Risk indicators for Newcastle Disease outbreaks in broiler flocks in Pakistan

ELSEVIER

PREVENTIVE VETERINARY

MEDICINE Preventive Veterinary Medicine 22 ( 1995 ) 6 l-69

Risk: indicators for Newcastle Disease broiler flocks in Pakistan

Saeed Akhtar*, Saleem Zahid

outbreaks in

Animal Sciences Institute, National Agricultural Research Centre, Islamabad 45500, Pakistan

Accepted 24 May 1994

Abstract

A prospective study was conducted to assess the risk indicators for the occurrence of Newcastle Disease (ND) outbreaks among broiler flocks in Islamabad Capital Territory. Data were collected between July 1989 and April 1990 from 13 1 flocks pertaining to their demography, management practices, prophylactic procedures and concurrent diseases. The attack rate of ND in the whole population was 16.0% (95% Confidence Interval (CI) = 12.8, 19.2). Multivariate logistic regression analysis of the data showed that vaccination against hydropericardium syndrome with an autogenous vaccine (adjusted Odds Ratio (OR) = 10.0; 95% CI= 1.2, 85.2), improper feed storage (OR=5.7; 95% CI = 1.7, 18.6) and poor experience of producers in broiler farming (OR= 3.4; 95% CI = 1.1, 10.5 ) were independently associated with the occurrence of ND outbreaks.

Keywords: Chicken; Epidemiology; Newcastle disease virus; Hydropericardium syndrome vaccine; Risk indicators; Pakistan

1. Introduction

Newcastle disease (ND) is a commonly reported poultry disease both in vaccinated ;and unvaccinated flocks in Pakistan. The annual incidence of ND outbreaks in commercial broiler chicken and in layer flocks up to 38% ( Anjum, 1990) and within flock mortality of commercial broiler chicken up to 50% has been reported (Siddique et al., 1986). Therefore, ND alone seems to be severely ham- pering the production performance of both layer and broiler flocks.

The methods of ND spread have been reviewed by Lancaster ( 1963a,b). The

* Corresponding author. Tel: 92-51-2408 13; Fax: 92-5 l-240909.

0167-5875/95/$09.50 0 1995 Elsevier Science B.V. All rights reserved XSDI0167-5877(94)00402-5

62 S. Akhtar, S. Zahid / Preventive Veterinary Medicine 22 (1995) 61-69

most important factors involved in the spread of ND within an area include the movements of live birds and mechanical transport of infection by normal trade practice (Dawson, 1973). Evidence has also been presented for the air-borne spread of ND among the flocks during an epidemic in Great Britain (Hugh-Jones et al., 1973).

An immunosuppressive effect of a naturally acquired subclinical infectious bursal disease virus infection was a probable causal factor of marked immunosuppression of ND vaccine in young chicks, thus making the flock vulnerable to a natural ND outbreak (Almassy and Kakuk, 1976). Furthermore, concurrent infection with infectious bursal disease virus also has been regarded as an important factor for ND outbreaks (Rathore et al., 1987; Singh et al., 1987).

The poultry industry in Pakistan in recent years experienced a devastating epidemic of hydropericardium syndrome (HPS), apparently a new disease. The syndrome is typically observed in 3- to j-week-old growing commercial broiler chickens and results in up to 60% mortality (Anjum et al., 1989; Cheema et al., 1989 ) . The aetiology of the disease is uncertain, although studies have indicated that in addition to an adenovirus some other viral agent is involved. The nature of this viral agent, however remains unknown (Afzal et al., 199 1). An autogenous vaccine comprised of formalized liver extract (Chishti et al., 1989; Afzal and Ahmad, 1990) was widely used for protection against the HPS in commercial broiler chickens at a flock age of 15 days. Reports on ND outbreaks were received from high-density broiler growing area of Islamabad Capital Territory following the use of HPS vaccine.

The objective of the present prospective study was to investigate the use of HPS vaccine as a risk indicator for ND outbreak after adjusting for the effects of demographic, management, hygiene methods, prophylactic procedures and concurrent diseases using a multivariate analytical technique.

2. Materials and methods

2.1. Study design

One hundred and live farmers willing to participate in the project were en- rolled by extensive field visits of four townships of Islamabad Capital Territory during June 1989. In the preliminary visits, the objectives of the study were ex- plained to farmers, who were requested to maintain all required records.

The unit of interest in this study was a broiler flock. It was decided to study at most two consecutive, non-overlapping flocks on each broiler production unit. The interview form was designed to include questions about factors which were known and/or hypothesized to influence the occurrence and spread of ND. The factors were compiled from the literature and from the observations of the tech- nical personnel. The interview form was pretested by questioning ten different broiler producers before the actual field investigation started. For consistency, the questions were grouped into four sections, i.e. demographic information,

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management and hygiene information, prophylactic procedures and disease variables. The interviews were conducted by means of personal interview of the owner/manager of the flock.

2.2. Disease diagnosis

Diagnosis of ND and other concurrent diseases was carried out by postmortem examination at the Disease Diagnostic Laboratories of the Poultry Development Centre, Rawalpindi. The producers were requested to select a few but not all dead birds for postmortem examination (in case mortality was too high on any day). A flock was considered affected by a particular disease in cases where mortality rate due to the disease in question was equal to or greater than 1%; otherwise the flock was regarded as unaffected by that disease.

2.3. Stat8istical methods

For aIl analyses the dependent variable, ND epidemic status of the flock had two categories: affected or unaffected flock. Other variables included in the analyses are defined in Tables 1 and 2. The attack rate of ND among study flocks and 95% confidence interval (CI) for this proportion were calculated, using exact binomial methods (Cox, 1970). The bivariate relationships between ND status and independent variables were examined using the chi-square and Fisher’s exact tests (Rosner, 1986), or Student’s t-test (Daniel, 1987) as appropriate. Multi- variate lmogistic model was used to estimate the effect of each study variable on the outcome, adjusting for other variables in the model (Kleinbaum et al., 1982). Variables with significant univariate relationships at P-C 0.1 were offered to the algorithm for final model selection. Model selection was performed by forward and backward stepping procedures. Models were selected on the basis of minimal goodness-of-fit x2. The improvement x2 test was used to distinguish between nested hierarchical models. The P values for F-to-enter and F-to-remove were set at 0.10 and 0.15, respectively. Variables with coefficients that did not signifi-

Table I Unadjusted analysis of some quantitative type of demographic variables for a prospective study of Newcastle disease (ND) outbreak affected (2 1) and unaffected ( 110) commercial broiler flocks in Islamabad Capital Territory, July 1989-April 1990

Variable ND status Mean (&SE)

Flock size Affected 2600f 128 Unaffected 2763f 112

Flock interval” Affected 80+23 (days) Unaffected 45+6

Minimum intertlock Affected 166f.52 distance (m ) Unaffected 343 + 40

a Minimum rest period (days) between two consecutive flocks.

t statistics

0.96

1.49

2.73

P value

0.34

0.15

0.009

64

Table 2

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Distribution of the qualitative variables for a prospective study of Newcastle disease (ND) outbreaks among 13 1 broiler flocks in Islamabad Capital Territory, July 1989-April 1990

Variable ND affected Total flocks x2 value P value

No. %

Demographic variables Farm location

Sihala Chak Shazad Lehtrar Tarli

Operation type Broiler Mixed

Number of flocks studieda Two One

Farming system All-in all-out Multiple age

Broiler strain Indian River Ross- 1 Hubbard Lohmann

Management variables Interviewee

Manager Farmer

Farming experienceb Experienced Inexperienced

Labour type Owner Hired Mixed

Wild birds proofing Adequate Inadequate

Visitors at the farm No Yes

Visit by the poultry crew No Yes

Light/heat source Kerosene oil Electricity

Feed storage method” Proper Improper

1 4 28 12.51 0.006 2 10 21

10 17 59 8 35 23

20 16 128 0.68 0.41 1 33 3

6 12 52 1.29 0.26 15 19 79

18 15 118 0.53 0.47 3 23 13

3 10 31 1.04 0.79 1 8 12

16 19 85 1 33 3

1 7 15 20 17 116

6 9 64 15 22 67

8 20 40 6 12 49 I 17 42

0.01 0.91

4.12 0.04

1.00 0.61

7 14 52 14 18 79

5 13 40 16 18 91

1 20

9 17

11 19

12 31

11 120

5 16

45 86

13 8

105 26

0.42 0.52

0.53 0.47

0.43 0.51

1.23 0.21

5.23 0.02



No. %

Management variables Feed transportation

Private Agency vehicle Hired

Feed chan,ged No Yes

2 15 4

13 8

Hygiene variables Litter changed

Yes No

Shed water-washed Yes No

Shed disinfected Yes No

Shed white-washed Yes No

Litter disposal Sold Used

Shed ventlation Adequate Inadequate

Carcass disposal Thrown loose Buried

18 3

17 4

19 2

20 1

7 14

19 2

12 9

Disease prophylactic measures Initial ND vaccination

Mukhteswar 11 Lasota 9 No 1

Booster ND vaccination Mukhteswar 9 Lasota 10 No 2

IBD vaccination Yes 2 No 19

HPS vaccl nation Yes 20 No 1

Drug use Yes 2 No 19

11 19 0.94 0.63 16 94 22 18

12 109 26 22

8.12 0.004

15 120 27 11

15 21

112 19

16 117 14 14

16 127 25 4

14 50 17 81

18 106 8 25

16 77 17 54

1.13 0.29

0.42 0.52

0.04 0.85

0.25 0.62

0.93 0.63

1.48 0.22

0.69 0.88

16 65 15 61 20 5

1 .oo 0.61

18 50 14 74 29 I

1.31 0.52

8 24 18 107

20 3

9 17

99 32

22 109

1.29 0.26

5.24 0.02

1.05 0.59

66 S. Akhtar, S. Zahid / Preventive Veterinary Medicine 22 (1995) 61-69

Table 2 (continued)


No. %

Concurrent diseases Early chick mortality

Yes 4 I 56 5.14 0.02 No 17 23 75

Coccidiosis Yes 7 22 32 1.07 0.30 No 14 14 99

Infectious coryza Yes 7 14 49 0.18 0.61 No 14 17 82

Escherichia coli infection Yes 5 19 26 0.25 0.62 No 16 15 105

Hydropericardium syndrome Yes 8 13 61 0.12 0.40 No 13 17 70

’ Number of consecutive flocks studied at the same property. b Experienced: if farmer/manager had raised previously more than three commercial broiler flocks.

Inexperienced: if raised three flocks or less. ’ Proper: if feed bags stored on wooden slabs covered with plastic sheets. Improper: if feed bags were

kept directly on cemented or uncemented floor and not covered with plastic sheets. d Whether the same feed brand (from same manufacture) was used during the entire life of the flock

or changed to another feed brand.

cantly affect the goodness-of-lit were eliminated from the model (Hosmer and Lemeshow, 1989). Logistic analysis was carried out with the aid of a statistical package LIMDEP (Greene, 1988). After fitting the final model, logistic coefficients and their standard errors were used to compute adjusted odds ratios (ORs) and corresponding 95% CIs.

3. Results

A total of 13 1 flocks from 105 broiler production units were investigated during the study period. The attack rate of ND in this population was 16.0% (2 1 / 131) (95% CI= 12.8, 19.2%).

Minimum interflock distance (Table 1) and six categorical variables (Table 2 ), none of which was from the hygiene category, had significant (PC 0.1) bivariate relationship with flock ND status and were offered to the logistic model for final selection.

3.1. Multivariate logistic model

Table 3 presents the tinal multivariate logistic regression model. Vaccination with an autogenous formalized killed HPS vaccine was significantly (P= 0.035 )


Table 3 Multivariate logistic model of risk indicators for Newcastle disease outbreaks among 13 1 commercial broiler flocks in Islamabad Capital Territory, July 1989-April 1990

Variable b SE (b) P value Adjusted odds ratio

Point estimate

95% confidence limits

EXPE 1.21 0.58 0.04 3.4 1.1, 10.5 FEDS 1.74 0.6 1 0.004 5.7 1.7, 18.6 HPSV 2.31 1.09 0.04 10.0 1.2, 85.2 Constant -4.83 1.30 0.000

Log likelihood = -48.59 Model x2 = 18.15 P= 0.0004 ( 3 d.f. )

EXPE, (experience of the farmer/manager in broiler farming (0 = experienced, if farmer/manager had raised previously more than three flocks; 1 = inexperienced; if farmer/manager raised three flocks or less; HPSV, vaccination of the flock against hydropericardium syndrome at the age of 15 days (0 = no; 1 = yes) ; FEDS, feed storage method ( 0 = proper, if feed bags stored on wooden slabs covered with plastic sheets; 1 = improper, if feed bags were kept directly on cemented or uncemented floor and not covered with plastic sheets).

associated with ND outbreak status even after adjusting for possible differences in the experience of the farmers and feed storage method. Improper feed storage method (feed bags stored directly on uncemented or cemented floor and not covered with plastic sheet ) and poor experience of producers in broiler farming (previously raised three or less commercial broiler flocks) also increased the odds of ND outlbreak.

4. Discussion

The methods of spread of ND virus have been adequately documented (Lan- caster, 1963a,b). The ND epidemic in any poultry flock could occur due to some faulty poultry husbandry method followed by a rapid mechanical spread to other immunologically unprotected or suboptimally protected flocks in the area.

HPS (an apparently new disease) has caused colossal economic losses to the broiler industry in Pakistan (Afzal and Ahmad, 1990). The aetiology of the disease is uncertain. An adenovirus has been detected in infected liver by electron microscopy (Cheema et al., 1989 ) . Aetiological studies (Afzal et al., 199 1) have indicateid that in addition to an adenovirus some other agent is involved. A formalized liver homogenate vaccine, when administered to a 15-day-old commercial broiler flock, provided good protection against experimental challenge (Chishti et al., 1989) and in field trials (Afzal and Ahmad, 1990). Many private entrepreneurs as well as public institutes started producing and marketing this prototype formalized liver homogenate vaccine without the required level of

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professional supervision and quality control procedures. Therefore, it might well be that liver from birds having subclinical ND was used to prepare HPS vaccine and that ND virus escaped inactivation procedures, resulting in field outbreaks. An alternate and/or simultaneous explanation could be based on the following observation. The HPS vaccine is prepared from liver homogenate, and the pres- ence of cell debris and subcellular proteins in this vaccine has been shown to cause transient immunosuppression for up to 5 days in the flock following its use (Naeem, 1992). Any exposure to ND virulent virus during that period might have resulted in overt ND epidemic and/or ND vaccination carried out during that period may have left the flock unprotected due to poor response to ND vaccination and such flocks succumbed to field exposure.

Improper feed storage methods significantly increased the risk of occurrence of ND epidemic in a flock in this study. This observed relationship could be ex- plained based on results of previous studies. Feedstuffs are liable to be contami- nated by fungi during storage because the mould spores are ubiquitously distrib- uted. During feed storage temperature, percent moisture and percent humidity influence the growth of fungi and development of mycotoxins (Chaudhry et al., 198 1). Improper feed storage methods favor the growth of Aspergillus frauous which produces the aflatoxins B and G in the feed (Afzal et al., 1979; Chaudhry et al., 198 I ) . Aflatoxins are known to trigger immunosuppression (Muneer et al., 1988), and thus, resultant poor immune response of the flock to vaccination in- creases the vulnerability to ND virus infection. ND outbreak in a large broiler farm associated with immunosuppression caused by aflatoxin excess in feed at 0.5-0.6 ppm (mg kg-‘) has been previously documented (Rao, 1987; Rathore et al., 1987).

Farmers with poor experience in broiler farming in the present study had an increased likelihood of having ND epidemics in their flocks. This association might have occurred due to one or both of the following reasons: (i) the failure of many immunization programs and procedures has been attributed frequently to inadequate vaccination techniques of the farmers (Rathore et al., 1987; Roth, 199 1); (ii) these farmers might not have observed proper bio-security measures, which led to the exposure of the immunologically unprotected or suboptimally protected flock to the virulent ND virus. Similar management errors in flock management have been reported to be associated with the spread of ND (Bur- ridge et al., 1975).

The epidemiologic analysis of the data in the present study helped to develop information about the mode of dissemination and other risk indicators for the occurrence of a ND epidemic. It can be used by the poultry farmers to establish preventive measures in their management practice to avoid any ND outbreaks in future.

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Risk indicators for Newcastle Disease outbreaks in broiler flocks in Pakistan