Equine African Horse Sickness Study Guide

Overview and Clinical Importance

African Horse Sickness (AHS) is a highly infectious, non-contagious, arthropod-borne viral disease of equids that is endemic to sub-Saharan Africa. It is caused by the African Horse Sickness Virus (AHSV), an Orbivirus of the family Reoviridae. AHS is characterized by respiratory and circulatory impairment with mortality rates reaching up to 95% in naive horses. The disease is listed as a notifiable disease by the World Organisation for Animal Health (WOAH/OIE) due to its severity and potential for rapid global spread.

AHS has significant economic consequences for the equine industry, affecting international horse trade, sporting events, and equestrian activities. Climate change and the expanding geographic distribution of Culicoides midge vectors have raised concerns about potential outbreaks in previously AHS-free regions including Europe and North America.

Etiology

Causative Agent

African Horse Sickness Virus (AHSV) is a member of the genus Orbivirus within the family Reoviridae. The virus is closely related to Bluetongue Virus (BTV), another Orbivirus that affects ruminants.

Viral Characteristics

Epidemiology

Geographic Distribution

AHSV is endemic to sub-Saharan Africa, extending from Senegal in the west to Ethiopia and Somalia in the east, and south to South Africa. All 9 serotypes circulate in southern Africa, while serotypes 9, 4, and 2 are most common in North and West Africa.

Historical Outbreaks Outside Africa

Transmission and Vectors

AHS is NOT directly contagious between horses. Transmission occurs primarily through the bite of infected Culicoides biting midges (Diptera: Ceratopogonidae).

Host Susceptibility and Reservoirs

Pathogenesis

Following the bite of an infected Culicoides midge, AHSV replication occurs in a predictable sequence that determines clinical manifestation:

• Primary Replication: Regional lymph nodes at the bite site
• Primary Viremia: Virus disseminates via bloodstream (associated with erythrocytes and monocytes)
• Secondary Replication: Target organs - lungs, spleen, other lymphoid tissue, and endothelial cells
• Secondary Viremia: Marked viremia leading to severe vascular damage
• Vascular Injury: Endothelial damage causes increased vascular permeability, leading to edema, hemorrhage, and effusions

The incubation period is typically 3-14 days (usually less than 9 days). Viremia in horses typically lasts 4-8 days but can extend to 21 days.

Clinical Forms and Signs

Four clinical forms of AHS are recognized, which may represent a continuum of disease severity based on viral virulence and host immunity:

Memory Aid - AHS Clinical Forms: 'PFMH' = Pulmonary (Peracute, Frothy), Cardiac (Fat head), Mixed (Most common), Horsesickness fever (Happy ending - usually survives)

Diagnosis

Clinical and Epidemiological Diagnosis

In endemic areas with characteristic clinical signs and seasonal vector activity, a presumptive diagnosis can be made based on:

• Characteristic clinical signs (supraorbital edema, respiratory distress, frothy nasal discharge)
• Seasonal occurrence (late summer/autumn when vectors are abundant)
• Geographic location (endemic region or recent introduction risk)
• Vaccination history and immune status

Laboratory Diagnosis

Laboratory confirmation is ESSENTIAL for definitive diagnosis, as clinical signs can overlap with other equine diseases.

Differential Diagnosis

Postmortem Findings

Pulmonary Form

• Severe pulmonary edema - lungs heavy, distended, wet
• Frothy, serofibrinous fluid in trachea, bronchi, and nostrils
• Interlobular edema and alveolar flooding
• Hydrothorax (yellow, serosanguinous pleural effusion)
• Gastric fundus congestion

Cardiac Form

• Hydropericardium - yellow, gelatinous pericardial fluid
• Petechial/ecchymotic hemorrhages on epicardium and endocardium
• Subcutaneous and intermuscular edema (head, neck, thorax)
• Gelatinous yellow infiltration of subcutis, especially around jugular vein and ligamentum nuchae
• Submucosal edema of cecum, large colon, and rectum
• Ascites; lungs usually normal or only slightly congested

Mixed Form

Combination of pulmonary and cardiac lesions; often the most common presentation at necropsy.

Other Histopathological Findings

• Lymphoid depletion and necrosis in germinal centers
• Vascular endothelial damage
• Exudative pneumonia with fibrin and inflammatory cells

Treatment and Supportive Care

There is NO specific treatment or cure for AHS. Management is entirely supportive and often unsuccessful given the rapid disease progression.

Prevention and Control

Vector Control

• Stabling horses from dusk to dawn in insect-proof housing (Culicoides are crepuscular/nocturnal)
• Use of insecticide-treated nets/mesh on stables
• Topical insect repellents (pyrethroids) - limited efficacy
• Environmental management: eliminate midge breeding sites (wet, organically rich soil)
• Note: Environmental insecticide application is generally impractical due to extensive breeding sites

Movement Control and Quarantine

• Strict movement restrictions on equids from affected areas
• Quarantine and testing of imported horses
• Establishment of control zones (free zone, surveillance zone, protection zone)
• Euthanasia of infected animals may be required in outbreak situations in free countries

Vaccination

Memory Aid - LAV Limitations: 'RAVVAT' = Reversion to virulence risk, Abortifacient/teratogenic, Viremia (vector can become infected), Vaccine-field reassortment possible, Africa only (not licensed elsewhere), Takes multiple years for full protection