Anaplasma ovis in sheep: Experimental infection, vertical transmission and colostral immunity

Abstract

Ovine anaplasmosis is caused by the obligate intraerythrocytic bacteria Anaplasma ovis and the disease is characterized by anaemia, weight loss and weakness. A severe outbreak of ovine anaplasmosis was diagnosed for the first time in Spain in 2014 and, subsequently, a research was developed in order to increase de knowledge of this not-well-known disease.An experimental infection with Anaplasma ovis was carried out in three lambs and the evolution of the disease was assessed during a period of three years and two production cycles (gestation, birth and lactation). Clinical and haematological parameters were analyzed through all the period and detection of Anaplasma ovis by qPCR and Giemsa staining of blood smears was performed periodically as well as cELISA test for the detection of antibodies against Anaplasma spp. Finally, Anaplasma ovis vertical transmission and transfer of immunity through the colostrum were studied and evaluation of the immune response against infection of lambs born from infected dams was likewise analyzed by means of a new experimental infection.The results showed that A. ovis induces a severe normocytic and normochromic anaemia, however, evident clinical signs were not observed until the first lambing. Molecular identification of Anaplasma ovis by qPCR revealed that the three animals started to test positive between day 4 and 8 post-infection and afterwards, the bacterial load increased progressively until reaching a peak on day 36 post-infection. The presence of antibodies against A. ovis was detected from day 10 after inoculation in the three animals.Ruminal atony, paleness of mucous membranes, weakness and fever were observed after the first delivery, followed by a chronic condition with a relevant loss of body weight that led to a state of cachexia. The three animals showed severe anaemia after delivery, coinciding with a peak in the bacterial load and relevant decrease in the humoral response. In the second production cycle, the animals presented very slight clinical signs, with minor variations in the erythrocyte count, however, the high percentage of inhibition of the humoral response was not modified at the time of delivery, indicating in this case, a good immune response of the animals against A. ovis.All the blood samples taken to the lambs born from infected dams tested negative to A. ovis by qPCR in all the analyzed period, discarding the vertical transmission of the disease. Further, the analysis performed by cELISA test to the colostrum taken from the infected sheep evidenced the presence of antibodies against A. ovis, thus, all the lambs born from these ewes and naturally nursed showed antibodies against A. ovis after colostrum intake.Finally, in the experimental infection carried out in lambs born from infected ewes no clinical findings were observed and only a slight anaemia with a fast erythrocytic regenerative response was detected. In contrast with the first experimental infection, these animals tested antibody positive by cELISA the first day after infection.The studies shown in this article shed some light on how A. ovis infection occurs and how immunity is generated at the flock level. However, new studies are needed with a greater number of animals to be able to corroborate all these promising results.