A foreign animal disease investigation was oficially launched.   Later that evening, two regulatory veterinarians, one state and one federal, but both trained as foreign animal disease diagnosticians, came to the farm, took detailed histories and blood samples from all the live animals and the necropsy samples you had collected.  The material was sent via courier to the Foreign Animal Disease Diagnostic Laboratory (FADDL) at Plum Island.  Technicians greeted the courier at the airport, whisked the samples into containment, and began their diagnostic testing.   Preliminary rapid diagnosis using PCR indicated the presence of African horse sickness virus.  This was confirmed within two days by the more lengthy procedure of viral isolation.  Click here for more information about diagnosis.
African horse sickness is foreign to the United States.  Click here to see the distribution and the vector of the disease.
Back on the farm, once the diagnosis was reached, all infected animals were destroyed and the area was treated thoroughly to destroy all Culicoides.   Neighboring farms were placed under quarantine and surveilance undertaken.
How does the African horse sickness virus cause disease?
The virus gets in and replicates in endothelium.  Endothelium of lung and heart are affected preferentially, becoming leaky and resulting in heart failure and overwhelming pulmonary edema.  However, endothelium in any organ can be affected.  Consequently, there may be edema in any area of the body.  Another preferential area to be affected is the deep musculature around the ligamentum nuchae.   Abnormal vascular permeability here results in a lemon jello type of appearance:
Lemon jello appearance
Hey, we didn't examine this area when we necropsied the mare in the field!  Uh oh.   But that is not surprising as this deep cervical musculature is rarely examined during a routine necropsy.  If this were the only area of the body affected by the African horse sickness virus, we would surely miss the diagnosis!  This happens occasionally.  African horse sicknes can be very challenging to recognize!
Another group of blood vessels that can be affected and cause confusing clinical signs is the vasculature of the gut.  Horses affected in this manner show colic and nothing more - wouldn't this be a hard case to diagnose!!?  Well, whaddya know, one of the horses in the barn had signs of colic, didn't it?