Fig. 8.1
Relationship between lameness (gait score) and liveweight of 3 genotype groupings of 13 genotypes at 54 days of age fed a non-limiting (NL) or Label Rouge (LR) diet. Reproduced with permission, from Kestin et al. (2001). Regression coefficient 1.262 (P<0.001). A similar relationship was found at 81 days of age
Fig. 8.2
The curvilinear relationship between Gait Score (lameness) and body weight. The lamest birds (GS 4 and 5) cannot access feed and water freely
8.4.2.3 Husbandry Influences on Lameness
Research has shown that several variables can affect the expression of lameness in a flock of broilers. It is important to note that many of these affect liveweight and that often any benefits are due primarily to this weight reduction. Thus, to determine whether changes have a value per se, correction must be made for alterations in body weight. Lameness increases in prevalence and severity with increasing stocking density (Hall, 2001; Sørensen et al., 2000). The free floor space available for exercise decreases with increasing stocking density and could account for some of the increase in lameness. Several studies support the hypothesis that reduced exercise increases leg problems (Reiter and Bessei, 1998; Bizeray et al., 2000). The effects of light on broiler welfare are complex, however certain lighting patterns may reduce the incidence of leg disorders (reviewed by Gordon, 1994; Sorensen et al., 1999). Improvements in leg health may be due both to the stimulation of bouts of activity and also to periods of high quality rest and improved metabolic health (Gordon, 1994). This explanation could equally apply to the reported benefits of providing feed in meals rather than ad libitum (Su et al., 1999). To reduce leg problems effectively, feed restriction generally has to be severe enough to cause long-term growth reduction whether applied for short periods at a young age in broilers (e.g. Su et al., 1999) or throughout life in broiler parent stocks. It then becomes difficult to balance the resultant welfare benefits with the welfare problems. In the case of broilers other methods should preferably be employed to improve leg health. Because use of perches by broiler is so low, particularly after about 4 weeks of age, they have no effect on leg health. Most trials using environmental enrichment to stimulate activity also have negligible effect on health although they may alter behaviour.
Recent experiments have begun to examine effects on lameness and other responses to combinations of husbandry measures such as lighting schedules and stocking density (Sanotra et al., 2002). There is now a need for the combined effects of all factors known to affect lameness and other aspects of broiler welfare to be studied in a systematic way in commercial scale trials.
8.4.2.4 Causes of Lameness
In simplistic terms, modern broilers have been selected to have the potential for extremely rapid muscle growth. If this growth of soft tissue occurs at a very young age, then it may exceed the capacity of the skeletal system to support it. Moreover, broilers have been selected for increased breast muscle size. This confers biomechanical disadvantages owing to the width of the breasts in much the same way as the enlarged udders of modern dairy cows force abnormal gait of their hind limbs. Reiter (2002) contrasts the rolling gait of broilers, which need to shift the centre of gravity over each leg in turn with each step, with layers that walk normally because both feet are beneath their centre of gravity.
The principal musculo-skeletal disorders associated with lameness in broilers were reviewed in 1993 by Thorp. Although these are conveniently considered separately from infectious causes, reviewed by Butterworth (1999), there may be overlap. For example, abnormal growth plate development, or wear and tear in distorted joints, may predispose to colonisation of the synovial fluid and membranes by opportunist infectious agents such as staphylococci or reoviruses. Following acute infection, chronic conditions such as tenosynovitis and arthritis may develop. The inflammation and pain associated with infectious causes of lameness generally results in a substantially reduced ability and inclination to walk, that is to a gait score classification of 4 or 5 (Jordan, 1996, Kestin et al., 1994). From post mortem examination, it estimated that staphylococcal tenosynovitis accounts for 3–4% of cases of lameness (Pattison, 1992, Reece, 1992) and the incidence in broiler breeders is higher, since the disease has longer to develop.
8.4.2.5 Consequences of Lameness
The disabling consequences of lameness have particular significance for broiler welfare if they cause behavioural frustration, prolonged discomfort or pain.
Many studies have shown significantly altered behaviour patterns, particularly reduced activity, in lame and heavy birds (Murphy and Preston, 1988; Newberry et al., 1988; Blokhuis and van der Haar, 1990; Bessei, 1992; Weeks et al., 1994). More recently, Weeks et al. (2000) observed that, on average, lame broilers (GS 3) lay down for 86% of their time, which was significantly longer than the 76% of sound birds (GS 0). Walking declined with age but occupied an average of 3.3% of the time of broilers approaching slaughter weight. Lameness significantly reduced this to a minimal 1.5% in the worst affected birds. For lame birds, the time spent on their feet idling or preening was significantly less than sound birds, and this could indicate a reduced quality of life. More importantly, there was evidence of frustration of normal feeding behaviour. When the feeders were set lower than usual, lame broilers lay down to eat for almost half their feeding time, whereas sound birds predominantly chose the usual standing posture for eating. Moreover, detailed observations using video records revealed that lameness altered the feeding strategy of broilers. The sound birds visited the feeder an average of more than 50 times in 24 h, but the number of visits to the feeder was reduced with increasing lameness to an average of around 30 visits in the lamest broilers. However, meal duration was adjusted to give no overall differences in time spent feeding per day (Fig. 8.3).
Fig. 8.3
With increasing lameness from none (GS0) to pronounced (GS3), broilers reduce the number of feeding bouts (solid grey) but increase the duration of each bout (hatched) so that overall time spent feeding remains similar (after Weeks et al., 2000)
The alterations of the time budget, in particular the reductions in activities performed whilst standing, and the different feeding strategies adopted, are consistent with lameness imposing a cost on the affected broilers to the detriment of their welfare. In an experimental study of dustbathing behaviour, Vestergaard and Sanotra (1999), found that lame broilers with tibial dischrondroplasia (TD) dustbathed on significantly fewer days and showed reduced dustbathing behaviour. These birds also had longer periods of tonic immobility when tested at six weeks of age than birds without TD, and the authors suggested that an inability to dustbathe might increase the sense of fear. Their study also indicated that it was the pain associated with lameness that reduced dustbathing behaviour. Very severe lameness may result in reduced ability to access food and, more critically, water drinkers – particularly if nipple and cup drinkers are set at a height which requires that birds have to “stretch” to reach them. Birds with this degree of lameness should always be humanely culled. The consequences of not doing so were revealed in a study carried out in the UK by Butterworth et al. (2002) who associated chronic dehydration with severe lameness (see Fig. 8.4).
Fig. 8.4
The effect of time of dehydration on plasma osmolality in domestic fowl. A best fit line x–y has been drawn for the data available from previous work (Arad et al., 1985; Knowles et al., 1994; Robinson et al., 1990; Stallone and Braun 1986).The mean plasma osmolality values for the high gait score (H) and low gait score (L) birds in this case study are also indicated, as are error bars for the standard error of the mean
There is increasing evidence that broilers in high gait score categories, many of which will be in the acute stages of bacterial chondonecrosis (BCN), are in pain (Duncan et al., 1991; Gentle and Thorp, 1994; Thorp, 1996; Danbury et al., 1997; Pickup et al., 1997; McGeown et al., 1999; Danbury et al., 2000). Osteomyelitis in human beings is recognised to be a painful and debilitating condition (Whalen et al., 1988), and chickens have been used as an experimental model of haematogenous osteomyelitis in human beings (Emslie et al., 1983). Thus we propose that BCN and osteomyelitis should be considered to have a substantial adverse welfare impact on affected birds.
8.5 Injuries and Disease Associated with Poor Husbandry
8.5.1 General
All forms of disease may be associated with poor stockmanship, but more widespread is the inadequate provision of quantity rather than quality of care. It is important to identify sick birds swiftly. Removal of diseased animals promptly is likely to reduce the spread of any infectious disease throughout a flock. Diseased birds are exhibiting poor welfare through distress and pain, and should be treated or culled without delay. However, in the majority of modern, intensive, production systems this is virtually impossible to achieve owing to the thousands of birds under each person’s care.
On a typical site there could be a total of 160 thousand broilers in several houses, but only 2 stock-people. If each bird were to be visually examined daily, then even if both people spent 8 h, this would allow only 0.28 s per bird. In reality there are many other tasks to fill the working day and so sick birds get overlooked, especially in systems such as multi-tier cages for layers where it is physically very difficult to actually see into every cage. The potential for poor welfare is built into such systems where the number of human carers is dictated by economic rather than animal welfare considerations. In the EC, future legislation may force an improvement: the current proposals for a new broiler welfare directive indicate that all birds should be inspected twice daily from a distance of less than 3 m.
8.5.2 Bone Breaks
These are a major cause of poor welfare in laying hens and a concern during their handling. The number of freshly broken bones found in live birds prior to slaughter and the number of old healed breaks found at slaughter are unacceptably high (Knowles and Wilkins, 1998). The bones of most layers, especially those housed in cages, are weakened by osteoporosis which increases their susceptibility to breaks. Birds from more extensive laying systems often have stronger bones (Gregory et al., 1991; Fleming et al., 1994) and suffer fewer breaks during depopulation, but they can have a greater prevalence of old healed breaks. For example, Gregory and Wilkins (1996) reported a rate of healed bone fractures of 23% at 72 weeks in one study. The old breaks may occur as a result of collisions due to poor design within these housing systems. The number of breaks occurring just prior to slaughter can be reduced by increasing bone strength, and by handling birds with more care. For example, Gregory et al. (1993) found the incidence of broken bones in end-of-lay hens removed by two legs from cages was less than half that in birds removed by one leg. The numbers of breaks occurring during lay can be reduced by better design of housing systems and the physical environment within them.
8.5.3 Bumble Foot
This condition is essentially an acute infection of the soft tissues (pad) underneath the foot and arises from minor skin abrasions that enable the entry of bacteria such as Staphylococci spp. The body’s immune response produces considerable amounts of pus and often inflammation, which may be accompanied by pain. Because of the relatively poor circulation of the foot, the condition tends to persist for weeks. If treated individually by lancing and cleaning the foot or antibiotic therapy, bumble foot can be cured. This is seldom done on modern large production units to the detriment of bird welfare. Affected birds find it difficult to walk and they may approach feed and water less and thus have reduced production. Prevention lies in separating the bird from its faeces (as in conventional wire battery cages) or, preferably, in housing on a clean soft substrate. The condition is increasing in prevalence as cages modified by perches and alternative laying systems become more common (Tauson and Abrahamsson, 1994).
8.5.4 Foot Pad Dermatitis and Hockburn
The type of litter substrate and its moisture levels and ammonia content affect the incidence of these skin lesions which are a form of contact dermatitis. Su et al. (2000) found lower incidences of foot burn and of lameness on wood shavings compared with chopped straw. Tucker and Walker (1992) also reported reduced hockburn on drier, friable litter.
8.5.5 Respiratory and Eye Conditions Associated with Aerial Pollutants
In general, intensive poultry housing has the largest concentration of aerial pollutants of all farm animal housing, with inhalable dust concentrations of up to 10 mg per m3 and respirable dust (particles small enough to enter the lungs) of 1.2 mg per m3 (Hartung, 1998). This survey also found concentrations of endotoxins in laying hen housing up to 860 ng per m3 of inspirable dust. Overall mean inhalable and respirable dust concentrations in a different survey were 3.60 and 0.45 mg per m3 in poultry buildings, with the high concentrations in broiler houses and in percheries for laying hens giving particular concern for both stockmen and animal health and performance (Takai et al., 1998). Dust concentrations are higher where litter is present.
Ammonia concentrations may vary over time and location within a house as well as between houses. Typical European mean values are 21 ppm in broiler houses and 3 ppm in cage layer houses (Seedorf and Hartung, 1999). Exposure to ammonia may reduce poultry welfare by causing irritation to mucous membranes in the eyes and respiratory system, increasing susceptibility to respiratory disease and reducing productivity (Kristensen and Wathes, 2000). At concentrations above 60–70 ppm, ammonia causes irritation of mucous membranes and of the respiratory tissues resulting in keratoconjunctivitis, tracheitis and oedema in the upper airway, and damage to the cilia in the trachea. High levels of ammonia also seem to depress the birds’ appetite (Jones and Roper, 1997). At low concentrations it is likely that ammonia contributes to the severity of respiratory disease caused, for example, by Infectious Bronchitis (IB), or Mycoplasma infection.
Many viral, bacterial, fungal and parasitic disease organisms rely on aerial transmission to other birds and human beings. Steps should be taken to reduce shedding, spreading and concentrations in the air. For example, the risk of contamination by Salmonella may be reduced where air flow rates over litter or manure deposits exceed 15.6 m per minute (Mallinson et al., 2000).
The use of formalin vapour to control aerosol pathogens within the hatching chambers in broiler and pullet hatcheries has welfare benefit in terms of reduced incidence of yolk sac infection, usually caused by E. coli infection during the first days of life, but the highly irritant formalin vapour environment into which the chicks hatch is likely to have an impact on the birds in terms of upper respiratory and ocular irritation during the hatching period.
8.6 Infectious Diseases
Whilst it is tempting to give high priority to the “big gun” diseases in poultry, it is the low level, chronic conditions that have the most insidious impact on global poultry welfare because these conditions are often sporadic, they are hard to resolve, and there may not be strong economic pressure to resolve them. But, for the individual bird, of which there are estimated to be approximately 40 billion meat birds each year, and 5 billion layers at any given time, the combined impact of these conditions is very great. For this reason, the following summary of selected infectious diseases includes some which would not normally be considered as “big guns”, but by their common, low level nature, are likely to have substantial impact on global poultry welfare.
8.6.1 Parasitic Diseases
Through evolution, the majority of birds reached a state of equilibrium with parasites that caused tolerable debility, but rarely severe disease. The extremely rapid intensification of the industry during the last half century has disrupted this balance in favour of the parasites and at the potential disadvantage of their bird hosts. Antihelmintics provided a short term solution, but concerns for human health, and resistance of parasites to the drugs is forcing new strategies for controlling disease. Parasitism tends to be worse in free-range and large group floor housing systems that otherwise have many potential welfare benefits. Since morbidity and mortality can reach unacceptably high levels, solutions need to be found. This is likely to be a key area of concern for the welfare of poultry in many alternative systems of husbandry.
8.6.1.1 Coccidiosis
Coccidiosis is a significant poultry disease found universally in chicken, turkeys, ducks and game birds worldwide. Species of the protozoan parasites Eimeria and Tyzzeri, which cause coccidiosis are often very host specific, and site specific within each host. The disease is variable in its severity, but may result in frank enteric disease, due to damage to duodenum, caecum or rectum, with diarrhoea, which may result in increased mortality, depression and anaemia. Alternatively, the disease may not present any clinical signs, but show as poor growth. Because coccidia produce resistant oocysts, which can persist in the environment for long periods, coccidiosis is very difficult to eliminate from intensively farmed poultry, and control relies on vaccination, or use of scrolling generations of antiparasitic drugs which become ineffective as the parasite develops resistance. The welfare impact of coccidiosis on poultry is substantial because the potential for enteric disease or poor thrift is universal. The need for vaccination and use of coccidiostats in intensive systems ties animal health to the ability of the pharmaceutical industry to keep one step ahead (or at least keep pace) with the malleable resistance of the parasite.
8.6.1.2 Arthropod Ectoparasites
The developments in housing, and the increasing size of flocks have led to changes in the relative importance of different ectoparasites, and some previously uncommon parasites have become common, whereas some previously problematic conditions have almost disappeared. For example, the Red Mite (chicken mite) does not as readily infest layers in metal cage systems because of the absence of wood, roosts and static litter. Many arthropod parasite lifecycles are stopped by the short production cycles seen in broiler chicken, or by repeated house disinfection and cleaning. However, in systems which take the birds to greater ages, or which cannot easily be cleaned (free range, wooden houses), then arthropod parasites can become a very significant cause of disability in the birds. Intensively farmed poultry are affected by Lice (especially caged layers), Red Mite (broiler breeders and free range units), Northern Fowl Mite (turkeys and broiler breeders) and Scaly Leg Mites (older birds) along with fleas, ticks and flies. Lice and mites cause irritation, feather loss, and skin damage. In systems where there are parasite challenges and dustbathing is not possible, it might be assumed that prevention of this normal behaviour which can decrease the burden of parasites, reduces welfare.
8.6.2 Chronic Diseases
8.6.2.1 Cellulitis
Localised skin infection in poultry is common. In broilers, breast blisters and cellulitis result from breaks in the skin and subsequent colonisation by opportunistic bacteria such as E. coli or Staphylococci spp.. E. coli cellulitis may secondarily cause subcutaneous and cutaneous skin infection with inflammation and oedema, particularly in the thigh and lower abdomen (Randall et al., 1984; Peighambari et al., 1995a; Onderka et al., 1997). In welfare terms, the likely impact of these skin conditions can be assessed by evidence provided by, for example, reports from slaughterhouse meat inspection (Yogaratnam, 1995).
8.6.2.2 Egg Peritonitis
This can be an important cause of sporadic death and poor welfare through disease in layers and breeding birds. Impacted egg material, or material from the oviduct may enter the peritoneal cavity, eventually causing localised abscesses, salpingitis and peritonitis. The cause appears to be a combination of hormonal effects and bacterial infection within the peritoneal cavity.
8.6.2.3 Mycoplasmosis
Mycoplasmas, most notably M. gallisepticum and to a lesser extent M. synoviae, cause significant respiratory disease in broilers and “egg drop” in broiler breeders or hens when the bird is subject to simultaneous infection or immunosuppression with infectious bronchitis (IB) or infectious bursal disease (IBD) (Jordan, 1996). Uncomplicated infections may cause no clinical signs or mortality, but it would appear that at the present time, Mycoplasmae are becoming a more significant cause of welfare insult through respiratory disease and lameness in intensively farmed birds of over 45 days of age.
8.6.2.4 Pendulous Crop
In chickens, the formation of an over-large, fluid-filled crop leads to poor growth and chronic ill thrift. Small birds will be culled on farm, or may suffer problems at slaughter as a result of the setting up of the stunning equipment for bigger birds. This common but sporadic problem appears to have a genetic component. Feed type, whole grain feeding, litter type and previous exposure to disease also have a role to play in development of pendulous crop.
8.6.3 Contagious Diseases
8.6.3.1 Avian Influenza Viruses (Fowl Plague)
These viruses are a cause of intermittent serious disease, with the potential for high morbidity and mortality in intensively reared poultry. Influenza A (H5N1) virus is a highly pathogenic and contagious influenza virus affecting birds. The first outbreaks of H5N1 occurred in Cambodia, China, Indonesia, Thailand and Vietnam in 2003, and 100 million birds were destroyed in order to control the outbreak. Highly pathogenic H5N1 is now found in a number of European countries, and low pathogenicity varieties are found in wild bird populations in North America. The general trend has been for improved control through vaccination and biosecurity in farmed birds – with a reduction in the number of outbreaks, but a gradual increase in the pathogenicity in wild birds. The number of human cases of Avian Influenza is 433 cases (262 deaths) to June 2009 (WHO 2009).
For an individual bird which becomes infected, the disease has severe and profound consequences with depression, coughing, respiratory distress, nasal and ocular discharge, a swollen face, diarrhoea and finally, paralysis and death in up to 80% of cases where compulsory slaughter has not intervened. From a welfare perspective, along with the direct effects of disease, a significant welfare “threat” to farmed birds is the potential for poorly controlled destruction of birds. In a disease outbreak situation it is possible for the normal standards of handling and humane slaughter to be overwhelmed by the sense of “urgency” to protect people and other farmed birds. Recent video and news footage shows very poor regard to care and humane treatment for the birds in the culling area. Whilst it is clear that robust disease control measures are important in preventing the uncontrolled spread on AI, it should not be forgotten that a measure of “humanity” is how we treat animals in time of “crisis” – and the evidence provided by the response in some countries has not been encouraging in this respect.
8.6.3.2 Chicken Anaemia Virus
Chicken anaemia virus (CAV) is a common worldwide infectious disease of chickens caused by a Circovirus. If breeder birds are exposed to CAV before they come into lay, the disease is sub-clinical, but, if they are exposed when they first come into lay, the virus, transmitted via the egg, leads to destruction of the bone marrow and the thymus, spleen and bursa of Fabricius in their offspring. Young birds show signs of this immune depletion from about 10 days of age, and mortality is usually around 10%, but can be up to 60%. Infected bird show haemorrhages under the skin and in the muscles, and gangrenous dermatitis may occur. Because of the anaemia and immunosuppression which results from lymphoid tissue damage, birds which recover are more susceptible to concurrent or secondary disease.
8.6.3.3 Gumboro Disease
Gumboro disease (Infectious Bursal Disease, IBD) is a highly infectious global disease that affects young chickens, including layer and breeder stock, turkeys and ducks, usually before the age of six weeks. Strains of the IBD virus (IBDV, genus Birnavirus), which vary from continent to continent, affect B lymphocytes and macrophages in the focal lymphoid tissue, particularly in the bursa of Fabricius (BF), tonsils and spleen. Some genotypes and strains of bird appear to be less susceptible (e.g. White Leghorn), as do older birds in which lymphoid tissue has become involuted. Birds shed the virus in their faeces, and, because of the resistant nature of the IBD virus, it is readily mechanically transmitted between farms.
Damage to lymphoid tissue by the virus in early life reduces the birds resistance to concurrent or secondary disease challenges, and may also affect the birds ability to mount effective responses to vaccines for diseases such as Marek’s disease, Newcastle disease, IB and coccidiosis. In acute disease, birds become rapidly depressed, inactive, sitting with ruffled feathers, trembling, anorexic, dehydrated and soiled by watery diarrhoea. Vent pecking may become common for a period. Initial (acute) morbidity can be up to 100%, but in many cases infection may be subclinical, and “worse case” mortality is usually less than 20%. Even if the initial challenge does not produce clinical disease, the bird is subsequently permanently immunosuppressed, and this is the major animal welfare and economic impact of the disease.
Vaccination, including that of parent stock to provide maternally derived antibody, has reduced the impact of Gumboro disease worldwide, but strain variation makes complete protection complex. The immunosuppression resulting from Gumboro infection contributes to many cases of respiratory and enteric disease in chickens. The effect of Gumboro disease on the global population of (particularly) broiler chickens, cannot be overstated, as IBD has a significant impact on bird susceptibility to other disease organisms, including gut parasites.
8.6.3.4 Infectious Bronchitis
Infectious bronchitis (IB) is a highly infectious viral disease caused by the infectious bronchitis coronavirus (IBV). IB affects layers and broilers to cause initial respiratory distress, sneezing, gasping, facial swelling, and malaise and retarded growth with low mortality, but high morbidity. If the birds are infected at between 3 and 6 weeks of age, the infection may additionally damage renal tissues causing depression and mortality of up to 30%. However, in some outbreaks the initial disease can be asymptomatic, but in laying birds, damage to the oviduct results in reduced egg production, or can cause “blind” layers – where the egg is passed into the body cavity due to damage to the oviduct. IB is controlled to a large extent by aerosol and water vaccination (see section on Vaccination below).
8.6.3.5 Marek’s Disease
Marek’s disease, caused by avian herpes viruses, commonly affects birds from 6 weeks or more. Six sub-classifications of Marek’s disease have been suggested, although the disease may take a form which presents a mixture of the following forms (Herenda and Franco, 1996);
(a)
Per-acute, manifested by sudden death.
(b)
Anaemia, in 3–6 week old chickens.
(c)
A “classical” presentation with thickening of peripheral nerves and subsequent nerve damage, resulting in progressive spastic paralysis of the legs and wings, sometimes with torticollis (head and neck twisting) and sometimes with respiratory distress. Lymphomatous tumours appear in the skin, gut, eye, ovary, lungs, heart and liver. Total mortality in the “classical” form is usually less than 15%, with a low incidence at any one time, and disease appearing over many months.
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