Modulation of leukocyte populations and immune responses in sheep experimentally infected with Anaplasma (formerly Ehrlichia) phagocytophilum

Anaplasma phagocytophilum infection in sheep is characterized by an immune suppression as indicated by impaired antibody response, reduced lymphocyte response and reduced oxidative burst. The effect of A. phagocytophilum infection on leucocyte populations, especially lymphocytes, was therefore investigated in six sheep experimentally infected with A. phagocytophilum, and compared with leucocyte populations from control animals.To investigate the ability of the infection to interfere with the cellular and humoral responses to specific antigens, the animals were vaccinated with commercial vaccines at the time of experimental infection, and monitored for 56 days.There were reduced percentages of gammadelta T-cells and CD4+ T-cells in peripheral blood of infected animals throughout the study period, and these cell populations showed a down-regulation of CD25 expression; while there was a relative increase in CD8+ T-cells. The reduction in CD25+ gammadelta T-cells involved a subpopulation of WC1+ gammadelta T-cells. During the first 2 weeks of the study there were reduced percentages of B-cells and leukocytes expressing MHC II and CD11b, though this decrease changed to a relative increase later in the study. The relative reductions in leucocyte populations corresponded with the observed leucopenia during the first 3 weeks post-infection, which involved lymphocyte, neutrophil and eosinophil subsets [Vet. Immunol. Immunopathol. 86 (2002) 183]. There was a reduced expression of CD11b and CD14 on granulocytes during the first 2 weeks of the study, which corresponded with the previously reported leucopenia involving neutrophils and eosinophils. Antibody responses to vaccines, lymphocyte in vitro proliferative responses to antigens and mitogens, and in vitro IFN-gamma responses to antigens were reduced up to 4 weeks after infection.     

Persistence of Ehrlichia phagocytophila infection in two age groups of lambs

Tick-borne fever (TBF) is caused by the rickettsiae Ehrlichia phagocytophila and is a common disease in sheep in tick (Ixodes ricinus) infested areas in Norway. Earlier investigations have shown that some sheep could remain infected for several months after the primary infection. In this study, the persistence of E. phagocytophila after experimental infection was investigated in 2 age groups of lambs. Six lambs (1-2 weeks old) and 14 lambs (6-8 months old) were inoculated intravenously with an ovine strain of E. phagocytophila and thereafter examined clinically (including daily body temperature recording) and by haematological and serological (E. equi antibodies) methods for the next 4 months. At the end of this period, the lambs were examined for a TBF infection by blood smear investigation and blood inoculation studies. The infection was demonstrated in 19 (95%) of the 20 lambs.     

Response of sheep to experimental concurrent infection with tick-borne fever (Cytoecetes phagocytophila) and louping-ill virus

The pathogenesis of concurrent Cytoecetes phagocytophila and louping-ill virus infection was studied in two experiments. In the first experiment 18 four- to seven-year-old rams were used. Ten were infected with C phagocytophila and five days later eight of these animals and the remaining eight sheep were infected with louping-ill virus. The two rams infected with C phagocytophila alone developed no clinical signs apart from a transient pyrexia, while only three of the eight rams infected with louping-ill virus alone showed mild clinical signs. In marked contrast, all eight dually infected sheep developed severe clinical signs with pronounced depression and dysentery and three died and five were killed in extremis. They developed higher titres of viraemia and the antibody response was depressed while necrotising lesions affecting a variety of organs were detected at post mortem examination. Rhizomucor pucillus was recovered from these lesions in seven of the eight sheep. A second experiment using 10 sheep, five aged seven months and five aged two to three years, confirmed the findings of the first experiment indicating that the age of the animal had not significantly influenced the initial result. It was concluded that C phagocytophila infection could enhance the pathogenicity of louping-ill virus and that, operating together, the two pathogens facilitated fungal invasion. It is postulated that sudden deaths in sheep recently transferred to tick-infested pastures may be due to this newly described syndrome.     

Effects of the dose of Ehrlichia phagocytophila on the severity of experimental infections in lambs

Twenty-one lambs were used to investigate whether their response to an infection with Ehrlichia phagocytophila was dose-dependent Four groups of four lambs were infected intravenously with a dilution in physiological saline of E phagocytophila-infected sheep blood containing either 1.3 x 10(5) infected cells, or approximately 43 infected cells, 4.3 infected cells, or 1.3 infected cells (mean values) and four lambs were left uninfected. The incubation period was significantly shorter in the lambs infected with the highest dose of E phagocytophila. However, the clinical and haematological changes observed, and the weekly weight gains of the lambs were independent of the dose of E phagocytophila. As little as one Ephagocytophila infected cell may be enough to transmit the infection.     

The serum glucose and beta-hydroxybutyrate levels in sheep with experimental Fasciola hepatica and Fasciola gigantica infection

The influence of Fasciola hepatica and Fasciola gigantica infection on serum glucose and beta-hydroxybutyrate (beta-HOB) in sheep was evaluated. This was done by setting up two groups of sheep. The first group (n=13) was split in two sub-groups, one experimentally infected with F. hepatica (n=9) and the other (n=4) as uninfected control. A second group consisting of a sub-group experimentally infected with F. gigantica (n=9) the other sub-group (n=6) left as uninfected control was also set up. The results of weight gain, parasitological and serum liver enzymes activity (glutamate dehydrogenase [GLDH] and gamma glutamyltransferase [gamma-GT]) used in monitoring the infection showed that all infected animals developed fasciolosis. It was observed that a reduction in serum glucose levels was significantly lower (p<0.05) in F. hepatica infected sheep than in uninfected control sheep starting from 5 weeks post-infection (wpi) to the end of the experiment. Similar reduction was recorded in F. gigantica infected sheep between 8 and 19 wpi. In contrast, serum beta-HOB levels were elevated in F. hepatica infected sheep between 6 and 16 wpi and in F. gigantica infected sheep between 7 and 15 wpi. It would appear from these serum glucose and beta-HOB levels that fasciolosis does lead to energy deficiency (low glucose) and ketosis (increased beta-HOB). The decrease in serum glucose and increase in serum beta-HOB levels in infected sheep may help in understanding the interaction between fasciolosis and nutritional status of infected ruminants especially in young growing animals.     

Prophylactic efficacy of buparvaquone in experimentally induced Theileria annulata infection in calves

The antitheilerial activity of buparvaquone (BW 720C) was evaluated in experimentally induced Theileria annulata infections in cross-bred male calves. T. annulata infections were induced by injecting a suspension of infected ground tick tissue suspension (GUTTS) equivalent to two ticks subcutaneously into each calf. Buparvaquone at a dose of 2.5 mg kg-1 body weight was given as a single injection (intramuscularly) on Day 0 (Group 1), Day 8 (Group 2) and Day 12 (Group 3) post-infection. The animals in Groups 4 and 5 were untreated and challenged controls, respectively. All of the recovered animals from Groups 1-4 were challenged with a lethal dose of T. annulata at 6 weeks post-infection. The immunized animals were resistant to the homologous challenge, which killed three of four control animals (Group 5); the controls showed typical antemortem and post-mortem lesions of theileriosis.     

Sheep as a new experimental host for Babesia divergens

Babesia divergens was cultivated in sheep erythrocytes in RPMI 1640 supplemented with 10% Fetal Calf Serum (FCS) or sheep serum. In vitro cultures in sheep red blood cells were initiated with human erythrocytes infected in vitro with B. divergens Rouen 1987 or with gerbil blood infected with several isolates from bovine origin. After the first subcultures on sheep erythrocytes, a ten-fold multiplication of the parasites was obtained within 48 h. Erythrocytes from three splenectomized sheep were infected in vitro with B. divergens; when parasitaemia reached 10%, the animals were inoculated with homologous parasitized erythrocytes. All sheep expressed hyperthermia with a peak between the 6th and the 9th day post-infection (p-i) and a transitory parasitaemia 10 days p-i. In vitro primary cultures were performed on two of these sheep, demonstrating the parasite persistence at very low parasitaemia in the infected animals. Splenectomized sheep can be used as a new model for B. divergens chronic infection.     

Sequential testicular and epididymal damage in Zebu bulls experimentally infected with Trypanosoma vivax

Six Zebu bulls aged between 31 and 34 months exhibiting good libido were used to study sequential testicular and epididymal damage in Trypanosoma vivax infection. Three bulls were infected with T. vivax, while the other three served as controls. All infected bulls became parasitaemic by day 5 post-infection and developed clinical trypanosomosis with rapidly developing anaemia. Representative bulls, one from each of the infected and control groups, were sacrificed on days 14, 28 and 56 post-infection. Testes and epididymides from these animals were studied histopathologically after processing and staining with haematoxylin and eosin (H and E). Testicular degeneration developed in all the infected bulls characterized by depletion of spermatogenic cells and destruction of interstitial tissue. The most severe testicular degeneration occurred in the bull that was sacrificed 56 days post-infection. Epididymal sperm reserves were 36%, 4% and 0%, respectively, in infected bulls that were sacrificed on days 14, 28 and 56 post-infection. The 0% epididymal sperm reserve may suggest complete cessation of spermatogenesis. It was concluded from this study that T. vivax infection of Zebu bulls could cause severe testicular and epididymal damage that may result in infertility or even sterility of the affected animals at early infection stages not previously thought.     

Experimental infection of weaner sheep with S strain Mycobacterium avium subsp. paratuberculosis

We sought to determine whether infection of recently weaned 12-16-week-old Merino lambs with an Australian S strain M. a. paratuberculosis, at doses consistent with natural exposure, could be detected in the first few months post-inoculation. Such detection would facilitate the use of weaner sheep as sentinel animals for the presence of infectious doses of M. a. paratuberculosis on pastures. In controlled pen trials, oral doses of approximately 10(7)-10(8) viable organisms were demonstrated to be infective, whereas doses below 10(4) organisms failed to produce detectable infection. Mycobacterium avium subsp. paratuberculosis (M. a. paratuberculosis) was isolated from intestinal and/or lymphoid tissues collected at necropsy 7 or 14 weeks after first infection, but there were no associated gross or microscopic lesions. Skin testing with intradermal Johnin detected all three infected lambs at 13 weeks post-infection, and one of the three infected lambs at 6 weeks post-infection, with 100% specificity. Results for whole blood IFN-gamma assay showed some correlation with infection status but lacked specificity. One infected lamb gave a positive result in an ELISA for antibodies to M. a. paratuberculosis, 14 weeks post-infection and 1 week after skin testing. This was the first demonstration of experimental infection with S strain M. a. paratuberculosis in Australian Merino sheep at doses likely to be representative of natural infection. Culture from tissues in the first few months post-exposure could facilitate the use of naive weaner sheep as tracer animals to detect heavy contamination of pastures with M. a. paratuberculosis, but low-level contamination may not be detected in such a system.     

Hematology and clinical pathology of experimental Fascioloides magna infection in cattle and guinea pigs.

The hematologic and clinico-pathologic response to Fascioloides magna infection in cattle and guinea pigs was investigated. Twelve calves (six infected and six controls) were monitored for 26 weeks after inoculation with 1000 metacercariae. All calves remained healthy and there were no significant differences in weight gains between infected and control groups. Flukes (mean = 9.2, range 1–32) were recovered from the liver and abdominal cavity of all infected calves. The only significant response observed in the complete blood counts was an eosinophilia present in the infected calves extending from Weeks 2 to 26 post-infection. There were no significant differences in serum levels of aspartate aminotransferase and only minor increases in the levels of gamma-glutamyl transferase and sorbitol dehydrogenase.

A total of 48 infected and 48 control guinea pigs from three separate experiments were monitored for 16 weeks after inoculation with 20 metacercariae of Fascioloides magna. Infected guinea pigs died between 7 and 114 days after infection, and flukes (ean = 2.5, range 0–13) were recovered from the liver, abdominal cavity, lungs, thoracic cavity, skeletal muscle and subcutaneous tissue. There were no differences in weight gains between infected and control guinea pigs. Complete blood counts showed increases in white blood cell, monocyte and neutrophil counts from between the third and fourteenth weeks post-infection; however, the differences were not consistently significant. Infected guinea pigs developed a significant eosinophilia and basophilia from 2 to 16 weeks post-infection. There were no significant changes in the serum levels of alanine aminotransferase or gamma-glutamyl transferase. There was an increase in the serum levels of aspartate aminotransferase beginning at 5 weeks post-infection. The response observed in the guinea pigs was similar to that reported in sheep, suggesting the suitability of the guinea pig as a model for Fascioloides magna infection in the sheep.     

Effect of porcine reproductive and respiratory syndrome virus infection on the clearance of Haemophilus parasuis by porcine alveolar macrophages.

Porcine reproductive and respiratory syndrome virus (PRRSV) infection in young piglets is frequently associated with secondary infection due to various pathogens, especially those of the respiratory tract. One of the most important mechanisms in respiratory diseases is related to the alteration of function of porcine alveolar macrophages (PAMs). The objective of this study was to determine how PRRS virus infection affects the capabilities of PAMs in the phagocytosis and destruction of Haemophilus parasuis. Phagocytosis percentages were determined in vitro and ex vivo, after collected PAMs were directly exposed to the virus of if PAMs were collected from piglets previously infected with PRRSV. In vitro experiments demonstrated that H. parasuis uptake by PAMs is only increased in the early stages of PRRSV infection (2 h post-infection). In contrast, in the ex vivo experiments it was shown that PAMs from PRRSV-infected piglets do not seem to change in their phagocytic rate until the later stages of infection. Together with a decrease in the phagocytic rate, a marked decrease in the functional ability of PAMs to kill bacteria was observed 7 d post-infection. It is hypothesized that when animals are exposed to PRRSV, there is a marked decrease in the functional ability of PAMs to kill bacteria through the release of superoxide anion, indicating a possible negative effect of the virus, at least at the macrophage level.     

Responses of Fasciola hepatica infected sheep to various infection levels

The response to Fasciola hepatica was studied in sheep infected with 5, 30, 150 metacercariae. The animals were necropsied 12 weeks post-infection (p-i) for counting and measuring flukes. Cellular and humoral responses were detected by peripheral eosinophil count, peripheral blood lymphocyte proliferation with excretory-secretory products (FhESP) and ELISA. All sheep were infected at necropsy except one sheep which was infected with 5 metacercariae. Mean parasitic intensities were 40%, 44% and 27% of the infection dose in sheep infected with 5, 30, 150 metacercariae respectively. FhESP-specific lymphocyte responses of the 3 infected groups were significantly enhanced in weeks 3 and 4 p-i (p < 0.05). The kinetics of the specific humoral response were similar for the 3 infected groups but the antibody level was significantly lower in animals infected with 5 metacercariae than in the 2 other infected groups from week 5 p-i to week 12 p-i (p < 0.05). Peripheral eosinophil count was significantly enhanced (p < 0.05) in infected groups. The numbers of peripheral eosinophils were significantly different between the 3 infected groups in week 3, 4 and 6 p-i and were related to infection level. These results confirm that sheep are highly susceptible to F. hepatica infection, even when infection pressure is very low. Peripheral eosinophilia was dependent of the infection level. The immune response was similar in sheep infected with various numbers of flukes.     

Modulation of juvenile brook trout (Salvelinus fontinalis) cellular immune system after Aeromonas salmonicida challenge

In fish, the first line of defence against infectious microorganisms is based on non-specific cellular immune mechanisms (innate immunity). In this study, we measured the non-specific immune parameters (natural cytotoxic cells (NCC) activity, lymphoproliferation, percentage of phagocytosis and phagocytic activity) in brook trout (Salvelinus fontinalis) infected by a virulent strain of Aeromonas salmonicida. Eight days post-infection, the mortality of infected fish reached 70%. A transient immunostimulation of the NCC activity was noticed 24h post-infection, but there was no significant difference at 48 h. Then, infection of brook trout with A. salmonicida induced a biphasic immune response. At 24h post-infection, lymphoproliferation was drastically depressed but returned to control level at 96 h. A slight increase in the percentage of phagocytosis and the phagocytic activity was noticed throughout the experiment. Conversely the cell mortality was significantly higher in infected fish compared to control. The modulation of immunological parameters might reveal important clues on how innate immunity might protect fish from bacterial infections.     

Indirect fluorescent antibody test for the detection of antibodies to Echinococcus granulosus in experimentally infected pups

Adult parasites of Echinococcus granulosus of buffalo origin were used as antigen in the indirect fluorescent antibody test (IFA) for detection of antibodies to E. granulosus in experimentally infected pups. The technique permitted detection of antibodies on Day 5 post-infection (p.i.) and up to Day 80 p.i. in infected animals. The antigen-antibody reaction was characterized by the appearance of a specific brilliant greenish yellow fluorescence on the embryophore of mature eggs present within and outside the gravid segment of the cestode. A maximum antibody titre of 1:320 at Day 50 p.i. was observed in the infected pups, coinciding with maturation of adult worms in the intestine of the host.     

In utero infection with PRRS virus modulates cellular functions of blood monocytes and alveolar lung macrophages in piglets

The putative immunosuppressive effect of PRRS virus (PRRSV) on innate immune responses was studied in piglets infected in utero with PRRSV. Phagocytosis and oxidative burst capacities in 2-, 4- and 6-week-old in utero infected piglets were investigated and compared with age-matched control piglets. Phagocytic capacity of blood monocytes against Salmonella bacteria was investigated by flow cytometry. Oxidative burst in blood monocytes and in alveolar lung macrophages was investigated by luminol- and lucigenin-enhanced chemiluminescence, respectively. Decreased phagocytosis against Salmonella was found in blood monocytes from 4- and 6-week-old infected piglets compared to controls. In contrast, 2-week-old infected piglets showed phagocytic responses comparable to age matched control piglets. While oxidative burst capacity was increased in blood (PBMC) from in utero PRRSV infected piglets, the oxidative burst capacity of alveolar lung macrophages was decreased, especially in 2- and 4-week-old piglets, compared to age-matched control piglets. The present results indicate that in utero infection with PRRSV inhibits phagocytosis against Salmonella in blood monocytes as well as the oxidative burst capacity of alveolar macrophages. These observations indicate that PRRSV in utero infection induces at state of immunosuppression in piglets paving the way for enhanced secondary infections.     

Effect of chronic FIV infection, and efficacy of marbofloxacin treatment, on Mycoplasma haemofelis infection

The purpose of this study was to investigate the effect of chronic feline immunodeficiency virus (FIV) infection, and efficacy of marbofloxacin treatment, on Mycoplasma haemofelis infection. Six cats chronically infected with FIV-Glasgow8 (Group X) and six FIV-free cats (Group Y) were infected with M. haemofelis on Day 0 by intravenous blood inoculation. From Day 0 until Day 86 post-infection (pi), blood samples were collected for M. haemofelis and FIV provirus quantitative real-time PCR and haematology. Three of the six cats in each of Groups X and Y were randomly selected to receive marbofloxacin treatment (2 mg/kg PO q24 h) from Day 16 to 43 pi, with the remaining cats being untreated controls with no antibiotic treatment. The M. haemofelis copy numbers and haematological data were compared between Groups X and Y, and between marbofloxacin-treated and control cats using a Mann-Whitney U-test. M. haemofelis infection was associated with development of macrocytic hypochromic anaemia. In some cats, marked variation in M. haemofelis copy number over time (>100,000-fold difference within 48 h in some cats) and/or cycling of copy number was seen. No correlation was found between FIV provirus copy number and M. haemofelis copy number or haematological variables. No significant effect of chronic FIV infection on M. haemofelis copy number kinetics or haematological changes due to M. haemofelis infection was found, other than MCHC (P=0.03). Marbofloxacin treatment was associated with a significant decrease in M. haemofelis copy number (P=0.002), although consistent clearance of infection was not demonstrated. This study reveals the presence of marked fluctuations in M. haemofelis copy number kinetics in vivo and a significant response to marbofloxacin antibiotic treatment.     

Differential haematological effects of tick-borne fever in sheep and goats

Tick-borne fever (TBF) is a rickettsial disease of domestic and wild ruminants in temperate climates. It is characterized by high fever and severe leukopenia. In the present study, the possible difference in the severity of disease in sheep and goats was investigated by inoculating one group of eight goats and one group of eight sheep with the Old Sourhope (OS) strain of Ehrlichia (Cytoecetes) phagocytophila. All sheep and goats experimentally infected with E. phagocytophila reacted with fever and rickettsiaemia, but there were significant differences between goats and sheep in the severity of clinical disease, the duration and magnitude of fever, the magnitude of rickettsiaemia and the patterns of reduction in the number of total leucocytes. Sheep reacted with fever significantly earlier than goats and the febrile period lasted for a significantly longer period. In contrast, the magnitude of rickettsiaemia was significantly higher in goats than in sheep. Infection with TBF was characterized by a transient increase in the number of neutrophils, which was quickly followed by an acute reduction in the number of lymphocytes and a prolonged reduction in the number of neutrophils in both sheep and goats. In both groups of animals, infection with TBF was also characterized by significant reductions in the total number of red blood cells (RBCs), thrombocytes and packed cell volume (PCV) and the concentration of haemoglobin (Hb). However, the mean corpuscular volume (MCV) and mean corpuscular Hb (MCH) were significantly increased in sheep only.     

Influence of naturally acquired feline leukemia virus (FeLV) infection on the phagocytic and respiratory burst activity of neutrophils and monocytes of peripheral blood

The purpose of this study was cytometric evaluation of phagocytic and oxidative burst activity of neutrophils and monocytes in cats naturally infected with FeLV. To conduct the study, the peripheral blood was obtained from 33 cats naturally infected with FeLV. The control group consisted of 30 FeLV-, FIV-, clinically healthy cats. The percentage of phagocytizing neutrophils of peripheral blood was lower in FeLV+ than in FeLV- cats. The percentage of neutrophils and monocytes in which an oxidative burst occurred was lower in FeLV+ than in FeLV-animals. Also an oxidative product formation in neutrophils after E. coli and PMA stimulation was lower in FeLV+ than in FeLV-animals. Obtained results allow to conclude that diminished phagocytic and oxidative burst activity of peripheral blood leukocytes may cause impairment of innate immunity in cats infected with FeLV.     

Comparison of blood non-specific immune parameters in Bovine virus diarrhoea virus (BVDV) persistently infected and in immune heifers

Several data from different authors show that Bovine virus diarrhoea virus (BVDV) could be a key component in multiple-etiology diseases, indeed a lower leukocytes number and their impaired functions decrease the resistance to infections. However, most of the information on the impairment of immune function during BVDV infections arise from circumstantial evidence and from experimental infection studies, and few from field data. To assess the effects of BVDV on blood cells parameters, cellular and humoral functions under field conditions, we designed a controlled study in commercial dairy herds, comparing persistent infected (PI) and healthy heifers. A total of 45 heifers were considered, the PI animals were nine, the control animals were 34, while two controls were considered as acute infected animals. The comparison of the mean values in PI calves showed a significant decrease for leukocytes and granulocytes, while platelets showed a significant increase, when compared with control animals. The total number of lymphocytes decreased not significantly in PI animals, while the proportion significantly increased. The number and proportion of monocytes was significantly reduced in PI animals, when compared with controls. The data collected on markers of cellular immunity during our study cannot be compared with the literature because there are no reference values. The presence of a persistent infection affected the cellular enzymes: NAGase, lysozyme and respiratory burst showed a large statistically significant decrease in PI animals when compared with controls. The presence of a persistent infection with BVD virus influenced blood cells number and impaired some blood cell functions. Such impairment confirms that PI animals represent a threat to the herd not only because they could spread BVDV, but also because they are more susceptible to other infectious diseases.     

Comparison of Blood Non‐Specific Immune Parameters in Bovine Virus Diarrhoea Virus (BVDV) Persistently Infected and in Immune Heifers

Several data from different authors show that Bovine virus diarrhoea virus (BVDV) could be a key component in multiple‐etiology diseases, indeed a lower leukocytes number and their impaired functions decrease the resistance to infections. However, most of the information on the impairment of immune function during BVDV infections arise from circumstantial evidence and from experimental infection studies, and few from field data. To assess the effects of BVDV on blood cells parameters, cellular and humoral functions under field conditions, we designed a controlled study in commercial dairy herds, comparing persistent infected (PI) and healthy heifers. A total of 45 heifers were considered, the PI animals were nine, the control animals were 34, while two controls were considered as acute infected animals. The comparison of the mean values in PI calves showed a significant decrease for leukocytes and granulocytes, while platelets showed a significant increase, when compared with control animals. The total number of lymphocytes decreased not significantly in PI animals, while the proportion significantly increased. The number and proportion of monocytes was significantly reduced in PI animals, when compared with controls. The data collected on markers of cellular immunity during our study cannot be compared with the literature because there are no reference values. The presence of a persistent infection affected the cellular enzymes: NAGase, lysozyme and respiratory burst showed a large statistically significant decrease in PI animals when compared with controls. The presence of a persistent infection with BVD virus influenced blood cells number and impaired some blood cell functions. Such impairment confirms that PI animals represent a threat to the herd not only because they could spread BVDV, but also because they are more susceptible to other infectious diseases.