The effect of selection pressure on the cholinesterase (AChE) activity of two strains of Boophilus microplus (Canestrini) resistant to coumaphos was monitored. Total AChE and protein was determined from three generations of resistant ticks and a susceptible strain. The effect of an AChE inhibitor, coroxon (the oxygen analog of coumaphos), was also determined. The resistance of the susceptible strain (Escondido) to coumaphos remained relatively unchanged throughout the study. The Tuxpan strain lost some of its resistance to coumaphos as the generations proceeded (AChE increased instead of decreased). The Tuxtla strain became more resistant to coumaphos as the generations proceeded (AChE increased).

The cholinesterase (AChE) activity and total protein in homogenates of unfed larvae of Amblyomma americanum (L.), A. cajennense (Fabricius), A. maculatum Koch, Anocentor nitens Neumann, and Boophilus microplus (Canestrini) were determined weekly in ticks 1-6 wk of age. There was a considerable variation in total protein, AChE, and the ratio of AChE to protein as the ticks aged. However, AChE activities were constant or increased with age and total protein levels were constant or decreased. No direct relationship was detected between AChE activity and total protein levels. The ratio of AChE to protein increased as the unfed larval ticks aged.

Journal Article Cholinesterase Insensitivity: A Mechanism of Resistance in Mexican Strains of Boophilus microplus (Acari: Ixodidae) Against Coumaphos Get access F. C. Wright, F. C. Wright 2U.S. Livestock Insects Laboratory, USDA-ARS, P.O. Box 232, Kerrville, Tex. 78029 Search for other works by this author on: Oxford Academic PubMed Google Scholar E. H. Ahrens E. H. Ahrens 3Cattle Fever Tick Research Laboratory, USDA-ARS, P.O. Box 969, Mission, Tex. 78572 Search for other works by this author on: Oxford Academic PubMed Google Scholar Journal of Medical Entomology, Volume 25, Issue 4, 1 July 1988, Pages 234–239, https://doi.org/10.1093/jmedent/25.4.234 Published: 01 July 1988 Article history Received: 08 September 1987 Accepted: 11 March 1988 Published: 01 July 1988

Rabbits infested with Psoroptes ovis (Hering) or with P cuniculi (Delafond) were injected IM or subcutaneously with ivermectin or with vehicle. A single injection of ivermectin (200 micrograms/kg) either IM or subcutaneously was inadequate to eliminate all mites in the ears of the rabbits. However, a single injection of ivermectin at 400 micrograms/kg, regardless of the route of injection, eliminated all P cuniculi mites, but eliminated P ovis mites in only 50% of the rabbits. Ivermectin was more effective against P cuniculi than against P ovis at both doses tested, regardless of the route of injection.

AbstractIvermectin injected subcutaneously into calves at the rate of 200 μ/kg body weight completely eliminated females ofPsoroptes ovis(Her.) in 22 days. Injection of ivermectin also adversely affected the number and hatch of eggs and the proportion of adult males forming attachment pairs. The reduced number of eggs recovered in skin scrapings was apparently the result of a significantly lower fecundity among females that temporarily survived on treated calves. Females that were exposed to ivermectin for only three days did not recover their full reproductive potential even though they were transferred to calves not injected with ivermectin. The proportion of ovigerous females among survivors was not significantly affected five days after injection. Since the number of eggs recovered in skin scrapings was substantially reduced after only three days, the effect of ivermectin on female fecundity was not primarily gonadal. Reduction in the number ofP. oviseggs three days after treatment probably occurred because of a combination of mortality of adult females and reduced oviposition by ovigerous females surviving in the population.

ABSTRACT Psoroptes ovis (Hering) originally obtained from cattle were reared in the ears of domestic rabbits for 2.8 years. The P. ovis mites thrived on the rabbits and exhibited no morphological change as measured by adult male I4 setal lengths, body length, or width, or adult female body length or width during this test period; nor did they resemble P. cuniculi (Delafond) from rabbits. The P. ovis reared on rabbits remained infestive to cattle during the test period.

Cattle scabies mites, Psoroptes ovis (Hering), surviving on cattle injected subcutaneously with ivermectin were capable of infesting to unprotected cattle 3, 5, 7, and 9 days after injection. All recipient cattle exposed to mites from vehicle-injected cattle developed reproductive colonies in 5 weeks. All recipient cattle exposed to mites from ivermectin-injected cattle at 3 and 5 days after injection developed reproductive colonies in 5 weeks. By 8 weeks, 3 of 4 and 2 of 4 recipient cattle exposed to mites from ivermectin-injected cattle 7 and 9 days after injection, respectively, developed reproductive colonies. To prevent the spread of mites, injected cattle should be isolated from uninfested cattle for longer than 9 days.

Sweatman (1958, Can. J. Zool. 36: 905-929) reported that the location of Psoroptes mites on the host animal, i.e., ear or body, was one of the determining factors for separation of the psoroptic mites into species. He also confirmed the observation of Hirst (1922, Brit. Museum Nat. History, Econ. Ser. No. 13) that the setae on the opisthosomal lobes of the adult male were of diagnostic value. Sweatman concluded that the outer opisthosomal setae (OOS) or 14 setae (Fain, 1970, Rev. Zool. Bot. Afr. LXXXI: 95-100) was the primary factor in separation of the species and that Psoroptes ovis and P. bovis were the same species even though the host animals were different. Observations in our laboratory have indicated a wide overlap in the lengths of the 14 setae of adult males of psoroptic mites in our collections from various hosts and geographic locations. Sweatman also noted this overlap. The objective of our study was to compare lengths of the 14 setae of male psoroptic mites collected from various hosts from different geographic locations and to determine the reliability of the length of 14 setae as a characteristic to differentiate between species. Most of the mites studied were collected in recent years; however, additional mounted specimens were obtained from the collection of the Parasite Research Laboratory, Albuquerque, New Mexico, now located at Kerrville, Texas, as the Scabies and Mange Mite Research Unit of the U.S. Livestock Insects Laboratory. Mites collected from the ears of rabbits, goats, mule deer, white-tail deer, and ibex were considered to be P. cuniculi. Mites collected from the body of cattle and domestic sheep were considered to be P. ovis. Mites collected from bighorn sheep were considered to be P. ovis or P. cervinus. The mites were mounted on glass slides in Hoyer's medium for measurements. Setal length was measured in micrometers (,um) with a Zeiss microscope equipped with differential interference-contrast, x 10 eye pieces, a x 40 objective, and a calibrated measuring reticule. Each 14 seta measured was considered an observation. Data were transformed to log 10 values and analyzed by one-way analysis of variance (ANOVA) for unequal sample sizes (Sokal and Rohlf, 1969, W. H. Freeman and Co., San Francisco, California, 776 p.). The differences between the means of the lengths of the 14 setae from the mites from various collections were analyzed by the Student-Neuman-Keuls (SNK) test for unequal sample sizes (Sokal and Rohlf, 1969, loc. cit.). The log 10 mean values were converted to their antilog value before being reported in the test. The lengths of the 14 setae of the adult male mites of Psoroptes spp. collected from the various hosts and geographic locations were compared statistically (Table I). The lengths of the 14 setae of mites (Mission strain) collected from sheep, of mites from bighorn sheep (Idaho), and of mites from cattle (Brazil) were all significantly different (P < 0.05) from the lengths of 14 setae from all other collections of mites regardless of host, body location, or geographic location. There were many differences within the same host animal. For example, the lengths of 14 setae from mites collected from rabbits from New Mexico, Kerrville, and England were all significantly different from each other. The lengths of 14 setae of mites collected from goats in Texas and New Mexico were significantly different from each other. The lengths of 14 setae of mites collected from cattle in the USA (Albuquerque, Kerrville, Hereford, and Colorado) were significantly different (P < 0.05) from those of mites collected from Holstein cattle from Brazil. The lengths of 14 setae from mites of the Mission strain from sheep were significantly different from those of all other strains of mites from sheep at Albuquerque, England, and South Africa. The lengths of 14 setae from mites of the mixed strain and the ADP strain were significantly different from those of the Beltsville-2 strain and England strain also. There was no consistency between the

Ten stanchioned heifers, heavily infested with Psoroptes ovis (Hering), were each given a single subcutaneous injection of 200 micrograms of ivermectin/kg. Another 10 heavily infested heifers (stanchioned) were injected subcutaneously with the solvent system (vehicle) used to formulate the ivermectin. Treatment with the vehicle did not reduce mite populations or total egg counts. The 10 heifers treated with ivermectin had an immediate reduction in live mite and egg counts (an increase in the percentage of mortality of mites) and were completely free of psoroptic mites by 20 days after treatment. Ivermectin was equally effective against adult and immature psoroptic mites.

Reciprocal cross-matings of Psoroptes ovis and P. cuniculi occurred and produced viable offspring through the F2 generation. Hybrids of these crosses were infective to both rabbits and cattle. Mean lengths of the male l4 setae of the parental species differed significantly from each other as well as from those of the hybrid males. These data show that P. ovis and P. cuniculi are not reproductively isolated. Because viable, fertile hybrids are produced, cross-matings of P. ovis and P. cuniculi are not suitable as a suppression method of cattle scabies.

SUMMARY Hereford calves were exposed to ca 200 active adult and mymphal psoroptic mites at intervals of 3 or 4 days after a single im injection of 200 μg of ivermeciin/kg of body weight, after dipping in 0.54% toxaphene, or no treatment. Mites established reproducing colonies on 11 of 13 untreated calves, on all of the toxaphene-dipped calves exposed more than 8 days after treatment, and on all, but 1, of the ivermectin-injected calves exposed more than 21 days after treatment. Mites did not survive long enough to establish reproducing colonies on any of the ivermectin-injected calves exposed through 21 days after treatment, on any of the toxaphene-dipped calves exposed through 8 days after treatment, or 2 of the control calves.

Studies were conducted during the winters of 1978 to 1979 and 1979 to 1980 to determine if Psoroptes ovis (Hering) affects cumulative weight gains (CWG) of young Hereford steer calves. In the first test, no significant difference was found in the mean CWG between the control and P. ovis-infested calves, but after the infestation had reached a mean scabies rating of 3.3, the mean CWG of the infested calves began to decrease and then increase as the scabies rating declined. One calf died due to a heavy mite population. In the second test, a significant difference (P < 0.05) was found between the mean CWG of the control and the P. ovis-infested calves. The same trend as in test 1 was seen between the control and the P. ovis-infested calves. The mean CWG was lower in the infested calves than in the control calves from 6 through 10 weeks postinitial infestation (PI) (P < 0.025) and 12 through 14 weeks PI (P < 0.05). To prevent animal mortality in the second test (two of the calves were in poor condition due to heavy scabies infestation), the infested calves were dipped in toxaphene. The moderately and heavily infested calves, (scabies ratings of 3 and 5, respectively) showed a lower CWG than the control calves and the lightly infested calves (scabies rating of 2) at 8 weeks PI. After dipping, calves began to gain weight, thus indicating that P. ovis infestation had been limiting the mean CWG. It is clear from these data that the mean CWG of young Hereford steer calves is affected by P. ovis infestation.

Washed rabbit erythrocytes labeled with radioactive chromium in vitro were injected into 3 groups of 2 adult rabbits each. One group was heavily infested with the sheep scab mite, Psoroptes ovis; the 2nd group was infested with Psoroptes mite from Mexican Mountain Sheep, and the 3rd group was infested with the ear canker mite, P. cuniculi. After 36 h, mites and scabs were harvested and the radioactivity determined. At time of collections, mites from all groups had an average of 8.9 ± 2.0 μl of blood per 100 mg of mites. More than 70% of the consumed hemoglobin had been digested by the mites. The data suggest that the ingestion of whole blood is not specific to a particular mite species but may be common to Psoroptes mites when feeding on rabbit hosts.