The equine adenovirus is present worldwide. It is a DNA virus from the Adenoviridae group. For the most part it is sub-clinical but exists in the upper respiratory tract of adult horses where it can be spread to other animals through airborne droplets or as respiratory or ocular infection acquired from close contact. Its typical isolates are Equine Adenovirus Type 1 and Equine Adenovirus Type 2, although the latter is seen more rarely.
Arabian foals with Equine Severe Combined Immune Deficiency, caused by the absence of B and T lymphocytes, are subject to high levels of morbidity and mortality from this virus. The necrotising bronchiolitis it causes in these foals nearly always proves fatal.
Once acquired, the virus replicates in epithelial cells in the respiratory tract, with cell sloughing and lysis that brings on a hyperplastic response in remaining cells. Most animals recover within 10 days, but infection with other pathogens may engender more serious disease. Adenovirus may also infect the gastrointestinal cells and is shed in feces.
Clinical Signs of Equine Adenovirus
Adenovirus infection in adult animals may cause upper respiratory disease with purulent nasal discharge, conjunctivitis with discharge, GI tract infection with loose feces, and bronchopneumonia. There may be enlarged sub-mandibular nodes as well as coughing with exercise and lassitude is also observed occasionally.
In immunodeficient foals the characteristic course of the infection is rhinitis, intermittent fever, conjunctivitis, diarrhea and pneumonia. Foals may appear normal at birth but develop symptoms before six weeks of age and die within two weeks.
Foals that are immunocompetent may also be affected, showing dyspnea, coughing, fever and diarrhea. In healthy foals it is rarely fatal unless complicated by other pathogens.
Diagnosis of Equine Adenovirus
In the acute stage of the infection the virus may be isolated from nasopharyngeal and conjunctival specimens. Electron microscopy can detect the virus in feces. Serum neutralization of hemagglutination inhibition (HI) can detect the virus in samples taken 10-14 days apart.
In Arab foals diagnosis at necropsy includes findings of necrotizing bronchiolitis and interstitial pneumonia with fibrin and hyalin membranes in the alveoli. Also seen is atelectasis and lobules that are consolidated within the cranioventral region. The airways show mucopurulent exudate. The thymus, spleen, and lymph nodes are small with no germinal centers. Abscess formation from bacterial infection are often seen.
In histological findings, sloughed debris shows bronchiolar obstruction as well as large basophilic intranuclear inclusion bodies in respiratory epithelial cells. These are sometimes also seen in epithelial cells of the salivary glands, the conjunctiva, the bladder, small intestines and pancreas. Adenovirus can be isolated from lung tissue, although primary equine kidney cells are the best choice for virus isolation.
Treatment of Equine Adenovirus
For foals with equine adenovirus no treatment exists and death usually occurs within 2 weeks of infection. Healthy adult horses rarely evidence significant disease, but may be supported with antibiotic therapy to preclude secondary infection, as well as mucolytics and anti-inflammatories if necessary. Otherwise healthy horses almost always recover within 10 days.