Neosporosis

Disease caused by Neospora caninum, a parasitic protozoan phylogenetically close to Toxoplasma gondii, with which it was often confused before it was identified in 1988. Neospora caninum occurs worldwide and infects dogs, ruminants, and many other animals. In cats, only infection of artificially immunosuppressed animals has been described. In dogs, it causes neurological and muscular disorders, and in bovines it causes abortions and neonatal mortality. There is so far no evidence that it is pathogenic in humans.

Dogs and other wild canids are both definitive and intermediate hosts. In natural infection, dogs become infected by eating contaminated meat, but repeated congenital transmission can also occur. Females with a subclinical infection can transmit the parasite to their foetuses so that successive litters from the same animal may be born with the infection. Estimates suggest that 3% of puppies born to seropositive mothers have clinical neosporosis. Many aspects of the transmission and spread of this parasite are still poorly understood. Infection is most frequent in young dogs, and congenital infection is the most severe form. Estimates suggest that half of all infected animals can be cured if the disease is diagnosed and treated early. The infection is sometimes subclinical, and can reactivate later in the animal's life.

In intermediate hosts, the parasite is found in two morphologically different infectious stages: tachyzoites, which are free forms, and bradyzoites, which are located in tissue cysts. A third stage - unsporulated oocysts - occurs in definitive hosts. Tachyzoites destroy cells and proliferate in muscle tissue, including the myocardium, and also in nervous tissue. They also infect the skin, eyes, liver, lungs, lymph nodes, spleen and adrenal glands. The parasite causes an inflammatory response followed by the formation of mononuclear cell infiltrates, central areas of necrosis, and granulomas. The most common histopathological alterations are polyradiculoneuritis, multifocal encephalomyelitis, polymyositis, and occasionally myocarditis and hepatitis. In muscles, severe interstitial fibrosis is observed in chronic cases, accompanied by degeneration of nerve fibres and demyelination. Bradyzoites are confined to tissue cysts, and as such elicit virtually no reaction in the host. These cysts will probably reactivate in immunodepressed animals, giving rise to clinical neosporosis. Dogs shed oocysts into the environment, contaminating the food and drinking water of farm animals. Oocysts sporulate within three days and contain two sporocysts, each with four sporozoites.

Symptoms

Symptoms, in order of frequency, are:

• Paresis, hyperextension and progressive ataxia of the hind limbs. May progress to tetraparesis.
• Myositis and pain on palpation of the lumbar muscles and/or quadriceps.
• Muscle atrophy and flaccidity in animals with chronic infection. Dogs in which posterior paralysis is the only abnormality may remain alert and survive for several months.
• Multifocal manifestations of central nervous system disorders, neck weakness, difficulty swallowing, jaw paralysis, head tilting.
• Myocarditis and sudden death (rare).

Interpretation of laboratory tests

General tests:

Blood and biochemical alterations are not specific to the infection.

• Complete blood count. Unremarkable in many cases. Non-regenerative anaemia, neutrophilic leucocytosis, lymphocytosis and eosinophilia have been observed in some animals.
• Serum biochemistry ALT, AST, LDH, and CPK may be elevated, especially in dogs with acute muscle and liver necrosis.

Specific tests

• Identification of the parasite.
  Tachyzoites may be found in CSF, tracheal or bronchoalveolar lavage fluid, lung aspirates, tissue biopsies (especially of affected muscles), and in impressions of skin lesions.
  Identification techniques.
  • Optical microscope: not useful for definitive diagnosis, since it cannot differentiated Neospora caninum from Toxoplasma gondii.
  • Electron microscope: differentiates between Neospora caninum and Toxoplasma gondii by studying their ultrastructure.
  • Immunocytochemistry / immunohistochemistry: special immunoperoxidase stains are often used. One of the most widely used and most reliable methods.
  • PCR: Highly sensitive technique for identifying the parasite.
• Determination of antibodies in serum or CSF.
  Direct Agglutination Test (DAT): Used to quantify the level of virus-specific antibodies. It is a simple test that has a specificity similar to IIF, but lower sensitivity.
  Indirect immunofluorescence (IIF): Usually considered the reference test, and is the most widely used. Values of 1/20 are positive.
  Test limitations:
  • Persistence of very high titres for years in clinically healthy dogs.
  • Occasionally, neosporosis can be confirmed histologically with titres below the positive values.
  Other techniques. Other diagnostic techniques based on ELISA, Western blot and RIA methods are being developed.
• Detection of oocysts in faeces. Neosporosis oocysts are morphologically identical to those of other non-pathogenic protozoa, although techniques have been developed to identify them using PCR. The test is of more epidemiological than diagnostic value.

Complementary tests:

• Cerebrospinal fluid (CSF) analysis: Shows an increase in total protein and mixed monocytic-polymorphonuclear pleocytosis, typical of non-suppurative meningoencephalitis.
• Electromyography: Spontaneous activity and high-frequency discharges characteristic of myopathy and peripheral neuropathy with lower motor neuron impairment.

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