This is a highly contagious epitheliotropic enterovirus with a predilection for rapidly dividing cells. Hence the leucopenia and gastroenteritis seen in adults and myocarditis of puppies (). The first outbreak of acute parvovirus infection occurred in 1978 when dogs were affected in one of two ways: (1) sudden deaths and acute myocarditis in unweaned puppies with a very high mortality rate in affected litters; and (2) weaned puppies and adult dogs showed signs of acute gastroenteritis which was rapidly fatal if left untreated in the early stages. The first syndrome is now rare, and is only seen when totally susceptible unvaccinated dams pick up the infection and pass it to the fetus. After ingestion the virus lodges in the pharyngeal lymphoid tissues and thymus and then establishes itself in the rapidly dividing crypt cells of the intestinal villi. Joint infection with parvovirus has also been documented ().
A very heavy loss of good breeding animals occurred in the early years but the development of acquired immunity followed by vaccination has resulted in a sharp fall in mortality. It is now rare to observe acute myocarditis in puppies but isolated outbreaks and individual cases of gastroenteritis still occur. The problem appears to be worst in stray dogs of poor vaccinal status and in those with low maternal protection or under stress.
The gastroenteric form usually starts with anorexia and marked depression which is followed by persistent vomiting, initially productive but rapidly yielding only gastric secretion and occasionally blood. Faeces are usually fluid or gelatinous in consistency and red/brown in colour. They have a fetid smell and may contain sloughed mucosal casts. There is frequently tenesmus and severe dehydration rapidly develops. The temperature may be high initially (103—105°F) but quickly falls to below normal, so the dog becomes shocked and will die if not treated quickly.
The diagnosis is often suspected on clinical grounds but may be confirmed by haematology, serology and virus isolation. Haematology shows a severe leucopenia and neutropenia with total white cell counts as low as 0.9 x 109/litre. This level of immunosuppression often results in secondary bacterial infection. Virus may be isolated from the faeces () or faecal haemaglutination may be useful in demonstrating the virus. Fluorescent antibody tests on tissues can also be used to detect the virus. Serology to demonstrate a rising titre is essential. Single estimates of parvovirus titres vary considerably from dog to dog and are not diagnostically reliable. Finally histological examination of small intestinal biopsy samples will reveal villus atrophy and viral involvement of the crypt cells suppressing epithelial renewal. Changes associated with the large intestine are rare.
Canine parvoviras: Treatment
These are real emergencies and require intensive fluid therapy if they are to survive. A compound lactated Ringer solution is required and should be maintained until the dog is observed to be able to retain oral fluids. This may involve intravenous fluids for 4 to 7 days. If fluids are stopped prematurely then the dog will often relapse and die.
All food and oral fluids should be withdrawn. Metaclopramide (Emequell; SmithKline Beecham Pharmaceuticals) should be given intramuscularly at 0.5mg/kg twice daily and appears to be the only effective method of reducing the vomiting and preventing further exhaustion and electrolyte loss.
Broad spectrum antibiotics in the form of ampicillin (Amfipen; Gist-Brocades Pharmaceuticals) at 5-10mg/kg daily by intramuscular injection are required to prevent secondary bacterial infection. Oral fluids and kaolin based preparations should only be started once the dog stops vomiting. Lectade (SmithKline Beecham Pharmaceuticals) should be the oral fluid of choice initially followed by water to maintain fluid balance.
Prevention is much better than cure and this can easily be achieved by adequate vaccination. Where dogs are kept in close confinement, strict hygiene and adequate vaccination are essential. Carrier states do exist which make control more difficult. The virus is very resistant and can last in the environment for up to many months. Sodium hypochlorite or one of the new parvocidal disinfectants should be used to keep environmental contamination low ().