Muscle stiffness, tremors and seizures all have many possible underlying causes, but a tiny parasite could actually be the source of the issue if your dog is experiencing these symptoms. Neosporosis is a very uncommon medical condition caused by the protozoal parasite Neospora caninum. Dogs and coyotes are the definitive host for this disease. While cats can be infected experimentally, no natural cases have been seen.
Because neosporosis is so uncommon, it’s not really understood all that well. In fact, until relatively recently, it was confused with toxoplasmosis, the well-known condition that affects our cat population. They are very similar, indeed, and much of what we know about neosporosis is inferred from the feline counterpart.
The lifecycle and transmission of this protozoa are not very well understood. The most common mode of transmission seems to be transplacental, meaning that infected moms pass the organism on to the fetus while pregnant. Infection in these puppies is variable—some, none, or all of them may be infected, but only a small amount of these pups will develop clinical disease.
Another possible mode of transmission is through the ingestion of infected tissues. N. caninum eggs that are passed in the stool of infected dogs may go on to infect intermediate hosts, like sheep and cattle, where they form tissue cysts. Ingestion of raw meat from these intermediate hosts can pass the organisms onto the definitive host, the dog.
In dogs, neosporosis causes clinical signs due to infection of the central nervous system and muscular tissues. Clinical signs are usually seen in young dogs, but we do occasionally see them in older dogs, too. Signs may start with hind limb weakness which gradually progresses to include all four limbs. Muscle stiffness can become so severe that the limbs are unable to relax, and when this occurs it is usually non-reversible.
Both young and older dogs can present with neurologic signs due to neosporosis, including muscle atrophy, tremors, exaggerated gaits, tongue paralysis, and trouble chewing. Seizures, blindness, and sudden behavior changes can also be seen if the central nervous system is affected.
Though the organism that causes neosporosis is shed in the stool of infected animals, it is rare to actually find it when the pet is sick, making in-house fecal tests unreliable for detecting disease. Instead, the condition can be diagnosed by finding antibodies in the blood or cerebral spinal fluid, or by finding the organism itself in muscle biopsies or cerebral spinal fluid samples.
Information on effective treatment of neosporosis is lacking due to its rarity, though treating the condition the way we treat toxoplasmosis in cats seems to work well if started early in the progression of disease. Once muscle stiffness or paralysis occur, improvement is not likely. It is a good idea to treat all members of a litter once one puppy has shown clinical signs.
There is no way to prevent transmission of the disease to pups in utero, so it is best not to breed infected dogs. To prevent disease in adult dogs, refrain from feeding them raw meat.