HOME HELP FEEDBACK SUBSCRIPTIONS ARCHIVE SEARCH TABLE OF CONTENTS		QUICK SEARCH:   [advanced] Author: Keyword(s): Year:  Vol:  Page:

This Article Abstract Full Text (PDF) Alert me when this article is cited Alert me if a correction is posted Services Similar articles in this journal Similar articles in Web of Science Similar articles in PubMed Alert me to new issues of the journal Download to citation manager Citing Articles Citing Articles via HighWire Citing Articles via Google Scholar Google Scholar Articles by Willis, W. L. Articles by Ibrahim, S. A. Search for Related Content PubMed PubMed Citation Articles by Willis, W. L. Articles by Ibrahim, S. A.

Performance Assessment of Broiler Chickens Given Mushroom Extract Alone or in Combination with Probiotics¹

Department of Animal Sciences, North Carolina Agricultural and Technical State University, Greensboro 27411

² Corresponding author: willisw{at}ncat.edu

	ABSTRACT

TOP ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS DISCUSSION REFERENCES

 
A study was conducted to evaluate the effect of combined Shiitake mushroom (Lentinus edodes) extract with probiotics (PrimaLac) on the growth and health of broiler chickens. In trial 1, 540 d-of-hatch chicks were randomly assigned to 6 treatment groups, replicated 3 times, with 15 males and 15 females per pen for 3 wk. Dietary probiotics and mushroom treatments were as follows: 1) control feed + ad libitum tap water; 2) control feed + skip-a-day mushroom water; 3) control feed + ad libitum mushroom water; 4) probiotic feed + ad libitum tap water; 5) probiotic feed + skip-a-day mushroom water; 6) probiotic feed + ad libitum mushroom water. Body weight gain, feed consumption and efficiency, mortality, bursa, liver, and spleen relative weights of chicks were taken. In trial 2, the performance of broilers 3 to 7 wk withdrawn from the mushroom extract was evaluated along with the comparative level of fecal biofidobacteria in the control and mushroom extract treatment (trt). Mortality, weight gain, feed consumption and efficiency, carcass yield, fat pads, bursa weights and fecal bifidobacteria were measured in trial 2. In trial 1, significant differences (P < 0.05) in female weight gain (trt 4–0.62 vs. trt 1–0.54 kg) and male spleen weights were observed. In trial 2, significant differences were observed in male weight gain (trt 2–2.40 vs. trt 4–1.12 kg), male and female fat pads, male bursa weights (trt 3–0.15 vs. trt 6–0.39), female carcass yield percentage (trt 1–77.8 vs. trt 4–66.4), and feed consumption and efficiency. Body weights were severely depressed in the male broilers receiving the probiotics feed in treatments 4, 5, and 6, but not in the female broilers. These results indicate that performance differences in gender occur with additives during different grow-out periods, and mushroom extract promotes bifidobacteria growth in broiler chickens after 4 wk of withdrawal. It appears that probiotics and mushroom extract offered no combination potential for weight gain, which was compromised in this study, but possible health-enhanced attributes.

Key Words: probiotic • broiler chicken • mushroom extract • performance • bifidobacteria

	INTRODUCTION

TOP ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS DISCUSSION REFERENCES

 
The use of subtherapeutic antibiotics has been a cornerstone of the poultry industry for decades, enabling producers to lower overhead by raising animals in confinement. As new antibiotic-resistant strains of pathogens emerge, the routine use of antibiotics in animal feed is less common. Many nontherapeutic uses of antibiotics in poultry rearing are being phased out. In light of this trend, alternate means for preventing and treating bacterial infection common to poultry are needed.

Probiotics present a promising alternative. A probiotic is a live microbial food supplement that benefits the host animal by improving its intestinal microbial balance (Isolauri et al., 2001). These health-promoting bacteria are increasingly being used in poultry feed, in place of antibiotics, as an alternative approach to controlling the growth of unfavorable microorganisms.

Other health-enhancing compounds are also being examined. Of particular interest are extracts derived from shiitake mushrooms. Mushrooms are known to have considerable health-promoting benefits based on the multitude of compounds with antioxidant and antibacterial properties they contain. Wang et al. (1998) reported antimicrobial activities, immune enhancement, and stress reduction in farm animals given natural medicinal products from fungi and herbs. Yuan et al. (1993) reported antibacterial action from the use of Shiitake (Lentinus edodes) mushrooms in various experiments. Recent work by Guo et al. (2004) reported that the population of bifidobacteria and lactobacilli were significantly increased with the addition of shiitake mushroom extract (Lentinus edodes). Based on these findings, it is reasonable to assume that the inclusion of some combination of probiotics and mushroom extract may have considerable health promotion benefits for poultry. Given this potential, an important step in poultry is to determine if probiotics can act synergistically with mushroom extract to enhance the health and growth of broiler chickens. Based on this objective, the purpose of this research was to conduct a systematic investigation involving feeding probiotics, mushroom extract, or a combination of the 2 to assess the health and growth effects on broilers.

	MATERIALS AND METHODS

TOP ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS DISCUSSION REFERENCES

 
Experimental Design and Husbandry

In trial 1, on the day of hatching, 540 commercial (Ross x Ross) male and female broiler chicks (270 each) were obtained from a commercial hatchery. The chicks were weighed, and distributed randomly into 6 treatment groups, replicated 3 times, with 15 males and 15 females per pen for 3 wk. The pens were 1.524 m x 3.6576 m with 2 hanging tube feeders and 1 suspended drinker in floor pens containing wood shaving on a concrete pad. The mushroom extract was given in separate plastic drinkers. The 6 feeding conditions of dietary probiotic (PrimaLac) and Shiitake mushroom (Lentinus edodes) treatments were as follows: 1) control feed + ad libitum tap water; 2) control feed + skip-a-day mushroom water; 3) control feed + ad libitum mushroom water; 4) probiotic feed + ad libitum tap water; 5) probiotic feed + skip-a-day mushroom water; and 6) probiotic feed + ad libitum mushroom water. Chicks were fed a starter/grower diet from 1 to 21 d in trial 1 and continually from 22 to 49 d in trial 2. The control and experimental diets were purchased from a commercial feed manufacturer. Nutrient concentrations met or exceeded minimum requirements according to the National Research Council (1994). The chicks were initially started at 35°C; the temperature was gradually decreased by 5°C each wk to 25°C by the end of wk 3. During the 3-wk experiment, continuous lighting was provided. Feed and water were provided ad libitum, except for the skip-a-day mushroom additive in the water treatments. Chicks were weighed by replicated pen, and feed consumption was measured for the 3-wk experiment. Cumulative weight gain and feed/gain ratio were calculated. Mortality was recorded daily per pen replicate and calculated for the 3-wk period by treatment. For organ weights, 5 birds per pen replicate treatment were pooled, weighed, and euthanized by cervical dislocation. Liver, spleen, and bursa were excised and weighed.

In trial 2, the growth and health of birds from 21 to 49 d of age were assessed following termination of being fed the mushroom extract. A total of 10 males and 10 females per pen from trial 1 were weighed and subjected to a slightly modified treatment regimen. Treatment conditions were the same as for trial 1, except that in all conditions the chicks received tap water with no mushroom extract additive. The same parameters were measured as for trial 1, with the addition of measurements of carcass yield percent and fat pads. Five males and 5 female chicks per pen from each treatment replicate were pooled weighed, stunned, and killed by severing the carotid artery and jugular vein. After bleed-out, the broilers were defeathered in a rotary drum picker, eviscerated, and appropriate organs removed, after which they were reweighed to determine carcass yield percentages.

Pooled samples of feces were collected from broilers at 49 d from each replicate of treatments 1 and 3 to assess the level of bifidobacteria. Samples were transported in a cooler to the laboratory and analyzed within 6 h.

Mushroom Extract and Probiotics

Shiitake mushroom was obtained from the outdoor log cultivation demonstration run by the Mushroom Biology and Fungal Biotechnology Laboratory at North Carolina Agricultural and Technical State University Farm (Greensboro, NC). A known weight (100 g) of dried shii-take mushroom was extracted with 1 L of distilled water and kept at 4°C before use as an additive in the water that was given to the chicks starting at d 1 and continuing until d 21.

The commercial microbial cultures (PrimaLac, Star Labs Inc., St. Joseph, MO) contained sources of live naturally occurring microorganisms that were incorporated into the feed ration. The analysis was a minimum 1.0 x 10⁸ cfu per g (Lactobacillus acidophilus, Lactobacillus casei, Bifidobacterium bifdium, Enterococcus faecium). The PrimaLac was incorporated into the ration at the level of 907.2 g/t of feed.

Microbiological Fecal Analysis

The population of bifidobacteria in fecal samples was determined using the standard laboratory method (Ibrahim and Salameh, 2001; Brown et al., 2005). Fecal samples (10 g) were diluted with 99 mL sterilized 0.1% peptone water and homogenized using Stomacher 400Lab System 4 (Seward, Norfolk, UK) for 2 min, and 100 µL of appropriate dilution was surface plated onto modified BIM 24 agar (Ibrahim and Salameh, 2001). The level was determined at the serial dilution of 10^–5. Plates were incubated at 37°C for at least 3 d to allow for bifidobacteria cell growth and a total count of the bacterial population. Additionally, the Gram stain technique was used to facilitate microscopic examination of morphological characteristics of bifidobacteria. Fructose 6-phosphate phosphoketalase activity was measured to confirm the identity of bifidobacteria.

Statistical Analysis

Data were statistically analyzed using the GLM procedure of SAS (1990). Differences among groups were determined using Duncan’s multiple-range test. Statements of statistical significance were based on P < 0.05.

	RESULTS

TOP ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS DISCUSSION REFERENCES

 
Trial 1

Body Weight, Mortality, Feed Consumption/Efficiency, and Organ Weights. Male weight gain, mortality, feed consumption/efficiency, liver, bursa, and female spleen weights were not affected by the broiler probiotic diet or mushroom extract water additive over the 3-wk period (P < 0.05; data not shown). There were significant differences (P < 0.05) in female weight gain and male spleen weights over the 3-wk period (Table 1). Female weight gain was lower in chicks fed a probiotic ration and given mushroom extract on the skip-a-day water additive in treatment 5 when compared with the control fed a conventional diet with the skip-a-day water additive treatment 2. Weight gain was not affected in male broilers given the control feed and skip-a-day water extract. Treatment 4 containing the probiotic without mushroom water additive showed the highest weight gain of 0.62 kg in broiler chicks at 3 wk of age. Male relative spleen weights were higher in treatment 4 with probiotic feed and ad libitum tap water when compared with the other treatments.

Body, Bursa, and Fat Pad Weight, Mortality, Feed Consumption/Efficiency, Fecal Bifidobacteria, and Carcass Yield. There were no significant differences (P < 0.05) found with regard to mortality, female weight gain, male carcass yield, and female relative bursa weights (data not shown). Male body weight was significantly depressed in treatments 4, 5, and 6 when compared with the controls with no probiotic (Table 2). This observation was absent in the females receiving the probiotic feed in this trial. Chicks in treatment 4 receiving the probiotics and formally subjected to the skip-a-day mushroom water additive had the most severely depressed body weight gain of 1.12 kg. The relative bursa weight differed between treatments, with treatment 3 having the lowest at 0.15 g and treatment 6 having the highest at 0.39 g (Table 2). The carcass yield percentage for females differed with a low of 66.4% for treatment 4 and a high of 77.8% for treatment 1 (control). Fat pad relative weights percentage for males and females was affected by treatments, ranging from a low 0.30 in treatment 6 to a high in treatment 2 of 0.94 for the males (Table 2). Female pads were higher for all treatments when compared with those observed in the males. Feed consumption and efficiency differed between treatment groups with the control groups having a higher consumption level that coincided with a good feed efficiency (Table 2). As shown in Figure 1, a significant higher level of bifidobacteria was shed via the feces in the former treatment involving a control feed with ad libitum mushroom extract when compared with the former control feed without mushroom extract.

View larger version (15K): [in this window] [in a new window]   Figure 1. Counts of bifidobacteria (cfu/g) recovered from broiler chicken feces at 7 wk of age. Means are based on triplicate samples with duplicate measurements for each sample (n = 3; d = 2). a,bMeans with different letters are different (P < 0.05). rep = repetition.

	DISCUSSION

TOP ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS DISCUSSION REFERENCES

Body Weight Gain and Spleen Weights. In trial 1, female broiler chickens responded differently than their male counterparts with regard to weight gain while receiving the various treatments. The performance of chicks provided probiotics in the feed of treatment 4 was comparable to that in treatment 2 (control). This finding was not observed in the male broiler chicks. Gender performance differences exist with probioticsfed chickens, but weight gain is not well documented in the literature. The observation cited in this research is in agreement with the finding of Kabir et al. (2004). They reported increased weight gain in females receiving probiotics and concluded by stating that females utilized their feed more efficiently, causing an increase in weight gain. The other factors studied in this trial had no differential gender effect. The scant information in the literature about the mechanism is lacking probably because probiotics are viewed as contributing to improved health rather than to improved weight gain performance. It is widely recognized that good health, especially in the gastrointestinal tract, leads to positive production attributes. It has been demonstrated that probiotics inhibit the in vitro growth of many enteric pathogens (Fioramonti et al., 2003). Significant differences were observed in male relative spleen weights; however, they were not viewed as a health issue due, primarily, to the consistency of the treatment data. Based on the response variables evaluated, the treatments did not pose a threat to chick health.

Trial 2

Body Weight Gain, Bursa Weights, Carcass Yield, Fat Pads, Fecal Bifidobacteria, and Feed Consumption/Efficiency. Statistically significant observations at the end of trial 2 were many. Body weight gain at 49 d of age in males was suppressed in all treatments subjected to probiotics, whereas no differences were observed at 21 d of age with males in trial 1. The continual use of the starter grower probiotic ration is believed to cause suppression of male weight; however, females did not exhibit the same depressed weight at 49 d of age. The starter/grower diet was utilized throughout both trials to reduce possible changes in feed management practices that influence intestinal metabolism and microbial populations in broilers. Despite efforts to reduce variabilities in this experiment, they occurred with BW of the sexes. Many advances in microbial molecular biology are available, but the full understanding of the biological actions of direct-fed microbials is incomplete (Mai, 2004). Studies are needed to describe the effects of probiotics on intestinal metabolism. Recently, Chichlowisk et al. (2007) provided data suggesting that direct-fed microbials like PrimaLac increase metabolic efficiency via changes in intestinal physiology and metabolism. By helping to manipulate organ tissue energy expenditures, selected probiotic cultures may potentially increase performance parameters of the avian genders. Moreover, the immunological challenges that increase caloric demands as suggested by Martin et al. (2003) could influence the performance of the male and female broiler chickens. In a study conducted by Netherwood et al. (1999), their observation implies that probiotics have to be continuously supplied in the diet to provide beneficial effects. They found no significant difference in growth. Some researchers have reported improved performance and feed utilization in poultry with the inclusion of Lactobacillus-based direct-fed microbials (Jin et al., 1998; Schneitz et al., 1998; Zulkifli et al., 2000), whereas others reported no beneficial effects (Goodling et al., 1987; Maiolino et al., 1992; Owings, 1992). The disparity in age, gender, duration, and method of feeding is documented in a report by Tolkamp (2005). These data support the observation that the mushroom extract did not impede weight gain because it was removed from treatments at 21 d of age. This is further supported by the work of Guo et al. (2004) that showed increase BW gain in broilers with the use of mushroom and herb polysaccharides.

The relative bursa weights of the males did differ significantly (P < 0.05) at 49 d of age, whereas no differences were observed at 21 d of age. Some measure of stress or lack of nutrients was apparently exerted on the male during this period; however, at this time the cause is not known because the probiotic-fed broilers had some of the highest bursa weights. Fat pads were lower in males than in females. This may be due to the females being more efficient in their feed conversion process. Carcass yield percentages were higher for the nonprobiotic-fed female broilers than for the control. This points to the fact that probiotic causes suppression in production and processing performance traits. The feed consumption and efficiency followed an expected trend based on previous supporting data. Broilers consumed less probiotic feed than nonprobiotic, ultimately resulting in a reduction of BW gain and carcass yields. This observation was more notable for gender differences. The findings from this study clearly show that gender is affected by probiotics, but some production and processing attributes may be compromised for health when using probiotics for broiler chickens. Probiotics fed chickens did not perform adequately, with or without mushroom extract in the early production stages of this study, thus exhibiting no combination complement.

As shown in Figure 1, it was interesting to observe a substantial continual shedding of bifidobacteria in the feces after 4 wk postremoval. This may provide an indication that bifidobacteria proliferation is enhanced with the mushroom extract. As demonstrated by another study (Guo et al., 2004), mushroom and herb extract diets reduced Bacteroides spp., enterococci, and E. coli numbers, but increased numbers of bifidobacteria and lactobacilli. In conclusion, the inconsistencies found in this study are consistent with other studies. Thus, mushroom extract applications in broiler research certainly warrant further investigation because plant extracts have yielded contradictive results; however, enough evidence exists to suggest that they may aid in enhancing health in poultry.

	ACKNOWLEDGMENTS

 
The authors wish to thank Celestine Murray, Anthony Hooks, and Osman Hassan for technical assistance. Appreciation is extended to Veronica Carmichael for secretarial assistance in many aspects of this project. The authors also thank the Golden Leaf Foundation for some financial contributions.

	FOOTNOTES

 
¹ Research was supported by Evans-Allen funding through the USDA Cooperative State Research Service (USDA/Cooperative State Research, Education and Extension Service, 1400 Independence Ave., S.W. Washington, DC 20250).

Received for publication January 16, 2007. Accepted for publication June 9, 2007.

	REFERENCES

TOP ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS DISCUSSION REFERENCES

 
Brown, A. C., A. Shovic, S. A. Ibrahim, P. Holck, and A. Huang. 2005. A non-dairy probiotic’s (poi) influence on changing the gastrointestinal tract’s microflora environment. Altern. Ther. Health Med. 11:58–64.[Web of Science][Medline]

Chichlowisk, M., J. Croom, B. W. McBride, L. Daniel, G. Davis, and M. D. Koci. 2007. Direct-fed microbial PrimaLac and salinomycin modulate whole-body and intestinal oxygen consumption and intestinal mucosal cytokine production in the broiler chick. Poult. Sci. 86:1100–1106.[Abstract/Free Full Text]

Fioramonti, J., V. Theodorou, and L. Buero. 2003. Probiotics: What are they? What are their effects on gut physiology? Best Pract. Res. Clin. Gastroenterol. 17:711–724.

Goodling, A. C., G. J. Cerniglia, and J. A. Herbert. 1987. Production performance of white leghorn layers fed Lactobacillus fermentation products. Poult. Sci. 66:480–486.[Web of Science][Medline]

Guo, F. C., B. A. Williams, R. P. Kwakkel, H. S. Li, X. P. Li, J. Y. Luo, W. K. Li, and M. W. Verstegen. 2004. Effects of mushroom and herb polysaccharides, as alternatives for an antibiotic, on the cecal microbial ecosystem in broiler chickens. Poult. Sci. 83:175–182.[Abstract/Free Full Text]

Ibrahim, S. A., and M. M. Salameh. 2001. Simple and rapid method for screening antimicrobial activities of Bifidobacterium species of human isolates. J. Rapid Methods Automation Microbiol. 9:52–63.

Isolauri, E., Y. Sutas, P. Kankaanpaa, H. Arvilommi, and S. Salminen. 2001. Probiotics: Effects on immunity. Am. J. Clin. Nutr. 73:444S–450S.[Abstract/Free Full Text]

Jin, L. Z., Y. W. Ho, N. Abdullah, and S. Jalaludin. 1998. Growth performance, intestinal michrobial populations and serum cholesterol of broilers fed diets containing Lactobacillus cultures. Poult. Sci. 77:1259–1265.[Abstract/Free Full Text]

Kabir, S. M. L., M. M. Rahman, M. B. Rahman, and S. U. Ahmen. 2004. The dynamic of probiotics on growth performance and immune response in broilers. Int. J. Poult. Sci. 3.5:361–364.

Mai, V. 2004. Dietary modification of the intestinal microbiota. Nutr. Rev. 62:235–242.[Web of Science][Medline]

Maiolino, R., A. Fioretti, L. F. Menna, and C. Meo. 1992. Research on the efficiency of probiotics in diets for broiler chickens. Nutr. Abstr. Rev. B62:482–486.

Martin, L. B., II, A. Scheuerlein, and M. Wikelski. 2003. Immune activity elevates energy expenditure of house sparrows: A link between direct and indirect cost? Proc. Biol. Sci. 270:153–158.[Abstract/Free Full Text]

National Research Council. 1994. Nutrient Requirements of Poultry. 8th ed. Natl. Acad. Press, Washington, DC.

Netherwood, T., H. J. Gilbert, D. S. Parket, and A. G. O’Donnell. 1999. Probiotics shown to change bacterial community structure in avian gastrointestinal tract. Appl. Environ. Microbiol. 65:5134–5138.[Abstract/Free Full Text]

Owings, W. J. 1992. Nutritive effects of a direct-fed microbial preparation on growing turkey toms. Poult. Sci. 71:932–935.[Web of Science]

SAS Institute Inc. 1990. SAS/STAT User’s Guide. Version 6, 4 ed. Vol. 2 SAS Inst., Cary, NC.

Schneitz, C., T. Kiskinen, V. Toivonen, and M. Nasi. 1998. Effect of BROILAC on the physiochemical conditions and nutrient digestibility in the gastrointestinal tract of broilers. Poult. Sci. 77:426–432.[Abstract/Free Full Text]

Tolkamp, B. J. 2005. Effects of qualitative reed restriction during rearing on the performance of broiler breeding during rearing and lay. Poult. Sci. 84:1286–1293.[Abstract/Free Full Text]

Wang, R. J., D. F. Li, and S. Bourne. 1998. Can 2000 years of herbal medicine history help us solve probelms in the year 2000? Pages 273–291 in Biotechnology in the Feed Industry. Proc. Alltech’s 14th Annu. Symp. Univ. Pres, Nottingham, UK.

Yuan, Y. L., B. T. Fang, and Y. X. Zhang. 1993. Efficacy of 8301 polysaccharides as an adjuvant against avian pasteurellosis in chickens. J. Trad. Chin. Vet Med. 3:6–10. (in Chinese)

Zulkifli, I., N. Abdullah, N. Mohad Azrin, and Y. W. Ho. 2000. Growth performance and immune response of two commercial broiler strains fed diets containing Lactobacillus cultures and oxytetracycline under heat stress conditions. Br. Poult. Sci. 41:593–597.[Web of Science][Medline]

This article has been cited by other articles:

				K. W. Lee, S. H. Lee, H. S. Lillehoj, G. X. Li, S. I. Jang, U. S. Babu, M. S. Park, D. K. Kim, E. P. Lillehoj, A. P. Neumann, et al. Effects of direct-fed microbials on growth performance, gut morphometry, and immune characteristics in broiler chickens Poult. Sci., February 1, 2010; 89(2): 203 - 216. [Abstract] [Full Text] [PDF]

				I. Giannenas, I. S. Pappas, S. Mavridis, G. Kontopidis, J. Skoufos, and I. Kyriazakis Performance and antioxidant status of broiler chickens supplemented with dried mushrooms (Agaricus bisporus) in their diet Poult. Sci., February 1, 2010; 89(2): 303 - 311. [Abstract] [Full Text] [PDF]

				W. L. Willis, K. King, O. S. Iskhuemhen, and S. A. Ibrahim Administration of mushroom extract to broiler chickens for bifidobacteria enhancement and Salmonella reduction J. Appl. Poult. Res., December 1, 2009; 18(4): 658 - 664. [Abstract] [Full Text] [PDF]

				S. P. Li, X. J. Zhao, and J. Y. Wang Synergy of Astragalus polysaccharides and probiotics (Lactobacillus and Bacillus cereus) on immunity and intestinal microbiota in chicks Poult. Sci., March 1, 2009; 88(3): 519 - 525. [Abstract] [Full Text] [PDF]

				W. L. Willis, I. Goktepe, O. S. Isikhuemhen, M. Reed, K. King, and C. Murray The Effect of Mushroom and Pokeweed Extract on Salmonella, Egg Production, and Weight Loss in Molting Hens Poult. Sci., December 1, 2008; 87(12): 2451 - 2457. [Abstract] [Full Text] [PDF]

This Article Abstract Full Text (PDF) Alert me when this article is cited Alert me if a correction is posted Services Similar articles in this journal Similar articles in Web of Science Similar articles in PubMed Alert me to new issues of the journal Download to citation manager Citing Articles Citing Articles via HighWire Citing Articles via Google Scholar Google Scholar Articles by Willis, W. L. Articles by Ibrahim, S. A. Search for Related Content PubMed PubMed Citation Articles by Willis, W. L. Articles by Ibrahim, S. A.