Integrated control of ticks and tick-borne diseases of cattle in Africa.

The problems caused by tick and tick-borne diseases for livestock particularly cattle on the African continent are described and discussed. The control of ticks and tick-borne diseases must receive high priority in Africa with regard to both research and control application because of their widespread distribution in areas of high livestock potential and productivity. The conventional methods of tick and tick-borne disease control are discussed and are found to be inadequate in the conditions prevailing in Africa. Methods of integrated control are suggested and discussed in light of recent development in control methods and those still under development. Any one of these methods may not be adequate to control the problem on its own but when several of the methods are combined an economic and robust integrated control is likely to result. Encouragement is given to attempt this approach in Africa to solve what must be the largest animal health problem of livestock remaining in the world.

Cytotoxic T-cells elicited in cattle challenged with Theileria parva (Muguga): evidence for restriction by class I MHC determinants and parasite strain specificity.

The MHC restriction and parasite strain specificity of cytotoxic cells elicited in a group of Theileria parva (Muguga)-immunized cattle following homologous challenge, were investigated. The cytotoxic cells were specific for parasitized target cells and in 9 of the 10 animals examined, they were clearly genetically restricted. Cytotoxicity could be inhibited by monoclonal antibodies (MoAb) to class I MHC molecules but not by MoAb to class II molecules, indicating that a large component of the response was restricted by class I MHC determinants. Low levels of inhibition of cytotoxicity were also obtained with a MoAb to the T-cell subset marker BoT8, suggesting that at least part of the response was mediated by BoT8+ lymphocytes. When cytotoxic cells from individual cattle were assayed on panels of parasitized target cells, there was a close correlation between susceptibility of the target cells to lysis and sharing of BoLA-A locus-encoded specificities with the effectors. This observation, taken together with the knowledge that within several of the sets of BoLA-A-matched targets the relevant BoLA-A specificities were on different MHC haplotypes, indicated that the responses were restricted predominantly by BoLA-A products. In individual cattle there was a striking bias in the restriction of the response to one or other BoLA-A specificity. Among the six specificities represented, responses restricted by w6, w8 and KN18 consistently predominated over responses restricted by w7, w10 and w11. In the three cattle tested for parasite strain specificity, two showed complete specificity and one partial specificity for cells infected with the parasite stock used for immunization, T. parva (Muguga).

Bovine alloreactive cytotoxic cells generated in vitro: target specificity in relation to BoLA phenotype.

Cytotoxic cells of bovine origin were generated in primary MLC using stimulator cells of BoLA w8/w11 phenotype. Bovine lymphoblasts transformed by the protozoan parasite Theileria parva parva acted as target cells in studies of the specificity of cytotoxicity. When responder cells in MLC did not share w8 or w11 with stimulator cells, cytotoxicity was evident with all targets bearing w8 or w11, or both, and was almost entirely restricted to these products of the BoLA-A locus. When responder and stimulator cells shared both w8 and w11, cytotoxicity was also generated. Whether this was specific for the products of other putative Class I loci in cattle, or for the products of a Class II region, remains to be determined. These results suggest that the determinants recognized by appropriately generated bovine alloreactive cytotoxic cells are identical with, or closely related to, determinants characterized by BoLA w8 and w11 defining alloantisera.

Detection of antibody to Theileria parva schizonts and cell surface membrane antigens of infected lymphoblastoid cells by immunoperoxidase techniques.

Peroxidase-labeled antibody procedures were described for detecting bovine antibodies reactive with intracellular Theileria parva schizonts and cell surface membrane antigens of infected lymphoblastoid cells. Indirect tests were performed where the reacting bovine antibodies were localized with affinity purified rabbit-anti-bovine IgG coupled to horseradish peroxidase. A 4- to 8-fold increase in sensitivity for detecting bovine antibodies was obtained with unlabeled rabbit-anti-bovine IgG which in turn was detected with peroxidase labeled goat-anti-rabbit IgG. The T. parva infected cells used as antigen were attached to poly-l-lysine treated glass slides and all reaction steps were performed on the slides. The intracellular schizonts and cell surface staining reactions were dependent upon the status of the cells; acetone-fixed cells were required for schizont reactions and viable unfixed cells for cell surface membrane reactions. Sera from cattle stimulated in various ways with T. parva were examined by the techniques. Cattle infected by stabilate inoculation or inoculated with infected autologous lymphoblastoid cells developed relatively high levels of antibody to schizonts, but no detectable antibody to cell surface membrane antigens. This would indicate that parasite antigens do not occur on the surface of infected lymphoblasts. Cattle inoculated with infected allogeneic lymphoblasts developed low-levels of antibody to schizonts and readily demonstrable antibody to cell surface antigens. The immunoperoxidase procedures have certain advantages over immunofluorescence in that light microscopy is used; therefore, the reactions do not fade which permits a more detailed examination and provides a relatively permanent record, the preparations can be counterstained, and the reagents may be used for immunoelectron-microscopy. The procedures could provide suitable alternatives to immunofluorescence methods for East Coast fever investigations and other systems having intracellular and/or cell surface membrane antigens.

A histocompatibility barrier to immunization against East Coast fever using Theileria parva-infected lymphoblastoid cell lines.

Histocompatibility may be a barrier to the infection of cattle when Theileria parva parva-infected tissues or in vitro cultured macroschizont-infected lymphoblastoid cell lines are used for immunization. By inoculating 10(3) and 10(5) infected cells into autologous recipients infection was achieved and immunity engendered. Cell lines inoculated into BoLA matched recipients did not produce patent infections but some recipients developed antibodies to the parasite and 3/5 were immune to challenge. No evidence of infection or immunity was found in BoLA half matched or mismatched cattle. This result suggests that there is an histocompatibility barrier to infection using T. p. parva-infected lymphoblastoid cells.

Bluetongue virus serotype 20 infections in cattle of breeding age.

Ten cattle (6 heifers and 4 bulls) were inoculated with bluetongue virus (BTV) type 20. Clinical signs, antibody responses, and capabilities of these animals to replicate and maintain virus were assessed. The cattle showed no clinical signs of disease, although they did develop antibodies to BTV which showed long-term increasing titers. Virus was intermittently isolated from samples of blood, but not beyond day 21. Virus was not detected in the semen of the bulls, nor isolated from the tissues of the cattle at necropsy except from the genital tract of the 2 bulls which had been killed within 30 days after their inoculation. Gross and microscopic pathologic changes were not seen in any tissues that were attributable to BTV infection. There was no evidence of damage to the reproductive organs or of the development of a latent carrier state in either the heifers or the bulls.

Bluetongue virus serotypes 20 and 17 infections in sheep: comparison of clinical and serological responses.

The clinical, virological and serological responses of sheep infected with an Australian bluetongue virus (BTV) isolate (serotype 20) were compared to responses in sheep inoculated with an American bluetongue isolate (serotype 17) with which it had shown cross-reactions in serum neutralization tests. In sheep inoculated with BTV 20, clinical signs were very mild and viremia was first detected by day 5; virus was isolated intermittently for a further 2 to 3 days. Neutralizing and precipitating antibodies were first detected in the serum of the sheep between 2 to 3 weeks following inoculation. In contrast, sheep inoculated with BTV 17 showed pyrexia and severe hyperemia of the nasolabial area and oral mucosa from day 7 to 17. Viremia was first detected on day 3 and extended to day 20, while the appearance and titers of serum antibodies was similar in both groups. After challenge with BTV 17 the sheep in both groups remained clinically normal, and virus was not detected in the blood; however, serum neutralizing antibody titers to both viruses increased 2 weeks after challenge and the mean titer of the two groups ranged from 1:250 to 1:640.

Isolation of an exotic serotype of bluetongue virus from imported cattle in quarantine.

In 1980, 60 zebu cattle from Brazil were admitted into quarantine in Florida for 150 days. During the 30 days between their last test in Brazil and their first test in Florida, four animals developed antibody to bluetongue virus detectable by agar gel immunodiffusion test. Within 62 days after arrival in Florida, three more seroconverted and one more was positive by the 86th day. Virus neutralizing titers of serums from the first four cattle were highest against bluetongue virus serotype 4 and 20; both of these serotypes are exotic to the United States. A bluetongue virus serotype 4 was isolated from one of these animals. The eight positive reactors were slaughtered; the other 52 cattle, which did not develop detectable antibody titers to bluetongue virus, were released into the United States.

Antibodies to bluetongue viruses in animals imported into United States zoological gardens.

Three hundred forty-five serum samples from 30 zoological animal species which had been imported into the United States were examined retrospectively for the presence of antibody to bluetongue viruses. Ninety eight (28.4%) were positive for antibody to bluetongue group antigen by the bluetongue agar gel immunodiffusion test. Bluetongue antibodies, most of which were against serotypes exotic to the United States, were detected in 13 animal species from Africa not previously reported to be infected by bluetongue virus. The lack of virus neutralizing antibody to any of the 20 known bluetongue virus types in four of the 28 positive serums studied may indicate the existence of new bluetongue virus serotypes, cross reactions with other orbiviruses or a more rapid decline of neutralizing than precipitating antibody. The possibility of recrudescence of bluetongue virus infection from some inapparently infected zoological animals and existence of a known bluetongue vector (Culicoides variipennis) in the United States would suggest that further assessment of bluetongue in zoological animals be made.

Experimental transmission of African swine fever virus by Ornithodoros coriaceus, an argasid tick indigenous to the United States.

Three ticks, indigenous to the United States, were assessed for their ability to maintain and transmit African swine fever virus (ASFV). Amblyomma americanum and A cajennense adults and nymphs maintained virus for 4 to 7 days after engorging on infected swine blood; however, virus was not carried through the subsequent molt of nymphs and it was not transmitted vertically to the eggs and larvae of the infected females. On refeeding of the ticks, infection was not transmitted to healthy swine. Ornithodoros coriaceus adult females also did not show transovarial transmission of two strains of ASFV. Nymphs, however, maintained virus for 77 to 118 days through a molting stage and transmitted one strain of the virus (Lisbon 60) to healthy swine.

Immunity to foot-and-mouth disease virus in guinea pigs: clinical and immune responses.

Clinical and immune responses were determined for guinea pigs infected with different doses of foot-and-mouth disease virus (FMDV) type A12, strain 119, administered by different routes. Vesicles developed on the tongue or heel pad 1 day after these areas were intradermally inoculated with FMDV. However, vesicles did not develop on the feet and tongue until 3 to 4 days after the intradermal inoculation of FMDV in the flank skin or after intracardiac or subcutaneous inoculation. Infected guinea pigs developed neutralizing antibody, immediate skin reactivity of the Arthus type (4 h), and delayed skin reactivity. In addition to a delayed skin response, the presence of a cell-mediated immune response to FMDV was shown by the specific production of macrophage migration inhibition factor by peritoneal exudate cells in response to FMDV. Kinetic studies showed that neutralizing antibodies were detected at 3 days postinfection, and Arthus and delayed skin reactivity were detected at 4 days postinfection. Some guinea pigs developed either mild or subclinical infections. Regardless of the dose of infectious virus, the route of inoculation, the severity of disease, or the time of clinical onset of disease, infected guinea pigs developed similar immune responses.