Histological findings in experimentally infected male calves with chronic besnoitiosis

The histological findings associated to Besnoitia besnoiti infection were exhaustively studied in target tissues from experimentally and chronically infected calves. Calves were inoculated with 10⁶ bradyzoites via intravenous, subcutaneous and intradermal route. Visible pathognomonic sclera cysts were observed in all infected animals. Tissue cysts were more abundant and lesions were more frequent in calves inoculated via intradermal. The most parasitized tissues were skin, including scrotum (40.81% of positive samples), nostril and nasal turbinate. Tissue cysts were already fully developed as the average tissue cyst diameter was 181.20 μm. Microscopic lesions were mainly detected in skin samples, followed by reproductive and upper respiratory tracts. Mild lesions compatible with both acute (thrombus, oedema and inflammation) and chronic besnoitiosis (skin lesions, hyperkeratosis and dilated sweat glands) coexisted. Vascular damage and inflammation were more frequently observed in skin (including scrotum) followed by testicular parenchyma, epididymis and pampiniform plexus. Histological findings evidenced a subclinical chronic besnoitiosis.

A model for chronic bovine besnoitiosis: Parasite stage and inoculation route are key factors

In this work, an experimental model for chronic besnoitiosis in bovine was developed and characterized. Using a previously established calf model, two new variables (parasite stage and inoculation route) were combined and used. Twelve Holstein Friesian 3-month-old male calves were randomly divided into four groups of three animals each. Bradyzoites were obtained from a chronically infected bull and used for inoculation via three different inoculation routes. Three groups were inoculated with 10⁶ bradyzoites by intravenous (G1), subcutaneous (G2) and intradermal (G3) routes, and a non-infected control group (G4) was inoculated with PBS. The trial lasted for 90 days and included daily clinical monitoring as well as weekly skin biopsies and blood sampling. Sera were obtained to analyse both cellular and humoral responses. Once the calves were euthanized, tissues from the skin, eyes, respiratory and reproductive tracts, among others, were collected to study presence of the parasite. Clinically, the infection was classified as mild to moderate for the acute stage since all infected calves showed lymphadenopathy from four days post-infection (pi) and fever from one week pi until 24 days pi. However, the most relevant results were achieved during the chronic stage that was classified as moderate to severe. In fact, pathognomonic conjunctival cysts were observed in all infected calves from 40 days pi onwards and were more abundant in G3. Moreover, one calf from this group developed skin lesions (49 days pi). The microscopic tissue cysts and Besnoitia DNA were detected primarily in skin, reproductive tract and respiratory tissue samples, and parasite load was higher in G3. In conclusion, the parasite stage (bradyzoite) and the inoculation route are key factors that influence the outcome of an infection. In particular, the intradermal route led to more severe clinical signs of the chronic phase in the inoculated calves.

The route of Besnoitia besnoiti tachyzoites inoculation does not influence the clinical outcome of the infection in calves

In a previous attempt, an experimental model of bovine besnoitiosis was established in calves that were intravenously inoculated with different doses of Besnoitia besnoiti tachyzoites. Despite the fact that all infected calves developed the acute stage of disease, only microscopic findings characteristic of chronic besnoitiosis were reported. In the present study, calves were inoculated by subcutaneous and intradermal routes with B. besnoiti tachyzoites with the aim of developing clinical signs and macroscopic lesions characteristic of chronic besnoitiosis. Nine 3-month-old male calves were randomly distributed into three groups of three animals each. Next, 10⁶ tachyzoites were inoculated by either the subcutaneous (G1) or intradermal route (G2). The negative control group (G3) was inoculated with PBS. Daily clinical monitoring and regular blood collection were performed. At 70 days post-infection (pi), animals were euthanized, and tissues were collected to investigate lesions and parasites. Infected animals developed mild-moderate acute besnoitiosis characterized by lymphadenopathy from four days to 47 days pi, and sporadic fever peaks were only observed in one calf from G2. However, other clinical signs and macroscopic lesions characteristic of chronic besnoitiosis were not detected. Only nine tissue samples were B. besnoiti-DNA-positive, eight of which belonged to reproductive and respiratory tracts tissues from G1. Finally, the kinetics of the immune responses were similar in both infected groups. However, delayed and lower cellular and humoral immune responses were observed in G1 followed by G2 and were compared with intravenously inoculated calves. The differences observed among the three inoculation routes could be due to different effector mechanisms of the host early innate immune response against B. besnoiti. Accordingly, the inoculation route of B. besnoiti tachyzoites does not significantly influence the clinical outcome of the infection in calves. Thus, a further refinement of this experimental model of bovine besnoitiosis is needed to reproduce macroscopic lesions characteristic of chronic stage disease.