Department of Clinical Sciences, Faculty of Veterinary Medicine, Utrecht University, Yalelaan 108, 3584 CM, Utrecht, The Netherlands.
Clinica Veterinaria Lago Maggiore, Corso Camillo Benso Cavour, 3, 28040, Dormelletto, NO, Italy.
Parasit Vectors. 2020 Oct 1;13(1):501. doi: 10.1186/s13071-020-04376-9.
Dirofilaria immitis is responsible for heartworm disease in dogs in endemic areas worldwide. Screening for this infection is done by blood tests. Antigen testing is the most sensitive method to detect an infection with adult (female) worms. Microscopic examination of a blood smear or Knott's test can be used to detect circulating microfilariae, the infective larvae. To increase the sensitivity of the antigen test by decreasing the false negative test results, heating of the blood sample has been recommended in recent guidelines. Heating is believed to remove blocking immune-complexes. Circulating microfilariae are not specific findings for heartworm infection, as other nematodes (among others, Acanthocheilonema dracunculoides) can also result in microfilaremia. Although the type of microfilariae cannot be determined by microscopy alone, real-time PCR can reliably identify the infecting nematode species. Correct identification of the parasite is of major importance, as an infection with D. immitis requires antiparasitic therapy, whereas A. dracunculoides is thought to be a clinically irrelevant coincidental finding. The present case report describes a microfilaremic dog where the initial antigen test for D. immitis turned positive after heat treatment, whereas real-time PCR revealed that the microfilariae were A. dracunculoides (syn. Dipetalonema dracunculoides).
A circa 5-year old, asymptomatic Spanish mastiff dog was referred for heartworm therapy because microfilariae were found via a screening blood test. The dog was recently imported to the Netherlands from Spain, where it had been a stray dog. Antigen tests on a plasma sample for D. immitis were performed with three different test kits, which all turned out to be negative. However, heat treatment of two of these samples were carried out and both of them led to a positive antigen test result. Real-time PCR showed that the circulating microfilariae belonged to A. dracunculoides species. Three administrations of moxidectin spot-on at monthly intervals resulted in a negative antigen and a negative Knott's tests one month after the last treatment.
We conclude that heat treatment of initially negative blood samples for D. immitis could lead to false positive antigen test results if the dog is infected with A. dracunculoides.
犬恶丝虫会导致世界范围内流行地区的犬类感染心丝虫病。通过血液检测对这种感染进行筛查。抗原检测是检测成年（雌性）虫感染的最敏感方法。通过血液涂片的显微镜检查或 Knott 试验可以检测到循环微丝蚴，即感染性幼虫。为了通过减少假阴性检测结果来提高抗原检测的灵敏度，最近的指南建议加热血样。加热被认为可以去除阻断免疫复合物。循环微丝蚴并不是心丝虫感染的特异性发现，因为其他线虫（包括棘唇属线虫）也可能导致微丝蚴血症。尽管仅凭显微镜检查无法确定微丝蚴的类型，但实时 PCR 可以可靠地识别感染的线虫种类。正确识别寄生虫非常重要，因为犬恶丝虫感染需要驱虫治疗，而棘唇属线虫被认为是临床上无关的偶然发现。本病例报告描述了一只微丝蚴血症犬，最初的犬恶丝虫抗原检测在加热处理后呈阳性，而实时 PCR 显示微丝蚴为棘唇属线虫（即旋尾属线虫）。
一只大约 5 岁、无症状的西班牙马士提夫犬因通过筛选性血液检测发现微丝蚴而被转诊接受心丝虫治疗。这只狗最近从西班牙进口到荷兰，在那里它曾是一只流浪狗。对来自该犬的血浆样本进行了三种不同的犬恶丝虫抗原检测试剂盒检测，结果均为阴性。然而，对其中两个样本进行了加热处理，这两个样本的抗原检测结果均为阳性。实时 PCR 显示循环微丝蚴属于棘唇属线虫种。每月间隔三次给予莫昔克丁滴剂治疗，最后一次治疗后一个月，抗原和 Knott 检测均为阴性。
我们的结论是，如果狗感染了棘唇属线虫，最初对犬恶丝虫的阴性血样进行加热处理可能会导致假阳性的抗原检测结果。
