Epidemiologic Analyses of 

Virulent Newcastle Disease in 

Backyard Birds in California

August 2018 Report

United States

Department of

Agriculture

Animal and Plant

Health Inspection

Service

Veterinary Services
Epidemiologic Analyses of Virulent Newcastle Disease in Backyard Birds in California Counties August 2018

ii USDA APHIS VS

Suggested bibliographic citation for this report: 

USDA-APHIS (2018). Epidemiologic analyses of virulent Newcastle disease in backyard birds in 

California (August 2018). USDA:APHIS:VS:STAS:Center for Epidemiology and Animal Health. 

Fort Collins, CO. August 2018. M&M Doc #447.0718. 27 pgs.

For more information, contact: 

Amy Delgado, DVM PhD

Director, Monitoring and Modeling

Center for Epidemiology and Animal Health

USDA:APHIS:Veterinary Services

Center for Epidemiology and Animal Health

2150 Centre Avenue, Bldg. B-2E7, Fort Collins, CO, 80526-8117

Phone (970) 494-7276 | Mobile (970) 420-2567 | Fax (970) 494-7228

Email: Amy.H.Delgado@aphis.usda.gov

Web: http://www.aphis.usda.gov/animal-health/ceah

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offices/veterinary_services/ceah.shtml

M&M Document #447.0718
Epidemiologic Analyses of Virulent Newcastle Disease in Backyard Birds in California Counties August 2018

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Contents

Executive Summary.............................................................................................................................2

Introduction........................................................................................................................................3

A. Disease Overview.....................................................................................................................................3

B. Description of Outbreak ..........................................................................................................................4

C. References...............................................................................................................................................4

II. Phylogenetic Analysis and Diagnostics..........................................................................................7

A. Virulent Newcastle Disease Virus............................................................................................................7

Methods ......................................................................................................................................................7

Results.........................................................................................................................................................7

B. Comparison to Other Viruses/Lineages...................................................................................................8

C. Diagnostics...............................................................................................................................................8

III. Population at Risk........................................................................................................................9

A. Predicting Areas of Backyard Bird Ownership.........................................................................................9

Methods ......................................................................................................................................................9

Results.........................................................................................................................................................9

Summary ...................................................................................................................................................11

B. References.............................................................................................................................................12

IV. Epidemiologic Risk.....................................................................................................................13

A. Case Control Study.................................................................................................................................13

Methods ....................................................................................................................................................13

Results.......................................................................................................................................................13

Summary ...................................................................................................................................................15

B. Summary of Historical Epidemiologic Risk Factors................................................................................15

C. References.............................................................................................................................................15

V. Estimating Disease Spread .........................................................................................................17

A. Overview................................................................................................................................................17

B. Summary of Epidemiologic Analysis......................................................................................................17

C. References.............................................................................................................................................19

VI. Acknowledgements ...................................................................................................................20

Appendix A: Questionnaire................................................................................................................21
Epidemiologic Analyses of Virulent Newcastle Disease in Backyard Birds in California Counties August 2018

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List of Figures and Tables

Figure 1. Counties with confirmed findings of virulent Newcastle Disease from 17 May 2018 to 31 July 

2018 in the United States...............................................................................................................................5

Figure 2. California vND case detection curve based upon the date the case definition was met for a 

presumptive positive flock by day from 17 May 2018 to 31 July 2018..........................................................6

Figure 3. Preliminary socio-demographic variables used in the model to predict the presence of backyard 

poultry in a census block in southern California. .........................................................................................10

Figure 4. Predicted number of backyard flocks by census block group in southern California. .........................11

Table 1. Number of vND confirmed positive premises by California counties and dates of earliest 

confirmation in each county as of 31 July 2018. ............................................................................................6

Table 2. Characteristics of backyard premises that are cases (confirmed/presumptive positive for vND) or 

dangerous contact (DC) premises or control premises (C). Odds ratios (OR) and p-values calculated by 

univariate logistic regression (dangerous contacts excluded). ....................................................................16

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EXECUTIVE SUMMARY

In May of 2018, virulent Newcastle Disease (vND) was reported in backyard exhibition chickens

that had clinical signs. An owner presented sick chickens to a California veterinary clinic; 

samples were sent to the California Animal Health and Food Safety (CAHFS) Laboratory where 

vND virus (vNDV) was detected. The National Veterinary Services Laboratories (NVSL) 

confirmed vND on 17 May 2018. As of 31 July, seventy-two premises had been confirmed with 

the virus.

Once initial response efforts were in place, a series of epidemiologic investigations and studies 

were initiated. This is a report of the findings available to-date, and these analyses are intended 

to assist in understanding disease introduction and transmission pathways. These studies were 

undertaken collaboratively with bird owners, state agriculture personnel, and the Animal and 

Plant Health Inspection Service.

The outbreak predominantly affects chickens in an area that crosses three counties in Southern 

California: San Bernardino, Riverside, and Los Angeles counties. Preliminary genetic analysis 

supports a single introduction followed by secondary spread. Lack of epidemiologic data 

regarding the index premises, and temporal and geographical gaps in available genetic data

contribute to the uncertainty surrounding the origin of the outbreak.

This area has a high density of backyard bird ownership, but such flocks are not typically 

registered and their locations are unknown. Using a Bayesian hierarchical model, previously 

identified socio- economic and demographic variables found to be associated with urban 

poultry ownership were used to estimate the probability of backyard flocks in this area. Results 

suggest that a single 10-km zone could have as many as 4,000 backyard flocks, and that the 

greater Los Angeles area may have over 11,000 backyard poultry premises. These modeled 

backyard ownership maps will help inform ongoing surveillance and response efforts.

Analysis of surveys conducted at case, control, and dangerous contact premises identified flock 

size, ownership of exhibition birds, a high proportion of roosters in the flock, and housing that 

allows contact with other domestic and wild birds as risk factors for vND in this population. The 

percent of premises reporting the use of Newcastle vaccine was low overall.

Initial results from disease spread and control simulations suggest that local area spread is 

likely to become more important as the outbreak increases in size. This type of disease spread is 

distance-dependent and represents mechanisms of spread that are difficult to trace, such as 

movement of free ranging birds, wildlife, or fence-line contact. Good biosecurity practices and 

measures are the best way to reduce local spread, but completely preventing this type of spread 

is difficult. Overall, the likelihood of spillover into commercial flocks is low, but spillover was 

observed in 7% of simulated outbreaks. 
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INTRODUCTION

USDA-APHIS and the State of California have initiated epidemiologic and genetic investigations

in response to the virulent Newcastle disease (vND) outbreak in backyard premises in southern 

California. These investigations will provide a better understanding of factors associated with 

vND virus transmission among backyard chickens and other susceptible species. These 

investigations include the following:

• Analysis of the phylogenetic characteristics of the virus,

• Estimation of the probability of homes in southern California owning backyard birds,

• A field-based study of backyard case and control premises using data collected through 

site visits and interviews with backyard chicken owners, and

• An epidemiologic disease spread simulation model of vND spread among bird-owning 

households in southern California.

This report includes the preliminary results from these investigations, in an effort to provide 

producers, industry, and other stakeholders with current epidemiologic information. The 

report will be updated as more information becomes available in the future.

A. Disease Overview

Newcastle disease (ND) is the cause of regular, frequent poultry epizootics throughout Africa, 

Asia, Central America, and parts of South America. The disease is caused by strains of avian 

paramyxovirus-1, also known as Newcastle disease virus, which can be classified into three 

pathotypes based on their virulence in chickens. The World Organization for Animal Health 

(OIE) defines ND as infection caused by highly virulent strains of APMV-1 viruses. This virulent 

form of ND (vND) is considered a Foreign Animal Disease (FAD) in the United States.

Clinical signs vary and can include respiratory, neurological, reproductive, and intestinal signs. 

Clinical signs seen in chickens during this outbreak include loss of appetite, difficulty breathing, 

nasal discharge/ocular discharge, swelling around the eyes, diarrhea, blue combs, and death.

Morbidity of unvaccinated chickens infected with vNDV strains can reach 100 percent, and 

mortality ranges from 70-100 percent. The severity of disease produced varies with the host 

species and the strain of the virus. Many other avian diseases present with clinical signs similar 

to vND; therefore, laboratory testing is necessary to distinguish between diseases. 

Newcastle disease is transmitted by inhalation or ingestion, and birds shed the virus in both 

feces and respiratory secretions. The virus can infect many species of domestic and wild birds. 

Chickens are highly susceptible; other gallinaceous birds such as turkey, quail, and guinea are 

also susceptible. There are two species-adapted viruses that are genetically distinguishable from 

those found in poultry in the absence of direct transmission: one is maintained in pigeons and 

doves, and another in double-crested cormorants (Brown and Bevins, 2017). Parrots have been 

reported to be infected with virulent viruses and have the potential to shed virus for long 
Epidemiologic Analyses of Virulent Newcastle Disease in Backyard Birds in California Counties August 2018

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periods of time without clinical signs; however, data supporting virus maintenance in these 

species is lacking. A detailed summary of susceptible wild bird species is available in Appendix A.

Vaccination of commercial poultry against ND is common in the Americas, including the United 

States. The classical vaccine strains are distinguishable from other viruses by genome 

sequencing. Widespread vaccination of poultry was implemented in Mexico and several Central 

American countries in the early 2000’s, and since this time divergence of subgenotypes 

circulating in vaccinated poultry has been documented (Susta et al., 2014; Garcia et al., 2013).

Outbreaks of vND occurred in California, Nevada, and Arizona in 2002-2003 and in Texas in 

2003. 

B. Description of Outbreak

16 May 2018 to 31 July 2018

On 16 May 2018, the California Department of Food and Agriculture (CDFA) reported vND in 

sick backyard exhibition chickens presented to a veterinary clinic in Los Angeles County (Figure 

1). The National Veterinary Services Laboratories (NVSL) confirmed vNDV on 17 May 2018. 

This was the first case of vND, formerly referred to as exotic Newcastle disease, in the U.S. since 

2003. An owner presented sick chickens to a California veterinary clinic; samples were sent to 

the California Animal Health and Food Safety (CAHFS) Laboratory where vNDV was detected.

CDFA responded to the incident by creating 3km control areas around the premises associated 

with the index case and began targeted surveillance and outreach including to feed stores and 

known exhibition bird premises. On 24 May 2018, NVSL confirmed vNDV in a backyard 

premises in San Bernardino County. On 26 May 2018, a USDA-APHIS incident management 

team joined the unified incident command in California. By this time, seven premises had been 

confirmed in San Bernardino County and two in Los Angeles County. On 30 June 2018, a

premises in Riverside County was confirmed. Between 16 May 2018 and 31 July 2018, 72

confirmed premises were identified in three California counties (Figure 2, Table 1).

C. References

Brown V.R., Bevins S.N., A review of virulent Newcastle disease viruses in the United States and 

the role of wild birds in viral persistence and spread. Vet Res. 2017; 48: 68. 

Garcia S.C., Lopez R.N., Morales R., Olvera M. A., Marquez M. A., Merino R., Miller P. J., Afonso C.L., 

Molecular epidemiology of Newcastle Disease in Mexico and the potential spillover of 

viruses from poultry into wild bird species. Appl. and Environ. Microbio. 2013; 79:4985-

4992.

Susta L., Hamal K.R., Miller P.J., Cardenas-Garcia S., Brown C.C., Pedersen J.C., Gongora V., Afonso 

C.L., Separate evolution of virulent Newcastle Disease viruses from Mexico and Central 

America. J Clin Micro. 2014; 52:1382-1390.
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Figure 1. Counties with confirmed findings of virulent Newcastle Disease from 17 May 2018 to 31 July 2018

in the United States. 

San Bernardino

Riverside

Los Angeles
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Table 1. Number of vND confirmed positive premises by California counties and dates of earliest 

confirmation in each county as of 31 July 2018.

County Confirmed Premises Earliest Confirmation Date in County

Los Angeles 5 17 May 2018

Riverside 7 30 June 2018

San Bernardino 60 24 May 2018

Total 72

Figure 2. California vND case detection curve based upon the date the case definition1 was met for a 

presumptive positive flock by day from 17 May 2018 to 31 July 2018.

 

1 Case definitions 

• Suspect case: domesticated bird or flock having clinical signs compatible with vND; or detection of APMV-1 

by rRT-PCR; or epidemiological information indicating exposure to vNDV

• Presumptive positive case: a suspect case with detection vNDV by the fusion-target rRT-PCR test at a 

laboratory designated by the Secretary of Agriculture

• Confirmed positive case: domesticated bird or flock from which vNDV has been identified at the NVSL as 

presumptive positive with confirmation of multiple basic amino acids (either directly via protein or by

deduction through sequencing) in the fusion gene at the C-terminus of the F2 protein and phenylalanine at 

residue 117, which is the N-terminus of the F-1 protien. The term ’multiple basic amino acids’ refers to at 

least three arginine or lysine residues between residues 113 and 116; and/or the vNDV has an intracerebral 

pathogenicity index (ICPI) in day-old chicks (Gallus gallus) of 0.7 or greater.

May 20 May 25 May 30 Jun 4 Jun 9 Jun 14 Jun 19 Jun 24 Jun 29 Jul 4 Jul 9 Jul 14 Jul 19 Jul 24 Jul 29

0

1

2

3

4

5

6

7

Count 

1 1 1

2

1

4

1

2

1 1

2

1 1

2

1

6

1

4

1

2 2

4

1 1

5

1

2

1 1 1 1 1 1 1 1 1

2

1 1

2 2

1

2
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II. PHYLOGENETIC ANALYSIS AND DIAGNOSTICS

A. Virulent Newcastle Disease Virus

This section describes viruses characterized from the 2018 vND events in California (CA2018). 

The index case is chicken/California/18-016505-1/2018, which has an amino acid cleavage site 

of PGGRRQKR/FVGAII. The intracerebral pathogenicity index (ICPI) conducted on selected 

isolates in accordance with OIE guidelines ranges confirms virulence and ranges from 1.67-

1.752. Chickens have been predominantly affected; other species from which the virus has been 

recovered include turkey, peafowl (peacock), duck, goose, and pigeon. Preliminary studies with 

the CA2018 index virus at the Southeast Poultry Research Laboratory suggest that it is highly 

chicken adapted and very infectious for chickens. 

Methods

Genetic sequence data from the virus is used to determine the cleavage site, which serves as 

disease confirmation. Additionally, full genomic sequence data are generated and analyzed to 

monitor virus evolution and to inform epidemiologic investigations. Genetic data are also used 

to confirm that diagnostic assays are fit for purpose. 

Results

The CA2018 virus (genotype Vb) is related to older Mexican-lineage viruses from Central 

American village poultry (Belize 2008, Honduras 2007), and the U.S. (smuggled parrot 1996, 

CA2002), which represent viruses from birds with low or no vaccine coverage. Preliminary 

genetic analysis of CA2018 virus isolates supports a single introduction followed by secondary 

spread based upon the high identity among available sequences from 41 chickens and 1 each 

from duck, goose, peafowl, pigeon, and turkey, representing 41 premises. Lack of epidemiologic 

data regarding the index premises, and of contemporary sequence data (the most recent 

available related sequences are from 2008) contribute to the uncertainty surrounding the 

origin of the outbreak. Evolutionary analysis of available sequences with the CA2018 and 

CA2002 viruses suggest ongoing circulation of the virus; however, where and in what 

population remains unclear.

NOTE: The outcomes of phylogenetic analysis should be interpreted in context of all 

available virus and epidemiologic information and should not be used directly to infer 

transmission.

 

2 The World Organization for Animal Health (OIE) defines Newcastle disease as an infection with a virulent APMV-1 

virus (vNDV) characterized by either an intracerebral pathogenicity index (ICPI) of 0.7 or greater in day-old chickens, 

or the presence of multiple basic amino acids at positions 113- 116 of the C-terminus of the fusion (F2) protein 

(either arginine (R) or lysine (K)), plus phenylalanine (F) at residue 117 of the F1 protein.
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B. Comparison to Other Viruses/Lineages 

The CA2018 virus is not related to classic Newcastle disease vaccine strains, nor to available 

strains from vaccinated poultry in Mexico (2000-2010). The virus is also unrelated to the 

species-adapted virus from columbids (pigeons, doves), and is not closely related to virulent 

viruses endemic to double-crested cormorants.

C. Diagnostics

Testing avian swabs/tissues for APMV-1 involves screening assays (real time reverse 

transcription polymerase chain reaction [rRT-PCR]), virus isolation, and characterization of the 

virus (sequencing and in vivo tests). The National Animal Health Laboratory Network (NAHLN) 

tests swab/tissue first by the APMV-1 matrix-target rRT-PCR test3

, best suited to detect Class II 

viruses that contain low and highly virulent pathotypes, including vaccine viruses. Detections 

by the matrix-target test are subsequently tested by a fusion gene-target rRT-PCR test, which is 

designed to allow rapid identification of virulent viruses reportable in poultry. This approach

does not provide the genetic or geographic lineage of the virus. Sequence analysis of the virus 

compared to the assay primers and probes confirmed high identity between the CA2018 virus 

sequences and the fusion gene-target rRT-PCR test. A negative fusion gene-target test in the 

face of clinical signs requires further testing including virus recovery, sequence and/or ICPI 

testing.

Under normal surveillance, all poultry samples with a non-negative test result by APMV-1 PCR 

or virus isolation are forwarded to NVSL for confirmatory testing; for the current CA vND event, 

the NAHLN laboratory is using the highly matched fusion-target assay. The NVSL uses Sanger 

sequencing protocols to generate partial fusion gene sequence directly from the sample for 

virulence determination, where sufficient viral RNA is present. Whole genome sequencing is 

conducted on all isolated viruses, and select viruses are further characterized by ICPI in specific 

pathogen-free chickens. 

The NVSL confirms the virus lineage and virulence through molecular sequencing. Where no 

virus can be recovered nor sequence obtained directly from sample(s), the virulence is 

determined by the clinical presentation of the flock compared to the USDA vNDV case 

definition.

 

3 PCR results from the NVSL are reported as “detected” or “not detected” and include the cycle threshold (Ct) value. 

The lower the Ct value, the more viral nucleic acid was detected.
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III. POPULATION AT RISK

A. Predicting Areas of Backyard Bird Ownership

The distribution of backyard bird flocks in the United States is currently unknown. However, 

statistical modeling can be used to estimate the likely locations and densities of backyard flocks 

in a given geographic area using socio-economic and demographic variables that have been 

shown historically to be related to bird ownership. This approach was used to develop 

neighborhood-level, spatial data to facilitate the creation of risk maps to identify and prioritize 

areas for surveillance during the 2002-2003 outbreak of vND in the United States (Freier et al., 

2004, Freier et al., 2007). Building on that historical work, we aimed to identify areas with 

increased probability of backyard poultry ownership to inform surveillance response efforts for 

the current outbreak.

Methods

A Bayesian hierarchical model for spatial areal unit data was used to analyze socio- economic 

and demographic variables that have previously been found to be associated with urban poultry 

ownership. Census block groups were used as the unit of analysis. The number of backyard 

flocks identified during the previous vND outbreak were tallied for each census block group.

During the 2002 outbreak all homes within 1 mile of an affected premises were queried about

backyard poultry ownership resulting in a near census of backyard poultry ownership for some 

census block groups. Census block groups within 1 mile of affected premises were then

assumed to have all flocks identified, and the total number of households reported in the census 

data was used as the total sample size. These census block groups were then used as data to fit 

the Bayesian model. 

The Bayesian model used a binomial likelihood conditional on historical socio- demographic 

and economic risk factors. The factors examined are listed in Figure 3. The model included a 

spatial random effect using a convolution model that allows for both weak and strong spatial 

autocorrelation with neighboring census block groups. Prior to model fitting 5% of the data was 

randomly withheld for out-of-sample model validation. The withheld data was identified using 

conditional Latin hypercube sampling. Models were fit using JAGS in R.

Results

Human population size, home value, education level, housing density, household income, and 

household size were all significant predictors. Figure 3 presents the predicted coefficients for 

the socio- demographic and economic predictors used in the model. Based on the model’s 

predictions, a single surveillance zone (10km) could have almost 4,000 premises with backyard 

poultry, while the greater Los Angeles area is predicted to have over 11,000 backyard poultry 

premises. Figure 4 presents the predicted distribution of backyard flocks by census block group 

within the current surveillance zone and control areas.
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The model performs generally well explaining 79.9% of the deviance in the spatial distribution 

of backyard poultry ownership during the 2002 outbreak. Comparison of the predicted number 

of households with backyard poultry with the out-of-sample data found a Pearson’s correlation 

of 0.67 indicating the model has good predictive capacity.

Figure 3. Preliminary socio-demographic variables used in the model to predict the presence of backyard poultry 

in a census block in southern California.
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Figure 4. Predicted number of backyard flocks by census block group in southern California.

Summary

The area where the outbreak is occurring is likely to have a very dense population of backyard 

poultry. In addition, there was a strong spatial pattern to the distribution of backyard poultry 

ownership, indicating that a spatially targeted approach may improve surveillance efficiency. 

Work is ongoing to improve the model and incorporate data and predictors related to the 

probability of vND is present in the census block groups. Formal model selection has not been 

implemented yet but may improve the predictive abilities of the model. The data used to fit the 

model was restricted to block groups in southern California to facilitate model fitting (i.e. limit 

cpu time). However, data is available throughout California, Nevada, and Arizona for the 2002 

vND outbreak. Including these data in the model may improve prediction and applicability to 

other regions of the United States.
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B. References

Freier, J. E., et al. "Spatially-targeted surveillance for Newcastle disease in southern California." 

Proc. GISVET 4 (2004): 3-5.

Freier, J. E, Miller, R. S, & Geter, K. D. (2007). Geospatial analysis and modelling in the 

prevention and control of animal diseases in the United States. Veterinaria italiana, 43(3), 

549-557.
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IV. EPIDEMIOLOGIC RISK

A. Case Control Study

A case-control epidemiological analysis was performed on confirmed and presumptive positive 

virulent Newcastle disease (vND) backyard premises, dangerous contact premises, and noninfected premises. Data were obtained from in-person interviews that used the CDFA NonCommercial Premises Virulent Newcastle Disease Epidemology Questionnaire, which were 

entered into the USDA’s Emergency Management Response System (EMRS).

Methods

Data were analyzed for a total of 202 premises: 60 confirmed or presumptive positive premises, 

13 dangerous contact premises and 129 non-infected premises. The analysis included 

questionnaires that were completed between May 16, 2018 and July 16, 2018 and includes all 

confirmed and presumptive premises with questionnaires entered in to the EMRS as of July 19, 

2018. The questionnaire form was updated in July 2018 with additional questions; 70 

respondents completed the original questionnaire and 132 completed the updated 

questionnaire. Questionnaires were not complete for all premises, such as in cases when 

owners refused to provide answers to certain questions. The number (n) of responding 

premises is noted in Table 1 that summarizes the results. Odds ratios, p-values and 95% 

confidence intervals for flock characteristics and other risk factors were calculated by 

univariate logistic regression using confirmed/presumptive premises as cases and non-infected 

premises as controls. Dangerous contacts were excluded from the regression analysis.

Results

Premises characteristics 

The reported flock sizes ranged from 1 to 800 birds (mean=59, median=14 birds). 33 percent of 

all backyard flock owners had multiple bird species on their premises. These premises 

primarily had backyard chickens (86 percent). Fewer premises had exhibition birds (13 

percent), and ducks/geese (14 percent). Other types of birds were reported on 32.5 percent of 

premises; the most commonly reported species were pigeons, turkeys, parrots, and cockatiels. 

Besides birds, 35 percent of owners had other livestock species on their backyard premises, 83 

percent had dogs/cats, and 18.5 percent reported other non-bird species.

Housing types 

Five percent of premises reported housing birds indoors. Forty-three percent reported housing 

birds outdoors in open top pens or enclosures, and 79.6 percent reported housing birds 

outdoors in cages or coops. Less than 2 percent reported keeping birds individually tethered,

and 37.6 percent reported having free-range birds.

Illness and mortality 

Over 70 percent of cases reported bird illness, and 71.2 percent reported mortality. The mean 

time between onset of illness and presumptive detection was 10.69 days (median=8.28, range 
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2-91 days). The mean time between onset of mortality and presumptive detection was 6.91 

days (median=4.85 days, range 1 to 30 days). As an indicator of background morbidity and 

mortality, the percent of control premises reporting illness was 15.7 percent and mortality was 

13.2 percent. 

Risk factors 

• Flock size

Case premises reported larger flock sizes than control premises. The odds of being a 

case were significantly greater for flock sizes greater than 100 birds (OR = 6.69, 

95% CI: 2.70-16.58) or from 20 to 99 birds (OR = 4.24, 95% CI: 2.03 – 8.86) when 

compared to flock sizes of less than 20 birds.

• Bird types

Case premises were more likely to report having flocks that included exhibition birds or 

other non-chicken bird species than control premises. The odds of being a case were

significantly greater for flocks reporting exhibition birds (OR = 3.69 , 95% CI: 1.47-

9.20) or the presence of other bird species (OR = 2.2, 95% CI: 1.14-4.25).

The odds of becoming a case were also higher when roosters comprised more than 50% 

of the adult birds in the flock (OR=5.7, 95% CI 2.12-15.14).

• Bird housing

Premises with free-roaming birds have significantly greater odds of being a case (OR = 

3.24, 95% CI 1.27-8.22). There were no other significant differences in housing 

types between cases and controls.

• Contact with other domestic and wild birds 

Both cases and controls reported high percentages of neighbors with birds (79 percent

and 66 percent, respectively); however, premises that reported that their birds visit 

neighboring properties (OR = 4.98, 95% CI 1.38-17.97) or had contact with wild 

birds (OR = 5.57, 95% CI 1.84-17.94) had greater odds of becoming a case.

• The use of Newcastle vaccine

The percent of premises reporting the use of Newcastle vaccine was low overall (10.2 

percent total). The percent of case premises that reported using Newcastle vaccine 

was more than double the percent of controls that reported using Newcastle vaccine 

(16.1 percent vs 7.6 percent, respectively); however, the risk of disease was not 

statistically greater among flocks which reported use of Newcastle vaccine (OR=2.6, 

p=0.061).

• Common sources of feed

Thirty-one local businesses were identified as a source for feed for 90 premises (32 

cases, 54 controls, 4 and dangerous contacts). Six businesses were reported as the 
Epidemiologic Analyses of Virulent Newcastle Disease in Backyard Birds in California Counties August 2018

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feed source for more than one case premises. One business was a common source 

for 12 total premises, 10 of which were cases.

Summary

These results suggest that flock size, ownership of exhibition birds, a high proportion of 

roosters in the flock, and housing that facilitates contact with nearby domestic and wild birds 

are risk factors for vND infection in this population. Some of these practices have been shown to 

be risk factors in other studies or previous vND outbreaks in the U.S., as summarized below. 

However, not all epidemiology questionnaires were complete, and it is likely there is 

misclassification bias for some of these results, such as the type and number of birds on 

premises, the number of owners, and use of Newcastle vaccine; therefore, results should be 

interpreted with caution. This analysis will be updated when additional information is 

available.

B. Summary of Historical Epidemiologic Risk Factors

An epidemiological study of backyard premises in the 2002-2003 California vND outbreak 

identified the following risk factors for vND infection on premises: presence of game fowl, 

presence of feral chickens on premises, flock sizes larger than 40 birds, and multiple owners of 

a flock. Epidemiological descriptions of infected backyard premises in the 1971-1974 outbreak 

identified contact with infected commercial layer farms as the primary source of infection, 

followed by active trading of birds among backyard flocks and purchases of infected exotic 

birds from dealers. Infection among commercial premises in the 1971-1974 outbreak, the 

severity of the outbreak was attributed to the high density of egg-laying premises and extensive 

contact among those premises. In both the 1971-1974 and 2002-2003 California vND 

outbreaks, a suspected risk factor for vND infection in commercial premises was movement of 

contaminated equipment such as egg carts.

C. References

Bulaga, L.L., Dargatz, D., Garber, L., Miller, R., Rhoads, G. (2004). Risk Factors for exotic 

Newcastle disease infection in non-commercial flocks, California. Unpublished Report

Burridge, M. J., Riemann, H. P., Utterback, W.W. (1975). Methods of spread of velogenic 

viscerotropic Newcastle disease virus in southern California. Avian Diseases, 19(4), 666-

678.

Speers, R., Webb, M., Grund, M, Howell, B., Hughes, C., Myrus, E., and, Silverman, J. (2004).

Reconstruction of response operations to eradicate exotic Newcastle disease in 2002-2003. 

Technical report produced for USDA by the CNA Corporation. 

USDA-APHIS (1978). Eradication of exotic Newcastle disease in southern California 1971-1974. 

APHIS-91-34 Technical Report

Utterback WW, Schwartz, J.H. (1973) Epizootiology of velogenic viscerotropic Newcastle 

disease in southern California, 1971-1973. J Am Vet Med Assoc. 163(9), 1080-1088.
Epidemiologic Analyses of Virulent Newcastle Disease in Backyard Birds in California Counties August 2018

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Table 2. Characteristics of backyard premises that are cases (confirmed/presumptive positive for vND) or 

dangerous contact (DC) premises or control premises (C). Odds ratios (OR) and p-values calculated by 

univariate logistic regression (dangerous contacts excluded).

Characteristic Level

n

Case DC Control OR p-value

Number of birds 1-19 20/60 7/13 92/129 Ref

20-99 24/60 4/13 26/129 4.2 <0.001

100+ 16/60 2/13 11/129 6.7 <0.001

Bird species on premises Backyard chickens 48/56 11/11 92/108 1.6 0.207

Exhibition birds 13/56 1/11 9/108 3.7 0.005

Ducks/geese 9/56 2/11 13/108 1.6 0.329

Other species 24/56 3/11 30/108 2.2 0.019

Multiple bird species on the same premises 31/56 5/11 31/108 3.4 <0.001

Adult birds >50% roosters 16/47 2/9 7/84 5.7 0.001

Non-bird species or wildlife on 

premises

Livestock

Dogs/cats

Other animals

23/51

45/52

7/39

5/10

11/11

1/5

38/128

104/169

12/64

1.94

1.54

0.95

0.0.51

0.348

0.919

Housing Inside home

Outdoor open top

Outdoor cage/coop

Individual tether

Free range

1/23

7/24

22/25

0/22

14/25

0/6

5/8

5/7

0/6

5/6

4/74

36/79

71/91

2/74

22/78

0.8

0.5

2.1

1.0

3.2

0.842

0.158

0.276

NA

0.013

Movement of new birds onto the premises within 30 

days prior to the interview 9/57 2/12 11/108 1.8 0.21

Movement of birds off the premises within 30 days prior 

to the interview 6/58 0/12 11/123 1.2 0.76

Give/sell eggs 1/26 2/8 14/93 0.2 0.09

Neighbors have birds 19/24 7/8 61/92 1.9 0.23

Birds visit neighbors 6/26 2/8 5/88 5.0 0.01

Wild birds have contact with domestic birds 23/27 7/8 46/92 5.8 0.003

Newcastle disease vaccine No 39/56 10/12 101/113 Ref

Yes 9/56 1/12 9/117 2.6 0.061

Unsure 8/56 1/12 7/117 2.9 0.049
Epidemiologic Analyses of Virulent Newcastle Disease in Backyard Birds in California Counties August 2018

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V. ESTIMATING DISEASE SPREAD

A. Overview

An epidemiologic scenario was developed in InterSpread Plus ® v. 6.01.44 (Stevenson et al., 

2013) to model the introduction and spread of vND from confirmed premises in San Bernardino 

counties in California. Commercial and backyard farm units from the Western U.S. (17 states) 

were incorporated into the model. The Farm Location and Animal Population Simulator was 

used to generate likely farm locations based on geospatial characteristics, with backyard farm 

locations adapted from current and historic outbreak-related data. Model parameters were 

developed to reflect the impact of sustained outreach activities, to incorporate preliminary 

experimental data on viral pathogenesis in chickens, to include current strategies for active 

surveillance of commercial operations, and to describe the potential geographic extent of 

disease spread during the silent-spread period. The model is updated regularly in order to 

provide timely results back to the response.

B. Summary of Epidemiologic Analysis

Summary outcomes for a 300-iteration scenario were generated from ten seeded-sites. The 

seeded sites were based on the latitude/longitude of initial confirmed cases of vND in San 

Bernardino County. Simulations include control activities implemented in the current vND 

response, including outreach, quarantine, euthanasia/depopulation of detected premises, 

movement controls, tracing, and active and passive surveillance. Note: These outcomes are 

based on a completely naïve poultry population. The variable levels of vaccination applied 

within backyard farms is not explicitly modeled in this scenario.

The summary of results and their potential implications for the current vND outbreak are as 

follows:

• Initial disease spread commonly involves direct movements of infectious birds, but local 

spread becomes more prevalent as outbreaks become greater than 50 infected 

premises.

Direct contacts associated with live animal movements accounted for 36 percent of 

spread for simulated outbreaks that resulted in less than 50 infected premises, and 

27 percent of spread for simulated outbreaks that resulted in 50 or more infected 

premises. 

Local area spread became more prevalent as simulated outbreaks became larger, being 

responsible for 56 percent of disease spread for simulated outbreaks that resulted 

in 50 or more infected premises.

Implications: As the number of detected premises continues to increase, outcomes 

from simulated outbreaks suggest that local spread of vND between premises 

may be responsible for additional infections. Local spread is associated with 

distance between infectious and susceptible premises and represents 
Epidemiologic Analyses of Virulent Newcastle Disease in Backyard Birds in California Counties August 2018

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mechanisms of spread that are difficult to trace such as movement of free 

ranging birds, wildlife, or fence-line contact. Good biosecurity practices and 

measures are the best way to prevent local spread (e.g., keeping outside birds in 

cages; moving cages away from neighboring fence lines; repairing 

damaged/missing fences; rodent control; covering/tarping cages to decrease 

wildlife/rodent/loose chicken exposure and the spread of vND viral particles

into the environment).

• Small backyard operations4 are the primary premises involved in outbreaks; large 

backyard operations or commercial poultry farms have a lower likelihood of becoming 

infected.

Across all simulated outbreaks, large backyard operations represented slightly less than 

1 percent of all infected premises, and commercial poultry farms represented 0.14 

percent of all infected premises. 

Virulent ND infected small backyard operations in 100 percent of all simulated 

outbreaks, large backyard operations in 22 percent of all simulated outbreaks, and 

commercial poultry farms in 7 percent of simulated outbreaks. 

All spread to commercial operations resulted from indirect contact (e.g. people or 

vehicles moving from operation to operation) with infected, primarily small 

backyard operations. 

Implications: Unless generated by indirect contacts with infected backyard 

operations, outcomes from simulated outbreaks suggest a low probability of 

spreading vND to commercial farms. 

• The extent of spread for simulated outbreaks is primarily in San Bernardino, Los 

Angeles, and Riverside counties. 

When considering disease spread within the silent period of the outbreak (three days 

prior to the first observation of clinical signs to the day of first detection), spread 

from infected premises in San Bernardino County to backyard chicken premises in 

Riverside County occurred in 66 percent of simulated outbreaks and to backyard 

chicken premises in Los Angeles County in 65 percent of simulated outbreaks.

 

4 In the model operations are defined as follows:

1) Commercial poultry farms: more than 75,000 table egg laying chickens, or more than 100,000 meat-type 

chickens, or more than 30,000 meat-type turkeys

2) Large backyard operations: more than 1,000 birds but fewer than the number of birds described for 

commercial operations

3) Small backyard operations: fewer than 1,000 birds
Epidemiologic Analyses of Virulent Newcastle Disease in Backyard Birds in California Counties August 2018

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In the current modeling scenario, 42 percent of simulated outbreaks involved 50 or 

more infected premises, and 19 percent of and outbreaks involved 100 or more 

infected premises. 

Implications: Simulated outbreaks suggest future detections in other counties in 

Southern California, most commonly Riverside and Los Angeles Counties. In 

addition, some infected premises may not be detected due to natural viral 

elimination from these premises (i.e., birds die and go unreported) and/or no 

new, naïve birds being brought onto previously infected premises.

C. References

Burdett, C. L., Kraus, B. R., Garza, S. J., Miller, R. S., & Bjork, K. E. (2015). Simulating the 

distribution of individual livestock farms and their populations in the United States: An 

example using domestic swine (Sus scrofa domesticus) farms. PloS One, 10(11). doi: 

10.1371/journal.pone.0140338

Stevenson, M.A., Sanson, R.L., Stern, M.W., O’Leary, B.D., Sujau, M., Moles-Benfell, N., Morris, R.S. 

(2013). Interspread plus: A spatial and stochastic simulation model of disease in animal 

populations. Preventive Veterinary Medicine, 109(1-2), 10-24. doi: 

10.1016/j.prevetmed.2012.08.015
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VI. ACKNOWLEDGEMENTS

We greatly appreciate the cooperation and support of the poultry owners for allowing us access 

to their properties, for providing information on their biosecurity and production practices, and 

for their cooperation with this investigation. 

We also appreciate the Federal and State animal health experts, as well as the many partners 

who assisted with this study while serving in the incident command for this outbreak. 

This report would not have been possible without a large group of researchers, epidemiologists, 

laboratory staff, and data scientists who participated in the various analyses and drafting and 

review of this report. We appreciate their dedication and professionalism in providing the best 

information possible to help bird owners reduce the risk of Newcastle disease spread.
Epidemiologic Analyses of Virulent Newcastle Disease in Backyard Birds in California Counties August 2018

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APPENDIX A: QUESTIONNAIRE

California Department of Food and Agriculture

CA VND 2018

Non-Commercial Premises 

Virulent Newcastle Disease Epidemiology Questionnaire

Investigator name: ____________________Date of Investigation: _______/_____/_____

Investigator name: ____________________

Quarantine # ______________________ Date Quarantine Issued: _____/_____/_____

1. Name of Premises Owner:

___________________________________________

(First) (MI) (Last)

2. Premises Address (location of birds):

________________________________________________

________________________________________________

Latitude: ___________________ Longitude: ___________________

3. Premises Owner Telephone #: 

a. Mobile: ___________________

b. Home: ___________________

c. Other: ___________________

If Premises Owner is the Bird Owner skip to Question 7

4. Name of Bird Owner:

___________________________________________

(First) (MI) (Last)

5. Bird Owner Address: ________________________________________________

_______________________________________________

6. Bird Owner Telephone #: ______________________

7. Other than the interviewee, how many other owners with birds 

are on this premises: #_______
Epidemiologic Analyses of Virulent Newcastle Disease in Backyard Birds in California Counties August 2018

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8. How many birds do you have on the premises today? # ______________

9. What percent of the adult chickens are: a) Roosters %____________________

b) Hens %_______________________

10. Which of the following birds are on the premises? Complete table below.

Type of Bird # Adults # Young birds Total

Backyard Poultry a b c

Exhibition Birds/gamefowl d e f

Ducks/Geese g h i

Other 

Specify j k l m

Other 

Specify n o p q

11. Which of the following animals are on the premises (potential fomites)? 

a) Livestock (Horses, Cattle, Swine, Sheep, Goats) 1 Yes 3 No

b) Dogs/Cats 1 Yes 3 No 

c) Other (specify _____________________) 1 Yes 3 No

12. Which of the following housing types are used to house birds? 

a) Inside the home 1 Yes 3 No

b) Outdoor open top poultry pen or enclosure 1 Yes 3 No

c) Outdoor cages or coops - fully enclosed 1 Yes 3 No

d) Individually tethered 1 Yes 3 No

e) Free range 1 Yes 3 No

f) Other (Specify__________________________) 1 Yes 3 No

13. Has there been an increase in illness in your birds 

on your premises? 1 Yes 3 No

a) If yes, how many days ago did the birds first show

signs of illness: ________days
Epidemiologic Analyses of Virulent Newcastle Disease in Backyard Birds in California Counties August 2018

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Which of the following clinical signs of illness have you observed? 

Check all that apply.

b) Not eating 1 Yes 3 No

c) Coughing/gasping 1 Yes 3 No

d) Depressed 1 Yes 3 No

e) Twisting of the neck 1 Yes 3 No 

f) Paralysis 1 Yes 3 No

g) Diarrhea 1 Yes 3 No

h) Swellings around the eyes and neck 1 Yes 3 No

i) Sudden death 1 Yes 3 No

j) Other (specify___________________) 1 Yes 3 No

14. Have there been any deaths in your birds on this premises 

during the past 30 days? 1 Yes 3 No

a) If yes, when did the first bird die? _____/___/______

b) If yes, how many birds died in the first 7 days? # ______________

c) If yes, how many birds have died in the past 7 days? # ______________ 

15. Do you keep any birds at another premises? 1 Yes 3 No

a) If yes, where are the birds housed?

________________________________________________

________________________________________________

16. Have you brought new birds onto this premises 

during the past 30 days? 1 Yes 3 No 

If Yes, list date and name the source and location of the new birds: 

 Date Source/Location

 

___/___/___a _________________________________b

___/___/___c _________________________________d

___/___/___e _________________________________f
Epidemiologic Analyses of Virulent Newcastle Disease in Backyard Birds in California Counties August 2018

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17. Have any of the following had contact with your birds, feed or water sources on your property in the 

last 30 days? 

a) Wild birds (e.g., pigeons, doves, sparrows) 1 Yes 3 No

b) Neighborhood/community chickens 1 Yes 3 No

c) Wild animals 1 Yes 3 No

18. Have any of your birds left these premises

during the last 30 days? 1 Yes 3 No 

If Yes, for what purposes listed below were the birds moved?

Purpose Date Destination (City/State) # of birds

Sale a b c

Show d e f

Competition g h i

Veterinary care j k l

Gift/trade m n o p

Other 

Specify q r s t

If Yes, did any birds leave and then return to these premises? 1 Yes 3 No

19. Do you give away or sell eggs from this premises? 1 Yes 3 No 

20. Do your neighbors have birds? 1 Yes 3 No

If No, skip to Question 23. 

If Yes, please note location(s) on the map at the end of the questionnaire.

21. When not cooped, do your birds ever visit the neighbor’s property? 1 Yes 3 No

22. Do your neighbor’s birds ever come onto your property? 1 Yes 3 No

a) If Yes, do the neighbors birds have contact with your birds? 1 Yes 3 No

23. Do you have family members or close friends 

who own/keep birds? 1 Yes 3 No

If Yes, do any of the following situations occur (evaluating direction of exposure): 

a) Your family or friends handle birds 

when they visit. 1 Yes 3 No

b) When visiting family/friends do 

you handle their birds. 1 Yes 3 No
Epidemiologic Analyses of Virulent Newcastle Disease in Backyard Birds in California Counties August 2018

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24. What is the name and location of the store(s) where you get 

feed and supplies for your birds?

Name Location (City)

__________________________a _____________________________b

__________________________c _____________________________d

__________________________e _____________________________f

25. Have the birds on your premises today been vaccinated 

with Newcastle vaccine? 1 Yes 2 Unsure 3 No

Vaccine does not protect against disease!

a) If Yes, at what age(s) were your birds vaccinated with Newcastle 

 vaccine? 

26. Have you seen any dead wild birds on your premises 1 Yes 3 No

in the last 30 days?

If Yes, what type of wild bird(s)?

____________________________a _______________________________b

____________________________c_ _______________________________d

Additional comments, observations and leads:

Insert Google Maps Image of the premises or draw a map and specify bird 

locations. Please indicate which neighbors, if any, have birds.

I __________________________________certify that I have ________birds on / / @ __________

 (owner signature) (number) (date and time)
Epidemiologic Analyses of Virulent Newcastle Disease in Backyard Birds in California Counties August 2018

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Epidemiologic Analyses of 

Virulent Newcastle Disease in 

Backyard Birds in California

August 2018 Report

USDA  APHIS  VS

Science, Technology, and Analysis Services

Center for Epidemiology and Animal Health

2150 Centre Avenue, Bldg. B, Fort Collins, CO, 80526-8117

Web: http://www.aphis.usda.gov/animal-health/ceah