Immune responses and resistance to Eimeria acervulina of chickens divergently selected for antibody responses to sheep red blood cells

Resistance to Eimeria acervulina was measured in two lines of chickens that had been divergently selected for high (H line) or low (L line) antibody (Ab) responses to SRBC, and in a randombred control (C) line originating from the same parental stock. Fecal oocyst output of cocks from the three lines from the 17th generation was estimated after primary and secondary infection with 2 x 10(5) oocysts. In addition, Ab responses to E. acervulina oocyst antigen and cellular immune responses in vitro to E. acervulina antigen were measured after primary and secondary infection with E. acervulina. No significant line differences were found with respect to fecal oocyst output after primary infection. Only at the end of the primary infection period, i.e., Day 15 postprimary infection, was a significantly lower fecal oocyst output found in the H line as compared to the C and L lines. After secondary infection, significantly higher fecal oocyst output was found in the C line. Significantly higher Ab response after primary and secondary infection were found in the H and C lines as compared to the L line. No line differences were found for cellular immune responses to E. acervulina oocyst antigen. These observations imply that selection for enhanced humoral immunity to SRBC did not result in enhanced resistance to E. acervulina in terms of fecal oocyst output. However, the H line might expel E. acervulina more rapidly than the other two lines. The absence of line differences in resistance to Eimeria is discussed with respect to the role of the humoral immune response.

Modelling the effect of surveillance programmes on spread of bovine herpesvirus 1 between certified cattle herds

For the eradication of an infectious agent, like bovine herpesvirus 1 (BHV-1), surveillance and certification can be used to reduce the transmission between herds. The goal of surveillance is that a certified herd that becomes infected is detected timely so that infection of several other certified herds is prevented. What counts is whether the reproduction ratio R, i.e. the average number of certified herds infected by one infected certified herd can be kept below 1. To support policy makers in making decisions about the minimal demands for a surveillance programme in an eradication campaign of BHV-1 in cattle, two mathematical models were investigated. With these models, the basic reproduction ratio between herds was calculated. The surveillance programmes were characterised with sample size, sampling frequency, test sensitivity, herd size, vaccination status, and contacts between herds. When R between herds is below 1, then the surveillance programme is sufficiently good to prevent spread of infection, provided that R is estimated well. In the model based on bulk milk testing sample size was replaced by a threshold at which bulk milk can be found positive. The R between herds was mainly influenced by the vaccination status, sampling frequency, and contacts between herds. Herd size moderately affected the outcome. Test sensitivity and sample size, however, were of minor importance. If herds of 50 cows became free of BHV-1 without vaccination, then spread of infection between herds might be prevented when animals within herds are sampled once a year (milk or blood samples). This frequency needs to be intensified, being twice a year, for larger herds and/or herds with extensive contacts with other herds. When bulk milk is sampled instead, sampling should be done at least every 5 months and more intensively, being each month, with larger herd sizes and more contacts between herds.

The serological BHV1 status of dams determines the precolostral status of their calves

Precolostral calves and their dams were serologically investigated for the presence of antibodies against Bovine Herpesvirus 1 in diagnostic tests with a very high sensitivity and specificity. Although the syndesmo-chorial type of placenta of ruminants does not transfer gamma globulins, a large number of calves had antibodies, in most cases in a very low concentration. Significant correlations were found between the serological status of the dam, the status of the calf, and the titre of antibodies. Oral intake of maternal blood by the calf at birth or transmission or leakage of maternal antibodies during pregnancy might be possible causes of precolostrally positive calves. From the results it is concluded that to reduce the risk of obtaining BHV1-positive calves, BHV1-negative dams should be selected for breeding purposes.

Risk factors for Salmonella dublin infection on dairy farms

In a case-control study of risk factors for Salmonella dublin on 126 dairy farms the following variables were significantly associated with infection. Standardized herd size and water surface area showed a positive association. Contact with cows from other herds during exchange was, unexpectedly, preventive for S. dublin infection. Purchase of cows from other herds was a risk factor. More infections occurred when the feed consisted of grass only compared to grass supplemented with maize or silage. S. dublin infection was highly associated with a liver fluke infection.

[Risk factors for Salmonella dublin infections on dairy farms]

A case-control investigation of the risk factors for Salmonella dublin infections on 126 dairy farms identified the following variables as being significantly associated with infection: standardized farm size and presence of water courses. Contrary to expectations, contact with cattle from other farms when the animals were turned out to grass was negatively associated with S. dublin infection. The purchase of cattle from other farms was a risk factor. There were more infections when feed consisted of grass alone than when grass was supplemented with maize or grass silage. S. dublin infections were strongly correlated with liver fluke infections.

Increased risk of abortion following Neospora caninum abortion outbreaks: a retrospective and prospective cohort study in four dairy herds

Explosive abortion outbreaks in 4 Dutch dairy herds during 1992 to 1994 are reported. In 50 of 51 fetuses submitted during the first 3 wk of the outbreaks characteristic histological lesions compatible for infection with Neospora caninum were seen. Diagnosis of infection was confirmed by immunohistochemistry in 40 fetuses (78%). No evidence for other abortifacients was found. The abortion risk of the herds was investigated in a prospective and retrospective cohort study. The prospective study showed that cows aborting during the outbreaks and N. caninum seropositive nonaborting cows had a two- to three-fold increased risk of abortion compared with N. caninum seronegative cows. Retrospective examination showed that seropositive nonaborting cows had an increased risk of abortion before the outbreaks, which may indicate that these animals were infected with N. caninum prior to the outbreaks. It is concluded that serostatus can be used for selective culling of cows to decrease future risk of abortion in dairy herds.

Quantifying risk factors of coccidiosis in broilers using on-farm data based on a veterinary practice

A study was done to find and quantify risk factors for coccidiosis. The study population consisted of 4774 broiler flocks kept on 177 farms. Flocks were considered a case when at least one bird in the flock showed microscopic presence of oocysts in intestinal scrapings in a grow-out cycle. Other flocks were defined as controls. This was done for three types of Eimeria: Eimeria acervulina, Eimeria tenella and Eimeria maxima. Logistic regression was used to assess variables that influence the occurrence of Eimeria species. There were 49 variables, based on animal, flock or farm level. There was an enhanced risk of coccidiosis due to environmental and management factors that increase the risk of introducing contamination or that are related to hygienic measures. These include lack of use of overalls by visitors, a farmyard which is difficult to clean, bad hygienic status, personnel who might also be working on other farms, presence of other animals on the farm, and feeding and drinking systems which are more difficult to clean. Also, the presence of other diseases on the farm and Eimeria species found in the previous flock increased the risk of coccidiosis.

Eimeria acervulina: influence of corticosterone-induced immunosuppression on oocyst shedding and production characteristics in broilers, and correlation with a computer simulation model

An experiment was conducted to investigate the effects of immune responsiveness on excretion of oocysts after E. acervulina infection and subsequent effects on production characteristics of broilers (Gallus domesticus). These effects were determined in broilers repeatedly infected with 2.85 x 10(3) oocysts of E. acervulina and treated with various dosages of corticosterone in the diet (0, 10, 20 and 30 p.p.m.). Corticosterone treatment did not have an effect on the peak oocyst excretion, although it was administered from 4 days before initial infection. The number of oocysts excreted shortly after the peak and the length of the excretion period were increased in corticosterone-treated groups. The absence of a difference in peak oocyst excretion was ascribed to the existence of a time-lag between first contact with the parasite and rate of development of protective immunity. In a recently developed computer simulation model this period was assumed to be 5 days. Assuming that immunosuppression, through corticosterone, is only effective when protective immunity is in operation, the results indicate a time-lag of at least a few days, which supports the inclusion of such a time-lag in the computer simulation model. General immunosuppressive effects of the corticosterone treatment, monitored by antibodies and mitogen-induced lymphocyte stimulation confirmed that immunosuppression occurred shortly after medication started. Infection did not have a significant influence on production characteristics in animals without dietary corticosterone. However, with increasing corticosterone levels the negative effects of infection on production also increased.

Effects of initial litter contamination level with Eimeria acervulina on population dynamics and production characteristics in broilers

The present experiment was done to obtain experimental evidence supporting the existence of an optimal initial contamination level of Eimeria acervulina with respect to broiler productivity, as suggested by simulation model outcomes. Three levels of initial (on Day 3) contamination of the litter were applied (low, intermediate and high). The peak of oocyst excretion during the flock cycle (grow-out) (Days 0-36) depended on the initial contamination level. Oocysts peaked at Day 15, 22 and 33 for the high, intermediate and low contamination level, respectively. Antibody titres and plasma carotenoid contents were not significantly affected by contamination levels. Average body weight at the end of the flock cycle (Day 36) and average daily body weight gain were significantly higher at the intermediate contamination level compared with the low and high contamination levels. Average body weight at Day 36 was 1681 g, 1712 g and 1674 g for the low, intermediate and high contamination level, respectively. Average daily weight gain was 45.7 g, 46.5 g and 45.5 g for the low, intermediate and high contamination level, respectively. Thus, the data from this experiment support the existence of an optimum initial contamination level for E. acervulina with respect to performance results.

Description of a model to simulate effects of Eimeria acervulina infection on broiler production

A simulation model for effects of Eimeria acervulina infection on technical and economic characteristics in broiler production is presented. The model describes development over time of the growth depression, feed intake reduction, and decrease in feed efficiency associated with infection. The model also shows a phase of compensatory growth during which earlier negative effects are counterbalanced, at least partly. Major assumptions made were: infection with E. acervulina occurs in each flock; production is affected in each flock; compensatory growth takes place because immunity develops and cell regeneration occurs. The results show that the pattern of development of the production characteristics during a flock cycle depends on the initial contamination level. Both a high and low initial contamination level results in a lower average daily gain, a worse feed to gain ratio, and a reduced net revenue compared to an intermediate contamination level.

Description of a simulation model for the population dynamics of Eimeria acervulina infection in broilers

A simulation model for the population dynamics of Eimeria acervulina infection in broilers is presented. The model describes the development of the numbers of parasites in the various life-stages during the growing period of broilers and the empty house period between grow-outs. The model includes assumptions with respect to development of immunity to E. acervulina infection and effects of application of anticoccidial drugs. The model consists of a set of difference equations that are solved numerically at 1 h intervals. Under constant conditions, an equilibrium level was reached after a few grow-outs during which infection always peaked around the 21st day in the growing period. Within a growing period, infection peaked earlier (later) than the 21st day in case initial numbers of sporulated oocysts were higher (lower) than the equilibrium number.

Rate and course of sporulation of oocysts of Eimeria acervulina under different environmental conditions

An experiment was conducted to determine the rate and maximum percentage of sporulation of Eimeria acervulina oocysts at various environmental conditions relating to temperature (21 versus 33 degrees C) and relative humidity (RH) (40 versus 80%). Measurements were made during 44 h after excretion of oocysts in 3 substrates: dry litter, clammy litter and pure faeces respectively. Maximum sporulation percentage in both dry (22.6%) and clammy litter (19.5%) was higher (P < 0.005) than in pure faeces (11.6%). Neither temperature nor RH had a significant influence on percentage of oocysts that sporulated. Under these simulated practical conditions approximately 25% of all oocysts sporulated, whereas sporulation under optimal conditions (29 degrees C, aeration, 2% K2Cr2O) showed a higher (68%) sporulation ability of oocysts. At 33 degrees C sporulation proceeded at a faster pace than at 21 degrees C (P < 0.005). With respect to RH and substrate, once sporulation started, the rate of increase to maximum percentage was not different. Time of onset of sporulation was influenced by temperature (P < 0.0001) and RH (P < 0.001). Time of onset occurred 15 h later at 21 degrees C compared with 33 degrees C and 5 h later at 40% RH compared with 80%. Also, an interaction effect (P < 0.01) was found with effect of RH being stronger at 21 degrees C compared with 33 degrees C. It was concluded that the most important aspect in the epidemiology of E. acervulina during a flock cycle is the time of onset of sporulation with temperature being the most important factor.

Multivariate epidemiological approach to salmonellosis in broiler breeder flocks

A retrospective, case-control study into risk factors of salmonellosis was undertaken using data from 111 broiler breeder flocks assembled during a 5-yr period. The results of both univariate and multivariate analyses are presented. Many different Salmonella species were detected. Multivariate models were created based on the outcome of univariate analyses. The following variables appeared to be the most relevant: disinfection tubs, hygiene barriers, the interaction of disinfection tubs by hygiene barriers, and feed mills. The final model indicated that flocks housed at farms without a disinfection tub, with poor hygiene barriers, and receiving their feed from a small feed mill had a 46.1 times greater risk of being Salmonella-positive than flocks housed at farms with a disinfection tub, with good hygiene barriers, and receiving their feed from a large feed mill. It is concluded that the application of quantitative epidemiological methods can be valuable not only to identify potential risk factors but also to quantify their contributory effect on the disease outcome. Hence, it may be a useful tool for application in "integrated food chain quality control programs".