Canine distemper

A century after its discovery and despite the existence of an effective vaccine, distemper remains a common disease in Europe and throughout the world, while other diseases caused by viruses belonging to the same family (e.g. measles, peste des petits ruminants) have been almost eradicated. Over time, the incidence of the acute form in young animals has decreased, but slowly progressive forms with predominantly nervous system signs have become more widespread. Although the direct mortality of these chronic forms is low, the prognosis of the disease remains uncertain because of the possible long-term, invalidating sequelae.

Despite the differences in pathogenicity (virulence, tropism), the strains of distemper virus isolated so far do not undergo appreciable antigenic modification. Vaccination with live attenuated vaccines provides dogs long-lasting immunity; nevertheless, the disease is reappearing, with numerous cases having been reported throughout Europe in  recent years. It is thought that various factors are contributing to this phenomenon: the interference of maternal antibodies in puppies vaccinated too early (e.g. before 12 weeks of age), poor compliance with performing booster vaccinations after the first vaccination, loss of immunocompetence in elderly animals, the sensitivity of vaccines to heat when refrigerated storage is inadequate and the variable quality of immunity conferred by different vaccine strains.

TRANSMISSION

The main route of transmission of distemper is direct, from one animal to another, in particular via the air-borne route (“from nose to nose”). The virus responsible for the disease is, however, present in most body secretions (faeces, urine, etc.). The spread of the virus starts 7 days after the  infection and lasts for several weeks, but rarely beyond 60-90 days. Transplacental vertical transmission is possible during the viraemic phase. It appears to be impossible to eradicate distemper because of the huge reservoir of the virus among wild animals.

AETIOPATHOGENESIS

Distemper is a multisystem disease caused by an RNA virus belonging to the family Paramyxoviridae, genus Morbillivirus. It predominantly infects dogs but can also affect ferrets, badgers and lions. Cats can be infected experimentally, but the disease in this species is self-limiting. The virus is inactivated by common disinfectants (quaternary ammonium salts, glutaraldehyde, phenols, sodium hypochlorite), by heat (T > 50/60° C for 30 minutes) and by desiccation.

The infection is caught by inhalation of aerosolised pathological material – respiratory, urinary and faecal excretions – from infected animals. The spread through the body starts from the lymphoreticular tissues, first local ones and then the systemic ones (spleen, thymus and bone marrow); the virus replicates in these sites causing  HYPERLINK "http://www.bibliovet.net/site/content/leucopenia-nel-cane-e-nel-gatto" leucopenia [2] (from destruction of both B and T lymphocytes) and viraemia. The subsequent evolution of the disease and its severity depend on the humoral and cell-mediated immune responses, which determine whether the infection develops into a clinical disorder of variable severity, with spread of the virus to epithelial tissues (cutaneous, respiratory, gastrointestinal, urogenital) and the central nervous system (CNS), or into a subclinical form until healing. Healed subjects shed the virus for 60-90 days and remain immune lifelong, although unfavourable conditions (concomitant diseases, immunosuppressive treatment, states of stress such as pregnancy and lactation) can weaken the protective immunity.

Localisation of the virus in the CNS causes an acute, non-inflammatory demyelinating encephalitis in young subjects if the host is not able to mount an immune response. In the case of an inadequate immune response, the virus can persist and cause inflammatory-based demyelination. In fact, histopathological studies can reveal perivascular sheaths of lymphocytes and plasma cells at the junction between white and grey matter and widespread demyelination. In elderly subjects the virus seems to be involved in the pathogenesis of subacute diffuse sclerosing encephalitis, or  encephalitis of the mature dog (also known as ‘old-dog’ encephalitis).

CLINICAL SIGNS

Distemper predominantly affects dogs aged between 3 and 6 months, corresponding to the period in which maternal antibodies decrease. There is great variability in the duration and severity of clinical manifestations, which can present in a classical, sequential manner although this does not always occur. Infections may be mild, subclinical, severe or fatal and there may or may not be clinical signs of CNS involvement.

• The classical form of distemper is manifested by acute conjunctivitis associated with a serous ocular discharge and a dry cough that appears after an initial fever which often goes unnoticed;
• There is progressive appearance of muco-purulent ocular and nasal discharges (Fig. 1) as a consequence of bacterial complications, and then rhinitis (Fig. 2), a productive cough with crackles and harsh breath sounds on chest auscultation as the lungs become involved, hyperthermia (>40° C), sensorial depression and anorexia;
• About 10-20 days after infection, localisation of the virus to the gastrointestinal tract leads to the onset of vomiting [5] and diarrhoea, which is often bloody, with possible secondary complications (when these signs predominate, the differential diagnosis includes parvovirus infection).

Ocular signs: distemper is the main infection causing ocular disorders in dogs. Keratoconjunctivitis sicca is common and has two hypothesized aetiological mechanisms. It is thought that the virus affects the tissue of the lacrimal gland directly, causing dacryoadenitis, but also that it is responsible for dysfunction of the trigeminal nerve with a consequent impairment in the palpebral reflex. The clinical manifestations are corneal ulcers, which rapidly evolve into multiple descemetoceles or corneal perforation in one or both of the eyes.

Within the eye, the virus can also cause multifocal chorioretinal lesions, particularly in the peripheral non-pigmented area. In rare cases these lesions can coalesce causing widespread retinal degeneration with consequent visual impairment. In the most severe forms, acute blindness and mydriasis can occur as a result of optic neuritis. Fundoscopy of animals that survive the infection may show multifocal  chorioretinal lesions and optic atrophy.

Neurological signs: because of infection of the central nervous system, some dogs develop neurological signs after the initial systemic manifestations; these neurological signs can appear from 2 weeks to many years after the acute infection, but can also follow a subclinical infection. Signs of neurological involvement include convulsions, depression, ataxia, paresis, paralysis and myoclonus of single muscle groups or widespread muscle twitching.

These clinical signs depend on the viral strain involved and the acute involvement of grey matter rather than subacute involvement of white matter. Convulsions and myoclonus with hyperaesthesia and depression predominate if the disease involves the grey matter; incoordination, ataxia, paresis, paralysis and muscle tremors in cases that the disease involves the white matter. Meningeal signs with hyperaesthesia and muscle rigidity are present in both cases.

One particular neurological manifestation of infection by distemper virus is the so-called encephalitis in the mature dog, which develops late after an infection. The characteristic signs are blindness of central nervous system origin, behavioural changes, dementia, compulsive head pressing and circling, a decrease in postural reflexes and an increase of spinal reflexes (differential diagnosis: encephalopathy or granulomatous meningo-encephalomyelitis).

Other manifestations: useful diagnostic indicators of previous infection are the appearance, 3-6 weeks after partial or complete resolution of the clinical manifestations, of hyperkeratosis of the truffle and foot pads, as well as hypoplasia of the dental enamel.

Distemper virus can also cross the placental barrier and cause abortion, perinatal mortality or the birth of diseased puppies.

Distemper virus has been recently documented in the bone marrow: on clinical examination the affected subjects are reluctant to move and have swelling and tenderness of the limbs involved as well as more typical symptoms such as fever and weakness. X-rays show signs of hypertrophic osteodystrophy with radiolucent lines at the metaphyseal regions of the long bones and new paraperiosteal bone formations. Macroscopic post-mortem findings include multifocal haemorrhages and bone necrosis, while histologically there is rarefaction of the cancellous bone and infiltration by mononucleated cells and neutrophil granulocytes.

IMMUNE RESPONSE

During the first week after infection, dogs develop lymphocytopenia and are immunodepressed. One direct effect of the infection seems to be depletion of T and B cells and necrosis of lymphatic tissue. Dogs that manage to produce vigorous humoral and cell-mediated responses have few clinical signs. Specific antibodies are produced from 6 to 20 days post-infection depending on whether they are raised against proteins of the core or envelope of the virus. Type M immunoglobulins (IgM) can be found from 8 days up to 3 months post-infection, depending on the severity of the infection and the strain of the virus.

DIAGNOSIS

The diagnosis in living subjects can be made on the basis of the clinical manifestations, but is often difficult because of the broad range of signs that can be confused with those of other respiratory or gastrointestinal diseases in dogs. Ideally, confirmation of the diagnosis should always be made using laboratory investigations although these, too, may be unable to demonstrate the infection. Care must be taken when deciding which type of sample to take in order to maximise the possibility of reaching a diagnosis (Tab. 1 e 2).

Clinical examination: in brief, there may be ocular and respiratory signs, sometimes gastrointestinal ones and, later, nervous system manifestations; fundoscopy can reveal multifocal chorioretinal lesions and optic atrophy; furthermore, hyperkeratinosis of the truffle and foot pads and hypoplasia of the dental enamel are indicators of previous infection.

Blood tests: in some cases there may be thrombocytopenia, lymphocytopenia and non-specific biochemical abnormalities (hypoglobulinaemia in puppies infected transplacentally); monocytosis is present occasionally.

The diagnosis can occasionally be made from the finding of inclusion bodies in the cytoplasm of leucocytes and red blood cells in a blood smear. These inclusion bodies can be found between 7 and 20 days after the appearance of the clinical manifestations of the disease and, when present, confirm the diagnosis.

The lack of demonstration of inclusion bodies, however, does not exclude the presence of the infection, given that the viral inclusion bodies may be complexed with specific antibodies, preventing them from being seen. The same phenomenon is also possible in immunofluorescence studies.

Chest X-rays: in the early stages of the infection there may be interstitial pneumonia which evolves to show an alveolar or mixed pattern.

Analysis of cerebrospinal fluid (CSF): this is often normal in the acute phase of the infection, while in the presence of neurological signs there is an increase in cellularity (large lymphocytes) and an increase in the protein content of the fluid (>25 mg/dl). The finding of inclusion bodies within the cytoplasm of mononucleated cells in the CSF enables the definitive diagnosis to be made (Fig. 5). The finding of antibodies against distemper virus in a sample of CSF not contaminated by blood is indicative that the infection has involved the brain given that these antibodies are produced locally (in the case of contamination by blood, the antibody titre in the CSF must be higher than that in the serum in order to indicate an active infection in the CNS). Inclusion bodies may also be demonstrated by an immunofluorescence assay.

Indirect immunofluorescence: this enables detection of distemper antigen in cytological preparations of the conjunctivitis, tonsil, respiratory and genital epithelium, buffy coat, CSF and urinary sediment. The reaction is usually positive in the early stages of the infection, before the production of neutralising antibodies. For studies of the CSF, a sample of the fluid must by centrifuged and the cellular component stained with an immunocytochemical polyclonal antiserum or monoclonal antibodies to the viral antigen conjugated with fluorescein.

Immunoperoxidase: this enables the demonstration of viral inclusions in skin biopsies.

Serological tests: seroconversion tests, enzyme-linked immunosorbent assays and immunofluorescence studies are routine methods which can show reactions of specific antibodies to the distemper virus. The titre of IgM in the serum indicates whether the infection is recent (appearance of antibodies at 7 days and decline at 4 weeks) or whether there is a subclinical evolution of the infection. The interpretation of an increased titre of IgG is more complicated since it could be mean a just overcome infection, a previous infection or an ongoing one; furthermore, IgG titres are influenced by the vaccination status of the animal.

Use of polymerase chain reaction analysis in the diagnosis of distemper: reverse transcription - polymerase chain reaction (RT-PCR) analysis is a sensitive and specific method for identifying canine distemper virus. Studies of a large number of samples had a sensitivity of 86-90%, even in cases in which serological or immunofluorescence tests were not able to demonstrate the presence of the virus. RT-PCR can be carried out on serum, whole blood, buffy coat, CSF, nasal swabs and broncho-alveolar lavage fluid. The one-step PCR and nested PCR methods are recent refinements of this technique giving greater sensitivity. The use of RT-PCR on urine, together with CSF studies, showed greater sensitivity, independently of clinical signs, histopathological findings, titres of neutralising antibodies and the distribution of the antigen in the various tissues of the infected animal.

Histopathological studies: from a histopathological point of view, the lesions are insignificant: the most frequent findings are thymic atrophy, interstitial pneumonia and lesions resulting from secondary bacterial infections. Macroscopically, there are no significant, characteristic lesions in the CNS.

Histological studies of brain sections can, however, show chronic non-purulent, demyelinating encephalitis. The lesions are predominantly present in the cerebellar and cerebral cortex; together with the lesions of the white matter, there are areas of intense inflammation, demonstrated by the formation of perivascular sheaths and infiltration of the surrounding tissue by mononucleated cells and macrophages. Immunohistochemical studies with antibodies against the distemper virus can demonstrate the presence of the viral antigen, particularly in glial cells, and mainly within the cytoplasm of these cells. Viral antigen is also present in ependymal cells, in perivascular areas, in the meninges, neurones, Purkinje’s cells and in granules of the cerebellum. Similar lesions are found in the spinal cord, particularly in the lateral funiculi.

Fluid therapy, expectorates, mucolytics, anti-emetics, artificial tears, prophylactic antibiotics given the frequent secondary bacterial infections of the respiratory and gastrointestinal tracts (aminosidine sulphate 17.5 mg/kg by deep intramuscular injection every 24 hours for 3-5 days) and anticonvulsants when required by the neurological symptoms.

In very early stages of the infection, it has been demonstrated that feline recombinant interferon (rFeIFN) is more effective than conventional treatments. The dose is 1-2 MU rFeIFN/animal administered subcutaneously or intravenously once a day for 3 days, on alternate days or once a week, depending on the severity of the symptoms. In a study of about 80 animals with clinically manifested distemper, about 70% recovered. The best results were observed when the rFeIFN was administered to animals who had shown clinical signs of the infection for no more than 3 days.

Although medical control of myoclonus and convulsions can give the patient an almost normal life, euthanasia is recommended in the case of severe neurological involvement. There are some ongoing prospective studies on the use of immunomodulatory drugs for these patients; in the future such drugs could be used to prevent and, perhaps, treat nervous system localisations of the disease.

PROPHYLAXIS AND CONTROL OF THE DISEASE

The veterinarian’s main role in the management of distemper is currently one of prevention. Despite the fact that vaccines against CDV have reduced the incidence of this disease over time, it has been noted that many of these vaccines do not yet offer sufficient cover against neurogenic strains. New vaccines are, therefore, being studied: these are based on recombinant DNA derived directly from the sequences of DNA from virulent strains of distemper. On the basis of preliminary studies it seems that these vaccines provide greater protection against the clinical manifestations of the infection.

PUBLIC HEALTH

For many years there were speculations on whether canine distemper virus could be a cause (or involved in the pathogenesis) of multiple sclerosis in humans. This hypothesis has not been supported by numerous publications over the last 15 years.

Suggested reading

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