Synonyms: Piroplasma equi, Nuttallia equi, Nuttallia asini, Nuttallia minor (?).
Hosts: Horse, mule, donkey, Burchell's zebra (Equus burchelli).
Geographic Distribution: Europe, USSR, Central Asia, North and South Africa, India, South America. This species is more widely distributed than B. caballi.
Morphology: This species is relatively small, being 2 u long. The trophozoites in the erythrocytes are rounded, amoeboid or most often pear-shaped. The last are usually found in a group of 4 joined together in the form of a cross. Because of this, some authorities prefer to use a separate generic name, Nuttallia, for this and similar species.
Life Cycle: Division in the erythrocytes is unlike that of most other species of Babesia in that 4 daughter trophozoites are formed at one time. The vectors are Dermacentor marginatus (syn., D. reticulatus), D. pictus, Hyalomma marginatum (syn., H.detritum), H. uralense and Rhipicephalus bursa in the USSR, H. anatolicum (syn., H. excavatum) and H. marginatum in Greece, H. dromedarii and R. sanguineus in North Africa, R. evertsi in South Africa, and H. marginatum, and R. sanguineus in central Asia. Transmission is thru the egg in H. anatolicum, and stage-to-stage in all the others. Intra-uterine transmission may also occur (Neitz, 1956).
Pathogenesis: This species is more pathogenic than B. caballi. Mixed infections are not rare, however, so that it is sometimes difficult to be sure which species is causing the symptoms. The incubation period following an infective tick bite is 10 to 21 days. The first sign of disease is a rise in temperature. This is followed by listlessness, depression, marked thirst, inappetence, watering of the eyes and swelling of the eyelids. The most characteristic sign is icterus. There is marked anemia, more than half the erythrocytes often being destroyed. Hemoglobinuria is present, but in contrast to B. caballi infections, posterior paralysis is absent. Edema of the head, legs, and ventral part of the body is sometimes present. Affected animals are constipated, passing small, hard balls of feces covered with yellow mucus; they lose condition fairly rapidly, and may become extremely emaciated. Hemorrhages are present on the mucous membranes of the nasal passages, vagina and third eyelid.
The disease usually lasts 7 to 12 days, but it may be peracute, with death occurring in 1 to 2 days, or it may be chronic and last for weeks. The mortality is generally not more than 10%, but may sometimes reach 50%. Recovery is slow, and it may be several weeks or even months before the animal returns to normal.
At necropsy, emaciation, icterus, anemia and edema are present. There are accumulations of fluid in the pericardial sac and body cavities, and the fat is gelatinous and yellow. The spleen is enlarged, with soft, dark brown pulp. The lymph nodes are swollen and sometimes inflamed. The liver is swollen, engorged, and brownish yellow; the hepatic lobules are yellow in the center and greenish yellow around the edges. The kidneys are pale yellow and may contain petechial hemorrhages. There are hemorrhages or red streaks on the mucosa of the intestine and stomach.
Immunity: There is no cross-immunity between B. equi and B. caballi. Young animals are less seriously affected than adults.
Diagnosis: Babesiosis can be diagnosed by identifying the parasites in stained blood smears. Examinations should be made as early as possible, since the parasites begin to disappear from the peripheral blood after the fifth day.
Treatment: Trypan blue is ineffective against B. equi. Acriflavine has been recommended; it is injected intravenously, 10 ml of a 2% aqueous solution being given per 100 kg body weight.