Piroplasms are parasitic apicomplexan protozoa which inhabit erythrocytes, and sometimes other cells of vertebrates, but do not form pigment from haemoglobin. They were the first infectious agent shown to be transmitted by an arthropod vector (1893) and are largely transmitted by haematophagus Ixodid ticks which may be single or multiple host feeders. All piroplasms are small and round or pear-shaped (erythrocyte forms) and are parasitic on fish, amphibians, birds and mammals.
The classification of the piroplasms like many groups of protozoa have been changed repeatedly as more is learnt about their structure and natural history. The single order Piroplasmidae is divided into 4 families of which Babesiidae and the Theileriidae contain species of major veterinary importance.
| Phylum | Apicomplexa |
| Class | Sporozoea |
| Order | Piroplasmorida |
| Families (4) | Babesiidae* Theileriidae** (Anthemosomatidae) (Haemohormidiiae) |
| Genera | Babesia* (111 species) Echinozoon* (1 species) Theileria** (39 species) |
Vaccines - A living vaccine against babesia has been in use for over 60 years initially using the "infect and treat" system where animals are infected with virulent organisms and the resulting disease controlled by chemotherapy. Since the mid 60's, particularly in Australia, an attenuated vaccine has been in use where the parasites (Ka strain) are attenuated by rapid syring passage through splenetomised calves. Both of these methods provide effective long term protection (> 4 years) although there are a number of disadvantages: a] Live vaccines require carefully controlled conditions for storage and distribution; b] Host components are present in the vaccine which pose a potential risk for autoimmune disease; c] Transmission of other contaminating cattle diseases is possible; and d] Selection occurs of babesia strains which posess antigens not present or immunodominant in the vaccine strain. In addition, immunisation against a single species or strain is of little used in tropical areas since it would be more cost effective to use acaricides which would reduce all tick born diseases.
Although Theileria infections are found world-wide the most serious and pathogenic species are confined to tropical and semi-tropical areas. This parasite is not transmitted tranovarially but it is transmitted by ticks which feed on more than one host during their life cycle.
The infective sporozoites which are injected with saliva when the tick feeds and rapidly infect mononuclear cells; lymphocytes (largely BoT4 and BoT8 and B cells) and monocytes and
induce a leukemia like proliferation of these cells. After internalisation of the parasite release of rhoptry and microneme contents is quickly followed by dissolution of the vacuole membrane and escape of the parasite into the host cell cytoplasm. In has been shown that parasite entry in T. parva can be blocked by antibodies against either MHC I or a 67kDa sporozoite glycoprotein. Within 72 hours the sporozoite enlarges to a 2µm pre-schizont and begins division. Theileria are unique in causing the proliferation of the host cell resulting in clonal expansion of the infected cell. The exact mechanism of this "reversible lymphocyte tansformation" is unknown. Initially the parasite divides synchronously with the host cell resulting in huge numbers of infected cells in the draining lymph nodes. This rapid clonal expansion results in the colonisation of all lymphoid tissue in a relatively short period of time (< 2 weeks). The
multiple nuclei of the initial macroschizont subsequently undergo rapid division followed by
development of unicellular merozoites (microschizont stage) which are released and infect red cells. The merozoite are pear shaped (1-2µm) and enter red cells where they undergo schizogny to produce merozoites which maintain an erythrocytic cycle. Levels of
parasitemia in red blood cells is generally low and may be difficult to detect.
Infected red blood cells lyse releasing the parasite which transforms into a ray-body (microgamonts) within 2-4 days leading to the formation of up to four gamete-like stages. Following syngamy a spherical zygote is formed which subsequently develops into a motile kinete which penetrates the gut wall and enters the salivary glands (only during redevelopment following a moult). The sporozoites develop within the acinar cells of the salivary gland and over 100,000 sporozoites may be present per cell.
Theilerosis is largely controlled by the use of acaricides usually applied to cattle by dipping or spraying. To be effective this approach requires consistent and regular treatment and tick resistance to insecticides is a problem, as is the environmental impact of organophosphates and synthetic pyrethroids as acaricides.
Chemotherapy is a only effective if used at an early stage of infection since none of the drugs are effective once respiratory symptoms have developed. The two most common drugs used are pavoquine and buparvoquine. Although treated cattle are usually immune some individuals become carriers.
Vaccines - Immunisation against theileria was first described by Theiler in the early part of this century using a suspension of lymphoid cells from infected animals. During the 60's continuous schizont infected lymphoblastoid lines were established which provided a source of attenuated organisms. This type of vaccine has been sucessfully used against T. annulata in China, Israel, Iran, Iraq, Morocco, Russia and Turkey. However, the only sucessful vaccine for T. para is based on the infect and treat system which was first demonstrated for this species in the 1950's. This approach uses inoculation of sporozoites extracted from infected ticks followed by the use of long acting tetracyclins.
Babesiosis and Theileriosis are both diagnosed largely on clinical signs and symptoms with
confirmation by parasitological observation of the parasites in the red cell or mononuclear cells.
Examination of stained blood films shows organisms within red cells, usually in pairs but
occasionally singly or rarely in fours (Maltese cross). Babesia species are divided into large
species (2.5 - 5µm) and small species (1.0 - 2.5µm). In Theileriosis levels of parasitemia in red
blood cells may be very low, particularly late in infection and parasitological diagnosis is usually made from stained smears of lymph node biopsies.
Macroschizont - intracellular stage of theileria undergoing schizogony (splitting).
Theileria
Diagnosis and Identification of Piroplasms
Piroplasm Glossary
Microschizont - intracellular stage in theileria undergoing merogony to produce merozoites.
Sporozoite - final uninucleate product of sporogony which is inoculated to the mammalian host.
Ookinete - the motile stages which develop from zygotes and enter the haemolymph before finding their way to the salivary glands.
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©David Humber 1996 - Last Modified: Tuesday, February 27, 1996 at 09:23 PM
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