Pathogenesis: Infected animals develop a high persistent fever becoming dull, listless and anorexic. Parasites cause extensive intravascular haemolysis (erythrocyte rupture) producing progressive signs of anaemia. Erythrocyte destruction may be as high as 75% in fatal cases and even milder infections produce severe anaemia. Haemoglobin clearance mechanisms become overloaded, resulting in jaundice and haemoglobinuria (red discolouration of the urine, ‘red water’ in bovine babesiosis). Haemolysis involves the release of many pharmacologically active agents (e.g. proteolytic enzymes) which affect microcirculation (vasodilatation, increased permeability) leading to hypotension and oedema, and affect blood (viscosity, coagulation, cytoadherence) leading to ischaemia (congestion and degenerative changes in tissues/organs). Infected animals may exhibit diarrhoea, abortion if pregnant, cerebral signs, muscle tremors, wasting, coma and death. Chronically infected animals remain weak, thin and out of condition for several weeks before recovery. Animals that recover are usually immune for life, sometimes thought to be due to complete cure (sterile immunity) but more often associated with the persistence of small numbers of parasites (premunitive immunity). There is an inverse age-resistance to infection and disease, with young cattle being less susceptible than older cattle. There is also a genetic component to resistance, with Bos taurus cattle being more susceptible than Bos indicus (zebu) cattle. Infections in humans have proven severe and fatal in asplenic individuals, with symptoms appearing 10-20 days after tick bite and presenting as a fulminant febrile haemolytic disease, characterized by general malaise, then fever, shaking chills, sweating, arthralgias, myalgias, fatigue, weakness, occasional hepatosplenomegaly, and jaundice.

Mode of transmission: Infections are transmitted by ixodid (hard-bodied) ticks which may be one-, two- or three-host ticks. Ingested parasites develop into large motile vermicules which migrate through the body of the tick and then undergo sporogony. Parasites undergo trans-stadial transmission, whereby infections persist during metamorphosis from larvae to nymphs to adults. They also undergo transovarian transmission, where parasites infect developing eggs in engorged female ticks, so nearly all the progeny are born already infected. Eventually, hundreds of small pyriform bodies (sporozoites) are formed within salivary cells and are injected into mammalian hosts during feeding.

Differential diagnosis: Infections are conventionally diagnosed by the detection of intraerythrocytic stages in smears of peripheral blood stained with any of the Romanowsky’s stains, notably Giemsa. However, once the acute febrile phase has passed, parasites may be difficult to find as they are rapidly removed from the circulation. Recourse has therefore been made to immunoserological tests to detect specific host antibodies against the parasites. Molecular biological techniques have also been developed to parasite DNA following the polymerase chain reaction (PCR) amplification of specific gene sequences.

Treatment and control: Timely chemotherapy is generally effective, although the small virulent species (such as B. bovis) are usually more difficult to treat than other less aggressive species. One of the first successful treatments for bovine babesiosis was the azonaphthalene dye, trypan blue, but it was not very effective against B. bovis. The most commonly used compounds are the diamidines (diminazene diaceturate, imidocarb, amicarbalide), and quinuronium and acridine derivatives. Macrolide antibiotics (clindamycin) and tetracyclines (oxytetracycline, chlortetracycline) have shown variable effects against human infections. Treatment can facilitate recovery, leaving latent infections or complete cure. However, elimination of all parasites may also eliminate premunitive immunity. Because young animals in endemic areas develop infection-immunity (premunition), this has been exploited for immunological control either through premunization/chemoimmunization (infect animals then treat them) or vaccination using whole parasites (attenuated strains) or subcellular (subunit) fractions. Several commercially available preparations have proven effective in regional areas, but a universal vaccine is not yet available. Prevention strategies involving tick control programmes have been relatively effective in several countries in controlling or eliminating infections in domestic stock.

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