Drought may lead to EHD return

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By Rick Greenwell


EHD update: These initials stand for Epizootic Hemorrhagic Disease Virus, not to be confused with blue tongue, which shows up mostly in sheep.  EHD is the problem we had a couple of years back that hurt our local deer population and even killed a few cattle.  The drought conditions this year could be setting us up for the same situation. Here is an article we received about this disease.

Epizootic hemorrhagic disease virus (EHDV) causes significant death loss in deer.  Some of these strains of EHD virus can cause disease in cattle, although it is uncommon, rarely fatal and usually associated with an epidemic in deer.  The virus is transmitted by biting midges in the genus Culicoides but some species of gnats and mosquitoes may transmit the virus as well.  EHD in deer has been correlated with droughts because the deer tend to concentrate around the few wet areas available and these are where the gnats breed.  

The gnats that can carry EHD tend to feed on the underbelly and lower legs, areas that are hard to treat effectively. Pour-on insecticides containing pyrethroid insecticides are distributed in skin oils and give good coverage but do get covered with mud or dust so they need to be reapplied.  Whole-animal sprays – particularly self-treating as the animal passes through a gate – can give good coverage that is applied frequently.

These gnats are not good fliers and tend to concentrate around waterholes in or at edges of woods and don’t get out into open pastures as much.  If practical, keeping cattle out of wooded areas, especially around wet spots can help decrease the risk of exposure.
EHD in cattle is usually subclinical (no symptoms of infection), but the morbidity (rate of sickness) may be as high as five percent and most cattle recover within a few weeks.  Typical signs include fever, ulcers in the mouth, drooling, lameness associated with inflammation of the coronary band and weight loss.  There are some reports of abortion and congenital defects in the fetus if pregnant dams are infected between 70-120 days gestation, but this is an uncommon finding.

Diagnosis in cattle is accomplished through several types of blood tests.  Virus isolation or real time PCR can detect the virus in whole blood (purple top tube).  Serum samples (red top tubes) are only useful if two samples are taken from the affected animal.  The first must be taken at the beginning of the illness and a second sample is taken three or four weeks later. Work with your veterinarian for proper diagnosis and treatment options, especially in areas with deer death loss.