Search

The occurrence of dermatophilosis on cattle in the tropics is closely associated with infestation by Amblyomma variegatum ticks. Animals that are more susceptible to the disease also tend to carry higher tick burdens. Longitudinal studies carried out on the Accra Plains suggest that N'Dama cattle acquire an enhanced degree of resistance to these ticks following initial exposure. Lesions on zebu-type Ghana Sanga cattle tend to be confined to the predilection feeding sites of adult A. variegatum even when relatively large numbers of ticks are present suggesting that these cattle are able to contain the effects of the tick. Further studies are required to investigate the nature of the apparent resistance of indigenous cattle to the tick and/or its effects and the potential for controlling the disease by immunoprophylaxis using tick derived immunogens. Current methods of controlling the disease depend on the control of ticks using chemical acaricides. Susceptibility to A. variegatum associated dermatophilosis varies between breeds of cattle therefore the level of tick control required is breed dependant. Rigorous tick control is necessary for the prevention of dermatophilosis on highly susceptible exotic breeds of cattle. The limited use of acaricides applied either at the predilection feeding sites of A. variegatum and/or at selected times when the level of challenge increases is sufficient to control the occurrence of the disease on indigenous cattle.

A close association was found between infestation with Amblyomma variegatum and the occurrence of dermatophilosis on cattle on the coastal Plains of Ghana. Animals belonging to breeds traditionally regarded as more susceptible to the disease also tended to carry higher tick burdens. The basis of the association between infestation with this tick and the occurrence of dermatophilosis would appear to involve tick-induced immunosuppression.

The proliferative response of lymphocytes from tick-infested Zebu type, N'Dama and Friesian cattle and acaricide-treated Zebu types and Friesians in concanavalin A (Con A) stimulated cultures was monitored regularly for periods ranging from 11 to 27 months. The numbers of ticks on the animals and the presence of dermatophilosis were also noted. The Friesian cattle carried most and the N'Dama fewest Amblyomma variegatum ticks. The tick-infested Friesians all developed severe clinical dermatophilosis within 5 months of becoming tick-infested. Dermatophilosis lesions on the tick-infested Zebu type and N'Dama cattle were less common and less severe especially in the N'Damas. The proliferative response of lymphocytes from tick-infested Friesians in Con A stimulated cultures fell to almost half that of the acaricide-treated Friesians soon after the former became tick-infested. The tick-infested Zebu types also developed a depressed response compared with the tick-free Zebu group over a 27 month study period. However, the responses of the N'Damas was similar to that of the tick-free Zebu types. The addition of autologous serum to Con A stimulated cultures of lymphocytes derived from the tick-infested Zebu types and N'Damas suppressed their proliferative response compared with that of similar cultures for the tick-free Zebu types.

The seasonal abundance of adult ticks on cattle and their association with dermatophilosis were investigated in five herds on the coastal plains of Ghana over a 26-month period. Four genera, Amblyomma, Boophilus, Rhipicephalus and Hyalomma were identified, A. variegatum being the predominant species occurring throughout the year with two peaks of infestation, one in April-May and the other in November. A significant positive correlation was revealed between A. variegatum and dermatophilosis in four of the five herds. Significant positive correlations were found between H. m. rufipes and dermatophilosis in two of the herds and between Rh. senegalensis and dermatophilosis in one herd. Negative correlations of statistical significance were observed between Boophilus species and dermatophilosis in three of the herds. Nevertheless, it was considered that A. variegatum was the most important tick factor involved in the pathogenesis of the disease.

The effectiveness of three tick control regimes on the occurrence of Amblyomma variegatum and dermatophilosis on cattle on the coastal plains of Ghana were compared. Animals at one kraal were sprayed with amitraz at predilection feeding sites of ticks every second week using a high concentration minimum volume technique. Animals at two other kraals were treated with a deltamethrin based pour-on acaricide; at one kraal it was applied once every month while at the other kraal it was used at strategic times based on the expected seasonal increases in the level of infestation with A. variegatum. Animals in a fourth kraal (control group) were treated, by the herdsmen, to control excessive tick build-up as practised under traditional management systems. Fortnightly treatment with amitraz reduced the level of infestation with A. variegatum and the prevalence of dermatophilosis dropped to a low level. The pour-on acaricide similarly depressed the prevalence of dermatophilosis.