Parasitology - Locomotor Parasites

Question 1

What are the main parasites of the locomotor system?

Answer 1

Sarcocystis spp (protozoa) - they form cysts (bradyzoite stage) in muscle

Trichinella (nematode)

Taenia (cestode) - this is considered also as a “nervous-system” parasite

Question 2

What are the main features of Sarcocystis spp? What type of parasite are they and what is special about its life cycle?

Answer 2

Sarcocystis spp., like Toxoplasma, is a cyst-forming coccidia in the protozoa parasite group.

It is OBLIGATELY HETEROXENOUS: It needs an intermediate host to complete its life cycle.

NB: Sarcocystis spp are host-specific for BOTH final hosts and intermediate hosts.

Question 3

What are the final hosts of Sarcocystis spp and what are the intermediate hosts?

Answer 3

Final hosts are carnivores and omnivores, including humans. Intermediate hosts are herbivores or omnivores.

Sarcocystis spp are host-specific for BOTH final hosts and intermediate hosts.

Question 4

Only gametogony & sporogony occur in the GIT epithelial cells of the final host with Sarcocystis. This means the merozoites become microgametes or macrogametes, and the oocyte becomes sporulated with two sporocysts.

In the intermediate host, tachyzoites enter circulation from meronts bursting in mesenteric lymph nodes or in endothelial cells.

What happens in the environment?

Answer 4

The sporulated oocyst becomes INFECTIVE in the environment, with the two sporocyts each developing two sporozoites, so there are four sporozoites per oocyst. The intermediate host eats this infective oocyst.

Question 5

How does the Sarcocystis spp life cycle differ from the Toxoplasma life cycle?

Answer 5

Toxoplasma:

Since Toxoplasma is facultatively heterexonous, cysts can develop in final-host tissue as it can in intermediate-host tissue.

Sporulation of oocyst occurs in the environment, not in the final host.

Basically, all the phases of the life cycle of Toxoplasma that happen in the intermediate host (tachyzoite infection, bradyzoite-filled cyst development) can occur in the final host.

Sarcocystis spp:

Since Sarcocystis is obligatively heteroxenous, schizongony and asexual replication of merozoites in to tachyzoites and bradyzoites & cyst-formation MUST occur in the intermediate host. After the final host ingests the tissue cysts, the bradyzoites become activated, invade the GIT epithelial cells where they turn into microgametes or macrogametes.

Sporulation of the oocyst - ie., the development of sporocytsts - takes place inside the final host, so that sporulated oocysts end up in faeces.

Question 6

How are the tissue cysts of Sarcocystis spp different from those of Toxoplasma?

Answer 6

Sarcocystis spp cysts are extremely large and can be seen by the naked eye - 0.5 mm to 15 mm.

They are compartmentalised by septa.

They have thick walls.

They are mostly found in muscle tissue.

******************

Toxoplasma cysts are small, about the size of strongyle eggs – 50-100 µm.

They are NOT compartmentalised.

They have very thin walls.

They are found mostly in the heart & nerves (CNS), and less in muscle tissue.

Question 7

What are the intermediate hosts and the final hosts in Sarcocystis cruzi?

Answer 7

Intermediate host is CATTLE.

Final host is DOG.

Question 8

What are the final and intermediate hosts of Sarcocystis neurona?

Answer 8

IH: Horse (aberrant, dead-end host); the normal IH are skunks or armadillos

FH: Oppossum

Horses end up getting: Equine protozoan myeloencephalitis (EPM)

Question 9

How do Sarcocystis cysts persist in meat?

Answer 9

They are easily destroyed by heating but can survive low temperatures above freezing:

Storage at 4-10°C: remains infectious

Freezing at -25°C: after 3 days killed

Heating >65°C: destroyed after 10 minutes
Smoking, salting or drying: ??? unknown effectiveness

Question 10

What are the intermediate and final hosts in Sarcocystis gigantea?

Answer 10

IH: Sheep (about 10% have it)

FH: cats

Question 11

What are the most pathogenic Sarcocystis spp for the intermediate host?

Sarcocystis spp in which the FINAL HOSTS are:

A. Cats

B. Dogs

C. Humans

Answer 11

B. Dogs

Question 12

What parasite group does Taenia belong to, and why is it considered a parasite of locomotion?

Answer 12

Taenia is a member of the cestode family. Its metacestode stage – the stage during which it develops in the intermediate host – is the cysticercus. See illustration.

The cysticercus of Taenia species are found in all different types of muscle tissue and can cause inspection losses.

Question 13

Where do the adult Taenia reside?

Answer 13

The adult tapeworm with scolex and proglottids resides in the small intestine of the final host. In the case of Taenia saginata and Taenia solium, the final host is the human.

Question 14

What are the intermediate and final hosts of Taenia saginata?

Answer 14

The final host is the human.

The intermediate host is cattle.

Question 15

What are the intermediate and final hosts of Taenia solium?

Answer 15

The intermediate host is the pig and the final host is the human.

Question 16

Which of the following Taenia species has an UNARMED scolex?

Taenia solium

Taenia saginata

Answer 16

T. saginata, which uses cattle as its intermediate host, has an unarmed scolex.

T. solium, which uses the pig as IH, has an armed scolex.

Question 17

What is the difference between Taenia species and Dipylidium tapeworm in terms of:

Shape of gravid segments

How eggs are dispersed

Intermediate hosts

Final hosts

Control

Answer 17

Shape of gravid segments:

Taenia’s are rectangular

Dipylidium are oval

How eggs are dispersed:

Taenia release single eggs when proglottid disintegrates so there’s no need for a uterine pore

Dipylidium eggs are dropped in packets along with flea eggs

Hosts:

Taenia’s intermediate hosts ruminants and rodents; final hosts are humans, dogs & cats

Dipylidium’s IH are FLEAS; final hosts are dogs or cats

**Control: **

Taenia control is anti-cestode: praziquantel, pyrantel

Dipylidium control is anti-flea (insecticides): imadocloprid, fipronil

Question 18

What is the commonest form of Taenia spp in the UK? What are the intermediate and final hosts? Where are the cysts found?

Answer 18

Taenia hydatigena

IH: Sheep

FH: Dog

The large, egg-sized cysticercus larvae are found in the peritoneum, mainly the omentum.

Question 19

What is the cause of “true gid” or “staggers gid”? What species is affected?

Answer 19

Taenia multiceps

Intermediate host is sheep: coenurus larvae develop in brain/CNS; Endogenous devt: oncosphere distributed via blood → CNS (neurotropism) → fertile w/i 3 months

Coenurus are worse than cysticercus because the former have about 200 protoscolices inside while the cysticercus only has one. See illustration.

Final host is the dog.

Question 20

How does the metacestode form of Taenia taeniaeformis differ from other Taenia species? What is are its final and intermediate hosts?

Answer 20

The metacestode phase is a strobilocercus, which is a pea- sized cyst comprised of a scolex joined by chain of segments to a fluid-filled cyst. See illustration.

T. saginata & T. solium have cysticercus metacestodes

T. multiceps & T. serialis have coenurus mestacestodes.

The final host of T. taeniaeformis is the cat and the intermediat host is the mouse.

Question 21

Discuss the veterinary significance of canine and feline Taenia spp in the UK.

Answer 21

Biotic potential:

A 20K-100K eggs per proglottid; up to 50 proglottids can be shed per day, causing widespread contamination of environment with highly resistant taeniid eggs

• *Spread**:
• Vectors such as birds, flies and mice and climate (wind, rain) can carry eggs or proglottids further

Longevity:

• Metacestodes can last for 3-5 months in the IH muscle and can thus have greater chance of infecting carnivore/omnivore final host when ingested.
• *Reinfection:**
• Cats and dogs can always become re-infected if they have little or no immunity.

Question 22

Why is Trichinella called the worm or parasite that thinks it’s a virus?

Answer 22

Trichinella is a nematode of the Trichinelloidea superfamily. It is non-bursate.

It is known as a worm that thinks it’s a virus because it INVADES HOST CELLS.

Especially important is the ZOONOTIC SPECIES,

Trichinella spiralis

Question 23

What is the common morphological feature of the adult Trichinella nematode?

Answer 23

The oesophagus is a column of doughnut-shaped cells.

Question 24

What is the domestic-animal host of Trichinella spiralis, and why is it considered a very important zoonotic?

Answer 24

Trichinella spiralis is found especially in free-range pigs. Pigs are asymptomatic.

However, Trichinella actually has a very wide host range and can be found in wild boar & horse. People end up eating these meats, undercooked, so they end up ingesting the larvae.

Question 25

What is different about the Trichinella life cycle compared with other nematodes?

Answer 25

They are viviparous! No oocyts!

• born LIVE in intestine
  • Invasion of epithelial cells by L1
  • Fusion of epithelial cells into a syncythium
• Migration via bloodstream to muscle, where they become encapsulated.
  • Four moults and development into adults
  • After 5-6 days: newborn larvae (viviparous)

• Production of 200-1500 larvae/female

The encapsulate larvae in the muscle is like a cyst that is infective to the next host.

Question 26

Where are Trichinella spiralis encapsulated larvae (nurse cells) usually found?

Answer 26

Tongue, diaphragm, masseter

Question 27

What are the clinical signs of Trichinellosis in humans?

Answer 27

ACUTE: myositis, myocarditis, facial oedema, encephalitis

DEATH: in coma 4th - 8th week

CHRONIC: severe muscle pains for 7 - 8 months


Question 28

What role does the sylvatic cycle play in the life cycle of Trichinella species?

Answer 28

The worm is transmitted among rats that feed raw, infested meat and ascends food-chain up to carnivores & omnivores (foxes) and some herbivores that accidentally eat contaminated feed. Some Trichinella species infect arctic marine mammals and polar bears (highly infective for humans).

Among rodents in the domestic and sylvatic cycle, there can be recycling of trichinella as the rodent hosts cannibalise each other or are eaten by omnivorous scavengers such as foxes (sylvatic) or pigs (domestic).

Question 29

What are the diagnostic methods used for detecting trichinellosis?

Answer 29

Diagnostic methods:

1. Trichinoscope: Microscopy of diaphragm muscle; being phased out as it’s too labour-intensive
2. Digestion: Acid-pepsin digestion of pooled samples (method of choice) - acid-pepsin digests away meat but not the parasite; still quite labour-intensive and costly
3. ELISA: automated, much cheaper than trichinoscope and digestion but used in epidemiological surveys only because it’s still too expensive for meat inspection. Cost: 0.12 – 2.5 euro per pig (250,000 pigs slaughtered per annum)