Do you have a dog or a cat? If you do, you probably have heard (if not suffered)
the diseases that we will be talking about in this step: canine parvovirosis and feline panleukopenia or parvovirosis. We will learn what are the clinical signs, why are they produced and how to diagnose the disease. We have already mentioned how hardy parvovirus are. Why? You probably remember it is mainly because they are naked viruses, but also because they are very small. They are shed mostly with the faeces, and they are able to endure the environmental conditions for long periods of time. Through direct transmission, the viral particles may reach other animals and infect them.
In addition, as they are so hardy, they stay infective when they adhere to inanimate objects, such as shoes, vehicle tires, and may infect animals which are not directly exposed to the virus. Finally, in cats, parvovirus may also cross the placenta and infect the foetus, producing a severe disease. Except in the vertical transmission, virus enter the animals through the oral route. They initially replicate in the oropharyngeal tissue, but soon reach the intestine.
The conditions are ideal for that: they are sturdy and may cross the stomach unaltered, and they require actively dividing cells, such as those in the intestinal crypts. As we have seen in a previous step, the result of the replication in these cells is destruction or lysis, leaving big areas of the intestine without lining. This is termed “enteritis” and the result is watery, sometimes haemorrhagic, pestilent diarrhoea as food, usually milk, is not digested. Viruses also reach the bone marrow, another site with rapidly dividing stem cells, which give rise to blood cells such as leukocytes, monocytes, neutrophils and others. Upon infecting the stem cells, there is a big decrease in the number of circulating leukocytes; this is termed “leukopenia”.
Leukopenic animals don’t have enough white cells to fight microorganisms, and they may succumb to secondary or opportunistic infections. The virus may also reach the heart and induce inflammation, or myocarditis, there. This happens in puppies younger than 1 month old, which may die fulminant without any other sign of the disease. As you might guess, any of these three processes are enough to cause the death of the parvovirus-infected dog. Parvovirosis is a very severe disease in this species, with a high morbidity and a very high mortality rates. Parvovirosis is also very severe in cats. The pathogenesis is very similar, responding to the requirement of parvovirus to infect rapidly dividing cells.
In cats, due to the infection of the bone marrow, leukopenia affects more blood populations and is usually called “panleukopenia”, as “pan” is a prefix which means “all”. Feline parvoviruses also infect enterocytes in the intestinal crypts and produce diarrhoea. Feline parvovirus may cross the placenta. If this happens early in pregnancy, it causes abortion. If it happens later in pregnancy, the virus may reach the cerebellum and the kittens might have ataxia or incoordination when they are born. Clinical signs in both dogs and cats may orientate diagnosis, which can be confirmed by laboratory tests.
Though the detection of antibodies presence in serum previously was the method of choice for diagnosis, nowadays it has been substituted by viral detection by PCR or, more frequently, by rapid, disposable tests. Serological methods have come into disuse, since most puppies and kittens are vaccinated and it is impossible to differentiate vaccinated from infected animals. In dead animals, post-mortem diagnosis is directed to detecting the typical lesions of necrosis in the enterocytes of the intestinal crypts, or isolating the virus or detecting its genome by PCR.
You can now understand how easy it is to predict the clinical signs in the infections by parvovirus, when you remember that they are very simple viruses, with a small genome, which require rapidly dividing cells to be able to replicate, such as those in the intestine, the bone marrow, or the foetus. Also remember that these diseases have high morbidity and mortality rates.