Association of lowered plasma carotenoids with protection against cecal coccidiosis by diets high in n-3 fatty acids

Effects of pre-starter and starter diets varying in amino acid density given to broiler chickens that received coccidiosis vaccination at hatch1

An experiment was conducted to evaluate broilers vaccinated against coccidiosis fed combinations of pre-starter and starter diets varying in digestible amino acid (dAA) density from 1 to 19 d of age on subsequent growth performance and meat yield. A total of 1,800 Yield Plus Ross × Ross 708 male broilers were allocated to 60 floor pens and assigned to 1 of the 6 treatments. Four pre-starter diets varying in dAA density [1.15% dLys, 1.25% dLys, 1.35% dLys, and Positive Control (1.15% dLys + Diclazuril)] were fed until 9 d of age. Then, 3 starter diets varying in dAA density [1.15% dLys, 1.25% dLys, and PC (1.15% dLys + Diclazuril)] were given from 10 to 19 d of age. All diets were formulated to similar dAA ratios to dLys; hence, resulting in increasing amino acid (AA) density among the dietary treatments. Birds were given common grower and finisher diets for the remainder of the experiment. At 19, 27, and 40 d of age, broilers that received the 1.35% and 1.25% dLys density pre-starter and 1.25% dLys density starter diets had increased BW gain (P ≤ 0.006) compared with broilers fed the 1.15% dLys density diets. Broilers that received the 1.15% dLys density pre-starter diet had a lower cumulative BW gain (P = 0.007) than those fed the 1.35% and 1.25% dLys density pre-starter diets. Broilers that received the 1.35% and 1.25% dLys density pre-starter and 1.25% dLys density starter diets had the heaviest (P ≤ 0.008) total breast weights. Increasing the pre-starter dAA density enhanced (P ≤ 0.03) total breast weight by 28 g (1.25% dLys density) and 51 g (1.35% dLys density). Results from this study indicated that increasing dAA density from 1 to 19 d of age enabled a 1 to 19 d dAA intake increase of 4.8 g, which enhanced cumulative growth and meat yield of broilers vaccinated against coccidiosis.

Temporal pattern changes in duodenal protein tyrosine nitration events in response to Eimeria acervulina infection in chickens

Intracellular generation of nitric oxide (NO) and superoxide anion (SOA) can result in the formation of 3'-nitrotyrosine proteins (NTp). Nitrated proteins usually are associated with significant perturbation in protein function, apoptosis, autophagy, and cell death. We undertook the present study to establish the temporal dynamics of NTp generation in cytokeratin-18-positive epithelial cells (ETCs) of broiler chickens in response to infection with Eimeria acervulina. Duodenal tissue was harvested from noninfected (NOI) and infected (INF) broilers on days (d) 1, 3, 6, 7, and 10 postinfection (PI) and fixed, embedded, and sectioned for quantitative image analysis, immunohistochemistry with antibodies specific to NTp and the SOA-generating enzyme xanthine oxidase (XO). The pixel density characteristics for NTp and XO representative of ETCs demonstrated that NTp and XO increased in intestinal villi as early as d1 PI (P < 0.05 vs. NOI). Progressive increases in NTp were evident in ETCs through d6 PI. For XO, increases in cell content increased only through d3. On d6 and d7 PI, high levels of NTp were present in immune infiltrating cells (IIC) where no XO was detected. The increases in ETC NTp occurred in a defined pattern, significant by villus-to-crypt location for day of infection, initiating in the distal villus and progressing down into the crypts. Two NTp patterns were observed for ETCs: a high level associated with ETCs harboring parasites and a low-level increase in ETCs not containing Eimeria but in proximity to such. The data suggest that NTp and XO responses may mediate some of the processes through which ETCs respond to Eimeria to limit the extent of infection by this pathogen.

Protection of Broiler Chicks Housed with Immunized Cohorts Against Infection with Eimeria maxima and E. acervulina

The use of live oocyst vaccines is becoming increasingly important in the control of avian coccidiosis in broilers. Knowledge of the mechanisms employed when chicks uptake oocysts and become immune is important for optimizing delivery of live vaccines. The current study tests the hypothesis that chicks not initially immunized may ingest oocysts by contact with litter containing oocysts shed by immunized cohorts. In Experiment 1, day-old broiler chicks were housed in pens containing clean litter. In Trial 1, 100% of chicks in some pens were immunized with 2.5 X 10(3) Eimeria acervulina oocysts while in other pens only 75% of chicks were immunized and remaining cohorts within the pens were not immunized. Other pens contained chicks that served as nonimmunized nonchallenged controls or nonimmunized challenged controls (NIC). On day 21, birds were given a homologous challenge of 6 X 10(5) oocysts. A second identical trial was conducted, except birds were immunized with 500 Eimeria maxima oocysts and were challenged with 3 X 10(3) E. maxima oocysts. In Experiment 2, 100% of chicks in some pens were immunized with 500 E. acervulina oocysts while in other pens either 75% or 50% of the birds were immunized. On day 14, birds were challenged with 1 X 10(6) oocysts. Trial 2 was identical to Trial 1 except that birds were immunized with 100 E. maxima oocysts and challenged with 1 X 10(6) oocysts. For all experiments weight gain, feed conversion ratio (FCR), plasma carotenoids, and litter oocyst counts were measured. In Experiment 1, the level of protection in groups containing 25% nonimmunized cohorts, as measured by weight gain, carotenoid level, FCR, and oocyst litter counts, was identical to groups containing 100% immunized chicks. In Experiment 2, pens where 50% or 75% of birds were immunized with either E. maxima or E. acervulina were not well protected from decreases in weight gain and plasma carotenoids nor from increases in litter oocyst counts following a challenge infection administered on day 14 relative to NIC. In addition, pens of birds where 100% of chicks were immunized were not well protected compared to NIC, and resistance to coccidiosis infection in immunized chicks was less than resistance in chicks challenged at 21 days. These results in total suggest that, when birds are challenged after 21 days, cohorts are protected from detrimental effects of challenge infection. However, when challenge infection is given at 14 days, cohorts are not well protected. The results support a conclusion that protection to coccidiosis is conveyed to cohorts by contact with oocysts shed into the litter by immunized chicks, but this resistance may take 14 days to develop.

Influence of grape seed proanthocyanidin extract in broiler chickens: effect on chicken coccidiosis and antioxidant status

Grape seed proanthocyanidin extract (GSPE) has been widely used as a human food supplement for health promotion and disease prevention. However, there was little information regarding its application in animal nutrition. The aim of the current study is to determine the effect of GSPE at different concentrations on chicken performance, and the status of antioxidant/oxidant system after the Eimeria tenella infection. In the first experiment, GSPE incorporated in the diet at 5, 10, 20, 40, and 80 mg/kg significantly decreased mortality and increased weight gain after the E. tenella infection, and the protective effect of GSPE was dose-dependent. The lowest mortality and the greatest growth gains were recorded in the group of birds fed with GSPE between 10 to 20 mg/kg. In the second experiment, 12 mg/kg of GSPE supplementation in the diet significantly reduced the mortality and lesion scores in birds after the infection with 5 x 10(4) and 1 x 10(5) oocysts of E. tenella. The weight gains also improved significantly. After the oral infection with 5 x 10(4) and 1 x 10(5) of E. tenella, analysis of the status of antioxidant/oxidant system revealed that plasma NO increased significantly from 7.11 to 21.31 micromol/L, plasma superoxide dismutase (SOD) decreased from 126.55 to 111.14 U/mL, and malondiadehyde increased, suggesting oxidative stress was increased in circulation. However, supplementation of 12 mg/kg GSPE reduced the level of plasma NO from 21.31 to 14.73 micromol/L and increased plasma SOD activities from 111.14 to 133.27 U/mL. The effects of incorporation of GSPE into the poultry diet on the concentration of plasma NO, malondiadehyde, and SOD indicated that the lower concentration of dietary GSPE was able to restore the balance of antioxidant/oxidant system that was exerted by the oxidative stress after the parasite infection. The current results suggested GSPE can act as an antioxidant in diet to improve the performance of broiler chickens and remedy the clinical symptoms caused by the oxidative stress of E. tenella infection.

Fatty acids and coccidiosis: effects of dietary supplementation with different oils on coccidiosis in chickens

This study was undertaken to investigate the effects of dietary oils on coccidiosis of chickens infected with Eimeria tenella. Five hundred and four 1-day-old male Arbor Acres chicks were randomly assigned to seven groups. Seven diets were supplemented with poultry oil at 25 and 45 g/kg, corn oil at 25 and 45 g/kg, fish oil at 25 and 45 g/kg, and without oils, respectively, from 1 to 42 days of age. Chickens were inoculated by gavage with 3 x 10(5) sporulated oocysts at 21 days of age. Diets supplemented with oils at 45 g/kg resulted in higher body weight gain than with oils at 25 g/kg or without oils. The packed cell volume in chicks fed with diets supplemented with poultry oil was lower than that in chicks fed with diets supplemented with fish oil, corn oil and without oil. Chickens fed with diets supplemented with poultry oil had higher mortality than that of chickens fed with diets without oil. At 7 days post infection, the chickens consuming fish or corn oil diets had lower levels of plasma carotenoids than those fed with poultry oil supplement or without oils. Fish oil supplementation increased serum interleukin-6 levels in chickens compared with poultry oil at 25 and 45 g/kg and corn oil at 25 g/kg. Fish oil or corn oil enhanced secretory IgA levels in the lumen of the caecum, and oil supplementation tended to decrease serum IgG levels. In conclusion, the diets supplemented with saturated fatty acids aggravated mortality in chickens infected with E. tenella. The deleterious effects on coccidiosis of the diets supplemented with poultry oils might be related to the decreased levels of caecal E. tenella antigen-binding-specific secretory IgA or serum IgG.

Effects of T-cell-mediated immune response on avian oxidative stress

We evaluated the oxidative cost paid by birds when coping with an immune challenge. We used the phytohaemagglutinin skin test (PHA) to assess the effects of the T-cell-mediated immune response on the concentration of reactive oxygen metabolites (ROMs), total antioxidant barrier (OXY) and total serum carotenoid concentration in wild nestlings of the Eurasian kestrel (Falco tinnunculus). Immunostimulation caused increased levels of ROMs, decreased OXY and increased circulating levels of carotenoids. These results suggest that an immune challenge can increase avian oxidative stress, and that carotenoids were remobilised from other tissues likely because their circulating levels were not sufficiently high to sustain an effective immune response.

Development of a Protective Index to Rank Effectiveness of Multiple Treatments Within an Experiment: Application to a Cross-Protection Study of Several Strains of Eimeria maxima and a Live Vaccine

Vaccination of chickens with live oocysts has become a more widely used method for controlling avian coccidiosis as resistance to anticoccidial medication increases. However, some coccidia strains are not useful in multispecies vaccines because antigenic variation has made them generally less protective. In order to experimentally test a number of strains for the best cross protection, we have devised an evaluation method using four independently measured variables: weight gain, lesion score, plasma carotenoids, and plasma NO2- + NO3-. These values, when measured at 6 days postchallenge, tend to be significantly correlated. A protective index (PX) is calculated for each chicken using the following algorithm: PX = (Ngain + Ncarotenoids) - (Nsqrls + N[NO2- + NO3-]), where the prefix N indicates values for a variable normalized against a mean of that variable from a control group. Nsqrls values are normalized values of the square roots of lesion scores. The PX can then be treated as a dependent variable. In this study, mean PX values of unchallenged groups cluster around 0. Mean PX values of protected chickens are statistically close to those from unchallenged groups, whereas unprotected chickens have highly negative mean PX values.

Effects of dietary vitamin E on chickens infected with eimeria maxima: observations over time of primary infection

Two trrials were conducted to define temporal changes in plasma D-alpha-tocopherol (AT) caused by infection with Eimeria maxima in chickens that consumed either low (25 ppm) or high (225 ppm) levels of dietary DL-alpha-tocopheryl acetate (VE-AC) from 1 day of age. In both trials, rates of weight gain were depressed between days 5 and 7 post-inoculation (PI) and were not influenced by the level of dietary VE-AC. Plasma AT was consistently depressed at 5 and 7 days PI in chickens consuming either level of dietary VE-AC. The pattern and degree of plasma AT depression correlated with those of plasma carotenoids. Plasma levels of NO2- + NO3- were significantly increased at 5 and 7 days PI. In trial 1, the average increase during that period was not as high in chicks consuming 225 ppm VE-AC, but in trial 2, diet had no effect on the degree of increase. Also, there were no consistent effects of dietary VE-AC on lesion scores or amount of oocysts shed. These results are in general accord with findings of earlier experiments, and we conclude that feeding high levels of VE-AC to broiler chicks from 1 day of age is not effective in mitigating the pathology, including weight gain depression and development of mucosal lesions, during E. maxima infections or in modifying immune response events associated with phagocytosis as indexed by plasma NO2- + NO3-. The likely basis for the ineffectiveness of feeding this fat-soluble form of vitamin E is that it is malabsorbed during E. maxima infection in the same manner as carotenoids and becomes less biologically available to infected tissues during the acute phase of infection.

Recent advances in biology and immunobiology of Eimeria species and in diagnosis and control of infection with these coccidian parasites of poultry

Avian coccidiosis, an intestinal disease caused by protozoan parasites of the genus Eimeria, occurs worldwide. It is considered to be one of the most economically important diseases of domestic poultry. For many years, prophylactic use of anticoccidial feed additives has been the primary means of controlling coccidiosis in the broiler industry and has played a major role in the growth of this industry, which now can produce about 7.6 billion chickens annually. However, development of anticoccidial resistance has threatened the economic stability of the broiler industry. Although there has been little effort by the pharmaceutical industry to develop new anticoccidials, the mounting problem of drug resistance of Eimeria species has prompted major research efforts to seek alternative means of control through increased knowledge of parasite biology, host response, and nutritional modulation. As a consequence, important advancements have been made, particularly in defining parasite antigens that have potential use in vaccines, defining the Eimeria genome, understanding the immunology of coccidial infections, and the practical applications of live vaccines. This review describes the progress in these areas, most of which has occurred within the past 10 to 15 years.

Interaction of dietary vitamin E with Eimeria maxima infections in chickens

In two trials, broiler chickens, processed similarly to those placed in commercial operation, were fed, from 1 d of age, a range (13 to 200 ppm) of DL-alpha-tocopheryl acetate (VE-AC) levels, and the effects on the pathology of Eimeria maxima infections were assessed at 6 d postinoculation (PI). In Trial 1, dietary levels of VE-AC had little significant effect on variables characterizing pathology except for the number of oocysts shed, which was significantly increased in chicks treated with higher VE-AC levels. The infection was judged to be mild based on moderate lesion scores (2.2+/-0.2), lack of significant effects on weight gain (7+/-1.6% decrease), moderate reduction in plasma carotenoids (21+/-2%) and small increases in plasma NO2-+NO3- (141+/-12%). In uninfected and infected chickens, plasma alpha-tocopherol (AT) increased with dietary levels of VE-AC; however, E. maxima infection caused a fairly constant decrease in AT of 35.3+/-3.2% across these levels. Plasma gamma-tocopherol (GT) levels were unaffected by dietary VE-AC or E. maxima infection. In Trial 2, pathology, again, was relatively unaffected by dietary VE-AC level. The infection was judged to be severe based on lesion scores (3.5+/-0.1), reduction in weight gain (30.7+/-3%), plasma carotenoids (72.4+/-1.5%), uric acid (16.3+/-3.4), albumin (37.8+/-2.8%), large increases (261+/-8%) in plasma NO2-+NO3-, and high numbers of oocysts shed per chick (4.12+/-0.4 x 10(7)). Plasma AT again increased with increasing dietary VE-AC levels in uninfected and infected chicks, but the mean decrease across VE-AC levels caused by E. maxima infection was 73.14+/-3.3%. GT levels were erratic and unrelated to dietary VEAC or infection. Thus, in processed broiler chickens, high dietary VE-AC did not prevent or lessen the pathology caused by mild or severe infections with E. maxima. The main effect of E. maxima infection appeared to be reduction in plasma AT levels. We postulate that this reduction may be due to malabsorption of AT, which results from physical damage to the absorptive mucosa, reduction in esterases required to hydrolyze the VE-AC, and a generalized lipid malabsorption, preventing movement of the free AT to circulating blood and infected tissues.

Effect of Eimeria acervulina infections on plasma L-arginine

As part of a program to study the pathological effects of coccidia infections on growth, we have examined the relationship of plasma L-arginine (ARG) levels to infective doses of Eimeria acervulina and infection-associated changes in weight gain, plasma carotenoids, and plasma NO2- + NO3-. Chickens consuming a starter ration containing 1.68% ARG were infected with a range of doses of E. acervulina. At 6 d postinoculation (PI), weight gains were significantly reduced by infections with 5 x 10(5) and 1 x 10(6) oocysts per chick (OPC). Gross lesion scores of chickens infected with 5 x 10(4) through 1 x 10(6) OPC were significantly greater than scores of chicks infected with 1 x 10(3) OPC. Compared with levels from uninfected controls, plasma NO2- + NO3- concentrations were significantly increased by infection with 5 x 10(5) and 1 x 10(6) OPC, plasma concentrations of ARG were significantly decreased by infection with 5 x 10(4) through 1 x 10(6) OPC, and plasma carotenoids were significantly decreased by all infection doses. Plasma arginine was significantly correlated with plasma carotenoids (P > 0.0187), but not with infection dose or weight gain; plasma NO2- + NO3- was positively correlated (P > 0.0043) with infection dose and negatively correlated (P > 0.0158) with weight gain. Regression analysis of the measured variables indicated that the strongest relationship existed between plasma ARG and carotenoids. This finding suggests that in this infection model, reduction in plasma ARG is most likely associated with nutrient malabsorption that accompanies infection and is likely not significantly impacted by synthesis of nitric oxide that is associated with the immune response.

Effects of treatments with cyclooxygenase inhibitors on chickens infected with Eimeria acervulina

Reduced weight gains and feed conversions are major sources of economic losses from avian coccidiosis. Experiments were conducted to determine possible involvement of up-regulation of cyclooxygenase (COX) activity during coccidia infections that might contribute to reduced weight gain. In a series of trials, young (3 to 5 wk of age) cockerels infected with Eimeria acervulina, a duodenal parasite, received oral treatments with indomethacin (IM), an inhibitor of both COX-1 and COX-2, or nimesulide (NIM), a specific inhibitor of COX-2. Infection doses varied with experiment (from 10(5) to 10(6) oocysts per chick). Effects of infection on weight gain, duodenal lesions, plasma carotenoids, and levels of NO2+ NO3- were determined at 6 d postinoculation. Total oocysts were enumerated in feces collected from 5 through 8 d postinoculation from separate replicate groups. In no trials did treatment with IM reverse effects of infection on weight gain or significantly reduce lesion scores. However, in all trials, IM treatment reduced oocyst output per chick (Trial 1, 15%; Trial 2, 19%; Trial 3, 53%; Trial 4, 29%; Trial 5, 28.5%). Supplementation of feed with 400 and 100 ppm NIM significantly reduced weight gain of, and increased oocyst output from, infected chicks, whereas supplementation with 50 ppm NIM had no beneficial effects on weight gain or lesion scores, but reduced oocyst shedding. The inability of the IM and NIM treatments to reverse infection-associated weight gain suppression suggests that this pathological effect is not linked to increased prostanoid synthesis as a result of COX-2 up-regulation during infection. The inhibitory effects of IM treatment on oocyst shedding suggest that COX-2 products may have immunosuppressive effects in coccidia infection at local sites of infection. The stimulating effects of high-dose NIM treatments on oocyst shedding suggest that this compound may inhibit synthesis of other prostanoids as well.

Eimeria acervulina infection elevates plasma and muscle 3-methylhistidine levels in chickens

To assess muscle breakdown during avian coccidiosis, the level of the nonmetabolizable amino acid 3-methylhistidine (3MH) was determined in muscle, plasma and excreta from chickens infected with Eimeria acervulina. The changes in 3MH levels during infection were assessed at 1-29 days postinoculation (DPI) in animals given 5 x 10(5) oocysts per bird. The effect of levels of parasitism were evaluated at 8 DPI in birds receiving 5 x 10(3), 5 x 10(4), 5 x 10(5) or 1 x 10(6) oocysts each. The 3MH levels of plasma, muscle, and excreta samples were determined by high-pressure liquid chromatography after derivatization with fluorescamine. Weight gains, breast muscle weight, eviscerated weight, plasma carotenoid levels, dry weight of muscle, and gross lesion scores were also determined. Infected birds had significantly elevated plasma and muscle 3MH at 4 and 8 DPI following a single dose of E. acervulina. The increase in 3MH levels had an inverse relationship with the time course of weight gain and plasma carotenoid levels. Plasma and muscle 3MH levels returned to control values by 15 DPI and remained unchanged from control values through the remainder of the experiment (29 DPI). Breast weight was decreased in infected birds, but the ratio of breast weight to eviscerated body weight was unchanged. Excretion of 3MH decreased relative to controls at 4 and 8 DPI and returned to control levels on 15 DPI. The plasma and muscle levels of 3MH were related to severity of infection; however, levels of excreted 3MH were not. The results suggested that muscle breakdown, as assessed by plasma and muscle levels of 3MH, increased during the acute stage of E. acervulina infection. The underlying causes for this muscle breakdown was unclear but could involve a physiological response to anorexia and decreased food intake during the acute phase of infection. Levels of excreted 3MH did not increase during infection and this may be the result of decreased excreta output during infection. Plasma and muscle levels of 3MH were correlated with severity of E. acervulina infections but may not be as sensitive an indicator of infection as plasma carotenoid levels or other physiological parameters.

Effects of dietary supplementation with n-3 fatty acid ethyl esters on coccidiosis in chickens

The ethyl esters of eicosapentaenoic (EPA) and docosahexaenoic (DHA) acids were added to a broiler starter diet singly or in combination [as bulk purified ethyl ester concentrate from menhaden oil (n3FAC)] in quantities similar to those found in a diet supplemented with 5% menhaden oil (MO). Diets were fed to chickens from 1 d of age through 3 wk of age. At 2 wk of age, the chickens were infected with Eimeria tenella, Eimeria acervulina, or Eimeria maxima. At 6 d postinfection (PI), the effects of the diets were assessed on weight gains, plasma carotenoids, gross lesion scores, and histological parasite scores in gut cross sections, or oocyst output. Significant ameliorating effects of diet on lesion scores and parasite scores were only seen in E. tenella infections and were only produced by the n3FAC and MO supplements. These two supplements, which contained higher molar concentrations of double bonds than the other supplements, also significantly reduced plasma carotenoids in uninfected chickens, indicating that they promoted a state of oxidative stress. These results are consistent with previous reports on the interaction of coccidiosis with dietary n-3 fatty acids and strengthen the hypotheses that dietary-induced oxidative stress is an effective deterrent against cecal coccidiosis in chickens.

Dietary modulation of avian coccidiosis

During the past several years, our laboratory has been investigating the anticoccidial activities of various natural products that have potential use as dietary supplements for coccidiosis control. Sources of fats containing high concentrations of n-3 fatty acids such as menhaden oil and flaxseed oil and flaxseed, when added to starter rations and fed to chicks from one day of age, effectively reduce lesions caused by the caecal parasite Eimeria tenella, but not lesions caused by Eimeria maxima. Our results are consistent with reports of effects of diets high in n-3 fatty acids on other protozoan parasites which suggest that the state of oxidative stress induced by these diets in the cells of both host and parasites is responsible for their parasitic actions. Artemisinin, a naturally occurring (Artemisia annua) endoperoxide and effective antimalarial significantly lowers lesions from E. tenella when given at low levels as a feed additive. The mechanism of its action is also considered to involve induction of oxidative stress. Diets supplemented with 8 p.p.m. gamma-tocopherol (abundant in flaxseeds) or with 1% of the spice tumeric, reduce mid-small intestinal lesion scores and improve weight gains during E. maxima infections. These compounds may exert their anticoccidial activity because they are effective antioxidants. Betaine, a choline analogue found in high concentrations in sugar beets, improves nutrient utilisation by animals under stress. When provided as a dietary supplement at a level of 0.15% it has enhanced the anticoccidial activity of the ionophore, salinomycin. Betaine may act as an osmoprotectant whereby it improves the integrity and function of the infected intestinal mucosa. In in vivo studies, betaine plus salinomycin significantly inhibit invasion of both E. tenella and E. acervulina. However, subsequent development of E. acervulina is inhibited more effectively with this combination treatment than development of E. tenella.

Production of free radical species during Eimeria maxima infections in chickens

Five experiments were conducted to investigate the production of nitric oxide (NO) and superoxide anion (O2-) during infections of chickens with the coccidial parasite, Eimeria maxima, in order to assess the importance of these free radical species in the pathogenesis of the infections. Nitric oxide production was estimated by analyzing NO2(-)+NO3-, stable metabolites of NO, in the plasma and intestinal mucosa. The potential for O2- production was estimated from activities of beta-nicotinamide adenine dinucleotide phosphate (NADPH) oxidase in mucosal homogenates. Levels of NO2(-)+NO3- reached maximum values at about 6 d postinoculation, a time when mucosal damage was high and oocysts were being shed. The activity of NADPH oxidase in infected mucosa was also increased. Thus, at that time, there was a potential for oxidative destruction of mucosal tissue from these free radicals and their reaction products. Levels of NO2(-)+NO3- did not increase in a stepwise manner with increasing infective dose, suggesting that production of NO may be regulated post-transcriptionally by other factors elaborated by the immune response to infection, or may be controlled by substrate limitations. A comparison of two E. maxima strains indicated that the virulence of a strain was not directly related to NO production. Increased production of O2- due to increased NADPH oxidase activity during infection may cause a reduction in levels of carotenoid pigments that is unrelated to malabsorption.