# Presentation: 236409

## Use of sentinel animals for West Nile virus surveillance

**CDC-Arbovirus Diseases Branch**
**Fort Collins, CO**

**Use of sentinel animals for West Nile virus surveillance**

**Notes:**
The purpose of my talk is to introduce the traditional types of bird-based surveillance for arboviruses, and present to you how they have been applied to WNV surveillance in the year 2000.

## WNV Surveillance Modalities

**WNV Surveillance Modalities**

- Mosquitoes
**Dead birds (especially crows)**
- Live wild bird sentinels
- Live captive sentinels (e.g. chickens)
- Veterinary surveillance
- Human surveillance

**Notes:**
Traditionally, sentinel birds have been used to monitor arbovirus transmission for SLE, WEE and EEE.  Wild birds are sometimes sampled as a type of sentinel, particularly in the states of Tennessee, Indiana, Illinois, Louisiana, Texas, and some others.  Chickens have been used extensively in states such as Florida, California, North Carolina, Louisiana and a few others.  
In both wild bird and chicken surveillance, the detection of seroconversion (from antibody-negative status to antibody-positive status) signals a transmission event. In this talk, I will present the yr-2000 experience with both these sentinel bird strategies for WNV surveillance. Later in the discussion, we will review the benefits and liabilities of each system.

## WNV Surveillance Totals, 2000-2001Reported to CDC thru 3/07/02

| Dead birds | 4305 | 7241 |
| --- | --- | --- |
| Mosquito pools |   515 |   905 |
| Horses
Other mammals |     63   
      6 |   720
      0 |
| Humans |     21 |     66 |
| Sentinel chickens
Wild birds  |     13 
      9 |   218
      3 |

_**2000**__**	**__**	**__**2001**_

**Notes:**
Although sentinel bird surveillance should be an effective tool, the numbers of positive chickens and hatch-year wild birds found positive for WNV antibodies in 2000 were smaller than all other types of surveillance indicators.  Clearly, sentinel bird surveillance must be increased and/or improved if it is to be used effectively.

## Why do we need sentinel surveillance data?

**Early detection**
**Quantification of transmission **
**Risk assessment**
**Prediction of disease (early warning)**
**Theory discussed in:**
**Komar N, 2001, Annals N.Y. Acad. Sci. 951:58-73 (available online)**

## Earliest detection per county (n=359), 2001

**Dead birds****	****	****295 (82.2%)**
**Horse cases****	****	****  39 (10.8%)**
**Mosquito pools****	****  16 (  4.5%)**
**Sentinel chickens****	****    6 (  1.7%)**
**Human cases****	****	****    2 (  0.6%)**
**Other****	****	****	****    3 (  0.8%)**

## Avian mortality surveillance

**What is the purpose of this type of surveillance?**
**Early detection of virus activity**
**Flag regions where quantitative surveillance should be conducted (e.g. mosquito infection rates, avian seroconversion rates)**
**Indicator of ongoing transmission**
**Possibly useful in predicting human cases**
**See Eidson et al 2001 Emerg. Inf. Dis. 7(4):662-664**

## Avian mortality surveillance

**What species to target?**
**Over 100 species have been affected since 1999**
**71% of positive carcasses were crows (2001)**
**Only 27% were crows in Florida**
**No positive crows in 22 of 63 Florida counties with positive birds**
**Mortality rate in crows is >90%**
**Ecology of WNV transmission may differ in new regions**

## Experimental WNV Mortality -1

**American Crow****	****	****13****	****	****  92****	****	****0****	****Blue Jay****	****	****	****  6****	****	****  83****	****	****0****American Magpie****	****	****  5****	****	****100****	****	****1****House Finch****	****	****	****  2****	****	****100****	****	****1****Common Grackle****	****	****10****	****	****  40****	****	****2****Fish Crow****	****	****	****11****	****	****  64****	****	****5****House Sparrow****	****  ****	****12****	****	****  25****	****	****5      ****Ring-billed Gull****	****	****  2****	****	****  50****	****	****1**
**Source: Unpublished data**

**Experimental WNV Mortality -1**

## Experimental WNV Mortality -2

_**Species**__**	**__**	**__**  **__**	**__** n**__**	**__**mortality rate     controls        **_
**European Starling****	****  ****	****  6****	****	****    0****	****	****2****	****Rock Dove****	****	****	****  6****	****	****    0****	****	****6****Chicken****	****	****	****18****	****	****    0****	****	****24****Ring-necked Pheasant****	****  3****	****	****    0****	****	****0****Canada Goose****	****	****  3****	****	****    0****	****	****0****American Robin****	****	****  3****	****	****    0****	****	****3****Red-winged Blackbird****	****  3****	****	****    0****	****	****0 ****Mallard****	****	****	****  2****	****	****    0****	****	****1****American Coot****	****	****  1****	****	****    0****	****	****0****Northern Flicker****	****	****  1****	****	****    0****	****	****0****Mourning Dove****	****	****  3****	****	****    0****	****	****3****Killdeer****	****	****	****  2****	****	****    0****	****	****0****Budgerigar****	****	****	****  3****	****	****    0****	****	****3****Monk Parakeet****	****	****  3****	****	****    0****	****	****3****Japanese Quail****	****	****  3****	****	****    0****	****	****3****Northern Bobwhite****	****	****  3****	****	****    0****	****	****3****Source: Unpublished data**

## Avian mortality surveillance

**How many to test?**
**Depends on transmission rates**
**Local decision**
**New regions should test as many as possible until local transmission dynamics are understood (more data is better than less data)**
**Determine lab capacity for testing**
**Meet capacity early in season**
**Triage specimens if necessary **

## Avian mortality surveillance

**Logistics .....**
**Who collects reports, and how?**
**Who picks up carcasses, where are they submitted?**
**Carcasses should be kept cold, ideally frozen**
**Triage of carcasses**
**Importance of species identification**
**Fresher the better (24 hr rule)**
**Pathologist’s examination not required**

## Which organs to test? 

**Refs: **
**Steele K., 2000, Vet. Path. 37: 208-224. **
**Panella N., 2001, Emerg. Inf. Dis. 7(4): 754-5.**
**Kramer L. & Bernard K., 2001, Ann. N. Y. Acad. Sci. 951:84-93.**
**20 crow carcasses collected in New Jersey, 1999**
**6 organs tested by Vero Plaque Assay and TaqMan RT-PCR: 10 WNV-positive carcasses**** **
_**       **__**Test**__**	**__**Brain**__**	**__**Heart**__**	**__**Kidney**__**	**__**Spleen**__**	**__**Lung**__**	**__**Liver**_
**Virus Isolation****	****    80%****	****   70%****	****  70%****	****  60%****	****50%****	****40%**
**RNA detection****	****  100%****	****   70%****	****  70%****	****  60%****	****70%****	****70%**
**Source: Panella N., 2001, Emerg. Inf. Dis. 7(4): 754-5.**

**Notes:**
In regards to the details of which birds to test in a dead bird surveillance system, there are a number of scientific, public health, and financial reasons for designing the system to be sensitive early in the season and more specific later on.  Among the information we’ve learned is that to have a good sensitive dead bird surveillance system, we should not restrict birds for testing based on pathology results, species, or lack of trauma.

## Streamlined testing

**Corvids shed large quantities of virus**
**Comparison of brain vs swabs in corvid carcasses:**
**Species (n)      ****	****	****Brain****	****Oral****	**** Cloacal**
** **
**American Crow (12)****	****	****7.1 ****1****	****6.6 ****	****6.9  **
** **
**Fish Crow (4)****	****	****	****5.8  ****	****6.1 ****	****6.6  **
** **
**Blue Jay (2)****	****	****	****6.3  ****	****5.7 ****	****6.7  **
**1. Mean PFU equivalents by TaqMan RT-PCR**

## Captive Sentinels

**What is the purpose of this type of surveillance?**
**Indicator of ongoing transmission**
**Useful in detecting increased transmission (risk)**
**Transmission monitoring, rather than early detection**
**Most useful in known transmission foci**
**Multiple years of data required to determine decision-making thresholds**

**Notes:**
Should chicken be used as WNV sentinels?  Chickens are well established as useful sentinels for other arboviruses, but what about WNV?  An effective sentinel animal for WNV must satisfy the following criteria:

It must be highly susceptible to infection, so that all infected animals develop detectable antibodies.

It must be resistant to severe infection, or else there will be problems with animal welfare groups and animal use committees.  CDC does not permit any research done on animals where mortality is a desired endpoint; sensitive animals would have to be monitored closely and euthanized after showing signs of illness.

It should not spread the infection to other animals within the sentinel flock, because then infection would not accurately reflect natural transmission.

It should not present a threat to handlers when infected.

It should not present a threat to the community at-large by developing infectious-level viremia that could pass the virus back to mosquitoes in the community.

Although not all answers to these questions are currently available, chickens score fairly well on these tests.

## Limitations to Sentinel Flocks

*Reference: **Komar N. Ann. NYAS 951:58-73; 2001*

## Captive Sentinels: Interpretation of Test Results

**How many seroconversions needed to indicate increased level of risk?**
**Local decision; years of data needed**
**Natural lag between transmission and positive test result**
**Possible cross-reaction in screening tests**
**Requires confirmatory testing**

## How is transmission measured?

**Seroconversion rates are skewed when infected birds are not replaced, because denominator of susceptible birds changes.**
**Best measure of transmission uses “Host-exposure-days” as denominator (needed for accurate quantification of transmission).**

## What is the best captive sentinel species?

**Chickens have been used extensively in SLE/WN monitoring programs in Florida in 2001, with >200 seroconversions detected**
**Other bird species?**
**Domestic mammals?**
**Horses?**
**Dogs?**

## Candidate sentinel birds that are incompetent for WNV: ChickenRing-necked PheasantBobwhiteJapanese QuailRock Dove (Pigeon)

**Notes:**
An as-yet-unpublished study at CDC evaluated infections in chickens by the natural route of mosquito-borne infection.  The findings of the USDA study were confirmed.  Chickens are essentially incompetent reservoir hosts for WNV.  The mean viremia measured in 13 adult birds was log 3.4, probably too low to infect most mosquitoes.  Interestingly, pigeons (a.k.a. rock doves) are similarly incompetent, whereas most passerine birds are competent.  In fact, this study in combination with the avian serosurvey data points to house sparrows as being important reservoir hosts, at least in urban areas such as northeastern Queens.

## Comparison of surveillance methods in NYC, 2001

**Collaboration among:**
**CDC**
**NYCDOH**
**NYSDOH**
**Green Street Scientific LLC**
**NIH and HSPH **
**Study conducted at 9 sites with history of WNV transmission in 1999-2000**

## Comparison of surveillance methods in NYC, 2001

**Sentinel chickens and pigeons were useful for monitoring transmission in enzootic study sites**
**Mosquitoes provided the best early detection**
**Avian mortality surveillance was slow in detecting WNV: effect of enzootic transmission?**

## Summary

**Avian mortality surveillance for early detection, especially in new regions**
**Useful for tracking geographic spread**
**Watch out for results of swab field tests**** **
**Captive sentinels useful for monitoring WNV activity in transmission foci**** **
**Free-ranging sentinel surveillance impractical (although wildlife serosurveys are important!!)**