Classification: Taxonomic ranks under review (cf. Illustrated Guide to Protozoa, 2000. Allen Press)
Protista (unicellular eukaryotes)
Babesia spp. [these species cause babesiosis (tick fever) in domestic and wild animals]
Parasite morphology: Intraerythrocytic stages appear singly as small round, ovoid or elongate trophozoites (2-4µm), in pairs as pear-shaped (= pyriform, hence piroplasm) merozoites, or in tetrads as cruciform merozoites.
have been detected in most domestic animals (cattle, sheep, goats, horses,
pigs, dogs, cats) and numerous wild animals (over 70 species) and humans.
Three species cause tick fever in cattle, B. bovis (B. argentina),
B. bigemina, and B. divergens. Some parasite species
are not host specific and can be transmitted among different mammals;
some are zoonotic. Infections in humans have been attributed to
B. microti from rodents in North America and B. divergens
from cattle in Europe. All parasite species are limited in their distribution
in accordance with that of their tick vectors.
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,
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.