Cross-sectional study of Anaplasma spp. in goats and sheep in Mongolia : a comparison between species in relationship to pasture conditions
Abstract: Anaplasma ovis, the main aetiology behind of ovine and caprine anaplasmosis, is a vector-borne bacterium of the order Rickettsiales, capable of infecting erythrocytes of small ruminants. The infection is generally a subclinical or mild condition, but stress-factors as co-infections, vaccinations, or transports may aggravate the disease. Severe cases of A. ovis infection may involve anaemia, abortion, and mortality. The pathogen is widely distributed, and is endemic in several tropical and subtropical areas. In addition to goat and sheep, several species of wild ruminants are known to be susceptible to infection, but their importance as reservoirs in the epidemiology of A. ovis is yet uncertain. The aim of this cross-sectional study was to investigate the prevalence of A. ovis in goats and sheep in Mongolia and to analyse the occurrences of changes in body condition, colours of mucous membranes and blood parameters due to infection. In addition, goats and sheep’s tendency of developing clinical signs was compared between different pasture conditions, analysing whether their species-related pasture adoptions as browser respective grazers may contribute as a stress-factor in insufficient pasture. Although a third of Mongolian population relies on small ruminant-dominated livestock for subsistence, few previous studies of A. ovis have been undertaken. Three regions were selected for the study, based on their respective pasture conditions. Samples from 80 sheep and 88 goats were collected and analysed by microscopic examination of stained blood smears, polymerase chain reaction (PCR) for the 16s RNA gene of Anaplasma spp. (IVM Ulaanbaatar, Mongolia) and msp4 PCR (IVM Ulaanbaatar, Mongolia) specific for detection of A. ovis. In addition, haemoglobin (Hb), haematocrit (HCT), Body Condition Score (BCS) and FAMACHA© (i.e. method for detecting anaemia by grading the colour of the lower inner eyelid) was recorded for each animal. The overall PCR-based prevalence of Anaplasma spp. in the study was 82.4 % with no significant difference between goats and sheep in any of the regions. The proportion of positive animals in South Gobi was significantly lower than in other regions. Typing of anaplasma subspecies was unsuccessful due to complications in the msp4 PCR, and A. ovis could therefore not be confirmed. There was a disagreement between the microscopic results and 16S rRNA PCR results, but all individuals with findings of more than eight inclusion-like structures during six minutes of microscopy were PCR-positive. Using this criterion, 34.2 % of PCR-positive individuals could be identified. Goats were generally over-represented compared to sheep regarding the occurrence of clinical parameters diverging from normal values. Neither low Hbvalues nor low BCS occurred to a higher extent in anaplasma-positive animals but there was a significant correlation between the occurrence of pale mucous membranes (FAMACHA©scores below 3) and anaplasma-positivity among goats in the South Gobi. In addition, goats were slightly paler than sheep in general, suggesting that FAMACHA© scale might be less accurate for goats. An interesting finding was the high occurrence of Hb values below reference among anaplasma-negative goats with in South Gobi. The aetiology behind this is still unknown. Regional comparisons between the infected and non-infected population were limited by the low occurrence of PCR-negative individuals in two of the regions. For the same reason, assessments about pasture influence on disease development were not possible.
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