# Presentation: 171575

## The Evolving 
West Nile Virus Epizootic
in Horses

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**The Evolving **
**West Nile Virus Epizootic**
**in Horses**

***R. L. Crom, DVM***
***USDA, APHIS, Veterinary Services***
**March 2002**

## Acknowledgments

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**Acknowledgments**

-    **Data on equine cases of West Nile virus (WNV) infection were acquired only through the efforts of private veterinary practitioners, clinicians, field investigators, laboratory diagnosticians, animal health and public health officials in all the States where suspect equine cases have been investigated to date.  **
**	**
**	****Laboratory confirmation of equine cases in 2001 was performed by USDA-APHIS' National Veterinary Services Laboratories (NVSL). ****	**

**Notes:**
This is to acknowledge all those whom have investigated, confirmed, and reported cases of WNV infection in equids.  Without them we would have no data to report.

## Equine WNV Cases, 1999

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**Equine WNV Cases, 1999**

**25 cases eventually detected, all on Long     Island (22 in Suffolk Co., 3 in Nassau Co.)**
*** ******19 premises affected***
*** ******dates of onset:***
*** *****first - 26 August (Riverhead, NY)**
** ****last - 18 October (Mattituck, NY)**
*** ******all horses, no other equids ***
*** ******9 horses (36%) died or were euthanatized***

## Equine WNV Cases, 2000

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**Equine WNV Cases, 2000**

**60 cases detected in 7 States**
*** ******NJ (27), NY (19), CT (7), DE (4), MA (1), ***
***    ******PA (1), RI (1)***
*** ******10 cases in Monmouth Co., NJ***
*** ******dates of onset:***
*** *****first - 17 August (Staten Island, NY)**
** ****last - 30 October (near Smyrna, DE)**
*** ******all horses, no other equids***
*** ******23 horses (38%) died or were euthanatized***

**Notes:**
Summary of 2000 for horses.  Much broader distribution than in 1999, most cases reported from NJ and NY.  Only semblance of a “cluster” of cases was in Monmouth County, NJ, where there were 10.

## Equine Surveillance Case Definition
2001

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**Equine Surveillance Case Definition**
**2001**

** ****Confirmed**** case – compatible clinical signs*  **
**    ****plus one or more diagnostic results:**
*** ******isolation of WNV from tissues  ***
*** ******>=4-fold change in PRNT titer (paired sera)***
*** ******IgM in serum/CSF, plus PRNT titer in serum***
*** ******IgM in serum/CSF, plus positive PCR in tissue***
*** ******IgM in serum/CSF, plus positive IHC in tissue***
*** ******positive IHC and positive PCR in tissue****** ***
** ***** ****ataxia or at least two other specified clinical signs**

**Notes:**
This is the “confirmed” case definition used by USDA-APHIS for official purposes (e.g., international reporting) in 2001.  A “probable” case was also defined but is not shown here.

## Equine WNV Cases, 2001

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**Equine WNV Cases, 2001**

**738 cases detected in 20 States**
*** ******States with >5 cases: FL (492), GA (68), MA (45), NJ (33), NY (22), MS (17), CT (11), LA (9), KY (8), PA (7), NC (6), VA (6)***
*** ******89 cases in Marion Co., FL***
*** ******dates of onset:***
*** *****first: 25 June, (Jefferson Co., FL)**
** “****last”: 21 December (Sumter Co., FL)**
*** ******all horses, no other equids***
*** ******at least 156 horses died or were euthanatized***

## Equine WNV Cases, 2001
 (continued)

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**Equine WNV Cases, 2001**
** *****(continued)***

**Information reported on the 738 cases**
*** ******County: 726 cases ***
*** ******Onset date : 702 cases***
*** ******Age, county and onset date: 641 cases***
*** ******Outcome, plus some other data: 470 cases***
*** ******Outcome, plus age, and some other: 455 cases***
**Of the 738 cases, at least 651 had diagnostic results at NVSL that met 2001 case definition**
*** ******Confirmed: 640 cases ***
*** ******Probable: 11 cases ***

## Slide 8

**Notes:**
The next 3 maps show the geographic progression of WNV in equine, by State, from 1999 to 2001.  All cases were in horses.  In 1999, the only equine cases were in Long Island, NY.

## Slide 9

**Notes:**
There were 7 States with equine cases in 2000....

## Slide 10

**Notes:**
...and 20 States in 2001.

## Slide 11

## Slide 12

**Notes:**
This is a log scale.  States are grouped by North vs. South, using the VA and NC border (extended) as the dividing line.

## <footer>

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**Notes:**
Compare this map to the next (showing attack rates) to get a better idea of where the risk was greatest for horses being infected with WNV as opposed to where the greatest population of horses were located.

## <footer>

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## Slide 15

**Notes:**
Combined epicurve, 1999 and 2000.  Appears to hae 2 peaks each year, one in early September and second in late September or early October.  We don’t know what this might mean, if anything...

## Slide 16

## Slide 17

**Notes:**
The greatest number of cases occurred in mid-September, but cases started early and never completely ended in 2001.

## Slide 18

**Notes:**
North means those north of the VA-NC border extended.

## Slide 19

## Clinical WNV Cases in Horses
1999-2000

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**Clinical WNV Cases in Horses**
**1999-2000**

**Case fatality rate:  38% (32/85)**
**No breed predilection:  11 or more affected**
**Slightly more males:  57% (48/84)**
**Age related to outcome**
*** ******all ages infected, but those that become ill ***
***    ******are older than those that do not***
*** ******those that die are older than those ***
***    ******that recover ***

**Notes:**
Here is some basic information on equine cases occurring in 1999 and 2000.  Males vs. females is not statistically significant.  Age factors are statistically significant, given the 1999-2000 data plus preliminary analysis of the large number of cases in 2001.  

## Clinical WNV Cases in Horses
2001

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**Clinical WNV Cases in Horses**
**2001**

**Case fatality rate:  33.2% (156/470), or lower?**
**No breed predilection:  >30 affected**
**Slightly more males:  53.7% (350/652)**
**Age related to outcome**
*** ******those that die are older than those ***
***    ******that recover ***

**Notes:**
Here is some basic information on equine cases occurring in 2001.  Males vs. females is not statistically significant.  Age factor is statistically significant.  

## Slide 22

**Notes:**
There is a statistically significant trend of an increasing case fatality rate by age group.

## Slide 23

**Notes:**
Horses of “unknown” outcome were younger than horses known to have recovered, suggesting the unknowns were mostly made up of horses that recovered.  If true, this would lower the case fatality rate overall.

## Slide 24

## Subclinical Infections
Case Premises vs. Non-Case Premises

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**Subclinical Infections**
**Case Premises vs. Non-Case Premises**

**1999: 18% of non-case horses tested were PRNT+ **
*** ******23% on case premises (33/146)***
*** ******8% on other equine premises (5/65)***
**2000: 2% of non-case horses tested were PRNT+ **
*** ******5% on case premises (23/489)***
*** ******<1% on other equine premises (8/1097)***

**Notes:**
There was a lower prevalence of subclinical WNV infections detected in 2000, as compared to 1999.  The difference existed both on premises with a clinical equine case and on premises that had no clinical cases in horses.  In both years, the prevalence of subclinical infections was greatest on premises where a clinical case of WNV was confirmed than on nearby equine premises.

## Case-Control Study / Spatial Data Analysis
Equine Infections - Year 2000

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**Case-Control Study / Spatial Data Analysis**
**Equine Infections - Year 2000**

**Case-Control Study: 49 “case” and 101 control premises in 7 States; 1,487 equids sampled   (64 seropositive)**
*** ******infected horses less likely housed in stalls at night; more likely used for pleasure purposes***
*** ******case premises more likely within 1⁄2 mile of blackbird roost or waterfowl congregation***
** ****Spatial Data Analysis:**
*** ******case premises clustered; within a cluster, exposure of individual equids appears to be chance event ***

**Notes:**
Case premises = one or more infected (seropositive) equids 
Control premises = within 5 miles of a case premises (mean distance=2.2 miles), no infected equids

## WNV Vaccine

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**WNV Vaccine**

**Killed virus product given conditional license    by APHIS-VS-CVB on August 1, 2001**
*** ******ensures safety and purity; “reasonable***
***    ******expectation" of efficacy***
**More than 1 million doses distributed to date**
**Administration:  2 doses IM, 3-6 weeks apart;   one annual booster**
**Antibody response:  no IgM; high levels of neutralizing Ab after 2****nd**** dose**** **

**Notes:**
(CVB = Center for Veterinary Biologics of USDA-APHIS-VS in Ames, Iowa.)
An equine vaccine has been approved by CVB.  It is up to individual states to decide what use will be allowed under a conditional license.  It is a killed virus product, so vaccinated horses may not necessarily be distinguishable from naturally infected horses.  However, information collected so far suggests that vaccinated horses do not develop detectable levels of IgM antibody, so horses found to have IgM were very likely infected naturally.  Horses are probably not protected from natural infection (do not have high levels of neutralizing antibody) until one or two weeks after the 2nd dose of vaccine is given.

## WNV Cases in “Vaccinated” Horses

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**WNV Cases in “Vaccinated” Horses**

**At least 132 horses developed cases of WNV after receiving vaccine**
*** ******113 after one dose; 19 after two doses***
*** ******States: FL (107), GA (18), NJ (2), CT, DE, IL, RI, VA   ***
**Of those occurring after 1****st**** dose of vaccine:**
*** ******ons******et appeared to be at random intervals after inoculation***
**Of those occurring after 2****nd**** dose of vaccine:**
*** ******14 had onset at 15 or fewer days after inoculation***
*** ******5 had onset at 20 or more days after inoculation:    ******37, 33, ~27,****** 21, and 20 days ***

## What should we expect in the future?

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**What should we expect in the future?**

**Areas that have had WNV in horses, will have it again (although in different horses).**
**Additional areas/states are likely to have WNV cases in horses.**
**Detecting equine cases of WNV will help define areas of WNV activity, but those cases may not be predictors of human cases. **
**The efficacy of equine vaccine, and the number of horses vaccinated, will determine the impact of vaccination on all of the above.**