Emerging Infectious Diseases

Introduction

The increasing globalization of the equine industry has diminished the significance of geographic boundaries in limiting the spread of infectious diseases. Infectious diseases of public health and veterinary significance can have a devastating effect on naïve populations. It is important that equine veterinarians be able to recognize clinical signs suggestive of the emergence of a new disease and be prepared to respond appropriately. This presentation will describe the basic clinical signs of four infectious diseases that have recently or might reasonably appear in horses in Europe.

Equine Infectious Anemia

EIA virus (EIAV) is in the Lentivirus genus of the family Retroviridae. Rapid antigenic variation of surface proteins enables the virus to evade immune responses and infection is considered life-long. EIAV is transmitted primarily through the bites of tabanid flies (e.g. horseflies and deerflies). Iatrogenic transmission may occur via contaminated needles or surgical instruments, contaminated blood transfusions, or improperly sterilized equipment.

Clinical signs of EIAV infection may be acute, chronic, or inapparent. In acute disease, fever (38.5oC - 40.5oC) occurs with lethargy and decreased appetite. Most horses are at least transiently thrombocytopenic, and some may become anemic. After recovery from acute disease, many horses never exhibit any additional clinical signs; other horses experience recurrent episodes of fever that may last days to weeks. The gradual decrease in number and severity of clinical signs is attributed to the ability of the horse’s immune system to control viral replication. Some horses progress to exhibit classic clinical signs of chronic EIA or “swamp fever” with fever, weight loss, dependent edema, lethargy, and depression. They are anemic, thrombocytopenic, hypoalbuminemic, and hyperglobulinemic. Petechial hemorrhages and epistaxis may develop. Most seropositive horses show no overt clinical signs of disease. They maintain normal body condition and can function in some athletic endeavors. Diagnosis is confirmed by serologic testing.

West Nile Virus

West Nile virus (WNV) is a member of the Japanese encephalitis serogroup of the Flaviviridae family. Mosquitoes are biologic vectors for WNV with a primary sylvatic cycle between birds and mosquitoes. Horses and humans are considered “dead-end” hosts due to their comparatively low level of viremia. The seasonal incidence of WNV in horses and people reflects regional vector activity. [...]