**4. Response to experimental anaemia**

It is difficult to distinguish whether, in the case of native sheep, the slightness of the degree of anaemia should be considered the cause or the effect of tolerance. However, it is certain that these animals have an unquestionable ability to maintain a good level of homeostasis during TBD evidenced from the data shown in table 4, particularly those concerning PCV, Hb and MCHC.

To the purpose, four sheep belonging to a sensitive and non tolerant breed (Romanov), and four sheep to a sensitive but tolerant breed (Altamurana) underwent regular bleeding for seven days, stopping when the decrease of the packed cell volume ranged from 35 to 40%, the same as usually observed in clinical ovine babesiosis caused by *B. ovis* (Yeruham et al., 1998).

Over time the quantity and quality of the evolution of the haematological response were checked. The regression analyses performed to compare the two breeds with respect to the various data sets, gave the following results (Tab. 6):


Of these two points, while the latter might have been expected as the trial was based on the assumption of difference between the two breeds, the former result opens new vistas in the evaluation of the phenomenon. The low variability in the response to the anaemization exhibited by Altamurana sheep might be the result of the selection pressure acted by the constant presence of anaemizing parasites. Conversely the variability of the Romanov sheep could be taken as the individual response to the impact of an unusual stress.

As a general consideration, the two groups were composed by animals which were profoundly different and constantly on different levels from the haematological point of view. Both situations observed seemed to represent different aspects of normality, particularly the Altamurana sheep are constantly "poor" in the absolute levels of PCV, Hb and RBC and constantly "richer" regarding the derived parameters, MCV, MCH, MCHC, the latter being constantly those expressing haematological "efficiency" in the face of

haemolysis. In our small experience the use of desametazone had a beneficial effect as it reduced the general response to the stimulation of the pathogen and particularly macrophage activity and improved red blood cell membrane response. Several years have elapsed since the first experiment and Lola, Lisa and Claretta got back to 'normal life' and, before their death, caused by old age, they showed no signs of disease which might have been related to haematological parasitosis. As to Lina, Gilda and Zoppina, they are back in the flock following a normal breeding and reproductive cycle. With no doubt the spleen naturally acts as an immunologically active filter-pad countering even severe red blood cell deprivation; its activity is particularly prominent in the presence of antibodies given that even after splenectomy these animals were still able to resist local diseases. There are grounds to believe that the animals may have a genetically derived tolerance to such instances based on active, diffuse and efficient structural systems which do not relate to one

It is difficult to distinguish whether, in the case of native sheep, the slightness of the degree of anaemia should be considered the cause or the effect of tolerance. However, it is certain that these animals have an unquestionable ability to maintain a good level of homeostasis during TBD evidenced from the data shown in table 4, particularly those concerning PCV,

To the purpose, four sheep belonging to a sensitive and non tolerant breed (Romanov), and four sheep to a sensitive but tolerant breed (Altamurana) underwent regular bleeding for seven days, stopping when the decrease of the packed cell volume ranged from 35 to 40%, the same as usually observed in clinical ovine babesiosis caused by *B. ovis* (Yeruham et al.,

Over time the quantity and quality of the evolution of the haematological response were checked. The regression analyses performed to compare the two breeds with respect to the

 the intrabreed correlation coefficients recorded for PCV, Hb and RBC, were statistically significant only in the case of Romanov sheep, testifying to high difformity in the anaemization response between Romanov individuals, while Altamurana sheep

the comparisons between the correlation coefficients obtained for PCV, Hb and RBC, in

Of these two points, while the latter might have been expected as the trial was based on the assumption of difference between the two breeds, the former result opens new vistas in the evaluation of the phenomenon. The low variability in the response to the anaemization exhibited by Altamurana sheep might be the result of the selection pressure acted by the constant presence of anaemizing parasites. Conversely the variability of the Romanov sheep

As a general consideration, the two groups were composed by animals which were profoundly different and constantly on different levels from the haematological point of view. Both situations observed seemed to represent different aspects of normality, particularly the Altamurana sheep are constantly "poor" in the absolute levels of PCV, Hb and RBC and constantly "richer" regarding the derived parameters, MCV, MCH, MCHC, the latter being constantly those expressing haematological "efficiency" in the face of

the two different breed groups were highly statistically significant.

could be taken as the individual response to the impact of an unusual stress.

sole organ.

Hb and MCHC.

1998).

**4. Response to experimental anaemia** 

various data sets, gave the following results (Tab. 6):

behaved almost the same;

anaemia (Pieragostini & Petazzi, 1999). So if it is a matter of fact that from the numerical point of view, the two breeds' responses to anaemization are to a large extent not very dissimilar, the greater efficiency of the local breeds is beyond doubt. It is not to be excluded that this may be identified in their greater capacity to cope with anoxemic stress, both by the production of red globules enriched with haemoglobin and maybe also by accelerating the turnover of older and less efficient red blood cells.


Table 6. Statistical significance of the differences between the correlation coefficients calculated by the regression analysis performed to compare the two breeds, Altamurana and Romanov, with respect to the hematological parameters RBC (Red Blood Cells), Hb (Haemoglobin) and PCV (Packed Cell Volume). \*\*\*P<0.001; \*\*\*\*P<0.0001; n.s., not significant.

The reading of these haematological aspects should be looked at without losing sight of the general aspect of the overall comparison between the two breeds. From this point of view, the considerable difference in absolute values, apparently almost negligible as regards the curve trends, becomes very striking in the comparison between the general situations of overall well-being of the two breeds compared. The Altamurana sheep continued to exhibit apparent good health when subjected to anaemization, at ease with their surroundings, ready to feed and drink. Conversely, the Romanovs exhibited a serious dulling of the senses and lack of reaction once anaemization was achieved; this necessitated support treatments with rehydrating solutions to allow them to overcome their state of anergy (Pieragostini & Petazzi, 1999). These results strongly support the hypothesis that, beyond the environmental factors such as stress, nutrition and other conditions, which in general facilitate infections (Agyemang et al., 1990; Bennison et al., 1998; Oppliger et al., 1998) and which are supposed to be particularly relevant in the case of non-native breeds, genetic predisposition plays a major role also in the pathogenesis of TBD.
