A copy of this Appendix has been sent to the International Commission on Zoological Nomenclature.
The Oxford dictionary defines nomenclature as the terminology of a science. The brief of this committee was to consider the terminology being used by those working on theileriosis in Africa so as to achieve consistency among different laboratories. This should help communication between laboratories and veterinary services in different countries and provide consistency in scientific publication. Scientists working on theileriosis have discussed nomenclature over the years and believe that the new techniques and types of study provide the opportunity for a much tighter nomenclature than has been available previously.
The following recommendations have been assembled for further discussion with appropriate authorities on theileriosis and the International Commission on Zoological Nomenclature.
I. Theileria Species
There does not appear to be a problem in classifying Theileria species. The following species infecting cattle have been recognized in Africa:
There may be one or two more species infective to cattle that have yet to be described and further work is required to characterize T. orientalis fully.
Theileria taurotragi is an important parasite with a wide host range and a high prevalence in countries in the East African region. Serological tests may not be specific for a particular Theileria species because of shared antigens. Serological cross-reactions have been shown between T. parva and T. annulata, and T. parva and T. taurotragi. A panel of monoclonal antibodies against Theileria antigens are useful for differentiating species, as are DNA characteristics and probes. New diagnostic reagents would be very useful for differentiating T. parva from T. taurotragi.
II. Sub-Species of Theileria parva
The sub-speciation of T. parva is an area in which change is urgently required. Recent results suggest that the sub-speciation of Theileria parva into T. p. parva, T. p. lawrencei and T. p. bovis, which was adopted for convenience rather than scientific accuracy, has no biological validity. It has been demonstrated that genetically these sub-species are not distinguishable. In one study it has been shown that the T. p. bovis parasite may be more closely related to one particular T. p. parva isolate than to other T. p. bovis isolates. In addition, it has been possible to transform T. p. lawrencei so that it behaves like T. p. parva in cattle. Further work is required to establish whether this change is due to a transformation in the behaviour of T. p. lawrencei in a different host or to selection of a sub-population of T. p. lawrencei that behaves like T. p. parva. Because many people working on Theileria adopted the trinomial system due to its convenience, this system became commonly used. It is recommended that the trinomial system be dropped. Descriptive terms, based on existing usage, could still be acceptable, such as "cattle-derived" and "buffalo-derived". This recommendation is believed to conform to international standards of nomenclature, but it is recognized that more discussion is needed to establish an acceptable system of suffixes to replace the present one, while retaining the convenience of its well understood shorthand.
III. Terminology Recommended for Handling and Characterizing Theileria Parasites
A. Isolate. Viable organisms, isolated on a single occasion from a field sample, in experimental hosts or culture systems, or prepared as a stabilate.
B. Stock. All the populations of a parasite derived from an isolate without any implication of homogeneity or characterization. Populations comprising a single stock thus include cell lines and tick stabilates and subsequent parasite preparations derived from them.
C. Line. A laboratory derivative of a stock maintained under defined physical conditions, such as a culture of parasitized bovine lymphoid cells.
D. Strain. A population of homogeneous organisms possessing a set of defined characteristics. Unambiguous characterization of a strain can be assured only if the population of organisms was initiated from a parasite clone.
E. Stabilate. A sample of organisms preserved alive (usually in replicate) on a single occasion.
F. Clone. Genetically identical organisms derived from a single cell by asexual division.
G. Parasite clone. Theileria line derived from a single parasite.
H. Cell clone. Theileria line derived from a single parasitized cell.
IV. Classification of Stabilates
It is important to get away from the system of classifying stocks by geographical areas of origin. Instead, the nature of the stock, established using an increasing number of tests for their characterization, should be emphasized. Because a parasite stock is isolated from a certain geographical area, such as the Kenya coast, does not mean that the stock is different from those isolated in other parts of Kenya or in other countries.
Once adequate in vitro characterization techniques have been developed and validated, it will be possible to advise those responsible for national or international disease control on the safety of particular stocks for use in a country. The removal of geographical designations will assist this process. This has already been done for trypanosomes and malaria. This information can be easily included in the database concerning the stabilate.
The stabilate reference should contain data on the following:
A. Species of Theileria
For this purpose, Theileria should be abbreviated to Th. to distinguish it from Trypanosoma. Single-letter suffixes will denote species, for example, Th.p., Th.t., Th.m., Th.a., Th.v. and Th.o.
B. The Laboratory that Isolates a Stock
The second identifier would be an abbreviation of the laboratory that isolates the stock. In Kenya, for example, three institutes may be producing stabilates: the National Veterinary Research Centre (NVRC), Muguga; NVRC, Kabete; and ILRAD. These would be abbreviated:
Hence, Th.t. IL would be used for an ILRAD T. taurotragi stabilate.
C. Heterogeneity of Stabilate
If there is evidence of a mixed Theileria species infection in the stabilate, such as T. parva and T. taurotragi at ILRAD, it should be identified as Th.t/Th.p. IL. If a T. taurotragi stabilate at NVRC, Kabete, was contaminated with Erhlichia bovis, it should be identified as Th.t./E.b. KA.
D. Reference to Stabilate
If a parasite that we now refer to as T. p. parva (Marikebuni) was the first stock isolated by ILRAD, it would be referred to as '1', for example, Th.p. IL 1. The first stabilate prepared from this stock would be referred to as Th.p. IL 1.1. It may be a reference stock and subsequent larger working stabilates would be referred to as Th.p. IL 1.2, and so on. If Th.p. IL 1.2 was cloned, it would then become Th.p. IL 1.2.1, and so on.
It is suggested that the use of the term "Muguga cocktail" be discouraged and be replaced by a trinomial designation of its three components. The removal of all references to the geographical origins of stabilates as mentioned above could bring the added benefit of facilitating the transfer of stabilates from one country to another. However, the movement of these stabilates must be with the authority of the governments involved and within the regulations on the movement of biologicals.
IV. Cloning Definitions
The following comments are included for consideration, but final recommendations must be made by a competent body of experts. This can be done only when studies have established which stages of the parasite are diploid and which haploid.
Cloning Theileria parasites is complicated because it is not known when meiosis occurs. Sexual reproduction occurs when micro- and macrogametes arising from piroplasms in erythrocytes fuse in the tick gut. Taking the example of Plasmodium, the parasite would continue through zygote and kinete stages as a diploid organism and reduction division would occur during sporogony, producing haploid sporozoites. Alternatively, but less likely, the sporozoite and schizont could remain diploid and reduction division might occur during merogony. Yet another possibility is that reduction division may occur at the zygote stage. Until this is known, care must be taken using the term "clone", remembering that a clone divides asexually and that the occurrence of meiosis may invalidate the genetic identity of a clone.
The possibilities for cloning at present are (1) inoculation of ticks with a single kinete, (2) selection of salivary glands with one infected acinus that by chance would be a product of one kinete, (3) the infection of cells with one sporozoite and (4) infected cell cloning. An ideal clone would be derived from a single kinete, a single sporozoite or a single schizont-infected cell. Any two of these methods used together are probably acceptable. The identification and stability of a clone can be checked by the methods of parasite characterization described earlier (see S.P. Morzaria, this meeting). An additional problem is that cloned theilerial parasites, as with malarial parasites, can produce both micro-and macrogametes and these parasites can be transmitted by ticks. This allows further genetic recombination.
Under these circumstances, the only "strains" that are likely to be produced from the heterogeneous stocks already isolated will have to be derived by cloning and then fully characterized. In infection-and-treatment immunization it will be essential to continue using well-characterized stocks. It may be possible to manipulate the parasites by cloning and obtain individuals that provide a wide spectrum of protection and a low pathogenicity, so rendering unnecessary the use of drug treatments.
VI. Tick Vector Species
The currently accepted vectors of the important Theileria species are:
|T. annulata||Hyalomma species|
|T. parva||Rhipicephalus appendiculatus complex|
|T. taurotragi||Rhipicephalus appendiculatus|
|T. velifera||Amblyomma species|
|T. mutans||Amblyomma species|
|T. orientalis||Amblyomma species|
In the last ten years some important revisions have been made in our knowledge of the ticks that transmit different Theileria species. Identification of tick vectors must be more critical than in the past and newer techniques, such as hydrocarbon profiles of cuticle and isoenzyme analysis of tick tissues, should be applied for the correct identification of closely related species.
It would be useful to assemble centrally any new data on the tick vectors of Theileria.
VII. Laboratory and Field Colonies of Tick Vectors
Interest in differences observed between laboratory raised ticks and field populations used in experiments has increased over the last few years. Enclosed and inbred laboratory populations may not represent the true type or behaviour of ticks in the field. Some laboratories use samples of local tick populations rather than laboratory stocks. Differences among laboratory populations have also been found. Studies of these differences should be encouraged and expanded. The word "strain" should not be used for a particular laboratory population unless it is fully characterized; "stock" should be used instead. In this way, consistency in terminology would be established between the Theileria parasites and their tick hosts. Thus, if the first field stock of R. appendiculatus established at Muguga was from the Trans-Mara, it would be referred to as R.a. MU.1.