Dr. Dave Samuel

Seems like every August-September we start hearing reports of dead deer.  Sometimes those reports center in one area with many hunters reporting lots of dead animals.  Most times the reports are scattered and dead animals are not that abundant.  So what’s the deal?  And why are we seeing more EHD in the past 20 years?

EHD has been around since it was first isolated in New Jersey in 1955.  It was probably around before 1955, but this was the first time it was confirmed.   As an aside, let me note that many hunters refer to EHD as “Blue Tongue (BT),” but in reality Blue Tongue is not the same viral disease as EHD and is  much less common than EHD.  These two viral diseases have almost identical symptoms, but they are different.  However, for all intents and purposes, and relative to deer dying and impacting your hunting, EHD and BT have the same effects.

The EHD vector is a midge that bites deer.  It was once more common in the southern US, but in the past ten years we are finding it in the North.  In addition every few years a new strains of EHD pops up and this further complicates the impact on deer.

As an example of EHD being found further North, note that in Michigan from 1955 to 2006 EHD was almost never found, but since that time deer have died there every year with a major outbreak killing around 15,000 deer in 2012.

One big question is why do we see EHD in some years?  Early on it was believed that deer density played a role in outbreaks, or even supplemental feeding, but now we know that EHD is not deer density dependent.  Deer do not give it to other deer so deer density is not a cause.  You might find more dead deer where deer populations are higher, but that is simply because when the midges are there, the more deer around, the more that get bit.

New research that there is a link between hot, dry summers and outbreaks of EHD.  In the past 20 years, we are seeing more persistent droughts in some areas.  Summer droughts are important because they exacerbate outbreaks of the biting midges that infect deer.  Longer and more severe droughts lead to more biting insects that exist in shallow mud flats.  Droughts also cause deer to go to water, and that’s where they get bit.  A perfect storm.

We know that EHD has been found further north since 2006.  The years 2007 and 2012 were big years for EHD...  Look where the big outbreaks occurred.  In 2007 most EHD occurred in southern Illinois, southern Indiana, southern Ohio, southeastern Pennsylvania, all of Kentucky, all of West Virginia, and all of Virginia.   But in 2012, outbreaks in that region were southern Minnesota, southern Michigan, all of Illinois, all of Indiana, northern Ohio and even northeastern Pennsylvania.  Researchers aren’t sure why this happened, but warmer fall weather allows the midges to be active longer, so that probably is a major reason.

Researchers at the Southeastern Cooperative Wildlife Disease Study compiled data on the incidence of EHD compared to a Drought Index and found that the Drought Index for 2007 and 2012 was very high.  Low drought years in 2000, 2004, and 2009 had lower outbreaks of EHD.

Researchers took this one step further and compared the Drought Index in each county to the occurrences of EHD.  Knowing that we get more midges in years of drought leads us to expect that in counties where we had drought, we should expect more reports of EHD.  Did this happen?

In fact it did.  Even further, there was also a correlation with the severity of the drought.  The worse the drought, the more EHD was reported.  This relationship was weak in the southern states, but occurred in middle tier and northern states.

We know that when EHD hits an area, some deer get the disease, but do not die and develop immunity.  Interestingly that immunity is passed from the doe to her unborn fawns and protects the fawn for 3-4 months.  However, if a new strain of EHD comes into an area, deer do not have immunity and must build it for the new strain.

If you google, “Climate Prediction Center” you can find drought predictions for the country.    Below the maps are written descriptions of predicted drought areas.  I know it’s too late to do this for this fall, but to predict EHD outbreaks in your area, go to this website around June 20 and see predictions for July, August, and September.  If drought is predicted in your area, EHD is definitely possible.  One last item.  At the bottom of that page is “Seasonal Drought Outlook Discussion.”  Click that and you can read what the experts think about their predictions for the next three months.  For each region of the country they rate the confidence in their predictions as low, moderate, or high.  Interesting reading.

In my home state of West Virginia we’ve had a ton or summer rain.  August too.  Does this mean low numbers of EHD deer?  Probably.