Department of Pathology, College of Veterinary Medicine, University of Georgia, Athens, GA, USA.
Metzger Animal Hospital, State College, PA, USA.
Parasit Vectors. 2019 Mar 19;12(1):115. doi: 10.1186/s13071-019-3347-5.
Cytauxzoon felis is a tick-transmitted apicomplexan that causes cytauxzoonosis in domestic cats (Felis catus). Even with intensive care, the mortality rate of acute cytauxzoonosis approaches 40% in domestic cats, while bobcats (Lynx rufus), the natural intermediate host of C. felis, remain clinically asymptomatic. However, multiple reports of domestic cats surviving acute disease without any treatment exist. One hypothesis for survival of these cats is infection with unique C. felis genotypes of lower pathogenicity. Prior studies have identified genetically distinct C. felis isolates containing polymorphisms within internal transcribed spacer regions (ITS) of the rRNA operon. However, these polymorphisms do not correlate with the clinical outcome of cytauxzoonosis, and so additional genetic markers are needed to test this hypothesis. We selected C. felis apical membrane antigen-1 (ama1) as a potential genetic marker of differential pathogenicity. AMA1 is a vaccine candidate for relatives of C. felis within Plasmodium spp.; however its historically high level of genetic polymorphism has resulted in escape from vaccine-induced immunity. While such diversity has hindered vaccine development, the expected polymorphism within the ama1 gene may be useful to evaluate population genetics.
A 677 bp sequence of the C. felis ama1 gene was PCR-amplified from 84 domestic cats and 9 bobcats and demonstrated 99.9% sequence identity across all samples. A single nucleotide polymorphism (SNP) was identified in domestic cats and bobcats with evidence for co-infection with both genotypes identified in two domestic cats. The prevalence of the two genotypes varied with geographical distribution in domestic cats. Nucleotide diversity (π) and haplotype diversity (H) were calculated for C. felis ama1 and ama1 of related apicomplexans to assess genetic diversity. Based on these values (π = 0.00067 and H = 0.457), the diversity of the C. felis ama1 gene region analyzed is considerably lower than what is documented in related apicomplexans.
In surprising contrast to related apicomplexans, our results support that the sequence of the C. felis ama1 gene is highly conserved. While lack of genetic diversity limits utility of C. felis AMA1 as a genetic marker for clinical outcome, it supports further investigation as a vaccine candidate for cytauxzoonosis.
猫泰勒虫是一种通过蜱传播的顶复门原虫，可引起家猫（Felis catus）的泰勒虫病。即使进行强化护理，急性泰勒虫病在家猫中的死亡率也接近 40%，而作为猫泰勒虫的天然中间宿主的美洲狮（Lynx rufus）则仍然没有临床症状。然而，有多个报道称家猫在没有任何治疗的情况下存活下来。这些猫存活的一个假设是感染了致病性较低的独特猫泰勒虫基因型。先前的研究已经确定了具有核糖体 RNA 操纵子内部转录间隔区（ITS）内多态性的遗传上不同的猫泰勒虫分离株。然而，这些多态性与泰勒虫病的临床结果无关，因此需要额外的遗传标记来检验这一假设。我们选择猫泰勒虫顶膜抗原-1（ama1）作为差异致病性的潜在遗传标记。AMA1 是泰勒虫属内疟原虫的候选疫苗；然而，其历史上高水平的遗传多态性导致了对疫苗诱导免疫的逃逸。尽管这种多样性阻碍了疫苗的开发，但预期在 ama1 基因内的多态性可能有助于评估种群遗传学。
从 84 只家猫和 9 只美洲狮中 PCR 扩增了猫泰勒虫 ama1 基因的 677 bp 序列，所有样本的序列同一性均为 99.9%。在家猫和美洲狮中发现了一个单核苷酸多态性（SNP），并在两只家猫中发现了两种基因型的合并感染证据。两种基因型在家猫中的流行程度随地理位置的不同而不同。核苷酸多样性（π）和单倍型多样性（H）用于评估遗传多样性，计算了猫泰勒虫 ama1 和相关顶复门生物的 ama1。基于这些值（π=0.00067 和 H=0.457），分析的猫泰勒虫 ama1 基因区域的多样性明显低于相关顶复门生物。
与相关顶复门生物形成惊人对比的是，我们的结果支持猫泰勒虫 ama1 基因的序列高度保守。虽然遗传多样性的缺乏限制了猫泰勒虫 AMA1 作为临床结果遗传标记的用途，但它支持进一步作为泰勒虫病的候选疫苗进行研究。
