Читать книгу Parasitology - Alan Gunn - Страница 93

Members of this genus are obligate intracellular parasites with a life cycle that involves two hosts – a herbivore intermediate host in which only asexual multiplication occurs and a carnivore definitive host in which sexual reproduction takes place. Most Sarcocystis species are very host‐specific and infect a limited number of closely related intermediate/final hosts (Table 3.3). You will not be surprised to hear that the taxonomy is confused, and one must take care when using the older literature. For example, some animals once thought to harbour just a single Sarcocystis species actually contain several species. In addition, some species are morphologically identical, and others have synonyms (e.g., Sarcocystis cruzi is also known as Sarcocystis bovicanis – a reflection that the intermediate and definitive hosts are cattle and dogs/ other canines respectively).

Table 3.3 Summary of the most important species of Sarcocystis in human and veterinary medicine.

Species of Sarcocystis	Synonym	Intermediate host	Definitive host
Sarcocystis bovicanis	Sarcocystis cruzi	Cattle	Dogs and other canines
Sarcocystis bovihominis	Sarcocystis hominis	Cattle	Humans
Sarcocystis bovifelis	Sarcocystis hirsuta	Cattle	Cats and other felines
Sarcocystis Suihominis	Isospora hominis	Pigs	Humans and some primates
Sarcocystis ovifelis	Sarcocystis tenella	Sheep	Cats and other felines
Sarcocystis hovathi	Sarcocystis gallinarum	Chicken	Dogs and other canines

The life cycle of S. bovicanis is typical of most Sarcocystis species. In this species, the definitive host is a dog or other canine, whilst the intermediate hosts are cows and other bovids (Figure 3.14). The life cycle begins when an infected dog sheds free sporocysts or oocysts in its faeces. A cow must then consume the sporocysts/oocysts, and this usually happens through contamination of food or water. When the sporocyts/oocysts reach the cow’s small intestine, they release the sporozoites. The sporozoites then invade the gut epithelial cells and make their way to the blood vessels. The bloodstream then distributes them around the body. The parasites invade the endothelial cells of the blood vessels that serve many of the body’s organ systems. Within the endothelial cells, the parasites transform into merozoites and undergo four cycles of merogony (asexual reproduction). After each cycle, the newly formed merozoites are released, and these infect new endothelial cells downstream of the initial infection. After the last cycle, the merozoites invade skeletal and cardiac muscle cells. Occasionally, smooth muscle, the brain, and spinal cord are also infected. Within these cells, the merozoites transform into metrocytes or ‘mother cells’ each of which divides asexually to form a structure called a sarcocysts (Figure 3.15). With repeated asexual division, a sarcocyst steadily becomes larger and larger and in some Sarcocystis species may become big enough to be visible to the naked eye. Eventually, the globular metrocytes cease producing new metrocytes and form crescent‐shaped bradyzoites. The time taken for this depends upon the species of Sarcocystis but can be around 2 months. During this time, the sarcocysts are non‐infectious since only bradyzoites can transmit the infection. Completion of the life cycle requires a dog to consume flesh containing the bradyzoites. Digestion of the sarcocyst within the dog’s small intestine releases the bradyzoites, and these become motile. The bradyzoites initially invade the gut epithelial cells and then make their way to the lamina propria region where they transform into either male or female gametes. After gamete fusion, the parasites undergo sporogony to form oocysts that contain two sporocysts. The oocysts are therefore already sporulated when shed and each contains four sporozoites. The oocysts are shed into the lumen of the gut and passed with the faeces. The oocyst has a thin wall and often ruptures when one is preparing faecal samples for microscopy. Consequently, one normally sees sporocysts (16.3 × 10.8 μm) in faecal samples. Sarcocystis seldom causes serious pathology in its definitive hosts.

Figure 3.14 Life cycle of Sarcocystis bovicanis. 1: Digestion of a sarcocyst within the dog’s small intestine, releases bradyzoites that invade the gut epithelial cells and then make their way to the lamina propria region where they transform into either male or female gametes. After gamete fusion, the parasites undergo sporogony to form oocysts that contain two sporocysts. The oocysts are shed into the lumen of the dog’s gut and pass with the faeces. 2: A cow consumes the sporocysts/oocysts, and these release the sporozoites that invade its gut epithelial cells and make their way to the blood vessels. The parasites invade the endothelial cells of the blood vessels, transform into merozoites, and undergo four cycles of merogony. After each cycle, the newly formed merozoites infect new endothelial cells. 3: After the last cycle, the merozoites invade skeletal and cardiac muscle cells and transform into metrocytes, each of which divides asexually to form a sarcocyst. Eventually, the metrocytes cease producing new metrocytes and form bradyzoites. Completion of the life cycle requires a dog to consume flesh containing the bradyzoites. Drawings not to scale.

Figure 3.15 Transverse section through a sarcocysts of Sarcocystis muris in the trachea of a mouse.