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

Revisiting the Economic Impacts of Eimeria and Its Control in European Intensive Broiler Systems With a Recursive Modeling Approach

Ionophore compounds active against Eimeria species are widely used in intensive broiler systems and have formed the backbone of coccidiosis control for almost 50 years. Producers, however, are under pressure to reduce ionophore use due to consumer concerns over antimicrobial usage in food animals, and antimicrobial resistance. Moreover, current vaccines against Eimeria are commonly considered to be less cost-effective in intensive broiler systems, especially in Europe where attenuated live vaccines are used. An economic assessment of the impact of Eimeria and the disease coccidiosis, including the cost implications of different efficacies of control, is therefore timely to provide evidence for industry and policy development. A mechanistic model of broiler production under varying infection and control states was used to construct a dataset from which system productivity can be measured. Coccidiosis impact increased rapidly as control efficacy decreased. In the total absence of control, median impact was found to maximize at between €2.55 and €2.97 in lost production per meter squared of broiler house over a 33 day growing period. Coccidiosis remains a major risk to intensive broiler systems and the model developed allows investigation of issues related to coccidiosis control, antimicrobial use and the development of antimicrobial resistance.

Re-calculating the cost of coccidiosis in chickens

Coccidiosis, caused by Eimeria species parasites, has long been recognised as an economically significant disease of chickens. As the global chicken population continues to grow, and its contribution to food security intensifies, it is increasingly important to assess the impact of diseases that compromise chicken productivity and welfare. In 1999, Williams published one of the most comprehensive estimates for the cost of coccidiosis in chickens, featuring a compartmentalised model for the costs of prophylaxis, treatment and losses, indicating a total cost in excess of £38 million in the United Kingdom (UK) in 1995. In the 25 years since this analysis the global chicken population has doubled and systems of chicken meat and egg production have advanced through improved nutrition, husbandry and selective breeding of chickens, and wider use of anticoccidial vaccines. Using data from industry representatives including veterinarians, farmers, production and health experts, we have updated the Williams model and estimate that coccidiosis in chickens cost the UK £99.2 million in 2016 (range £73.0-£125.5 million). Applying the model to data from Brazil, Egypt, Guatemala, India, New Zealand, Nigeria and the United States resulted in estimates that, when extrapolated by geographical region, indicate a global cost of ~ £10.4 billion at 2016 prices (£7.7-£13.0 billion), equivalent to £0.16/chicken produced. Understanding the economic costs of livestock diseases can be advantageous, providing baselines to evaluate the impact of different husbandry systems and interventions. The updated cost of coccidiosis in chickens will inform debates on the value of chemoprophylaxis and development of novel anticoccidial vaccines.

A model for the dynamics of a protozoan parasite within and between successive host populations

Parasite-host systems often include an obligatory environmental stage in the parasite life-cycle, which can be transmitted between successive populations. Complexity even increases if immunity only gradually develops upon re-infection. For a better understanding of such systems we study Eimeria spp. in chickens, a protozoan parasite transmitted through oocysts on the floor. This paper deals with dynamics within and between successive cohorts of chickens by coupling a within-host description of the parasite life-cycle (with immunity) to re-uptake of oocysts from the environment. First the initial environmental oocyst level is related to the maximum infection load within a cohort, as a measure of production damage, from which we conclude that minimum damage levels can be observed with intermediate oocyst levels. Then we relate the initial to the final oocyst level of a cohort, and study the dynamics between cohorts in relation to an oocyst cleaning efficiency after each cohort. The resulting unstable dynamics lead to the conclusion that it will often be impossible to minimize damage by repeatedly cleaning with the same effort: it may be necessary to artificially increase oocyst levels in the shed before each chicken cohort.

A model for predicting feed intake of growing animals during exposure to pathogens

A general model is proposed for predicting the effects of subclinical pathogen challenges of different doses and virulence on the relative feed intake (RFI) of animals. The RFI is defined as the feed intake (FI, kg/d) of the animal challenged by a pathogen divided by its FI in the same state had it not been challenged. Actual FI can be predicted from the RFI and the animal's state. The RFI was assumed to be affected only when animals were naïve to a particular pathogen (i.e., had not previously experienced it) and when the challenge dose was above a predetermined threshold. The model is for the period from recognition of a pathogen through acquisition and subsequent expression of immunity. The way in which RFI changes with time is described by 5 main parameters and is based on data for RFI during different pathogen challenges of a range of hosts. Lag time (L, d) is the delay from a pathogen challenge until any effects on RFI are seen. Reduction time (R, d) describes the time it takes for the lowest value of RFI (lambda) to be achieved. The duration time (D, d) describes the time that lambda is maintained for, and rho (RFI/d) describes the rate of recovery of RFI until RFI = 1. There is no compensatory intake, and RFI is always < or = 1. The effects of host resistance on the values of the model parameters are proposed. Attempts were made to parameterize the model; when data were scarce, initial parameter values were derived on conceptual grounds. Predictions of the effects of pathogen dose, virulence, and host resistance are described and discussed. When comparing the responses in RFI for different genotypes, it is crucial to define the pathogen challenge (in terms of dose and virulence) and the degree of resistance of different hosts. Possible interactions between dose, virulence, and resistance were explored. Feed intake of healthy and challenged animals, at a time, may be different once the challenged animal has recovered (RFI = 1). The issue of reductions in FI during pathogen challenges is important for nutritionist and animal breeders. The large variation that has been observed for reductions in FI during pathogen challenges may be a viable point of selection. The points highlighted will aid selection strategies by quantifying the effects of pathogen dose and virulence, and time, on the FI of challenged animals. The proposed model may be integrated with other models of growth to predict animal performance during exposure to pathogens.

Sporulation of Eimeria maxima oocysts in litter with different moisture contents

The aim of this study was to determine if the sporulation of Eimeria maxima oocysts was affected by the moisture content of the litter. Fresh feces were collected from chickens experimentally infected with E. maxima. The feces were mixed with dried wood shavings and different amounts of water to obtain final moisture contents of 16, 42, and 62% and a final oocyst concentration of 5,000 per g of mixture. The samples were kept at 23 C and 75% relative humidity and were thoroughly aerated every 12 h. Oocysts kept under ordinary laboratory sporulation conditions in 2% wt/vol aqueous potassium dichromate at 27 C were used as a standard for optimal sporulation. The proportion of sporulated oocysts was determined microscopically every 12 h. Sporulation of E. maxima was most efficient under the driest conditions studied (16% moisture content), and poorest in the samples with the highest moisture content (62%). Even though the differences may not have resulted from a direct effect of humidity on the oocysts, but more likely resulted from limited oxygen in the moister substrates, it is clear that sporulation is not favored by moist litter.

A compartmentalised model for the estimation of the cost of coccidiosis to the world's chicken production industry

A compartmentalised model is presented for the estimation of the monetary losses suffered by the world's poultry industry resulting from coccidiosis of chickens and costs of its control. The model is designed so that the major elements of loss may be separately quantified for any chicken-producing entity, e.g., a farm, a poultry company, a country, etc. Examples are presented and the sources, reliability and geographical relevance of the data used for each parameter are provided. Loss elements for specific geographical areas should be recalculated at appropriate intervals to take into account local and international fluctuations in costs of chicks feed, labour, financial inflation and world currency exchange rates. Equations are given for relationships among numbers of chickens, liveweights, weights of carcasses, feed consumptions, feed conversion ratio (FCR), prices of feeds, prices of anticoccidial therapeutic and prophylactic drugs, values of chickens, chicken rearing costs; and effects of coccidiosis on mortality, weight gain and FCR. Using these equations, it is theoretically possible for an international team of representatives, each using reliable local data, to calculate simultaneously each relevant loss element for their respective countries. Addition of these elements could give, for the first time, an accurate global estimate of the losses due to chicken coccidiosis. The total cost of coccidiosis in chickens in the United Kingdom in 1995 is estimated to have been at least GB pound silver 38 588 795, of which 98.1% involved broilers (80.6% due to effects on mortality, weight gain and feed conversion, and 17.5% due to the cost of chemoprophylaxis and therapy). The costs of poor performance due to coccidiosis and its chemical control totalled 4.54% of the gross revenue from UK sales of live broilers. This model includes a new method for comparing the profitabilities of different treatments in commercial trials. providing actual costs rather than the arbitrary numerical scores of other methods. Although originally designed for the study of coccidiosis, the model is equally applicable to any disease. It should be of value to agricultural economists, the animal feed and poultry industries, animal health companies, and to research scientists (particularly for preparing grant applications).

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 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.