12 Parasitic myositis
Parasitic diseases of the muscle are rarely pathological to the host, but are a critical part of the lifecycle of several important parasites. Generally speaking, the most important aspect of parasitic diseases of muscles in animals is their threat to human health.
12.1 Sarcocystosis
Sarcocysts are a very common protozoan parasite found in the muscle of herbivores. They have an in indirect life cycle. Carnivores are typically the definitive host, and become infected through consumption of muscle from an infected intermediate host. Most herbivorous species have their own Sarcocystis species (e.g. S. bertrami in horses, S. cruzi in cattle, and S. tenella in sheep), and sarcocysts are routinely found in the muscles of theses species, almost always with little to no associated pathology. Rarely, pathology in the form of myositis can occur. Even less commonly, cattle and sheep may develop eosinophilic myositis, which is thought to be caused by the degeneration of sarcocysts and which provokes a profound – and deadly – eosinophilic myositis. This grossly presents as distinctive, well demarcated areas of green discolouration the muscle.
12.2 Neospora caninum and Toxoplasma gondii
Although more commonly associated with abortion and neurological disease, N. caninum and T. gondii both occasionally present as a disease of muscle. Puppies and kittens tends to be affected most often. The disease manifests as a myositis with lymphoplasmacytic inflammation, myonecrosis, and atrophy.
12.3 Trichinellosis
Trichinellosis is an important parasitic disease of animals and humans. Several species of Trichinella exist. The most common is T. spiralis. The parasite infects a multitude of species, including pigs, dogs, cats, bears, rodents, and many other wild animals. The disease in humans in North America has largely become historical due to routine meat inspection, but does still affect people, and is still present in wild carnivores. Wild animals of the arctic, such as walrus and polar bears, are an important source of food for the Inuit, and represent a particularly important risk of Trichinella. Ingestion of undercooked wild meat is perhaps the most important risk factor for human trichinellosis.
The life cycle of Trichinella spp. is relatively simple. Nematode larva encysted in muscle are consumed, and released by the gastric juices of the host. The liberated larva molt into adults and reproduce. The females the penetrate through the intestinal crypts (the males die), and deposit larva into the lymphatics. The larva travel through the lymphatic system and into the systemic circulation, where they then preferentially encyst into muscles, and await the next cycle. Those that do not encyst in muscle are cleared by the immune system. Through an unknown mechanism, the larva encyst prefentially in the diaphragm, tongue, laryngeal muscles, and masseter muscles.
The pathology caused by Trichinella spp. is relatively mild. A single larva will enter into a single myofiber, where it enlarges and coils. The myofiber undergoes some changes, including enlargement of the nuclei, decrease in number of myofibrils, and an increase in the thickness of the basal lamina, to become what is known as a “nurse cell”. Rarely, lymphoplasmacytic inflammation is present. Larva can live in an encysted form for over 20 years, and T. nativa, the species found in northern animals, resists freezing.
Trichinellosis is zoonotic. In humans, clinical signs and symptosm related to trichinellosis include fever, myalgia, facial edema, rash, and occasionally chronic diarrhea.
12.4 Cysticercosis
The taxonomy and nomenclature of the tapeworms is confusing and frustrating. Adult taenids are classified separately from their intermediate form, the cysticerci. Cysticercosis is the result of consumption of teanid eggs, not the consumption of cysticerci. It is the pathology related to the development of cysticerci in the muscle, and is still an important disease in developing countries.
Like many parasites, the life cycle of tapeworms revolves around a predator-prey dynamic. Predators are the definitive host of the adult tapeworm and are infected through consumption of larval stages in the flesh of the prey. Larva of tapeworms go through several developmental cycles, one of which is a cysticercus, in the intermediate host. In some tapeworm species, the cysticercus has a predilection site for skeletal muscle; it is those species that interest us here, and which are described in Table 12.1.
| Tapeworm | Definitive.host | Cysticercus | Intermediate.host | Predilection.site |
|---|---|---|---|---|
| T. solium | Human | C. cellulosae | Pigs (and humans) | Heart, masseter, tongue |
| T. saginata | Human | C. bovis | Cattle | Heart, masticatory muscles |
| T. ovis | Dogs, wild carnivores | C. ovis | Sheep and goats | Heart, skeletal muscle |
The pathology in intermediate hosts is fairly minimal. Cysticerci form grossly visible cysts (1-2 cm) containing clear fluid and a larva. These may elicit a mild lymphoplasmacytic and eosinophilic inflammatory response, but otherwise do not harm the host. Dead larva become heavily calcified.
Occasionally, cysticerci can infect in an unusual intermediate host and form cysts in devastating anatomic locations. Consider the life cycle of T.solium and C. cellulosae. Adult tapeworms living in the definitive host, humans, pass eggs or proglottids in their feces, which are normally taken up by pigs, in which cysticerci form in muscle. Occasionally, however, humans can also come ingest the eggs, in which case the cysticerci develop not just in muscle, but also in brain and eyes, with devastating consequence. This condition can be devastating and is still prevalent in areas of high poverty and poor sanitation.