PREVALENT DISEASES AND MORTALITY IN EGG TYPE LAYERS M. Farooq, Department of Poultry Science, NWFP, Agricultural University, Peshawar, Pakistan. Summary Introduction Discussion References SUMMARY Mortality plays a major role in determining profitability of egg type layers, as it is a function of culled and dead birds. Negative association of mortality has been reported with net profit. Higher mortality and culling were reported due to severe outbreaks of infectious/non-infectious diseases, accidental deaths, substandard health and management practices and poor quality of chicks and feed. Filthy environment, wet bedding material and house temperature in the range of 20-280C favored sporolation of oocysts resulting in a higher incidence of coccidiosis. Overall mortality in egg type layers was in the range of 3.1 to 14.2%, however, higher mortality during brooding (26.23%), growing (24.56%) and laying period (49.2%) had also been observed. Newcastle (ND), Infectious bursal disease (IBD), yolk sac infections and coccidiosis were found to cause maximum mortality in egg type layers (>30%). Incidence of IBD was higher (32-76%) in between the age of 2-12 weeks age of chicken and concurrent infections of E.coli and coccidiosis also favored incidence of IBD. A drop in egg production by 10-40% was found with the incidence of infectious coryza, E.coli, mycoplasmosis, coccidiosis egg prolapes and aflatoxicosis. Eggs with pimple shells were associated with Mareks, mycoplasmosis and calcium deficiency. Salmonellae were abundantly found in bedding material of chicken (42%), drinkers (36%), feed (28%) and water tanks (17%) of the poultry farm. Feed toxicity and cannibalism resulted in 3-10% mortality in egg type layers. Infectious coryza, enteritis, Hydro-Pericardium (HP), collibacilloses, brooder pneumonia, lymphoid leukosis, Chronic Respiratory Disease (CRD), Fowl-pox, Fowl-typhoid, Fowl-cholera and infectious laryngotracheitis (IL) caused mortality within the range of 0.81% to 20% in egg type layers. Heating of eggs at a temperature of 460C prior to setting in incubator prevented vertical transmission of mycoplasmosis with a slight impact on hatchability. A loss of 127 million eggs and $7 million was found due to Mycoplasma gallisepticum. A decreasing trend in general prevalence of infectious diseases has been observed during the recent few years due to effective vaccination, better health care and appropriate management of the layers. Maintenance of a healthy environment in a poultry shed, protection of birds from extreme climatic conditions, restriction of visitors and wild animals, proper cleaning and disinfection of houses, equipment and workers, appropriate floor and house construction, assurance of healthy drinking water, antibiotic therapy, chlorine treatment and filtration of water tanks were reported as key factors in the reduction of loss due to diseases and mortality in egg type layers. INTRODUCTION Disease out-break, increased mortality and higher percentages of cull birds could adversely affect profitability of egg type layers. Farooq et al. (2001) reported a significant and negative association of mortality with net profit, suggesting that increase in mortality would result in a decrease of net profit. A higher percentage proportion of culls are a function of poor quality chicks and feed and inappropriate management or care of the flock. Similarly, higher death rates in egg type layers could be due to severe outbreak of diseases, substandard health measures and management practices, poor quality of chicks or feed and accidental deaths. Thus, due attention shall be given to infections, health care, management practices and predisposing factors in the avoidance of undue risks of mortality in chicken. Because, microorganisms deteriorating performance of chickens or resulting in morbidity or mortality could be abundantly found in and near the poultry sheds and any variation in rearing environment would provide a better chance for these microorganisms to invade chicken. Salmonellae, one of the bacterial species influencing higher losses in chicken, were abundantly found in bedding material of chicken (42%), drinkers (36%), feed (28%) and water tanks (17%) of the poultry farm (Sasipreeyajan et al., 1996). Majid et al. (1991) also reported higher prevalence of Salmonellosis in layer flocks maintained under poor management conditions in Faisalabad. These organisms contaminate feed and drinking water and result in severe economic losses. Diseases of chicken are mostly infectious in nature and therefore, wide variability in losses due to such diseases is expected in egg type layers. Infectious Bronchitis (IB) and IBD are currently the most prevalent diseases resulting in higher mortality among layers. Rikula et al. (1993) reported IB to be the most prevalent disease causing 67% mortality in chicken in Finland whereas, Amin et al. (1995) reported IBD to be a destructive force resulting in 40.4% mortality in egg type layers at Faisalabad, Pakistan since 1993. Because of its infectious nature, ND resulted in higher death losses in egg type layers (Anjum et al., 1993; 51.5% and Savic, 1999; 60%) however, Ghodasara et al. (1992) reported coccidiosis (35.26%) and yolk sac infections (31.45%) to cause severe losses in egg laying birds than any other disease. Qureshi et al. (1981) also reported higher losses due to ND (20%), Coccidiosis (20%) and yolk sac infections (30%) in egg type layers. Infectious coryza, enteritis, Hydro-Pericardium (HP) aflatoxicosis, collibacilloses, brooder pneumonia, lymphoid leukosis, fatty liver hemorrhagic syndrome, salmonellosis, Chronic Respiratory Disease (CRD), Fowl-pox, Fowl-typhoid, Fowl-cholera, Infectious Bronchitis (IB), Infectious Laryngotracheitis (IL) and Mareks disease were found to cause mortality within the range of 0.81% to 18% in layers. Preventive measures taken against infectious diseases; age of the bird, season and climatic conditions could influence mortality in chickens. Birds vaccinated against IBD before the 10th day of age were more adversely affected than those vaccinated after the 10th day of age (Anjum et al., 1993). Singh et al. (1994) reported that IBD, coccidiosis, E. coli and other bacterial infections to be more prevalent in between the age of 6-11 weeks than at later stages of life (18-22 weeks). Birds exposed to climatic extremes and seasonal conditions are characteristically more prone to all types of diseases than others. Anjum (1990) reported a higher incidence of mortality among the submitted cases in Faisalabad during spring (36.6%), than in summer (25.8%), winter (19.4%) and fall (18.3%) seasons. Tariq et al. (1989) also reported a higher mortality due to respiratory problem in Punjab, Pakistan during spring (41.43%) than in winter (31.43%), summer (29.31%) and fall season (6.43%). Such losses could be attributed to adverse climatic conditions to which the chickens were exposed, lowering the immune response against diseases. In fact, these losses are heavier and would contribute irreparable losses to flock owners if not properly handled, as increased level of mortality will result in poor economic gains. North (1984) and Kitsopanidis and Manes (1991) in Greece also reported a reduction in net profitability with increased mortality levels in chickens. Better hygienic measures, like maintenance of a healthy environment inside and outside the poultry shed, protection of birds from extreme climatic conditions, restriction of visitors and wild animals, proper cleaning and disinfection of houses, equipment and workers, and appropriate floor and house construction will help in reducing losses due to mortality. In addition, assurance of healthy drinking water (Khurshid et al., 1995), effective vaccination against disease, antibiotic therapy, chlorine treatment and filtration of water tanks would reduce mortality many folds (Mukherjee and Khamapurkar; 1994). Keeping in view, losses due to various diseases, this review paper was written with the objectives to highlight possible causes of mortality and report effective strategies for their control in future. DISCUSSION Findings regarding mortality and prevalent diseases in egg type layers in different parts of the world are reviewed and discussed under various sub sections as follows: OVERALL MORTALITY IN EGG TYPE LAYERS Overall mortality in layers was around 12% (Petek, 1999, and Amin et al. 1995). Singh et al. (1995) reported a little higher mortality (14.2%) in layers. However, contrary to the findings of Petek (1999), Amin et al. (1995) and Singh et al. (1995), higher mortality was reported by Ghodasara et al. (1992) in egg type layers during brooding (26.23%), growing (24.56%) and laying periods (49.2%). These losses were higher than the optimized level of mortality (8-10%) reported by North (1984) in egg type layers for better profitability. The higher mortality in egg type layers could be attributed to severe outbreak of infectious and non-infectious diseases, substandard management and health practices, poor quality of chicks or feed and filthy environment. Infectious diseases of commercial chicken are regarded as sweeping diseases and any variation in health coverage or management could result in higher mortality. A disease outbreak could result in severe economic losses within a shortest possible time before its medicated recovery is ensured. In all probability, it is the delicate nature of the commercial chicken, their susceptibility to diseases and undesirable conditions that would not allow them to retain more resistance against disease attacks. In commercial chickens, more emphasis had been placed upon its genetic potentials for higher production, rather than their acclimatization to odd environments or ability to resist diseases. Thus, they are to be reared in a healthy environment to avoid increased risks of mortality. Tolimir and Masic (2000) reported a lower mortality (6.8%) among egg laying birds in recent decades, as compared to previous periods. The decline in mortality with the passage of time (from 1992 through 1999) could probably be attributed to the awareness of people regarding disease prevention and the innovation of improved techniques and measures, than that prior to 1992. Assurance of healthy drinking water (Khurshid et al., 1995), appropriate and timely vaccination, antibiotic therapy, and filtration of water tanks reduced the incidence of mortality in India (Mukherjee and Khamapurkar; 1994). Better care of the flock, maintenance of a healthy environment inside and outside the poultry shed, protection of birds from extreme climatic conditions, restriction of visitors and wild animals, proper cleaning, and disinfection of houses, equipment, and workers, and appropriate floor and house construction were key factors in preventing higher mortality resulting from disease outbreak. DISTRIBUTION OF VARIOUS DISEASES Prevalence of various diseases in layers is discussed in this section under various subsections as follows: MAREKS DISEASE Mareks is one of the important diseases of chicken characterized by leg paralysis and lymphocyte infilteration of brachial and sciatic nerves (Nicholls, 1984), potentially causing 4.2-20.8% mortality in layers (Taylor et al., 1999). The disease could be more prevalent in layers lacking immunization and additionally, further risk exists with calcium deficiencies during the laying phase. Chickens are vaccinated against mareks at the hatchery before they are transported to the farms. Losses due to this disease therefore, are avoided through effective vaccination and eliminating calcium deficiency in the egg laying period. Calcium is vital for the eggshell and its insufficiency will not only result in poor shell eggs, but it could work as a predisposing factor for mareks disease (Taylor et al., 2000). In addition, the rearing of mixed age flocks increased risks of Mareks disease (Heier and Jarp (2000). The authors also reported a higher risk of Mareks in laying birds reared on the floor than those maintained in cages, probably due to the condition of a soiled environment. INFECTIOUS BURSAL DISEAS (IBD) Infectious Bursal Disease (IBD) also known as Gumboro had been reported to cause heavier losses in chickens (10-75%; Sah et al., 1995, and 80-100%; Chowdhury et al., 1986). The disease is characterized by lameness, severe morbidity and mortality in chicken. It is considered as AIDS of the chicken, because it adversely affects the chicken's immune system. Bursa fabricus, one of the organs responsible for antibody production in chicken was invaded by IBD virus and destroyed completely which in turn, resulted in higher losses in egg type layers (40.4%, Amin et al., 1995; 36.65%, Singh et al., 1994). Rao et al. (1990; 20%) and Farooq et al. (2000; 1.08±0.01%) reported a smaller mortality in layers than that reported by Amin et al. (1995) and Singh et al. (1994). Birds of all ages were susceptible to IBD however, losses in between the age of 2-12 weeks were higher (32-76%, Philip and Moitra, 1993; 20%, Rao et al., 1990 and 25%; Prabhakaran et al., 1997) than at any other stage of life. Unexpectedly higher losses due to IBD had also been observed in chickens at the age of 17 weeks (Philip and Moitra, 1993). The higher incidence of IBD in egg type layers could probably be due to poor vaccination and susceptibility of chickens to IBD (Anjum et al., 1993, and Farooq et al. 2000), filthy environment and predisposing factors like concurrent infections with E. coli, coccidiosis and other bacterial infections (Singh et al., 1994). Anjum et al. (1993) and Kouwenhoven et al. (1994) reported that vaccination against IBD at the age of 14-21 days partially controlled the problem, explaining that in spite of the vaccination, atrophy of bursa could not be protected even if there was a mild infection of IBD (Sultan and El-Sawy, 1997). Therefore, care must be taken to administer vaccines at stipulated times and successfully overcoming predisposing factors working as contusive media for outbreak of IBD. Prevention of concurrent infections like E. coli and coccidiosis and maintenance of standard hygiene will be helpful in reducing losses due to IBD in chicken. NEWCASTLE DISEASE (ND) Newcastle disease is one of the destructive diseases of chickens characterized by severe mortality, greenish diarrhea and thirst. The birds tend to drink more water and decrease their consumption of feed. Newcastle disease caused 60% losses in egg type layers (Savic, 1999); however, lower losses of 12.58% had also been reported by Srithar et al. (1997). On any standard these losses were high because the increase in mortality beyond 8-10% was not admissible in egg type layers (North, 1984). Aside from higher mortality, Newcastle disease caused a 15% drop in eggs of infected flocks and simultaneously resulted in 5% soft shell eggs (Lambert and Kabar, 1994). Newcastle disease was prevalent among layers in all seasons of the year and caused about 50% mortality in egg type layers (Anjum, 1990 and Anjum et al., 1993). Qureshi (1981; 20%), Mashooq (1981; 17.5%), Srinivasa et al. (1989; 17.21%), Siddique and Javed (1989; 16.4%) and Bhatti (1989; 9.35%) reported smaller mortality due to ND than that reported by Anjum (1990) and Anjum et al. (1993). The comparatively lower losses reported by some authors than others could probably be attributed to the implementation of effective measures for the prevention of diseases, such as vaccination against ND and maintenance of improved hygienic conditions. The higher losses due to ND could probably be due to the infectious nature of ND and its rapid spread from flock to flock within a shorter period of time. As it is a viral disease it can spread from one flock to another easily through the movement of workers from one farm to the another, wild animals and birds, visitors and transport vehicles used from farm to farm delivery. Thus, assurance of appropriate hygiene and effective and timely vaccination will be helpful in reducing losses. INFECTIOUS CORYZA Infectious coryza is also an important bacterial disease of chickens characterized by respiratory complications, swollen head syndrome, nasal discharge and severe drop in egg production. The most common cause is Haemophilus gallinarum. Conditions of poor hygiene, chilly environment and adverse climate exposure could work as predisposing factors for the onset of this disease. Chickens of all type and age were found susceptible to this infection and the disease caused 2-5% mortality and 35% drop in egg production (Sandoval et al., 1999; and Reece et al., 1986). El-Houadfi and Vanmarcke (1991) also reported adverse effects of coryza on egg production, and as the disease can spread slowly, results could be almost 100% morbidity (Bains, 1979). Protection of birds from extreme climatic conditions, maintenance of good hygiene and antibiotic therapy along with vitamin C or ascorbic acid were helpful in preventing losses due to coryza. INFECTIOUS BRONCHITIS (IB) Highly infectious viral disease characterized by respiratory symptoms, increased mortality and decreased egg production (Butcher et al., 1990). The disease could occur at any stage of the chicken's life and during any season of the year. However, it was found to be more prevalent (35.7%) in 7 days to 5 weeks of age with special reference to its higher incidence (66.6%) in the winter season (Javed et al., 1991). The higher incidence in young chickens was attributable to poor immunity development during the first few weeks of life. Similarly, winter conditions could have also favored the incidence of IB because of stressful conditions and chilly environment. Thus, protection of birds from extremely cold conditions and the maintenance of a healthy environment would further reduce incidence of this disease. AVIAN INFLUENZA Avian influenza is an important poultry disease that had emerged with higher mortality in the recent decades. This disease caused 90% morbidity and 80% mortality in 30 week aged chickens (Morgan and Kelly (1990). Pathogenisity of avian influenza was more in egg laying birds than in broilers (Swayne et al., 1994). The higher incidence of avian influenza in layers could probably be due to the incidence of avian influenza at later stages of life as layers were retained in a flock for a longer duration than broilers. MYCOPLASMOSIS Mycoplasmosis is a series of bacterial infections caused by bacterium mycoplasm of various types in egg type layers. Mycoplasmosis results in severe economic losses in egg type layers in terms of reduced egg production and higher mortality. Eggs with Pimpled shells were also associated with Mycoplasma infections (Branton et al., 1997). Flocks infected with Mycoplasma gallisepticum (MG) and Mycoplasma synoviae (MS) produced fewer eggs (Mohammad et al., 1987). The authors reported a loss of 127 million eggs and $7 million due to MG only. North (1984) reported 20-30% drop in egg production due to MS. These losses are very high and would narrow the margins between cost of production and net profit from commercial egg laying birds. Efforts shall always be made to reduce losses thereby increasing egg production. This could however, be possible through better health management and the application of improved husbandry skills. For effective control of MG, a vaccine has now been prepared and is administered in drinking water when the layers are 12 weeks old. However, the disease is mostly transmitted through eggs from the infected or carrier birds to the newborn chickens. Thus, it is imperative for the breeders to have a regular blood test program and eliminate infected or carrier birds from the flocks. An effective measure to prevent its vertical transmission has been developed by heating eggs prior to incubator placement at a temperature of 460C. At this temperature no mycoplasma will survive, however heating eggs at this temperature may have a slight impact on hatchability. SALMONELLOSIS Salmonellae, one of the bacterial species, are abundantly found in most of the areas where chances of contamination are greater; primarily poultry sheds and feed reservoirs. Salmonella had also been isolated from litter (42%), drinking water (36%), feed (28%) and water tanks (17%; Sasipreeyajan et al., 1996) in poultry premises. These organisms usually contaminate feed and drinking water thereby, resulting in poor economic gains and higher mortality. Salmonellae caused pullorum, typhoid, paratyphoid and other related infections in chicken resulting 50% losses (North, 1984). Salmonellae are vertically transmitted to the newborn chicks, therefore, regular blood testing of the parent flock and elimination of infected and carrier birds would be helpful in reducing its vertical transmission. In addition, preventing entry of rodents, vermin or other wild animals and the assurance of improved hygienic conditions would be helpful in reducing the incidence of salmonellosis. COCCIDIOSIS Coccidiosis a protozoan disease, is one of the major problems of the chicken industry, characterized by blood tinged feces, ruffled feathers, loss of appetite, poor growth and reduced egg production. Coocidiosis had been reported to result in higher mortality (51.38%; Demir, 1992) and economic losses ($35 to $200 million/year in USA; Hofstad et al., 1978). The most prevalent causative agents of coccidiosis among the coccidia species were sporolated oocysts of genus emeria that primarily invaded the small intestine and caecal pouches, leading to enteritis and thickening of the intestinal walls (Shukla et al., 1990). Oocysts are usually passed through feces by infected chickens, undergoing the process of sporolation when conditions are favorable. Unclean environment, wet bedding material and house temperature in the range of 20-280C favored sporolation of oocysts (Hofstad et al., 1978). Coccidia were found to be the most resistant type of protozoa, remaining viable for several months in poultry sheds (Stayer et al., 1995). However, deterioration of seeded oocysts started soon after a 24-hour period when sporolation conditions were not favorable (Williams, 1995). Coccidiosis could occur at any stage of the chicken's life and during any season of the year; however, it was found to be more prevalent in summer season (Boado et al., 1991), probably when higher summer temperatures and wet bedding favored rapid sporolation of oocysts. Bushell et al. (1989) reported affective use of live attenuated coccidiosis vaccine in controlling the problem. Assurance of a healthy environment and the elimination of moisture and increased heat conditions within the house were reported to reduce chances of a coccidiosis outbreak (Stayer et al., 1995). Addition of coccidiostates in the ration had been one of the best options for the control of coccidiosis; however, egg laying birds are given coccidiosate-free ration during the egg laying period and an outbreak of coccidiosis at that stage will not only result in massive death casualties, but it could lower egg production performance of the birds. The pullet should therefore, have complete immunity against coccidiosis before initiation of egg lay (North, 1984). Thus, management would be a key to avoid sporolation of oocysts when the layers are to be reared on the floor. With the introduction of cage systems, the coccidiosis problem has now been solved up to a greater extent. However, elimination of coccidiosis before the shifting of birds to cages should be ensured. YOLK SAC INFECTION It is one of the most common bacterial infections of chicken observed during the first few weeks of a chicken's life. Drowsiness, minimal mobility, vent pasting and the lack of interest of feeding in the chicken characterize yolk sac infection. There may be several predisposing factors such as poor hygiene and stressful conditions leading to this anomaly because, it is a general bacterial infection. Isolates of Staphylococci and E. coli were found to be the most common causes of yolk sac infection (Bains, 1979). Yolk (a reservoir of food for the embryo and chicken in the first few days after hatching as well) could easily become infected with the presence of any bacterium. The intact bacteria enter the inner content of the egg during the incubation process and cause infection of the navel area of chicken. Conversely, if the yolk is not affectively utilized after hatching, it could be easily infected and easily become rancid. The yolk usually becomes infected prior to hatching and during the first 48 hours after hatching. Yolk sac infection was found to cause 31.45% mortality in the early few days of a chicken's life (Ghodasara et al., 1992). North (1984; 10%) and Reece et al. (1986; 2.31%) however, reported smaller losses due to yolk sac infection in chicken than those reported by Ghodasara et al. (1992). As the infection is mostly transmitted through dirty shell eggs, frequent collection of eggs and keeping the conditions more favorable to obtain clean eggs will be helpful in the reduction of yolk sac infection. In addition, better management of the chicks during brooding; avoiding overcrowding and other stressful conditions will further reduce the incidence of yolk sac infection. Escherichia coli (E. coli) E. coli is one of the major problems in chicken production influencing heavier losses and severe drop in egg production. About 5.5% mortality and 10-20% drop in eggs was observed with E. coli infections in egg type layers reared in cages (Qu et al., 1997). Zanella et al. (2000) also reported 5-10% mortality due to E. coli infections with no pronounced signs, suggesting that the infection may be there but couldn't be easily detected until regular tests are performed for its proper diagnosis. The situation leading to mortality with no pronounced clinical signs will be more critical as it would result in heavier losses of reduced egg production prior to the investigations. E. coli will not only result in reduced egg production and mortality, it could be a predisposing factor for other complications like IBD as has been stated by Singh et al. (1994). Prabhakaran et al. (1997) associated IBD with concurrent infections of E. coli and coccidiosis. Thus, it is important to control E. coli infections in chickens, thereby preventing losses due to this disease and other associated infections. EGG PROLAPES AND CANNIBALISM Egg prolapes has become one of the major issues in egg type layers during the past few years. Egg prolapes could cause higher mortality and in turn, would result in huge economic losses (Tablante et al., 1994). The authors reported 9.4% egg prolapse cases in egg type layers. Abrahamsson and Tauson (1998) reported cannibalism as the picking habit of chicken, causing 4-20% mortality. North (1984) reported deficiency of fiber in feed and management faults as the major factors contributing to higher incidence of cannibalism. Damme (1999) reported that cannibalism could be effectively controlled through appropriate beak trimming. The author reported a smaller incidence of cannibalism (0.3%) in beak-trimmed birds than non-trimmed (7.5%). AFLATOXICOSIS Presently, aflatoxicosis is one of the major issues in chicken production. The common cause of aflatoxicosis is contaminated feed, resulting in higher mortality and severe drop in egg production. 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