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1. NAME Babesiosis

Bovine babesiosis also known as piroplasmosis, Texas fever, redwater, tick fever

Equine babesiosis

also known as equine piroplasmosis, biliary fever 2. DEFINITION

Babesiosis is an infectious disease caused by a protozoan- an intraerythrocytic tick-borne parasite that infects a wide variety of vertebrate hosts, typically inducing disease characterized by intravascular hemolysis leading to anemia and other variable clinical signs and lesions. This chapter will deal primarily with bovine babesiosis because, from an economic point of view, cattle are most significantly affected by the disease. Equine, ovine, caprine and porcine babesiosis are discussed briefly.

3. ETIOLOGY

Babesiosis is caused by protozoan organisms of the genus Babesia which was first described by Babés in Romania as a parasite of bovine red blood cells (RBCs). To date, more than 70 Babesia spp. have been recognized based on their morphology, serologic tests, and molecular characteristics. In nature, Babesia spp. are transmitted by vector ticks and only exceptionally by other means. A list of Babesia spp. that affect livestock, their vector ticks, and dimensions is presented in Table 1.

4. HOST RANGE

Although it is possible for a single Babesia species to infect more than one vertebrate host (e.g., B. microti affects rodents and humans; B. divergens and B. bovis affect cattle and humans), Babesia spp. are typically host specific.

Bovine babesiosis, also know as piroplasmosis, Texas fever, redwater, and tick fever is a febrile, tick-borne disease of cattle caused by 1 of at least 7 Babesia spp. It is generally characterized by extensive intravascular hemolysis leading to depression, anemia, icterus, hemoglobinuria, and, in the case of B. bovis infection, neurological signs. The two Babesia spp. of most concern as causes of

disease in cattle are B. bigemina and B. bovis, which are transmitted primarily by Boophilus ticks. B. bovis, B. bigemina, and their vector ticks once occurred in large areas of the U.S. and still occur in Mexico and throughout the tropical and subtropical areas of the Western Hemisphere.

Cattle are the main hosts, but the Asian water buffalo (Bubalus bubalis) and African buffalo (Syncerus caffer) may also become infected. Outbreaks of B. bigemina and B. bovis have been described in domestic water buffaloes in Northern Brazil where this animal is an important livestock species. It is possible that other ungulates are infected, but from a practical point of view such hosts are probably not important as reservoirs of infection.

B. bigemina and B. bovis are widespread in the cattle population and occur wherever Boophilus ticks are encountered, which includes North and South America, parts of Southern Europe, Africa, Asia, and Australia. Babesiosis also occurs in the Caribbean and South Pacific islands. Cattle and the tick hosts provide the major reservoir of infection.

Babesia divergens appears to be a serious pathogen for cattle in the United Kingdom and northern Europe, where it is spread by ticks of the Ixodes genus. The presence of Ixodes ticks in the U.S. suggests the potential for this babesia species to become established in the country. Babesia jakimovi is the cause of Siberian piroplasmosis in cattle. Babesia major affects cattle in the United Kingdom and northern Europe. It is essentially nonpathogenic but can be induced to produce clinical effects and even death by serial passage in splenectomized calves. Babesia ovata and B. occultans have been described in cattle from Japan and southern Africa, respectively. The latter babesia is transmitted by the Hyalomma marginatum rufipes tick.

Babesia caballi, B. equi or both are the cause of equine babesiosis, also known as equine piroplasmosis and biliary fever. Currently some authorities have postulated that B. equi should be reclassified as Theileria equi. Equine babesiosis is widely distributed throughout the tropics and subtropics and to a lesser extent is known to occur in temperate regions.

Babesia motas and B. ovis are reported causes of babesiosis in sheep and, occasionally, in goats. Information on these species is limited, and few serological and cross-immunological studies have been performed to clarify the identity of these intraerythrocytic parasites. They don’t seem to have the same epidemiological importance as the other babesias of domestic animals. More

recently B. sergenti, B. foliata and B. crassa were reported as causing disease in sheep, but these organisms are limited to Algeria, India, and Iran, respectively. Babesia trautmanni and B. perroncitoi infect swine, and, on occasion, are responsible for serious losses following infections. Swine babesiosis has been described in the former Soviet Union, southern Europe, and Africa.

Babesia jakimovi can infect the Tartarean roe deer (Capreolus capreolus), Asian elk (Alces alces), and reindeer (Rangifer tarandus). In Africa, the wild pigs (Potamochoerus porcus) are thought to be reservoirs of Babesia trautmanni and B. perroncitoi.

Babesiosis is a zoonosis, and cases of disease induced by B. microti, B. divergens and B. bovis have been described in humans. At one time these human infections were thought to occur only in splenectomized individuals, or in those who were otherwise immunocompromised. This is not, however, the case regarding the rodent parasite B. microti, which has been reported as causing disease in immunocompetent persons.

5. EPIDEMIOLOGY a. Transmission

The general mode of transmission of Babesia spp. is similar regardless of species, although minor differences exist. The disease is virtually always transmitted by ticks, but, as with most blood diseases, surgical procedures such as dehorning, castration and needle vaccination procedures are sometimes implicated in accidental transfer of blood from one animal to another, thereby transmitting infection.

Table 1 contains information on vector tick species implicated in the transmission of the various Babesia spp. Summarized here is the mode of transmission of B. bigemina and B. bovis. The infection occurs when nymphal and adult (for B. bigemina), and larval (for B. bovis) stages of the vector ticks feed on the host. After the inoculation as sporozoites, the parasite penetrates the RBCs in the definite host blood stream where they form a parasitophorous vacuole and evolve to trophozoites that later undergo binary division, usually forming a pair of merozoites. As merozoites leave the RBCs they cause rupture of their membrane and leakage of hemoglobin into the plasma (hemoglobinemia). Ticks acquire babesia infection during their feeding on infected animals. The infection is then passed on to the ovaries of the ticks, and thus the emerging larvae carry the infection.

Babesia caballi is transmitted by ticks of the genera Dermacentor, Hyalomma, and Rhipicephalus and is passed transovarially from one tick generation to the next. Experimental transmission of B. caballi under laboratory conditions has been reported using Dermacentor nitens and D. variabilis. The widespread prevalence of these ticks (D. albipictus and D. variabilis), plus the presence or past presence of B. caballi in the U.S., creates an unanswered question of why B. caballi has not become more widespread there. Transmission of B. equi appears only to occur transstadially. The vector or vectors of B. equi have not been identified in the Western Hemisphere.

b. Incubation period

The incubation period is 2-3 weeks for natural disease induced by both B. bigemina and B. bovis but can be as short as 4-5 (B. bigemina) and 10-14 (B. bovis) days in experimental inoculation depending on the size of the inoculum. The natural infection caused by B. bovis tends to present a longer incubation time than that caused by B. bigemina.

c. Morbidity

Several factors influence the infection and geographical distribution of bovine babesiosis. In general, the disease follows that of the vector ticks; this fact produces 3 epidemiological situations. In tick-vector free areas the disease does not occur and consequently cattle do not develop natural immunity. However, if tick-bearing and Babesia spp. carrier cattle are brought to these free areas, they can introduce the disease when the timing coincides with favorable weather conditions – thus allowing for the occurrence of a second generation of ticks infected by transovarian transmission. The larvae, nymphs or adults of this second generation will transmit the disease. Conversely, if cattle from free areas are introduced into areas where ticks are enzootic, the disease will occur unless the animals are vaccinated.

In areas of enzootic instability there is an alternation of warm and cold seasons. The cold periods prolong the free-living stages of the ticks, allowing cattle prolonged periods without tick contact. This results in a significant drop in antibodies due to the absence of babesia infection. When the warm periods return, the tick parasite load increases and outbreaks of the disease occur. In areas of enzootic stability (between parallels 32º S and 32º N) weather conditions allow the presence of ticks on cattle all year round. This fact confers high levels of lasting immunological protection to cattle.

Factors influencing the occurrence of babesiosis outbreaks include: (1) overinfestation by vector ticks resulting in a high inoculum of babesia; (2) long

periods without ticks with resultant loss of immunity and vulnerability to infection (this situation might occur due to the prolonged use of acaricides and maintenance of cattle in pastures free of ticks); (3) stress factors and nutritional deficiencies which can induce a drop in immunity and vulnerability to the disease.

Excluding situations related to a drop in acquired immunity, calves are more resistant to infection by Babesia spp. than adult cattle, and zebu breeds are more resistant to tick infestation and babesiosis than European breeds.

d. Mortality

Among fully susceptible older cattle, mortality rates of 5-10% are common, even with treatment, and lethality rates in nontreated cattle can be up 50-100% in the case of B. bovis infection. Among cattle raised in an area of enzootic stability, few, if any, losses occur even though infection takes place. This phenomenon usually reflects early exposure of the neonates when they are more resistant, thus resulting in varying levels of protection.

6. CLINICAL SIGNS a. Cattle

Bovine babesiosis caused by B. bigemina can occur as acute, subacute or chronic disease. In all cases affected cattle develop fever, depression, anorexia, paleness of mucous membranes, and hemoglobinuria. Icterus is also a clinical sign, mainly in the subacute form of the disease, but it can be minimal or absent in cases of peracute or acute disease. Cattle with the chronic form have marked emaciation, drop in milk yield, and abortions. Hematological findings in cattle with acute and subacute babesiosis are typical of hemolytic anemia and include hypochromic macrocytic anemia with excessive regeneration and high numbers of reticulocytes. Plasma of affected cattle has a tan-brown discoloration due to hemoglobinemia.

In many respects, infections of B. bovis resemble those seen with B. bigemina, but there are some characteristic differences. B. bovis tends to induce a more acute and severe disease in which peripheral circulatory disturbances with sequestration of parasitized RBCs in the peripheral circulation are unique features. Another unique feature of the disease caused by B. bovis infection in cattle are neurological disturbances characterized by incoordination, seizures, muscle tremors, opisthotonus, hyperexcitability, agressivity, blindness, head pressing, nystagmus, lateral recumbency with paddling limb movements, and coma. This form of disease is usually fatal.

The hematological findings observed in cattle with acute babesiosis by B. bovis are characterized by an intravascular hemolytic anemia with similar signs of erythroid regeneration as described previously for B. bigemina infection.

b. Horses

The severity of clinical disease in equine babesiois is variable, and in many cases spontaneous recovery may occur following a febrile response without marked hemoglobinuria or anemia. Clinical manifestations are characterized by depression, anorexia, fever, pale mucous membranes, icterus, hepatosplenomegaly and petechial hemorrhage in the mucous membranes. The prevalence of hemoglobinuria is controversial and described as commonly found by some and infrequent by others. Abortions have been also associated with equine babesiosis.

Chronic cases of equine babesiosis are characterized by weight loss, selective appetite and reduced performance. Horses with babesiosis develop normochromic normocytic anemia which becomes progressively more marked as the disease evolves, so the horses which do not die in the acute phase of the disease may become markedly anemic. Thrombocytopenia is another prevalent finding in equine babesiosis; it occurs in recurrent episodes that seem to be related to parasitemic peaks. Findings in blood smears are unremarkable because horses do not release reticulocytes in the blood stream. The plasma of affected horses can be markedly yellow (icterus) or have a tan-brown discoloration due to hemoglobinemia.

7. POST- MORTEM LESIONS