Bone marrow response in cattle with chronic dermatitis caused by Psoroptes ovis.

Six Hereford heifer calves were infested with Psoroptes ovis and compared to six uninfested control calves. Infested calves developed severe exudative dermatitis which covered 100% of the body surface by 7 weeks after infestation. They developed moderate nonregenerative anemia, decreased numbers of neutrophils and lymphocytes, and increased plasma protein and fibrinogen concentrations. Sternal marrow core biopsies taken 7 weeks after infestation were 75 to 90% cellular, while cores from uninfested calves were 10 to 20% cellular. Comparison of myelograms between the two groups of calves revealed increased M : E ratio, myelocyte and metamyelocyte hyperplasia with depletion of mature neutrophils and eosinophils in the P. ovis-infested calves. It was concluded that P. ovis-infested calves develop myeloid hyperplasia and that mange-related neu- tropenia is not due to bone marrow suppression by a mite or scab-associated toxin. Psoroptes ovis is a noninvasive, parasitic mite of ruminants. It is free-roaming over the skin and feeds by piercing the epidermis and ingesting the fluid and cellular ex~date.'.~ During the cool winter months, the mites become active, feed, and reproduce. This results in large increases in mite populations on infested an- imals and a severe exudative dermatitis.'2 The der- matitis usually begins over the withers and eventually spreads over the entire body. The extent of the der- matitis is determined by the population density of mites.I3 Cattle with 30% of the skin surface manifesting dermatitis have a significant decrease in performance, and those with 40% or more of the skin involved may die.5 Cattle develop resistance to P. ovis. Infested cattle have antibody to P. ovis antigens.'.* Previously exposed cattle have fewer mites and less extensive dermatitis than cattle with no prior exp0sure.l' Female mites from previously exposed cattle produce fewer eggs than those from naive cattle.1° Resistance to P. ovis may be me- diated by suppression of mite population expansion by the host. This is achieved by reducing the rate of oviposition in female mites.'O The pathogenesis of the dermatitis is at least partially determined by hypersensitivity to mite digestive fluid antigens.Iz The cause of death in severe cases, however, is unknown. Fluid, electrolyte, and heat loss through the dermal lesions have been ruled out.13 Infested an- imals in a previous study developed a moderate ane- mia caused by chronic inflammation, and a profound neutropenia. l3 Severely infested calves had signifi- cantly fewer neutrophils (X = 399 k 139/pl) than the uninfested controls (2 = 1,343 f 23/pl).I2 Neutropenia could be important in the pathogenesis of fatal bovine 36 psoroptic mange, by predisposing cattle to bacterial infection. The cause of neutropenia in cattle with P. ovis infestation is unknown. Because earlier investigators hypothesized that dis- ease in P. ovis-infested cattle might be related to a mite (or scab)-associated toxin, we considered the possibil- ity that the pathogenesis of the previously observed neutropenia might involve myeloid suppression. This study evaluates the bone marrow response in P. ovis- infested cattle to determine if mange-related neutro- penia is associated with marrow suppression.