Chronic bovine besnoitiosis: Intra-organ parasite distribution, parasite loads and parasite-associated lesions in subclinical cases

Abstract

Bovine besnoitiosis caused by Besnoitia besnoiti is a chronic and debilitating disease. The most characteristic clinical signs of chronic besnoitiosis are visible tissue cysts in the scleral conjunctiva and the vagina, thickened skin and a generally poor body condition. However, many seropositive animals remain subclinically infected, and the role that these animals may play in spreading the disease is not known. The aim of the present study was to assess the intra-organ parasite distribution, the parasite load and the parasite-associated lesions in seropositive but subclinically infected animals. These animals were seropositive at the time of several consecutive samplings, had visible tissue cysts in the past and, at time of slaughter, had detectable specific anti-Besnoitia spp. antibody levels, but they did not show evident clinical signs at culling. Thus, histopathological, immunohistochemical and molecular analyses of several samples from the respiratory tract, reproductive tract, other internal organs and skin from six cows were performed. The tissue cysts were located primarily in the upper respiratory tract, i.e., in the rhinarium and larynx/pharynx (four cows), followed by the distal genital tract (vulva/vagina) and the skin of the neck (three and two cows, respectively, out of the four cows with cysts in the respiratory tract). We were unable to detect any parasites in the two remaining cows. Cysts were associated with a significant non-purulent inflammatory infiltrate consisting predominantly of T lymphocytes and activated monocytes/macrophages in two cows. The parasite burden, estimated by quantitative real-time PCR, was very low. It is noteworthy that the only animal that showed a recent increase in the antibody titre had the highest parasite burden and the most conspicuous inflammatory reaction against the cysts. In conclusion, although these cows no longer displayed any visible signs of besnoitiosis, they remained infected. Therefore, cows without visible signs of disease may still be able to transmit the parasite.

Bovine besnoitiosis (Besnoitia besnoiti) in an Irish dairy herd

“…The herd investigated in the present study exhibited a decreased incidence of disease in the last two years, and some of the chronically infected animals have recovered at least partially, with skin lesions becoming much less obvious and milk production returning to optimal levels. This is also in accordance with previous studies (Kumi-Diaka and others 1981, Nobel and others 1981, Frey and others 2013), although it has been postulated that these animals remain infected.…”

“…Lymphadenomegaly was likely secondary to the heavy parasite burden. Sarcocystosis of the myocardium has been described in Besnoitia -infected animals (Wobeser 1976, Frey and others 2013) and may be coincidental.…”

Vector-borne transmission of Besnoitia besnoiti by blood-sucking and secretophagous flies: epidemiological and clinicopathological implications

“…Unfortunately, due to the method of fixation, the fine structure of these could not be discerned. However, this finding raises the possibility that cystozoites may reach the exterior not only by rupturing of mucosal cysts [1, 16], but also through sweating. This may theoretically provide the means of transmission by secretophagous flies (worthy of future investigation), if cystozoites are delivered onto the conjunctiva or mucosal surfaces which they are able to penetrate [13].…”

Naturally Acquired Bovine Besnoitiosis

“…3 In the subacute and chronic stages, bradyzoite proliferation occurs within mesenchymal host cells, resulting in typical tissue cyst formation in a variety of tissues. 3,4,19,35,36,38,49 Tachyzoites display different antigens compared with bradyzoites, which can be differentiated via serological tests. 17,47 However, differentiation of tachyzoites from bradyzoites in hematoxylin and eosin (HE)-stained histological sections is difficult, and there are no reliable (immuno)histochemical tools to specifically detect tachyzoites so far.…”