H. Canis

• Also reported from jackal, hyena, palm civet and cat.

Morphology

• Schizoints occur in the endothelial cells of spleen, liver and bone marrow. They are round or oval bodies, more or less filling the host cell and containing 30-40 nuclei.
• Gamonts are found in circulatory neutrophilis, ellipsoidal in shape about 11×4 µm, and are enveloped in a thick membrane.
• Schizoits include micromerozoites with defined nuclei, which in cross-section have ‘wheel-spoke’ appearance.

Fig: Wheel-spoke appearance of cs of schizonts including numerous micromerozoites

Life-cycle

• Life-cycle involve two hosts: Tick is final host in which syngamy occurs and dog is intermediate host in which asexual reproduction occurs.
• Dogs acquire infection when dogs ingest infected nymphs and adults of tick containing sporocyst in haemocoel.
• Sporozoites are liberated in intestine of dogs. Sporozoites penetrate intestinal wall and reaches spleen liver and bone marrow via blood stream.
• There they develop to schizonts. Two types of schizonts occurs.
• One produces few merozoites of large size (usually three and other produces large number of small merozoites, which later enter polymorphonuclear leukocytes.
• Merozoites forms gamonts in leukocytes which are elongate or rectangular bodies of size 8-12 µm x 3-6 µm surrounded by a delicate capsule. In citrated blood, these forms may be found free in plasma.
• Gaments in leukocytes are then ingested by R. sanguineus when feeds on infected dog.
• Gamonts are released in the gut of tick and undergoes sexual reproduction . Zygotes formed which are motile ookinetes.
• Penetrate intestinal wall of tick and enter haemocoel which grows to oocyst, with sporoblast and sporocyst.
• Each sporocyst produces 16 banana-shaped sporozoites . On ingestion of ticks, dog becomes infected.

Clinical signs

• Irregular fever
• Anaemia
• Loss of weigh
• Progressive emaciation
• Enlargement of spleen and lumbar paralysis also occurs
• Death occurs after 4-8 weeks of onset of clinical signs.

Diagnosis

• Based on clinical signs
• Based on PM findings: Enlargement of spleen, demonstration of parasite in myocardium of domestic cats.
• Demonstration of gamonts in blood smear.
• Demonstration of schizonts in spleen and bone marrow.

Treatment

• No treatment available

Prevention and control

• Control of ticks by regular grooming or use of ectoparasiticide.

Other species

a. H. americanum

• Gamonts present within neutrophils are ellipsoidal in shape, 8.8 x 3.9 µm with central compact nucleus and enveloped in a thick membrane.
• Cytoplasm stains pale blue and nucleus dark-reddish with Giemsa stain.
• Cyst in muscle are round to oval ; 250-500 µm in diameter, with outer portion composed of concentric layers of fine, pale staining laminar membranes, that give cyst ‘onion skin’ appearance.

b.  H. muris

• Meronts in liver are 10-30 µm in diameter.
• Gamonts in lymphocytes appears as elongated oval bodies in stained blood smears.