Immunisation of smallholder dairy cattle against anaplasmosis and babesiosis in Malawi.

A field study was conducted in the Southern Region of Malawi to evaluate the possible benefits of immunisation of improved dairy cattle against Anaplasma marginale, Babesia bigemina and Babesia bovis. Friesian crossbred heifers were immunised when they were being reared on Government farms. They were then issued to smallholder farmers, together with unvaccinated controls, where many of them were exposed to heavy tick infestation. Vaccination was shown to provide a significant degree of protection against babesiosis on the smallholder farms; 15/32 unvaccinated controls developed clinical disease as compared to only 3/28 vaccinates.

Integrated tick and tick-borne disease control trials in crossbred dairy cattle in Malawi.

Crossbred dairy heifers on a farm in an East Coast fever (ECF) endemic area in Malawi were immunised against Theileria parva, Anaplasma spp., Babesia bigemina, Babesia bovis and Cowdria ruminantium. They were treated at infrequent intervals with chlorfenvinphos to limit infestation with adult ticks, without providing complete tick control. In one trial, which tested a threshold dipping regimen, 20 heifers were dipped only once in 6 months to control a flush of Boophilus microplus. Unimmunised controls showed serological evidence of exposure to T. parva and B. bigemina, and one died of ECF, but there were no incidents of tick-borne disease in the immunised group. In a second trial, which tested a strategic dipping regimen, 107 animals were dipped 9 times over a 6 month period. Despite heavy challenge by B. bovis and moderate challenge by B. bigemina and Anaplasma spp. demonstrated serologically, there was only a single clinical case of babesiosis. The observations provide encouragement for the introduction of integrated tick and tick-borne disease control programmes in improved cattle in ECF endemic areas.

Field observations on the duration of immunity in cattle after vaccination against Anaplasma and Babesia species.

In an outbreak of Babesia bovis in a large herd of Friesian x Malawi Zebu cattle, which occurred after an interruption of intensive dipping, clinical or fatal babesiosis occurred in 54/299 (18.1%) animals which had never been vaccinated, as compared to 9/153 (5.9%) vaccinated animals. Eight of the nine affected vaccinates had been vaccinated more than 27 months previously. Sera were collected every 3-4 months from 33 Friesian x Malawi Zebu heifers maintained with intensive dipping and vaccinated with trivalent B. bovis, Babesia bigemina and Anaplasma centrale vaccine. After 2 years, 25% had become seronegative for B. bovis by indirect immunofluorescence, as compared to 97% for B. bigemina and 46% for A. centrale. Because of the evidence that immunity following vaccination against B. bovis declines after 2 years in the absence of tick challenge, it is recommended that tick control should be relaxed after immunity has been established, in order to save acaricide, reinforce immunity and avoid any need for revaccination.

Improving the specificity of indirect immunofluorescence for the serological diagnosis of bovine anaplasmosis.

Fluorescing inclusion appendages were detected consistently in preparations of Anaplasma centrale and Anaplasma marginale when they were used as antigen in indirect immunofluorescence serological tests for the diagnosis of anaplasmosis in cattle. The presence of the inclusion appendages made it possible to confirm the specificity of the immunofluorescent reaction and to determine end-points with accuracy.

Reactions to heartwater vaccination in crossbred Zebu cattle.

One thousand and ninety-four crossbred Zebu cattle were immunized against heartwater with a sheep-blood vaccine containing the Ball 3 strain of Cowdria ruminantium. Animals experiencing febrile reactions were treated at various stages of the reaction with tetracycline. Four hundred and sixty-two (42.2%) reacted, six (0.6%) of which died. Deaths were significantly less frequent in cattle treated on the first day of reaction (2/323, 0.6%) than those treated at a later stage (4/61, 6,6%). The mean incubation period was 15,6 d (range 7-23 d). Incubation period and frequency of reactions varied significantly between farms, between vaccine batches and between vaccine doses. Variations in frequency of reactions, incubation period and severity of reaction did not affect the efficacy of immunization, as assessed by seroconversion.

The serological response to heartwater immunization in cattle is an indicator of protective immunity.

A significant correlation was demonstrated in Friesian-cross steers between the serological response to previous vaccination with the Ball 3 strain of Cowdria ruminantium and the development of protective immunity against the Kalota isolate from Malawi. Of 10 animals which seroconverted after vaccination, all were completely or partially immune to challenge. Ten of the 14 animals which failed to seroconvert were immune but the proportion was not significantly different from that in the unvaccinated controls (4/10). Of 29 animals vaccinated and treated simultaneously with a slow-release doxycycline implant, 13 failed to seroconvert, and of these, four were completely susceptible to challenge.

Efficacy of an Australian Babesia bovis vaccine strain in Malawi.

Three calves vaccinated with the Australian Ka strain of Babesia bovis were fully protected against experimental infection with an isolate from a farm on which four of 210 vaccinated cattle had died from B bovis infection. A degree of cross protection against the isolate was demonstrated in one calf which had been infected previously with Babesia bigemina.

Use of serological response to evaluate heartwater immunization of cattle.

Frozen blood vaccine containing the Ball 3 strain of Cowdria ruminantium is prepared by an FAO/DANIDA Project in Malawi for the immunization of improved dairy cattle against heartwater. The immunogenicity of vaccine batches for cattle has been quantified and different regimens for immunization have been evaluated using indirect immunofluorescence to assess antibody responses. Infected endothelial cells grown in culture are used as antigen. The proportion of animals responding serologically has varied between different batches of vaccine tested in homogeneous cattle populations at the same time, presumably reflecting differences in immunogenicity of batches. The proportion of animals in a homogeneous population responding serologically to the same vaccine batch administered under different regimens has also varied. Indirect immunofluorescence testing has proved to be a useful method for assessing the immune response of cattle to immunization and has been adopted as a routine quality control procedure for heartwater vaccine production in Malawi.

Effect of trypsin treatment on the antigenic characteristics of plaque variants of type-O 1 and type Asia-1 foot-and-mouth disease viruses.

Antigenic differences were demonstrated between the large and small plaque variants of both types O1 and Asia-1 foot-and-mouth disease viruses. Treatment of the large and small plaque variants of the viruses with trypsin essentially abolished the observed antigenic differences. Thus, these plaque variants have antigenically different trypsin-sensitive determinants that may influence their immunogenicity and infection capabilities.