Important hemoprotozoan diseases of livestock: Challenges in current diagnostics and therapeutics: An update

Overview:

Babesiosis is caused by intraerythrocytic protozoan parasites of the genus Babesia. Transmitted by ticks, babesiosis affects a wide range of domestic and wild animals and occasionally people. Although the major economic impact of babesiosis is on the cattle industry, infections in other domestic animals, including horses, sheep, goats, pigs, and dogs, assume varying degrees of importance throughout the world.
Two important species in cattle
1. B bigemina 
2. B Bovis
are widespread in tropical and subtropical areas and are the focus of this discussion. However, because there are many common features of the diseases caused by different Babesia, much of this information can be applied to other species.

Transmission and Epidemiology:

The main vectors of B bigemina and B bovis are 1-host Rhipicephalus (Boophilus) spp ticks, in which transmission occurs transovarially. Although the parasites can be readily transmitted experimentally by blood inoculation, mechanical transmission by insects or during surgical procedures has no practical significance. The intrauterine infection has also been reported but is rare.
Transmission to the host occurs when larvae (in the case of B bovis) or nymphs and adults (in the case of B bigemina) feed. The percentage of larvae infected can vary from 0–50% or higher, depending mainly on the level of parasitemia of the host at the time the female ticks engorge. Under field conditions, the rate of tick transmission is generally higher for B bigemina than for B bovis.
In endemic areas, three features are important in determining the risk of clinical disease:
calves have a degree of immunity (related both to colostral-derived antibodies and to age-specific factors) that persists for ~6 mo,
Animals that recover from Babesia infections are generally immune for their commercial life (4 yr)
The susceptibility of cattle breeds to ticks and Babesia infections varies; eg, Bos indicus cattle tend to be more resistant to ticks and the effects of B bovis and bigemina infection than Bos taurus–derived breeds. At high levels of tick transmission, virtually all calves become infected with Babesia by 6 mo of age, show few if any clinical signs, and subsequently become immune. This situation can be upset by either a natural (eg, climatic) or artificial (eg, acaricide treatment or changing breed composition of the herd) reduction in tick numbers to levels such that tick transmission of Babesia to calves is insufficient to ensure all are infected during this critical early period. Other circumstances that can lead to clinical outbreaks include the introduction of susceptible cattle to endemic areas and the incursion of Babesia-infected ticks into previously tick-free areas. Strain variation in immunity has been demonstrated but is probably not of practical significance in the field.

Pathogenesis and Clinical signs:

B bovis is a much more virulent organism than B bigemina. With most strains of B bigemina, the pathogenic effects relate more directly to erythrocyte destruction. With virulent strains of B Bovis, a hypotensive shock syndrome, combined with generalized nonspecific inflammation, coagulation disturbances, and erythrocytic stasis in capillaries, contribute to the pathogenesis.
1. The acute disease generally runs a course of ~1 wk. 
2. The first sign is fever (frequently ≥106°F [41°C]), which persists throughout, and is accompanied 3. later by inappetence,
4. Increased respiratory rate
5. Muscle tremors
6. Anaemia
7. Jaundice, and weight loss; 
8. Hemoglobinemia and hemoglobinuria occur in the final stages. 
9. CNS involvement due to adhesion of parasitized erythrocytes in brain capillaries can occur with B bovis infections. 
10. Either constipation or diarrhoea may be present.
11. Late-term pregnant cows may abort, and temporary infertility due to transient fever may be seen in bulls.
12. Animals that recover from the acute disease remain infected for many years with B bovis and for a few months in the case of B bigemina. No clinical signs are apparent during this carrier state.

Diagnosis:

Therefore, confirmation of diagnosis by microscopic examination of Giemsa-stained blood or organ smears is essential. From the live animal, thick and thin blood smears should be prepared, preferably from capillaries in the ear or tail tip. 

The most commonly used are the indirect fluorescent antibody test and ELISA. A commercially produced ELISA for B bigemina is available.  

PCR and real-time PCR assays capable of detecting extremely low parasitemias, as occur in carrier animals, and differentiating isolates have also been described. A procedure that may occasionally be justified to confirm infection in suspected carrier animals is the subinoculation of blood (~500 mL) into a fully susceptible animal, preferably a splenectomized calf, and subsequent monitoring of the recipient for infection.

Treatment:

Drug of choice for Babesiosis Diminazene aceturate and imidocarb dipropionate.
Supportive treatment is advisable, particularly for invaluable animals, and may include the use of anti-inflammatory drugs, corticosteroids, and fluid therapy. Blood transfusions may be life-saving in very anaemic animals.

Control:

Vaccination using live, attenuated strains of the parasites has been used successfully in several countries. Although controlling or complete eradication of the tick vector can break the transmission cycle, this approach is rarely feasible in the long term and can lead to large, susceptible populations in endemic areas with a consequent risk of outbreaks of disease in naive animals.

Zoonotic Risk:

Several cases of human babesiosis have been reported.