The bio-ecology of the sheep scab mite Psoroptes ovis (Acari: Psoroptidae) Hering (1835)

English: Sheep scab, which is caused by the mite Psoroptes ovis, has been and is still today an impediment to sheep husbandry in many countries throughout the world. Despite the economic impact, sheep scab remains a serious veterinary problem in South Africa. The broad objectives of this study were to determine the adverse effects of P. ovis on the host, with special reference to Merino and Dorper sheep, and to examine various possible foci of infestation and host specificity. Furthermore, the effects of abiotic factors (relative humidity and temperature) on the off-host survival of P. ovis were determined. The nature and extent of sheep scab in small scale, predominantly black farming communities, were assessed and compared to commercial predominantly white farming communities. The results revealed the following: With the exception of neutrophils, haematological values remained within the normal range for infested Merino and Dorper sheep, during a 14 week assessment period. For both sheep breeds the albumin and globulin values were higher than the normal range during the entire 14 week observation period. Both sheep breeds displayed a slight decrease in albumin values and a slight increase in globulin values. At the termination of a 16 week assessment period, uninfested control Merino sheep gained a mean ·body mass of 3.44 kg which represented a Il. 60% increase in body mass, compared to the mean body mass decrease (6.4 kg) of infested Merino sheep. This represented a 23.43% decrease in body mass. The mean body mass of the infested Dorper sheep increased over the 16 week period by 4.56 kg (15.11%) compared to the 5.88 kg (18.28%) of the uninfested control Dorper sheep. Comparative studies on the rate of sheep scab lesion development indicated that there were profound differences between sheep breed and season. During the summer assessment of 1997 the mean lesion size on the Merino sheep expanded from 1.0 cm2 at two weeks post infestation to 342.2 cm2 at eight weeks post infestation, compared to the mean lesion size of 0.406 cm2 and 59.0 cm2, respectively, recorded at two and eight weeks post infestation on the Dorper sheep. Lesion growth for both sheep breeds were greater during the winter. Attempts to artificially infest Boer and Angora goats failed. No clinical signs of sheep scab developed on the goats during a nine week observation period. When a single artificially infested Merino and Dorper sheep were introduced during winter into a flock of nine uninfested sheep for each breed, it took 14 and 8 weeks, respectively, before all the sheep in the flocks displayed clinical signs of sheep scab. During summer it took 10 and 12 weeks, respectively, before all sheep in the two flocks displayed clinical signs of sheep scab. All developmental stages of the mites were found in proximal and distal parts of wool / hair tufts clipped from Merino and Dorper sheep. Immature and mature mites were readily transferred to wool/hair tufts placed onto, and later removed from infested sheep, confirming that direct contact between hosts is most probably the main means of transfer. A maximum mean off-host survival time of 15 days (T = 10°C and RH = 33% and 75%), 10.5 days (T = 10°C and RH = 75% and 90%) and 11.25 days (T = 10°C and RH = 90%) were recorded for nymphs, males and ovigerous females, respectively. The mean egg incubation time varied from 5.9 (±2.58) to 22.14 (±6.53) days. The longest time eggs took to hatch was 31 days (T = 10°C and RH = 75%). The longest mean larval longevity was 9.25 days (T = 10°C and RH = 90%). Under natural fluctuating conditions ovigerous females in glass vials containing Merino wool survived 20 days, compared to a maximum of 17 days of females in glass vials without Merino wool. A survey indicated that 36.36% of the sheep flocks in Botshabelo and 38.46% in Thaba Nchu, respectively, were infested with sheep scab. This high incidence was believed to be due to factors such as communal grazing systems, ignorance on the part of the farmers and the lack of financial means to purchase effective remedies to treat their sheep flocks.