出生后12至31周之间感染蜱。在两个处理组的母牛和犊牛血清以及初乳中均检测到考德里氏体特异性抗体，但感染蜱组的抗体滴度始终高于用杀螨剂处理过的组。由于初乳前血清滴度为阴性，两个组的犊牛中都存在可能源自母体的抗体，且在出生后至少8周内都有。产后24至52周期间，感染蜱组的犊牛抗体滴度与动物身上存在的希伯来花蜱数量显著相关。用杀螨剂处理过的组中的犊牛在出生60周后用心水病稳定株攻毒后，抗体滴度大幅上升。接种稳定株后，两组犊牛均未检测到临床心水病，得出结论两组犊牛对该病均具有免疫力。在用杀螨剂处理过的组中，免疫力与蜱传播感染的暴露情况无关。因此，我们得出结论，两组犊牛可能在出生前后通过垂直传播而感染。这种早期感染可能已被母体抗体或一种未知的免疫机制所控制。（摘要截选至400字）
