This study has shown that the total leukocyte count in cows with toxic mastitis is considerably lower than in healthy cows. Approximately 60.1% of the cows with mastitis had leukopenia and only 10.1% had leukocytosis. Segmented neutrophil counts were similar and were decreased in about 69.5% and increased in about 16.4% of the cows. Acute inflammation is generally accompanied by leukocytosis rather than leukopenia, but our findings are in agreement with published reports of clinical [1, 13, 19] as well as experimental acute mastitis [8–12]. The reason for the leukopenia is the low bone marrow reserve of neutrophils in cattle as opposed to other domestic animal species such as the dog  and the fact that the transit time from myeloblast to complete maturation is not reduced in the face of high neutrophil demand . In the dog the transit time can be reduced from 6 to 2 to 3 days with increased demand of segmented neutrophils but not in calves  and in all likelihood not in adult cattle either. With acute mastitis, large numbers of segmented neutrophils are moved from the circulation into the infected mammary tissue causing leukopenia. This is followed by an increase in leukocyte numbers after about 48 hours, often associated with a rebound neutrophilia . For these reasons, the leukocyte count and particularly the segmented neutrophil count were lower in the cows that were examined on the first day of illness compared with cows that were referred and sampled at a later stage of illness. Severe leukocytosis with cell counts exceeding 20,000 cells/µl occurred in only two cows (1.3%), which was in agreement with the observation that cattle as well as sheep and goats with acute inflammation have much lower leukocyte peaks than other domestic animal species . A leukocyte count between 20,000 and 30,000 cells/µl is considered extremely high in cattle  and only 16.4% of ill cows had neutrophilia in the present study. Stress is another cause of neutrophilia. Stress leukograms caused by endogenous or exogenous corticosteroids are common in cattle  and are characterised by neutrophilia, lymphopenia, eosinopenia and monocytosis [14, 20]. However, a stress leukogram is not associated with a left shift and therefore band neutrophils and metamyelocytes are not seen. In the present study, neutrophilia was always accompanied by a regenerative left shift in affected cows, which means that neutrophilia was due to inflammation and not stress. Degenerative left shift, characterised by an abundance of band neutrophils and metamyelocytes relative to segmented neutrophils, was more common than regenerative left shift and occurred in 44.5% of ill cows. Degenerative left shift with increased numbers of metamyelocytes is an alarming haematological finding and a poor prognostic sign when it persists for more than 3 to 4 days .
Lymphopenia was detected in 60.4% of all cows with toxic mastitis. Lymphopenia in cattle is most commonly caused by endogenous or exogenous corticosteroids , and in the present study was due to toxic mastitis. This was also observed after experimental intramammary infection with Pseudomonas aeruginosa  and Mycoplasma bovis . Interestingly, lymphopenia occurred as early as 24 hours after Pseudomonas aeruginosa inoculation but only 84 hours after Mycoplasma bovis inoculation. Lymphopenia is also seen in viral infections and diseases caused by Anaplasma spp. and other bacteria .
Metamyelocytes were not detected in blood smears from healthy cows, which was in agreement with the veterinary literature . The occurrence of metamyelocytes should therefore be considered pathological and indicative of a massive demand for leukocytes.
In 104 cows (62%), toxic mastitis occurred during late pregnancy or during the puerperal period, which most likely was related to impaired neutrophil function in the periparturient period [22, 23] described previously [24, 25]. It is believed that this is related to increased stress and thus increased glucocorticoid concentrations at this lactation stage . Furthermore, recruitment of neutrophils into the mammary gland and neutrophil function are altered in the periparturient period, which may also contribute to the occurrence of mastitis in early-lactation cows [22, 24, 27]. This is supported by the observation that experimental intramammary infusion of endotoxin results in more severe clinical signs in early lactation compared with late lactation in cows .
In contrast to other domestic animal species, cows have more lymphocytes than neutrophils in the circulation and therefore a relatively low NL ratio of about 0.5 [6, 14]; in the present study, the controls had a median NL ratio of 0.63. The significantly larger NL ratio of 0.97 in the cows with toxic mastitis was due primarily to lymphopenia (in 60.2%) rather than neutrophilia (only 16.4%). Similarly, cows with sole ulcer had a NL ratio of 1.04, which was significantly greater than that of healthy cows even though neutrophil and lymphocyte counts did not differ significantly between the groups . It was therefore recommended to include the NL ratio in the assessment of a blood cell count to emphasise the difference in absolute neutrophil and lymphocyte numbers .
Cytoplasmic basophilia combined with vacuolisation in all but 8.2% of the examined blood smears was indicative of toxic change . This confirms that the cows with mastitis had toxaemia because the changes are generated when maturation of neutrophils in the bone marrow is hindered because of acute and severe inflammation .
Cows with mastitis caused by gram-negative and gram-positive bacteria did not differ significantly with respect to total leukocyte, segmented and band neutrophil, monocyte and lymphocyte counts. Differences in cell counts between cows with gram-negative and gram-positive infections were significant in an earlier study but the number of cows was larger in that study . The observation that cows with gram-positive infection tended to have lower total leukocyte and segmented neutrophil counts and significantly lower eosinophil, basophil, monocyte and lymphocyte counts than controls indicates that mastitis caused by gram-positive pathogens constitutes a severe strain on the bone marrow. It should be remembered that differentiation of mastitis caused by gram-negative and gram-positive pathogens requires bacteriological examination and is not possible based on clinical signs alone .
Increased haematocrit and azotaemia in cows with toxic mastitis was in all likelihood attributable to prerenal causes. The increased fibrinogen concentration points to the role of fibrinogen as a positive acute-phase protein [18, 30]. Similar to haptoglobin, it increases rapidly in response to inflammation and was therefore higher in cows that were ill longer than three days compared with cows on the first day of illness. In cattle, hyperfibrinogenaemia is considered as good as or better than the neutrophil count for determining inflammation . As a general rule, the degree of increase in concentration of positive acute-phase proteins parallels the severity of inflammation. Fibrinogen is preferred by many because it is easy to measure.
We were surprised that only about 69% of all toxic mastitis cases were diagnosed by the primary care veterinarian. This may have been due to other clinical signs, such as poor general health status, poor rumen motility, low faecal output and positive percussion and/or ballottement and simultaneous auscultation, which were considered diagnostic of other disorders. This emphasises the importance of a comprehensive clinical examination that includes the assessment of the udder and the milk.