Hemorrhagic gastroenteritis (HGE) is a disease of dogs characterized by sudden vomiting and bloody diarrhea. The symptoms are usually severe, and HGE can be fatal if not treated. HGE is most common in young adult dogs of any breed, but especially small dogs such as the Toy Poodle and Miniature Schnauzer. It is not contagious. Cause
The cause is uncertain. Suspected causes include abnormal responses to bacteria or bacterial endotoxin, or a hypersensitivity to food. Pathologically there is an increase in the permeability of the intestinal lining and a leakage of blood and proteins into the bowel. Clostridium perfringens has been found in large numbers in the intestines of many affected dogs. Clinical signs
Profuse vomiting is usually the first symptom, followed by depression and bloody diarrhea with a foul odor. Severe hypovolemia (low blood volume) is one of the hallmarks of the disease, and severe hemoconcentration (concentrated blood) is considered necessary for diagnosis. The progression of HGE is so rapid that hypovolemic shock and death can occur within 24 hours. Disseminated intravascular coagulation (DIC) is a possible sequela of HGE. As a result, this disease can cause severe damage. Diagnosis
Clinical signs of HGE and canine parvovirus (CPV) are similar enough that they need to be differentiated. It may or may not be detected by a high or low white blood cell count, and there may be a low hematocrit. A negative fecal parvovirus test is sometimes necessary to completely rule out CPV.
Resistance
Resistance to aciclovir is rare in people with healthy immune systems, but is more common (up to 10%) in people with immunodeficiencies on chronic antiviral prophylaxis (transplant recipients, people with acquired immunodeficiency syndrome due to HIV infection). Mechanisms of resistance in HSV include deficient viral thymidine kinase; and mutations to viral thymidine kinase or DNA polymerase, altering substrate sensitivity. Microbiology
Aciclovir is active against most species in the herpesvirus family. In descending order of activity:
Herpes simplex virus type I (HSV-1)
Herpes simplex virus type II (HSV-2)
Varicella zoster virus (VZV)
Epstein–Barr virus (EBV)
Human cytomegalovirus (HCMV)  – least activity

Pharmacokinetics
Aciclovir is poorly water-soluble and has poor oral bioavailability (15–30%), hence intravenous administration is necessary if high concentrations are required. When orally administered, peak plasma concentration occurs after 1–2 hours. According to the Biopharmaceutical Classification System  (BCS),  aciclovir falls  under the BCS Class III drug i.e. soluble with low intestinal permeability. Aciclovir has a high distribution rate; protein binding is reported to range from 9 to 33%. The elimination half-life (t1/2) of aciclovir depends according to age group; neonates have a t1/2 of 4 hours, children 1–12 years have a t1/2 of 2–3 hours whereas adults have a t1/2 of 3 hours. Chemistry
Details of the synthesis of aciclovir were first published by scientists from the University at Buffalo. In the first step shown, 2,6-dichloropurine was alkylated with 1-benzoyloxy-2-chloromethoxyethane.