Acute phase response in the dog following surgical trauma

The levels of five acute phase reactants and serum glycoproteins were measured in dogs following surgical trauma. Canine C-reactive protein peaked 24 hours after surgery and peak levels of ceruloplasmin, haptoglobin and seromucoid were detected on the fourth to sixth days. Levels of serum glycoproteins varied in a similar manner to ceruloplasmin, haptoglobin and seromucoid. There was very little variation in the levels of alpha 1 antitrypsin.     

Acute phase response in calves following infection with Pasteurella haemolytica, Ostertagia ostertagi and endotoxin administration

The concentrations of bovine acute phase proteins were monitored in plasma following experimental infection with Pasteurella haemolytica and Ostertagia ostertagi and after endotoxin administration. Raised levels of haptoglobin, alpha 1 proteinase inhibitor and seromucoid were detected after pasteurella infection and endotoxin administration. Ceruloplasmin levels increased after endotoxin administration but not during pasteurella infection. Raised levels of the four acute phase proteins were found in eight of 19 calves infected with ostertagia but showed a variable pattern and did not correlate with plasma pepsinogen increases. Bovine alpha 1 antichymotrypsin and alpha 2 macroglobulin were identified as acute phase reactants.     

Acute phase response and mastitis in the cow

The levels of three plasma proteins, which are classed as acute phase reactants, were compared in a group of cows which suffered from mastitis with those in a group of cows which were clinically normal. The plasma levels of haptoglobin, ceruloplasmin and alpha 1 antitrypsin were higher in the cows with mastitis than in normal cows.     

Effect of transportation and commingling on the acute-phase protein response,growth, and feed intake of newly weaned beef calves

The objective of this study was to investigate the effect of transportation and commingling on measures of the acute-phase protein response in newly weaned beef calves. Thirty-two (Exp. 1; average BW = 266 +/- 20.8 kg) and thirty-six (Exp. 2; average BW = 222 +/- 34.6 kg) Brahman-crossbred calves were randomly allotted to one of four treatments (2 x 2 factorial arrangement [transportation x commingling] in a completely randomized design). Body weight and jugular blood were collected at weaning, after shipment, and 1, 3, and 7 d after transport for Exp. 1, and at weaning and 1, 5, 9, 13, 17, and 21 d after transport for Exp. 2. Feed intake within pen was recorded daily for Exp. 2. Plasma fibrinogen, ceruloplasmin, haptoglobin, and cortisol concentrations were determined for all collection times. Additionally, serum amyloid-A and alpha-acid glycoprotein concentrations were determined in Exp. 1 and 2, respectively. In Exp. 2, commingled calves tended (P = 0.13) to have a higher DMI than noncommingled calves (5.3 and 4.8 kg/d, respectively). Transported calves lost more BW than nontransported calves from the time of weaning to d 1 (2.0 and 3.1% more BW loss for Exp. 1 and 2, respectively). With the exception of haptoglobin in Exp. 1, each of the acute-phase proteins measured in these studies increased over each sampling day. In Exp. 1, transported calves had higher (P < 0.05) mean serum amyloid-A concentrations than nontransported calves (48.9 vs. 33.4 microg/mL). There was a significant sampling day x transportation interaction (P < 0.01) for fibrinogen, ceruloplasmin, and haptoglobin in Exp. 1; transported calves had higher concentrations of fibrinogen following transport and on d 2 and 3, and ceruloplasmin on d 3. Haptoglobin concentrations were higher (P = 0.04) in nontransported calves on d 1 and 2 of Exp. 1. In Exp. 2, overall mean haptoglobin concentrations were higher in nontransported vs. transported calves. The results of these studies indicate that stressors associated with transportation affect the acute-phase protein response in newly weaned beef calves. More research is needed to determine whether these proteins might be valuable indicators of stress following the weaning process.     

Acute phase protein response of ewes and the release of PGFM in relation to uterine involution and the presence of intrauterine bacteria.

The rate of uterine involution postpartum was monitored in 13 suckling mule ewes by using radio-opaque markers and radiography, and each ewe was also monitored for intrauterine bacterial contamination during the first week, using a sterile guarded swab. Peripheral plasma or serum concentrations of haptoglobin, seromucoid, ceruloplasmin and 15-keto-13,14-dihydro-prostaglandin F2 alpha (PGFM) were measured up to six weeks postpartum. The maximum reduction in the length of the uterine body and in the diameters of the horns occurred by 28 days postpartum, except in one ewe in which the size of the uterus continued to decrease for 42 days. Four ewes were positive for intrauterine bacterial contamination; Escherichia coli, clostridial species, Staphylococcus aureus, Streptococcus uberis and Enterococcus species were isolated in pure or mixed culture. The presence of intrauterine bacteria did not affect the time for the completion of uterine involution. No bacteria were isolated from the ewe in which involution was delayed, but it had a different acute phase protein response and was therefore excluded from further analyses. In the remaining 12 ewes the mean postpartum haptoglobin response increased, with peak concentrations occurring on day 1, and decreased slowly as uterine involution progressed, but the four contaminated ewes had a significantly greater response. There was no difference between the prepartum and postpartum concentrations of seromucoid in the eight sterile ewes, but significant increases were observed in the contaminated group; the concentrations of ceruloplasmin did not vary in either group. The concentrations of PGFM were higher during the early postpartum period in the ewes with contaminated uteri.     

Acute phase protein responses to uterine bacterial contamination in cattle after calving

Repeated ultrasonographic examinations and collections of blood samples and uterine lumenal swabs between seven and 28 days after calving were used to examine the relative effects of bacterial contamination and involution of the uterus on the concentrations of acute phase proteins in the blood of 26 dairy cows. The severity of bacterial contamination, as determined by the total bacterial growth score, was a significant variable for the concentrations of the acute phase proteins alpha1-acid glycoprotein (P < 0.0001), haptoglobin (P < 0.05) and ceruloplasmin (P < 0.0001). In addition, the concentrations of alpha1-acid glycoprotein and ceruloplasmin were increased in the cows from which Escherichia coli (P < 0.0001) and Arcanobacterium pyogenes (P < 0.05), respectively, were isolated from the uterine lumen. Uterine involution, as determined by the decreasing diameter of the previously gravid uterine horn, was associated with a decrease in the concentrations of alpha1-acid glycoprotein (P < 0.005), haptoglobin (P < 0.05) and ceruloplasmin (P < 0.01). However, the response of the acute phase proteins to bacterial contamination was independent of the day on which the samples were collected, indicating that their concentrations were increased by bacterial contamination in addition to the changes associated with uterine involution.     

Effect of Mannheimia (Pasteurella) haemolytica Infection on Acute-phase Proteins and Some Mineral Levels in Colostrum–Breast Milk-fed or Colostrum–Breast Milk-deprived Sheep

The aim of this study was to investigate the levels of acute-phase proteins and minerals as indicators for the reactivity in 1-year-old sheep. A total of 26 Chios breed sheep were fed colostrum-breast milk (control, n = 15)or were deprived afterseparation from their mother immediately after birth(experimental, n = 11). Mannheimia (Pasteurella) haemolytica serotype A1 was inoculated intratracheally and blood samples were taken in vacuumed sera on days 0, 1, 4, 7, 10, 13, 16, 19 and 22. Antibiotic treatment was initiated after blood sampling on day 22, and blood samples were taken on days 1, 4 and 7 after the treatment. The levels of C-reactive protein (CRP), haptoglobin, ceruloplasmin, fibrinogen, zinc, iron and calcium, which are the indicators of immune function and infectious diseases were analysed. No significant difference between the control and trial groups before and after the infection was determined. However, serum CRP, haptoglobin, ceruloplasmin and fibrinogen levels were increased in the course of the infection. These levels were restored to normal following treatment.     

Bovine and canine acute phase proteins

Acute phase proteins are serum proteins which increase in concentration during the acute phase response to inflammation or infection. The response occurs in all animals, but in different species the response of individual proteins can be significantly different. Of the numerous acute phase proteins which have been identified in humans, a number have been examined in cattle and dogs but usually on an individual basis with little reference to their part in the acute phase response. Biochemical, physiological and clinical investigations into haptoglobin, fibrinogen, ₁-proteinase inhibitor, ceruloplasmin, seromucoid and C-reactive protein of cattle and dogs have therefore been reviewed with the emphasis on their role in this response to tissue damage.     

Haptoglobulin in domestic mammals. IV. Experimental influencing of the haptoglobulin level]

The effects of turpentine oil, vaccines, pyrogens, corticotrophin, prednisolone, adrenaline and autologous haemoglobin on the haptoglobin content of plasma was investigated in cattle and pigs. In comparison with rabbits, relatively small doses of turpentine oil led to a considerable increase in haptoglobin, a 50% increase occurring within 4 to 6 days in pigs and only 1.5-2 days in cattle. Bacterial products also increased haptoglobin. It was confirmed that pigs react very rapidly (after 8 hours) to injection of corticotrophin, prednisolone or adrenaline with an increase in plasma haptoglobin. Following complete saturation of infused haemoglobin by haptoglobin, new haptoglobin was rapidly formed, so that the initial haptoglobin concentration was regained about 24 hours after infusion of haemoglobin.     

Development of a lameness model in sheep for assessing efficacy of analgesics

Objective To develop a lameness model to assess the efficacy of analgesics for alleviating pain, swelling and systemic signs of inflammation in sheep. Procedures The response to subcutaneous injection of 0.1 or 0.2 mL turpentine in a forelimb pastern (n = 4 ewes per dose) was examined at 0, 3, 6, 24, 48 and 72 h. In a second experiment, responses were measured at 0, 2, 4, 6, 8, 10, 12 and 24 h in ewes receiving 0.1 mL turpentine ± meloxicam 1 mg/kg IV at 0 h (n = 6 per group). Responses measured included forceplate pressure, skin temperature, limb circumference, nociception, leucocyte count, neutrophil : lymphocyte ratio, haptoglobin and daily feed intake. Results Turpentine injection caused a decrease in weight borne on the treated limb, increased skin temperature, increased sensitivity at the injection site and leucocytosis by 2 h and increased limb circumference by 4 h. Weight borne and sensitivity of the injected limb returned to control levels after around 24 h, whereas tissue swelling, elevated skin temperature and elevated haptoglobin levels persisted for at least 72 h. Treatment with meloxicam improved weight borne by and tolerance to pressure exerted on the turpentine‐injected limb. Conclusions The local and systemic signs of inflammation and pain, temporary reduction in function of the affected limb and partial amelioration of some of these changes by the dose of meloxicam used here suggest that injection of turpentine in the lower forelimb provides a suitable model for examining the efficacy of analgesics for alleviation of pain and inflammation in sheep.     

Effect of bovine lactoferrin feeding on lipopolysaccharide-induced metabolic and hormonal disturbances in preruminant calves

One of the biological functions of bovine lactoferrin (LF) is modulation of the host defense system, including cytokine production and immune response. The aim of the present study was to investigate the effect of oral administration of LF in calves on lipopolysaccharide (LPS)‐induced metabolic and hormonal changes in inflammatory response. Thirty Holstein calves at 4 day of age were given one of three oral doses of LF (0, 1, 3 g/day) for 10 days (?10 day to ?1 day). They were injected i.v. with LPS (50 ng/kg bodyweight) the day (day 0) after the end of LF treatment. Plasma samples were obtained on ?10, 0 day (immediately before LPS injection), and at 2, 6, 12, 24, 48, 72, and 96 h after LPS injection. Plasma tumor necrosis factor‐α concentrations at 2 h after LPS treatment were lower (P < 0.05) in LF 1 g/day‐fed claves compared with LF 0 g/day (control) calves. On day 0 there were no significant group differences in plasma LF concentration. Plasma concentration of haptoglobin in control calves was elevated by LPS injection. In LF groups, plasma haptoglobin concentrations slightly increased after LPS injection, but those levels at 6–24 h were lower (P < 0.05) than in the control group. The LF treatment inhibited (P < 0.05) the reduction of plasma ferrin concentration in calves following LPS challenge. The concentration of plasma aspartate aminotransferase in calves treated with LF was lower (P < 0.05) than in control calves at 24–96 h after LPS treatment. The concentration of plasma insulin‐like growth factor‐1 (IGF‐1) in all groups was decreased by LPS treatment, while in the LF groups the IGF‐1 level was higher (P < 0.05) than in the control group. Plasma adrenocorticotropic hormone and insulin concentrations in LF groups were lower (P < 0.05) than in control calves at 2 h after LPS injection. These data suggest that LF has a substantial anti‐inflammatory effect on the modulation of the host defense system in preruminant calves.     

Genetic effects on acute phase protein response to the stresses of weaning and transportation in beef calves

The objective herein was to estimate heterosis and breed effects in purebred and crossbred Romosinuano, Brahman, and Angus calves on acute phase protein response to weaning and transportation. Calves (n = 1,032) were weaned in September of 2002, 2003, and 2004 at approximately 7 mo of age. Approximately 28 d after weaning, steer calves (n = 482) were transported 1,800 km (20 h) to Oklahoma. Concentrations of 3 acute phase proteins (ceruloplasmin, fibrinogen, and haptoglobin) were measured in blood samples. Calves (steers and heifers) were sampled at weaning, and 24 and 72 h postweaning. For separate analyses, steers sent to Oklahoma were sampled before shipment, upon arrival, and 24 and 72 h after arrival. Combinations of the following fixed effects were investigated: sire breed, dam breed, sampling time, birth location, calf sex (weaning only), year, cow age, and interactions. Effects of special interest were sire breed x dam breed as an indication of breed group of calf, and the interaction of sire and dam breeds with sampling time. Weaning age and BW were investigated as linear and quadratic covariates. Sire of calf within sire breed was a random term. The correlation structure of repeated measures was determined by comparison of information criterion values for different structures within each analysis. In general, plasma acute phase protein concentrations in weaned calves increased with sampling time. Concentrations in the transported steers increased through sampling at 24 h after arrival, and were lower at 72 h. Significant estimates of heterosis were detected for Brahman-Angus haptoglobin concentrations at weaning (0.38 +/- 0.14 mg/dL x 100; 44%), and for Romosinuano-Angus fibrinogen concentrations at weaning (11.4 +/- 5.5 mg/dL; 10%) and in transported steers (22.5 +/- 8.4 mg/dL; 20%). The direct effect of Romosinuano was to increase (P <0.004) ceruloplasmin concentrations of weaned calves (4.1 +/- 0.9 mg/dL) and of transported steers (3.9 +/- 1.3 mg/dL). The direct effect of Angus was to lower ceruloplasmin concentrations in weaned calves (-3.9 +/- 1.2; P = 0.001). Significant maternal effects were detected at weaning for ceruloplasmin concentrations in Romosinuano (-1.4 +/- 0.5 mg/ dL) and Angus (1.6 +/- 0.7 mg/dL) and fibrinogen concentrations in Brahman calves (-17.7 +/- 8.8 mg/dL). These data imply that acute phase protein concentrations in response to weaning and transportation are impacted by cattle breed.     

Prognostic value of serum acute-phase proteins in dogs with parvoviral enteritis

Objective : To evaluate the acute-phase protein response in dogs with parvoviral enteritis as predictor of the clinical outcome. Methods : Canine parvovirus infection was diagnosed based on the compatible clinical findings and confirmed by the canine parvovirus antigen test in 43 dogs of less than six months of age. Blood samples for complete blood cell count and acute-phase proteins (C-reactive protein, haptoglobin, ceruloplasmin and albumin) were collected before treatment. Twenty-three dogs died during or after treatment (non-survival) and the rest recovered (survival). Five healthy dogs were enrolled as control. Results : Serum C-reactive protein, ceruloplasmin and haptoglobin levels in dogs with parvoviral enteritis were higher (P<0·001, P<0·01 and P<0·001, respectively), but serum albumin was lower (P<0·001) than those in controls. Mean C-reactive protein and ceruloplasmin values in non-survival were higher (P<0·01) than those for survival dogs. C-reactive protein was found to be superior to ceruloplasmin, haptoglobin and albumin for distinguishing survival from non-survival dogs. Values higher than 92·4 mg/l for C-reactive protein had a sensitivity of 91% to predict mortality. Clinical Significance : The magnitude of the increase in serum acute-phase proteins in dogs with parvoviral enteritis could be a useful indicator of the prognosis of the disease. In acute-phase proteins, C-reactive protein is a potent predictor of mortality in dogs with parvoviral enteritis.     

The stress of weaning influences serum levels of acute-phase proteins, iron-binding proteins, inflammatory cytokines, cortisol, and leukocyte subsets in Holstein calves

The purpose of our study was to investigate changes in immunological parameters induced by weaning stress (including milk restriction) in calves. Fifteen Holstein calves were subjected to weaning at 6 weeks of age. Blood samples were collected at -14, -7, -2, 1, 3, and 5 days post-weaning (DPW; 0 DPW = 42 days). Weaning caused significant (p < 0.01) increases in the neutrophil (NE):lymphocyte (LY) ratio at 5 DPW with a significant (p < 0.05) reduction of LYs. The concentration of acute-phase proteins (haptoglobin and serum amyloid A) also increased significantly (p < 0.05) at 3 and 5 DPW compared to -2 DPW. Levels of the iron-binding protein lactoferrin decreased significantly (p < 0.05) after weaning. Serum tumor necrosis factor-α and cortisol levels were elevated (p < 0.05) at 3 DPW, while those of serum interferon-γ decreased (p < 0.05) at 1 and 3 DPW compared to levels observed before weaning. Weaning significantly (p < 0.05) decreased the percentage of CD25⁺ T cells in the peripheral blood. In conclusion, weaning stress affected the NE:LY ratio along with the levels of acute phase proteins, lactoferrin, cortisol, and inflammatory cytokines in the peripheral blood of calves. Weaning stress may induce an acute phase response possibly through the elevation of cortisol production and modulation of inflammatory cytokines.     

Effects of dietary energy and starch concentrations for newly received feedlot calves: II. Acute-phase protein response

Two hundred forty five market-stressed bull and steer calves (205 +/- 14 kg) were received in January and September 2001 to determine the response of acute-phase proteins to dietary energy and starch concentrations and to determine whether acute-phase proteins could be used as a diagnostic or prognostic tool for calves affected by bovine respiratory disease. On arrival, calves were randomly assigned to one of two dietary energy levels (0.85 or 1.07 Mcal of NEg/kg of DM) and one of two dietary starch levels (34 or 48% of dietary ME from starch; n = 5 pens/treatment). All calves were weighed, and plasma and serum samples were collected from a subset of animals (n = 6 calves/pen; 30 calves/treatment) on d 0, 7, 14, 28, and 42 of the receiving period. This subset of calves (n = 120) was used for all subsequent analyses. Concentrations of fibrinogen (Fb), haptoglobin (Hp), and serum amyloid-A (SaA) were determined. In addition, samples were collected from the subset of calves when they received medical treatment and 7 d following treatment to measure serum concentrations of Hp and SaA. Serum concentrations of Fb, Hp, and SaA did not differ among dietary treatments, but decreased (P < 0.03) as day of the receiving period increased. Fibrinogen (P < 0.001) and the ratio of Fb:total blood protein were greater (P < 0.003) in calves treated multiple times than in calves never treated or treated once for bovine respiratory disease. In addition, on d 0 and 7, Hp concentration increased (antimicrobial treatment x day interaction, P < 0.03) as the number of antimicrobial treatments increased, and was greater on d 14 and 28 in calves treated multiple times than in calves never treated or treated once. Haptoglobin concentration was greater (P < 0.05) in calves on medical treatment days compared with recovery days (7 d after medical treatment). Although diet seemed to have little effect on acute-phase protein response, these results suggest that haptoglobin may be useful as a diagnostic tool to make management decisions regarding treatment protocols for calves with bovine respiratory disease.     

The effect of "Ferrodex", vitamin A and "Rovimix beta-carotene" on the iron and copper levels in blood plasma of calves

The effect of Ferrodex, vitamin A and synthetic beta-carotene on the content of iron and copper in the blood serum has been investigated in three groups of calves: I control group without vitamin supplements, II group received only once with the first colostrum dose 1.5 million IU of vitamin A, III group received Rovimix beta carotene containing 10 per cent of the synthetic beta carotene with Mlekomix from the 21st day to the 13th week of life. Moreover half of the calves from each group received Ferrodex intramusculary. Content of iron, total iron binding capacity by serum proteins, level of indirect and direct copper as well as the content of ceruloplasmin were determined in the blood serum of calves. Moreover hematocrit and hemoglobin content in the blood were determined. The results obtained show that a triple intramuscular injection of Ferrodex caused an increase of the level of iron, copper and ceruloplasmin in the blood serum of calves. However, a single administration of vitamin A to the calves or a continuous administration of Rovimix beta-carotene decreased significantly the level of iron and ceruloplasmin copper in blood serum as well as caused a more dynamic ceruloplasmin synthesis with a simultaneous increase in its utilization.     

Measurement of serum haptoglobin in neonatal farm-raised and bob veal calves using two immunoassay methods

Two immunoassays using an anti-bovine haptoglobin monoclonal antibody, Hap 1, were used to measure serum haptoglobin levels in neonatal farm-raised and bob veal calves. Bob veal calves were grouped into condemned, normal, and icteric groups based on the appearance of the carcass and viscera at postmortem examination. The competitive inhibition assay was more sensitive than the direct hemoglobin binding assay in detecting low levels of haptoglobin in all groups of calves. A significant number of bob veal calves with gross postmortem lesions other than icterus had detectable haptoglobin levels. The low levels of haptoglobin that were detected were not useful in distinguishing the relative severity of the inflammatory or degenerative process; however, the predictive values of a positive and negative test suggest haptoglobin measurement may be useful as a supplemental tool in evaluating the health status of the neonatal calf.     

Influence of selenium administration to dry cows on selected biochemical and immune parameters of their offspring

The study was performed on 16 Holstein‐Friesian calves divided into two groups of eight animals each. The first group was composed of calves whose mothers did not receive selenium supplements (Se0). The second group consisted of calves whose mothers were administered intramuscular injections of a selenium and vitamin E supplement containing 0.5 of sodium selenite/ml and 50 mg of tocopherol acetate/ml in a single dose of 30 ml (Se30) ml, 10 days before the expected parturition date (10 ± 2 days). The calves were fed 2.5 L of the mother's colostrum administered by stomach tube 2 hr after birth and another 2 L 6 hr after birth. Blood from all calves was collected 7 times from external jugular vein (day 0–before colostrum administration and on the 2nd, 3rd, 4th, 7th, 14th and 21st days of life) for analyses of selenium, ceruloplasmin, transferrin, lactoferrin, immunoglobulin G (IgG) concentrations and gamma‐glutamyl transferase (GGT) and lysozyme activity. Selenium concentration was significantly higher in calves whose mothers received selenium supplements than in the offspring of non‐supplemented cows until 72 hr after birth (p ≤ .05). Lysozyme and GGTP activity and IgG concentration were significantly higher in the S30 group during the entire experiment (p ≤ .05). Supplementation of selenium to the mothers did not influence the ceruloplasmin, lactoferrin and transferrin levels in calves. A single injection of a selenium supplement administered to cows during late pregnancy increases selenium levels in calves and enhances passive transfer from the mother to the offspring.     

Comparison of two automated spectrophotometric methods for ceruloplasmin measurement in pigs

Ceruloplasmin (Cp) determination could provide an objective measure of the health status of an animal and could be used as marker of animal health and welfare (Skinner, 2001) (Skinner, J. G., 2001. Special report. International standardization of acute phase proteins. Veterinary Clinical Pathology 30, 2-7.) but only manual methods have been reported to determine Cp concentrations in pigs (McCosker, 1961; Toussaint et al., 1995; Eckersall et al., 1996) (McCosker, P. J., 1961. Paraphenylenediamine oxidase activity and copper-levels in mammalian plasmas. Nature. 190, 887-889; Toussaint, M. J. M., Van Ederen, A. M., Gruys, E., 1995. Implication of clinical pathology in assessment of animal health and in animal production and meat inspection. Comparative Haematology International 5, 149-157; Eckersall, P. D., Saini, P. K., McComb, C., 1996. The acute phase response of acid soluble glycoprotein, alpha(1)-acid glycoprotein, ceruloplasmin, haptoglobin and C-reactive protein, in the pig. Veterinary Immunology and Immunopathology 51, 377-385). In the present study two automated methods based on the use of two different substrates for the determination of serum ceruloplasmin in pigs were developed, evaluated and compared. Both methods showed a good precision and detection limits, with no signs of inaccuracy and could be applied to biochemical autoanalyzers usually found in clinical laboratories using only minimal amounts of serum. Additionally the behaviour of Cp in experimental and clinical situations was studied showing an increase of around two fold after turpentine administration and significantly higher values in cases of pigs with inflammatory conditions when compared with healthy pigs.     

Alterations in bovine platelet function and acute phase proteins induced by Pasteurella haemolytica A1.

Platelet function was assessed by aggregometry in 10 Holstein calves before and after exposure to Pasteurella haemolytica (biotype A, serotype 1) by intrabronchial challenge. At 24 h after exposure the platelets had become more reactive to stimulation with known platelet agonists such as adenosine diphosphate (ADP) and platelet-activating factor (PAF) and the platelet aggregates that formed were more resistant to disaggregation. The activation of platelets was an early response in the challenged calves as platelet function had returned to pretreatment levels 72 h after exposure to the bacteria while the acute phase reactant proteins, haptoglobin and fibrinogen, were approaching their peak values and alpha 2-macroglobulin levels had also risen significantly (P < 0.05) at this time. The plasma levels of these proteins were still elevated and albumin levels were depressed 6 d post-treatment. At post-mortem all calves exhibited pneumonic tissue damage. When P. haemolytica leukotoxin was added directly to bovine platelet suspensions both spontaneous aggregation and an increase in the aggregation response to ADP and PAF stimulation were observed. The morphological appearance of the platelet aggregates exhibited the typical pattern for bovine platelets with 2 distinct zones of cells being visible within each aggregate. One zone contained platelets in which the cytoplasmic granules were still evident and the other zone contained irregularly shaped platelets devoid of granular content. In the latter zone, discrete gaps, or pores, were evident in the plasma membrane of numerous platelets. This pore formation is characteristic of leukotoxin action and is not observed in ADP or PAF induced aggregates.