Leg disorders in broiler chickens: a review of current knowledge

Abstract

Ensuring improved leg health is an important prerequisite for broilers to achieve optimal production performance and welfare status. Broiler leg disease is characterized by leg muscle weakness, leg bone deformation, joint cysts, arthritis, femoral head necrosis, and other symptoms that result in lameness or paralysis. These conditions significantly affect movement, feeding and broiler growth performance. Nowadays, the high incidence of leg abnormalities in broiler chickens has become an important issue that hampers the development of broiler farming. Therefore, it is imperative to prevent leg diseases and improve the health of broiler legs. This review mainly discusses the current prevalence of broiler leg diseases and describes the risk factors, diagnosis, and prevention of leg diseases to provide a scientific basis for addressing broiler leg health problems.

Keywords:

• Poultry
• lameness
• skeletal disorders
• prevalence
• diagnosis
• prevention and treatment

Introduction

Broiler farming is popular worldwide due to the fast growth and production cycle of chicken.¹ In poultry farms around the world, broilers are mostly raised in modernized and intensive production systems,² where the time from hatching to slaughter is about 35-40 days, resulting in improved production efficiency of commercial broilers.^3,4 However, the fast growth rate disrupts the original balance of tissue growth and development in broilers.⁵ Due to rapid weight gain and delayed skeletal development, skeletal diseases, especially in the leg bones occurr in broilers.^6,7 Currently, although large-scale intensive chicken production has improved the growth performance of broiler chickens, the issue of leg health still needs to be taken seriously as it has a detrimental impact on the economic benefits and welfare status of broiler farming.⁸

Generally, movement abnormalities in commercial broilers are referred to as leg diseases. Common leg diseases include viral arthritis (VA),⁹ bacterial chondronecrosis with osteomyelitis (BCO),¹⁰ tibial dyschondroplasia (TD),¹¹ and valgus-varus deformity (VVD).¹² Clinical symptoms are mostly characterized by leg weakness, reduced walking ability, unstable standing, frequent squatting, or lameness.^11,13 The causes of leg diseases may be infectious or noninfectious, which contribute to leg abnormalities in broiler chickens and result in a series of health and welfare problems for the birds. Therefore, this review focuses on the current prevalence, risk factors, diagnosis, and treatment of broiler leg diseases to highlight issues related to poultry leg health (Fig. 1).

Figure 1. Risk factors, diagnosis, prevention and treatment of leg disease in broiler chickens. (a) Risk factors for broiler leg disease. (b) Diagnosis of leg disease in broiler chickens. (c) Prevention of leg disease in broiler chickens. (d) Treatment of leg disease in broiler chickens. Rightsholder: Shucheng Huang, College of Veterinary Medicine, Henan Agricultural University, Zhengzhou, 450002, China.

Prevalence of broiler leg disease

More than 66 billion broilers are reported to be slaughtered annually worldwide, with the United States, China and Brazil being the largest producers of broiler chickens.¹⁴ Due to the increasing global population, the broiler industry faces pressure to improve efficiency and production. Consequently, breeding goals are excessively focused on broiler performance, leading to a rise in the incidence of leg disease in broilers, especially under large-scale farming conditions.¹

Table 1 summarizes the research on the incidence of leg disease in broiler chickens on relevant farms in different countries. In a study, conducted on 28 broiler flocks in Denmark (nearly 8% of the national flocks), the average prevalence of TD, varus/valgus deformations, toe curvature and foot pad burn, and ankle-plantar asymmetric development were found to be 57.1%, 37.0%, 32.6%, and 42.0%, respectively.^15,16 reported that statistics on the overall level of leg disease in broilers with locomotor disorders represented about 27.6% of the flock, with 3.3% of birds barely able to walk. In addition, a study by¹⁷ reported that 30% of broilers tested in four representative commercial broiler flocks in southwestern Finland had a gait score ≥3 and the overall average incidence of TD in these broilers was 3.5%. Besides, a survey of 50 commercial broiler farms in Norway found that approximately 19% of broilers exhibited moderate to severe lameness (gait score ≥3) and that the occurrence of lameness was strongly associated with an increased prevalence of hock burns and footpad dermatitis.^18,19 found a 10% prevalence of fractures due to chondromalacia in farm I, II, and III broiler flocks in fattening broiler flocks from seven farms in different regions of Bulgaria, of which about 7% occurred in the proximal tibia and about 3% in the proximal femur. On-farm V, the highest prevalence of TD was observed in crossbred strains A, B and C, and the highest prevalence was observed in strain C (27.7%), followed by strain B (26.5%) and strain A (24.22%). Furthermore, a recent study on VVD prevalence showed that the overall incidence of VVD was 1.75%, with 47.5% of cases in males and 52.5% in females.¹² Recently, studies conducted in various farms and countries have reported a prevalence of 5.5% to 48.8% in flocks with moderate to severe gait disorders (gait score of 3 or higher).^18,20–22

Table 1. Prevalence of broiler leg disease across various countries.

Country	Leg disease	Incidence rate	References
Denmark	TD	51.7%	(15)
	VVD	37%
	Toe curvature and foot pad burn	32.6%
	Ankle-plantar asymmetric development	42.0%
England	Movement disorders	27%	(16)
	Leg paralysis	3.3%
Finland	Gait score ≥ 3	30%	(17)
	TD	3.5%
Norway	Gait score ≥ 3	19%	(18)
Bulgaria	Chondromalacia	10%	(19)
	TD	24.22%-27.7%
China	VVD	1.75%	(12)

We have noticed that the incidence of leg disease in broiler chickens has not shown an increasing trend yet. In fact, more broilers are showing subclinical symptoms without showing obvious lameness, which breeders often overlook. At the same time, with the advancement of broiler scale and intensive breeding technology, the growth cycle of broilers will be further shortened to meet selling standards. However, rapid growth will further increase the load on broiler legs, thereby exacerbating issues related to broiler leg disease.¹³

Risk factors for broiler leg disease

Genetic factors

It has been suggested that the incidence of leg disease in broiler chickens has a genetic basis. Four purebred turkey strains were reared in different environments, with strains A and C reared on farms in the United States and strains B and D housed on farms in the United Kingdom. The results showed that the heritability of TD incidence was significantly different between strains B and D (0.06-0.07) and strains A and C (0.11-0.12) in the four turkey strains A, B, C, and D.²³ Additionally, comparisons between strains with different growth rates showed that slower-growing broilers strains had better walking ability and a lower incidence of contact dermatitis compared to faster-growing broilers.^8,24 Furthermore, a study on susceptibility to BCO in four broiler crosses indicated that crossbred broilers with rapid early growth had a higher incidence of BCO compared to slower-growing crossbred broilers.²⁵

Disease factors

Disease factors that cause leg diseases in broiler chickens include viruses,⁹ bacteria, fungi²⁶ and parasites,²⁷ all of which can affect leg health in broilers. A summary of the effects of different disease factors on broiler leg health is provided in Table 2.

Table 2. Effects of disease factors on the health of broiler legs.

Factors	Disease Name	Effect on broiler leg	References
Viral diseases	MD	Attacks the sciatic nerve and causes paralysis of the legs	(9, 28–31)
	AE	Encephalomyelitis
	VA	Swelling of the wing joint with damage to the gastrocnemius and toe flexor tendons
Bacterial diseases	BCO	Hematogenous colonization and physitis of the femoral and tibial growth plates	(10, 32,33,34, 35)
	Enterococcus cecum infection	Enterococcal spondylitis
	Mycoplasma synoviae infection	Infectious synovitis
	Salmonella enteritidis infection	Purulent arthritis
	Staphylococcus agnetis infection	Bacterial cartilage necrosis
Fungal diseases	AFB1 intoxication	Leg torsion TD	(26, 36)
Parasitic diseases	Eimeria spp. infection	Nutritional deficiency leg disease	(27, 37)
	Mite infection	Swollen hind metatarsal claws and scaly skin hyperplasia

Viral diseases

Viral diseases that cause leg disorders in broiler chickens include Marek’s disease (MD).^28,29 avian encephalomyelitis (AE),³⁰ and VA.^9,31 MD infection in broilers is capable of causing lymphoid tissue hyperplasia and tumors. Acute MD can cause nerve damage^28,29 and it is likely to attack the sciatic nerve, resulting in paralysis of the legs and affecting drinking and feeding. In addition, AE mainly affects chicks and can cause encephalomyelitis, with affected chicks showing head and neck tremors, limb incoordination, and inability to stand and walk normally.³⁰ Furthermore, VA is caused by avian reovirus, and clinical signs of VA infection in broiler chickens include swelling of the wing joint with damage to the gastrocnemius and toe flexor tendons (hypertrophy, sclerosis, or rupture of the gastrocnemius tendon), leading to lameness.^9,31

Bacterial diseases

Bacterial leg diseases are also prevalent in broiler farming. Infections withpathogenic bacteria such as Enterococcus cecorum, Mycoplasma synoviae, Salmonella enteritidis, and Staphylococcus can damage the leg health of broilers, resulting in significant economic losses for the farm. BCO, a leading infectious cause of lameness in broilers, causes bacterial infiltration, hematogenous colonization, and physitis of the femoral and tibial growth plates, necrosis and eventual lameness.^10,25 Besides, broilers infected with Enterococcus cecum exhibit the clinical features of enterococcal spondylitis, including light paralysis and paresis, making it difficult for them to stand or walk when their legs are stretched forward.³³ The Mycoplasma synoviae and Salmonella enteritidis infections mainly cause infectious synovitis and purulent arthritis in broilers, which pose a serious health risks to their legs.³² Furthermore, Staphylococcus infections in broilers commonly result in lameness and fever. Studies have shown that Staphylococcus agnetis can cause bacterial cartilage necrosis in broiler.^34,35

Mycotoxicosis diseases

Mycotoxicosis has a serious impact on the health of poultry. It should be noted that wheat and other grains in broiler feed may be contaminated with more than one mycotoxin. The additive or synergistic effects of mycotoxins may enhance the adverse effects on the poultry skeletal system.³⁸ investigated fungal contamination of wheat in five regions of Poland and found that only 6% of the samples were contaminated with a single toxin, followed by 25% with two toxins. In the remaining 69% of the samples, at least three toxins were found in each sample. Importantly, a study has shown that eating mold-contaminated feed not only has a toxic effect on the broiler’s vital organs but can also increase the probability of lameness in chickens from 2.3% to 25%.³⁹ Mycotoxicosis had negative effects on the broiler’s phosphorus metabolism and bone mineralization at 42 days of age. Exposure to aflatoxins reduces the strength of tibia and increases the incidence of leg torsion.²⁶ (36) found that aflatoxin B1 (AFB1) significantly impaired the development of the tibial growth plate, as evidenced by a significant reduction in both the hyperplastic and hypertrophic areas of the growth plate.

Parasitic diseases

Poor housing conditions on farms can lead to parasitic infections that are detrimental to the welfare of broilers. Parasites compete with their hosts for nutrients in the feed, affecting the body’s digestion and absorption of nutrients, causing nutritional deficiencies and ultimately increasing the risk of leg disease.⁴⁰ Studies have shown that Eimeria spp. parasite infection causes disruption of the intestinal environment and imbalance in homeostasis, which can lead to the multiplication of pathogens and result in increased susceptibility to leg disease in broilers.³⁷ In addition, mite infestations are a constant threat to the health of broiler chickens. Mite infections can cause swollen hind metatarsal claws and hyperplasia of scaly skin in broilers. Besides, it can lead to the development of knee osteoarthritis in broiler chickens.²⁷

Nutritional factors

Dietary energy and protein levels

Dietary energy levels and protein levels are closely related to the body’s metabolism. Excess protein content in feed, especially animal protein, leads to impaired purine metabolism in the body. This results in the conversion of purines into large amounts of uric acid and urates, which are then deposited in internal organs and joints. This can cause swelling and deformation of the toes and leg joints, leading to lameness. Therefore, managing dietary energy and protein during broiler growth is critical to meet the demands of broiler growth and prevent leg diseases.⁴¹ Dietary amino acid requirements for broilers have increased compared to the past in order to maintain increased protein turnover. Studies have shown that dietary amino acid levels of 3.17-3.48 g digestible lysine (Dlys)/Mcal during fattening feeding result in optimal protein gains in modern broiler lines.⁴² Formulating broiler diets on ideal protein improves problems associated with energy expenditure in protein and amino acid metabolism, and increases protein utilization efficiency. In addition,⁴³ have shown that low-protein diets supplemented with free amino acids can improve growth performance, and reduce pad damage in broilers, as well as nitrogen emissions in the henhouse to some extent. When fed on an ideal protein and amino acid based diet, broilers can utilize more available energy for productive purposes as their metabolic system performs optimally, without having to expend energy to eliminate excess nitrogen. This also helps to maintain body temperature homeostasis.⁴⁴ Moreover, there are various inconsistencies in the results of studies on dietary energy requirements for optimal performance of broilers.⁴⁵ investigated the effect of six dietary nitrogen-corrected apparent metabolizable energy (AMEn) levels ranging from 3000 to 3150 kcal/kg, with a difference of 30 kcal between each diet, and showed that performance parameters such as body weight (BW), feed intake (FI) and feed conversion rate (FCR) were not affected. In another study, feeding 55 kcal/kg AMEn resulted in better performance and yield benefits than other treatment groups during broiler growth and development.⁴⁶

Dietary calcium-phosphorus content and ratio

The development of bones in broiler chickens is closely related to nutritional factors, as two essential minerals for the body, calcium (Ca) and phosphorus (P), play a vital role in healthy bone growth.⁴⁷ Ca and P deficiency can lead to chondroplasia and/or osteoporosis in broiler chickens. A study reported that the incidence of leg abnormalities is lowest when the ratio of Ca to P in broiler chicken starter and grower diets is about 2:1.⁴⁸ Significantly, broilers have a relatively high utilization of calcium from different sources in the diet, but P is usually present in the form of phytate P, resulting in a low utilization rate. Therefore, non- phytate P sources are usually considered for incorporation into diets.⁴⁹ Studies have shown that the addition of high non-phytate P to diets during the starter period increases plasma P concentrations and increases tibial ash and P content in the tibial.⁵⁰

Trace element content

The trace elements associated with broiler leg disease are manganese (Mn), zinc (Zn), and copper (Cu). Mn deficiency has been reported to affect the development of the tibial growth plate, and some research suggests that dietary interactions with high levels of Ca and P may be a complicating factor.⁵¹ In addition, Zn deficiency can reduce the absorption of nutrients in the gut by reducing the diversity and abundance of microorganisms in the broiler’s gut, causing nutrient deficiency leg disease.^52,53 noted that broilers supplemented with Cu, iron (Fe), Mn and Zn were able to achieve significantly higher feed conversions and increased total ash content in tibia. Furthermore, the role of Cu is mainly related to the formation of substances necessary for the formation of collagen fibers, and low levels of Cu in the broiler’s diet can reduce the structure of the collagen network and decrease the strength of mineralization.⁵⁴

Vitamin content

Vitamins are necessary for the maintenance of life, as well as the growth and development of poultry. Any deficiency or excess can cause serious leg disorders.⁵⁵ Table 3 provides an overview of the physiological functions and effects of different vitamins on broiler bones. Studies have shown that a deficiency in vitamin B6 can affect the structural integrity of collagen in the bone matrix and alter fracture strength.⁵⁹ The bone matrix is responsible for the plastic deformation of tissues. In addition, vitamin D deficiency symptoms, such as chondroplasia and rickets, are caused by disordersin the body’s calcium and phosphorus metabolism. Insufficient levels of vitamin D in the diet can reduce the absorption of calcium and phosphorus.^64,65 demonstrated that both 1, 25-dihydroxyvitamin D3 (1, 25OHD) and 25-hydroxyvitamin D3 (25OHD) upregulate the expression of osteogenesis-related genes in broiler embryos. Vitamin E possesses strong antioxidant properties and helps maintain normal muscle structure and peripheral blood vessel function. Thus, a deficiency in vitamin E can lead to muscle dystrophy, resulting in impaired mobility, and an increased incidence of leg deformities, particularly lateral or medial deviation of the distal tibia or proximal talus.⁶¹

Table 3. Role of vitamins in the diet for the bone health of broiler chickens.

Name	Synonyms	Types	Physiological functions	Effect on broiler bones	References
Vitamin A	Retinoln	A1(Retinol) A2 (3-dehydroretinol)	Scavenging free radicals Maintaining vision Maintaining the integrity of skin mucosa Promoting immune function Regulation of cell growth and differentiation	VitaminA-excessive: TD, Osteoporosis, and Fracture Vitamin A-deficient: Skeletal dysplasia	(56, 57, 58)
Vitamin B	–	B1(Thiamin) B2(Riboflavin) B3(Nicotinic acid) B5(Pantothenic acid) B6(Pyridoxols) B7(Biotin) B9(Folic acid) B12(Cobalamin)	Reduces inflammatory response Promoting metabolism of amino acids, proteins and fats Maintaining normal organizational integrity. Promoting epithelial cell growth	Bone development retardation Deformity of the tibio-metatarsal joint TD Medial deviations of the tibio-tarsal and tarsal-metatarsal bones Curled-toe paralysis	(59, 60, 61)
Vitamin C	Ascorbic acid	–	Anti-Cancer Scavenging free radicals Promoting the biosynthesis of bone collagen Improved utilization of iron, calcium and folic acid Improving lipid metabolism and preventing cardiovascular diseases Enhancing stress resistance and immunity Strong Organization	TD	(62)
Vitamin D	–	Vitamin D3 (Cholecalciferol) Vitamin D2 (Ergocalciferol)	Regulation of calcium and phosphorus metabolism Promoting bone growth Regulation of cell growth and differentiation Regulating immune function	TD Rickets Chondropathy Varus and valgus deformities	(63)
Vitamin E	–	–	Anti-oxidation Scavenging free radicals Protect skin and mucous membrane	Muscular dystrophy Lateral or medial deviation of the distal tibia or proximal metatarsus	(61)

Management factors

Many factors during rearing can cause leg disease. Poor quality broiler bedding, sharp foreign objects in the bedding, and rough handling during immunization and weighing can lead to mechanical fractures or sprains in the legs of broilers.⁶⁶ Furthermore, environmental problems such as a high temperature and high humidity environment in summer, cold and wet bedding in winter coops, poor ventilation, poisoning by harmful gases, and excessive feeding density can increase the incidence of leg disease.⁶⁷ Studies have shown that excessive feeding density leads to a decrease in the length and strength of the tibia and an increase in the curvature of the tibia.⁶⁸ Additionally, high-intensity light can stimulate the development of tibial growth and reduce the occurrence of leg disease.⁶⁹ showed that higher light intensity during feeding may have a beneficial effect on calcium homeostasis and bone formation in young Lohmann Brown-Lite chicken. However, too long light duration, too high light intensity, and high chicken activity can also predispose to leg disease.⁷⁰ proved that the use of alternative lighting schedules (2 L: 1D) can significantly increase the leg bone length of broilers after hatching. In addition, the insecticide thiram is known to be capable of causing TD in broiler chickens, which is one of the main causes of TD caused by feed sources.⁷¹

Embryonic developmental factors

It is well known that embryonic development is completely dependent on the nutrients stored within the embryonated eggs laid by the hen, such as vitamin D, minerals and fatty acids. These specific nutrients play an irreplaceable role in early skeletal development.⁷² Therefore, nutritional deficiencies can adversely affect embryonic development. Furthermore, poor incubation temperature and ventilation can also affect the uptake of nutrients from yolk during embryonic development.⁷³ The development of connective tissue and skeletal calcification begins during embryonic development, with the fastest skeletal growth usually occurring around the time of hatching. The first weeks of the incubation or brooding period therefore place high demands on hatching conditions. If the incubation temperature is too low without adequate preheating of the embryonated eggs before hatching or without providing good ventilation, skeletal development will be affected and the incidence of toe deformities and leg skeletons will increase.⁷⁴ Similarly, higher temperatures and hypoxia during the later stages of incubation will slow the development of the skeleton and increase bone asymmetry between the legs.⁷⁵

Diagnosis of leg disease in broiler chickens

Clinical diagnosis

Leg disease in broiler chickens is usually not a single disease but a combination of factors, making the diagnosis process very difficult. Clinical diagnosis relies on observing the flock’s clinical signs and typical necropsy lesions to confirm the diagnosis. Still, the diagnosis of specific leg diseases also needs consideration of characteristic clinical symptoms. Autopsy is the most commonly used method for diagnosing animal diseases due to its convenience, feasibility, directness, and objectivity compared to clinical and laboratory diagnostics.⁷⁶ Autopsy plays an important role in diagnosing diseases in herd animals. In particular, during the early stages of an infectious disease outbreak, a diagnosis can be made based on the pathological changes seen in the first sick animals that are culled. This enables early diagnosis and prevention, minimizing the damage caused by the disease.

During the early stage of VA infection, severe edema of the gastrocnemius tendon in the upper part of hip joint and metatarsal joint of broilers could be observed. This is followed by leg muscle rupture and bleeding.⁷⁷ Swelling is also present around the top and bottom of the tarsal joint, and an incision of the skin reveals edema of the upper peroneal tendon in the affected chicken’s joint. Congestion or punctate hemorrhage within the synovial membrane is usually found, and the joint cavity contains yellowish or blood-like exudates.⁷⁸ In chickens infected with infectious synovitis, the phalangeal joints become swollen, along with thickening of the skin. There is a large amount of viscous, grey-white or cheese-colored exudates in the synovial bursa and tendon sheaths, and a pale yellow cheese-like exudate can also be present for a prolonged period.⁷⁹ Pathological histology reveals the presence of heterophilic leukocytes and fibrinous infiltrates.^76,80 In addition, broilers with gout may experience swelling in the legs, wings and toe joints. Incision of the joint capsule shows the flow of viscous white liquid and urate deposition can be observed in the in articular cartilage and surrounding tissues. The presence of urate worsens and often leads to ulceration of the joint surfaces or necrosis of the joint capsule.⁸¹

Technical diagnosis

Gait score

In broiler chickens, the gait scoring criteria of⁸² are mostly used as a reference for lameness studies. According to this criteria, GS0 (indicating normal walking) to GS5 (indicating inability to walk), a score equal to or greater than 3 is considered painful, indicating potential compromise of the welfare of broiler chickens.⁸³ The gait scoring method is a practical method to assess broiler lameness in the field. It has the advantage of being fast, does not requiring specialized equipment and being widely used on farms. However, the method is strongly subjective, serving as a research tool and the actual relationship between this impaired walking ability and specific leg problems remains unclear.

Kinematic analysis

Traditional kinematic measurements have provided quantitative means of assessing gait in poultry, using techniques such as plantar recorders,⁸⁴ force plates,⁸⁵ and piezoelectric pressure sensing pads⁸⁶ to collect kinetic data. However, these studies have identified technical shortcomings that have led to instability in broiler locomotion data. The development of kinematic analysis has shown promise in studying poultry gait, as it can detect subtle changes in gait patterns that are not easily quantifiable through visual observation alone. This method also allows for rapid assessment of data suitability and the ability to selectively collect useful data from broilers with different locomotor speeds. A study has showed that computer-aided 3D kinematic gait analysis can effectively record and quantify gait patterns. Specifically, the jungle chicken exhibits a short standing stage, less double support, less leg raising, and less back displacement, while the broiler displays more double support, more leg movement, and more vertical back movement.⁸⁷

Infrared thermography

Infrared thermography (IRT), also known as thermal imaging, is used as a noninvasive, noncontact diagnostic technique to determine the infrared radiation or surface heat of an object.⁸⁸ IRT has been widely used in animal studies, especially for detecting lameness in cows.⁸⁹ However, few studies have been conducted to assess and diagnose lameness caused by leg disease in broilers. Many of the health problems affecting broiler legs are associated with inflammation, and inflammation leads to an increase in tissue temperature.⁹⁰ Although IRT cannot identify specific pathologies, it can detect localized areas of increased heat due to inflammation or decreased heat caused by decreased blood flow.⁹¹ Studies have shown that the IRT technique can detect a decrease in surface temperature in broilers affected by BCO, which results from a decrease in oxygen saturation of the blood in the legs.^35,92,93 Therefore, IRT could be a new technique used for early detection of leg abnormalities or to predict subclinical foot lesions in broiler chickens.

Motion sensors

Motion sensors have been used to study different aspects of locomotion. In the case of broilers with imbalanced gait, piezoelectric crystal sensors were used to detect the asymmetry in peak force on each foot by measuring the maximum vertical force during walking. This method provides a real-time assessment of broiler gait.^94,95 used a 3-axis ADL335 accelerometer positioned within a laying hen to calculate landing force, jump height, and the timing of onset and landing during jumps from perches of varying heights onto lower landing surface. To validate the measurements, a pressure mapping system and camera were also installed as validation methods to measure landing force and record the jumps. This technology not only allows for the assessment of leg health through landing force in laying hens but also contributes to the improvement of housing and perch design, addressing longstanding issues in poultry housing systems and promoting laying hen welfare.

Biomarker

The biomarker is generally a characteristic biochemical indicator of a pathological or therapeutic process that can be objectively measured and evaluated, and its determination can inform the biological processes that the organism is currently undergoing.⁹⁶ The detection of biomarkers can not only accurately detect and reflect the health status of broiler legs but also contribute to the study of the pathogenesis of broiler leg disease.⁹⁷ Currently, few studies have reported on biomarkers of broiler leg health. However, the gut microbiota is closely related to the growth and development of bones,⁹⁸ therefore, the detection of gut microorganisms may have some significance for the diagnosis of leg health problems in broiler chickens. Research on broiler gut health has focused on multiple biomarkers, including DNA metabarcoding, metagenomic, metatranscriptomic, metaproteomic, and metabolomic approaches. Especially the combination of multi-omics technologies will be the future direction.⁹⁹ Studies have shown that noninvasive sampling of biomarkers, screening of fecal microbial markers, and metabolic biomarkers of intestinal microorganisms provide new avenues in the early diagnosis of TD in broiler chickens. The authors identified 4-hydroxybenzaldehyde as a key biomarker to differentiate TD broilers from normal broilers through interaction analysis of the fecal microbiota and metabolites by multi-omics techniques. They also elucidated that the molecular mechanism of broiler TD pathogenesis may be due to dysregulation of gut microbes and regulation of host fecal metabolism.¹⁰⁰ As biomarkers should have the advantages of reliability, specificity, and sensitivity, many researchers have embarked on the study of poultry candidate biomarkers in recent years.^101,102 However, the validation of biomarkers will be a major challenge due to the complexity of intestinal microbes.

Prevention and treatment of leg disease in broiler chickens

Genetic selection

Although it has been argued that good management strategies can alleviate leg problems better than genetic improvement.¹⁰³ However, the important contribution that genetic selection for broiler traits can make to improving leg health in modern broilers cannot be ignored. Long-term selection for improved leg health has been demonstrated in broilers.¹⁰⁴ Results from studies assessing genetic parameters of leg health and mobility in purebred turkeys and their genetic correlation with body weight have shown that leg health can be effectively improved in turkeys using balanced breeding objectives based on a balance of production traits and leg health traits.²³ In addition, a study comparing the degree of heritability of two broiler lines differing in growth capacity and adult body weight, foot pad dermatitis (FPD) and hock burn (HB), found relatively high heritability for FPD in female fast-growing broilers.¹⁰⁵ Therefore, genetic selection of broilers is necessary for the prevention and improvement of leg health.

Housing management

Housing management provides a suitable growth environment for chickens and an effective way to prevent leg diseases in broiler chickens. The relationship between temperature, humidity and ventilation should be addressed in both summer and winter to ensure that chicken houses are well ventilated, thus preventing the spread of diseases. In addition, maintaining a suitable feeding density is very important. When rearing chickens to expand the flock, it is recommended to keep broilers older than 3 to 4 weeks old at no more than 12 per square meter.¹⁰⁶ Moreover, it is advised to provide less feed and more frequent meals to increase the feeding and exercise time of broilers. Furthermore, the height of the water-feeding line should be adjusted in a timely manner according to the different ages of the chickens, and high-quality bedding should be selected to prevent bacterial growth, especially mold contamination. It is worth noting that ehen catching and immunizing chickens, it is essential to handle them gently to reduce stress and minimize the risk of leg deformities in broilers.²² Finally, a reasonable light program and light intensity should be maintained in the chicken house to ensure that chickens rest and move in a specific pattern, thus reducing mortality caused by leg diseases.

Nutritionally balanced diets

With the development of the broiler nutrition and feed industry, the formulation and processing of feed tend to be mature. The biggest trigger for leg abnormalities due to unbalanced diets may be problems with the quality of raw material components, digestion and absorption. Therefore, strict control of feed ingredient quality, meeting the minerals and vitamins required by broilers, is the key to reducing leg disease in broilers. The manganese (Mn) requirement for poultry production under normal ration conditions is generally considered to be 40 mg/kg. Mn deficiency leads to the synthesis of mucopolysaccharides, reduces the amount of mucopolysaccharides in bones, hinders the formation of a calcified matrix, and affects bone strength.¹⁰⁷ In addition, high concentrations of Ca and P can lead to the formation of calcium phosphate precipitates in the intestine, which adsorb Mn and excrete it with feces, exacerbating manganese deficiency. Therefore, to safely adapt to changes in Ca and P intake, 50 mg/kg Mn supplementation is required in the diet. The recommendations for the requirements of several different forms of essential trace elements that are important in bone formation are provided in Table 4. (57) reported that vitamin D3 supplementation in broiler diets was effective in preventing the development of TD. For broilers under 14 days of age, vitamin D3 requirements might be in the range of 35–50 μg/kg of feed to ensure proper cortical bone quality, provided that the calcium and effective phosphorus content of the diet is adequate. Furthermore, 25-hydroxycholecalciferol (25-HCC) is the active metabolite of vitamin D, which can enhance collagen maturation in the bones and facilitate bone calcification. Studies have shown that the addition of 250 μg/kg of 25-HCC in the diet can significantly reduce the incidence of TD in broilers.⁶²

Table 4. Broiler requirements for various forms of essential trace elements crucial for bone formation (mg/kg).

Trace elements	Inorganic	Organic	References
Manganese(Mn)	74.30	38.0	(108)
Iron(Fe)	80.0	80.0	(109)
Copper(Cu)	8.84	9.87	(110)
Zinc(Zn)	110.0	55.0	(111)
Selenium(Se)	0.30	0.30	(112)
Chromium(Cr)	1.60 ∼ 2.30	1.81	(113, 114)

Microbioecologics

Recently, microbioecologics (probiotics, prebiotics and synbiotic) have shown great potential in the preventing leg diseases in broiler chickens.¹¹⁵ It is well known that probiotics are closely related to the gut microbiota, and a few studies have reported that broilers fed probiotics exhibit better growth performance, immune response, intestinal morphology, and expression of immune-related genes.¹¹⁶ The possible mechanism of microbioecologics action in leg disorders is by regulating the composition of the poultry intestinal flora, indirectly improving bone metabolism, and promoting healthy bone growth by regulating calcium and phosphorus, hormones, and nutrients.¹¹⁷ Research has indicated that dietary supplements containing probiotics, prebiotics, and synbiotics significantly improved bone mineral metabolism.¹¹⁸ The addition of synbiotics (containing a prebiotic-fructooligosaccharides and a probiotic mixture of 4 microbial strains - Enterococcus faecium, Pediococcus acidilactici, Bifidobacterium animalis, and Lactobacillus reuteri) to the diet can significantly increase the bone mineral density, bone mineral content, and bone area of heat-stressed broilers, and reduce the gait score.¹¹⁹ In addition, it was found that synbiotics (Lactobacillus B94, Lactobacillus casei, Lactobacillus acidophilus) were able to significantly improve broiler performance intestinal histomorphology, cecum volatile fatty acids, meat quality, and leg bone strength.^120,121 demonstrated that Bacillus subtilis increased bone mass and density in broilers. Also, Lactobacillus rhamnosus JYLR-005 was able to prevent TD by improving bone-related growth performance in broiler chickens.¹²² Therefore, the addition of microbioecologics restorative nutritional supplements for bone damage may be an effective, safe, and inexpensive method for the prevention and treatment of leg diseases in broiler chickens.¹²³

Traditional Chinese medicine

Common skeletal metabolic disorders in broiler chickens are characterized by dysregulation of bone resorption and bone formation, low bone mass, and abnormal bone tissue structure. A variety of traditional Chinese medicines, such as polysaccharides, flavonoids, and glycosides, have been shown to be therapeutically effective in the prevention and treatment of broiler leg disease.^124–127 In addition, previous studies have confirmed that flavonoids have estrogen-like effects,¹²⁸ which can act synergistically with estrogen to promote intestinal calcium absorption, affect calcium and phosphorus metabolism, and protect tibial growth and development. Quercetin can improve broiler TD by regulating calcium and phosphorus metabolism.¹²⁹ Assemble flavone of Rhizome Drynaria can increase body weight, tibial weight, and tibial length, and reduce growth plate width in broilers with TD, indicating its skeletal strengthening effect.¹¹ Additionally, Icariin ameliorates broiler TD by regulating the expression of the wingless-type MMTV integration site family, member 4 (WNT4), and vascular endothelial growth factor (VEGF) genes.¹³⁰ Chlorogenic acid-like components of various herbs can ameliorate broiler leg disease by regulating the molecular mechanisms of Caspases, Beclin 1 expression and extracellular matrix (ECM) degradation.¹³¹ Therefore, the use of herbs for the prevention of leg diseases in broiler chickens is an effective and safe strategy.

Conclusion

Briefly, this review summarizes the risk factors that cause leg disorders, as well as the diagnosis, prevention, and treatment of leg diseases in broiler chickens. It is commonly considered that the main cause of leg disease in broiler chickens is their rapid growth, as their leg bones, joints, tendons, ligaments, and other tissues are weak and unable to support their weight, leading to the disorder. However, there are multiple factors associated with genetics, diseases, nutrition, and embryonic development. These risk factors may even interact with each other to influence the development of leg diseases. The key issue to reduce the incidence of leg diseases in broilers is to pay attention to poultry welfare, strengthen feeding management, create a comfortable and relatively stable growth environment for broilers, implement comprehensive control measures, and try to eliminate factors unfavorable for broiler growth. In addition, proper diagnosis and effective treatment are crucial for broilers with locomotor disorders, as the problem can be effectively addressed by identifying the factors that disrupt the function of the skeletal system.

Authors’ contributions

S.C.H. contributed to the conceptualization, resources, funding acquisition, and revision and editing of the manuscript. K.L.L. performed the literature review, and drafted the manuscript. Y.F.H and B.W.X contributed to the revision and editing of the manuscript. L.X.L., P.C., M.K.I. and K.M. collected the literature and reviewed the text. All authors contributed to the article and approved the submitted version.

Acknowledgements

The authors would like to thank Dr. Mujeeb Ur Rehman, from Hainan University, China for his generous time to edit the manuscript for language and spelling errors.

Disclosure statement

No potential conflict of interest was reported by the author(s).

Additional information

Funding

This study was supported by the National Natural Science Foundation of China (No. 32202876), the China Postdoctoral Science Foundation (No. 2023T160198), and the Key Scientific and Technological Project of Henan Province Department of China (No. 232102111046).