Resistance to the cattle tick Boophilus microplus was assessed in heifer herds of various breeds of beef and dairy cattle in spring and summer in Queensland. All cattle had had tick experience before assessment. Resistance levels were determined as the percentage of larval ticks that failed to survive to maturity following infestations with c. 20,000 larvae. Bos indicus Brahman beef cattle were the most resistant (99%), followed by B. indicus × B. taurus (95–97%) and B. taurus British cattle (85%). In the dairy breeds, B. taurus Jersey cattle (98%) were more resistant than Guernsey (93%), Australian Illawarra Shorthorn (89%), and Friesian (85%), but not significantly different from B. indicus × B. taurus Australian Milking Zebu (96%). Cattle were classified as having high (> 98%), moderate (95–98%), low (90–95%), or very low (< 90%) resistance. The frequency distributions of resistance in B. indicus × B. taurus cattle showed that 80% had moderate to high resistance and that culling of 20% of the cattle would about halve the mean tick population. Of B. taurus cattle, 80% had low to very low resistance. Supplementary information on sibling bull herds showed that their resistance levels and frequency distributions of resistance were similar to those of the heifer herds.
AbstracrA pattern of engorgement and dropping of the female cattle tick Boophilus microplus is described. Partl engorged females, which have grown to a length of 4-6 mm (10-30 mg), undergo rapid hal engorgement at night to reach a length of 8-11 mm (150-250 mg) and detach from cattle in the early hours of the morning. The minimum size of females which engorge varies with season. Differences in the engorgement and dropping pattern of ticks on cattle observed in summer and in winter, and on cattle held in open yards and in covered stalls, suggest an influence of the external environment, particularly temperature and light, on tick engorgement and detachment. Counting of ticks 4.5-8 mm in length on one day was shown to provide a reliable estimate of the numbers of engorged ticks dropping the following day and has been adopted for the assessment of tick numbers on cattle.
Anopheles hackeri, a mosquito commonly found breeding in nipa palm leaf bases along the Malayan coast, was demonstrated to be infected with Plasmodium knowlesi by the inoculation of sporozoites into an uninfected rhesus monkey. This was the first demonstration of a natural vector of any monkey malaria.
An Australian Illawarra Shorthorn herd of 24 cows was mated in three consecutive years with an AIS bull. The cows and their progeny were rated for tick resistance at frequent intervals from August 1959 to December 1965 by counting the numbers of semiengorged female ticks on the right side. The mean of log counts for all counts on a particular animal was adopted as the reference value for its degree of susceptibility. The ranking of cattle generally showed a high level of consistency with mean repeatability of counts (r = 0.47, P < 0.01). Discrimination between animals was more reliable (P < 0.01) in summer (r = 0.52) than in winter (r = 0.27). The repeatability of tick counts increased with mean count, from r = 0.27 when the mean count was 3 to r = 0.67 when it was 100. The reliability of counts on the cows decreased with age and with lactation. Supplementary information on a larger herd showed no effect of pregnancy on mean count or on discrimination between susceptible and resistant animals, but showed that there was a partial breakdown of resistance during lactation. In calves infested naturally, no effects of age or sex on tick counts or their repeatability were detected, though male calves yielded significantly larger numbers of ticks than females when infested artificially. The mean yield of mature female ticks on the cows following two artificial infestations with known numbers of larvae ranged from 0.2 to 27.4% of the potential. Natural and artificial assessments of susceptibility were closely correlated. The rank of the bull was similar to that of the more resistant cows. Mean estimates of the heritability of tick resistance based on single counts were 39 % from dam-calf correlations and 49 % from full-sib correlations. Estimates based on summer counts only were 42 and 64% respectively. These results provide strong encouragement for selecting for tick resistance.
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