*4.3.1.4. Serum amyloid A*

Serum amyloid A (SAA) is a small hydrophobic protein that belongs to the family of apolipoproteins associated with high density lipoprotein [157]. Different isoforms of SAA are expressed constitutively at different levels in response to inflammatory stimuli [158]. During inflammation, SAA1 and SAA2 are expressed principally in the liver, whereas SAA3 is induced in many distinct tissues, including the mammary gland [159]. The fourth isoform, SAA4, does not respond to external stimuli [160]. The main functions of SAA are the reverse transport of cholesterol from tissue to hepatocytes, opsonization, inhibition of phagocyte oxidative burst, and platelet activation [136]. The M-SAA3 isoform found in colostrum stimulates the production of mucin from intestinal cells and thus helps to prevent bacterial colonization [161].

In ruminants, SAA belongs to major acute-phase proteins which increases more in acute rather than in chronic conditions [162]. Intense changes in the concentrations of SAA were reported in dairy cows with various inflammatory diseases, including cows with endometritis, mastitis, as well as in lame cows [163–165]. It was raised also in cattle experimentally infected with *Mannheimia haemolytica* and bovine respiratory syncytial virus, or with bovine viral diarrhea virus [166, 167].

Eckersall et al. [168] found significantly elevated SAA concentrations in sheep with experimental caseous lymphadenitis induced by *Corynebacterium pseudotuberculosis*. Chalmeh et al. [169] observed in sheep, a rapid increase of SAA values during experimentally induced endotoxaemia by lipopolysaccharide from *Escherichia coli*. Another study conducted by El-Deeb [170] showed an increase in the concentrations of SAA in ewes with pregnancy toxemia. Marked increase of SAA concentrations was recorded also in sheep following experimental infestation with *Psoroptes ovis* [171]. After treatment, the SAA values decreased rapidly within 3 days and returned to the pre-infestation values for 10–14 days. The alterations in the acute-phase protein production during experimental caprine coccidiosis were evaluated by Hashemnia et al. [172]. They found markedly higher concentrations of SAA at day 7 after inoculation. Furthermore, the magnitude and duration of the acute-phase responses are correlated well with the severity of the clinical signs and diarrhea in goat kids.
