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SUMMARY

The development of Anaplasma marginale was studied in Dermacentor andersoni nymphs after they had fed on a calf with ascending Anaplasma infection. Gut tissues were collected on day 4 of tick feeding, from newly replete (fed) nymphs and on postfeeding days (pfd) 5, 10, 15, 20, and were processed for light and electron microscopy to determine density of A marginale colonies. Homogenates of gut tissues were prepared from nymphs collected on the same days and inoculated into susceptible, splenectomized calves to test for infectivity. Anaplasma colonies were detected in gut cells on pfd 5, 10, 15, and 20. Although colony density appeared to be higher on pfd 10 and 15, differences were not significant. Nymphal type-1 colonies were detected in highest numbers on pfd 5 and 10, transitional colonies were seen in highest numbers at pfd 10 and 15, and nymphal type-2 colonies were observed only on pfd 20. Gut homogenates that were collected from ticks at 4 days of feeding, when newly replete, and on pfd 20 caused anaplasmosis when injected into susceptible calves, but homogenates made from ticks collected on pfd 5, 10, and 15 were not infective. The data indicate that of the colony types of A marginale that develop in replete nymphs, nymphal type-1 and transitional colonies may contain organisms that are not infective for cattle.

Free access
in American Journal of Veterinary Research

SUMMARY

The development of Anaplasma marginale in midgut epithelial cells was studied in feeding, transmitting adult Dermacentor andersoni ticks. Laboratory-reared ticks experimentally infected as nymphs were allowed to feed from 1 to 9 days on susceptible calves. Gut tissues from ticks were collected on each day they fed (total, 9 days) and were processed for light and transmission electron microscopy. Colonies of A marginale were abundant during the first 6 days of feeding, after which numbers decreased. Colonies were adherent to the basement membrane of gut cells early during feeding, with resultant flattening of the colonies. Colonies also were seen in muscle cells on the hemocoel side of the basement membrane. Morphologic features of A marginale within muscle cells varied and were similar to those observed in gut cells. In addition, however, a large reticulated form in the colonies was observed in muscle cells and appeared to give rise to small particles by budding. Development of A marginale in muscle cells appears to represent an intermediate site of development between those in gut and in salivary glands.

Free access
in American Journal of Veterinary Research

SUMMARY

Development of the rickettsia, Anaplasmamarginale, in salivary glands of male Dermacentor andersoni exposed as nymphs or adult ticks, was studied indirectly by inoculation of susceptible calves with homogenates and directly by examination, using light microscopy and a DNA probe; some unfed ticks were incubated before tissues were collected. Salivary gland homogenates made from ticks in every treatment group caused anaplasmosis when injected into susceptible calves; prepatent periods decreased as the time that ticks had fed increased. Colonies of A marginale were seen only in salivary glands of ticks exposed as adults and not in those exposed as nymphs; the percentage of salivary gland acini infected in these ticks increased linearly with feeding time. However, the probe detected A marginale DNA in salivary glands of ticks from both groups; the amount of DNA detected increased as feeding time was extended. The amount of A marginaleDNA appeared to remain constant in gut tissues, but to increase in salivary glands. Salivary glands of adult-infected male ticks that were incubated, but did not feed a second time, became infected with A marginale, and the pattern of infection of acini varied with incubation temperature. Development of A marginale in salivary glands appears to be coordinated with the tick feeding cycle; highest infection rate was observed in ticks exposed as adults.

Free access
in American Journal of Veterinary Research

Summary

The development and transmission of Anaplasma marginale was studied in Dermacentor andersoni males. Laboratory-reared male D andersoni were allowed to feed for 7 days on a calf with ascending A marginale parasitemia. The ticks were then held in a humidity chamber for 7 days before being placed on 2 susceptible calves. Anaplasmosis developed in the calves after incubation periods of 24 and 26 days. Gut and salivary glands were collected from ticks on each day of the 23-day experiment and examined with light and electron microscopy. Colonies of A marginale were first observed in midgut epithelial cells on the sixth day of feeding on infected calves, with the highest density of colonies found in gut cells while ticks were between feeding periods. The first colonies contained 1 large dense organism that subsequently gave rise to many reticulated organisms. Initially, these smaller organisms were electron-lucent and then became electron-dense. On the fifth day after ticks were transferred to susceptible calves for feeding, A marginale colonies were found in muscle cells on the hemocoel side of the gut basement membrane. A final site for development of A marginale was the salivary glands. Colonies were first seen in acinar cells on the first day that ticks fed on susceptible calves, with the highest percentage of infected host cells observed on days 7 to 9 of that feeding. Organisms within these colonies were initially electron-lucent, but became electron-dense.

Free access
in American Journal of Veterinary Research

SUMMARY

Objective

To determine whether eprinomectin was effective against mange caused by Chorioptes bovis and Sarcoptes bovis in cattle.

Animals

80 cows naturally infested with C bovis and 30 cattle experimentally infested with S bovis.

Procedure

6 trials were performed to determine efficacy against C bovis, and 2 trials were performed to determine efficacy against S bovis. In each trial, a group of untreated animals or of animals treated with vehicle alone was compared with a group of animals treated with a 0.5% formulation of eprinomectin applied topically (500 μg/kg). Number of mites in skin scrapings was determined prior to treatment and at weekly intervals for 8 weeks after treatment. Severity of skin lesions was evaluated when skin scrapings were obtained. In 5 trials, animals were weighed before and 56 days after treatment.

Results

Mite counts for treated cattle were significantly less than counts for control cattle from day 14 onwards in trials to determine efficacy against C bovis and from day 7 onwards in trials to determine efficacy against S bovis. Mites were not detected in scrapings collected from treated cattle on day 56. Mean weight gain of treated cattle was not significantly different from mean weight gain of control cattle in trials evaluating efficacy against C bovis but was significantly greater in trials evaluating efficacy against S bovis.

Conclusion and Clinical Relevance

Eprinomectin was highly effective against C bovis and S bovis. Because eprinomectin can be administered to lactating cows, it may be useful for controlling mange in cattle. (Am J Vet Res 1997;58:1257–1259)

Free access
in American Journal of Veterinary Research