Bovine Dictyocaulosis

Abstract

A combined epidemiology and control investigation was performed with parasite-free calves turned out in May on a permanent pasture naturally contaminated with lungworm larvae the previous year. Before the start the field was divided into two plots. One plot was grazed by 12 calves after the first week of May. The other plot was grazed by 12 calves turned out two weeks later. Both groups as well as the plots grazed by them were subdivided six weeks after turning out. Based on a predicted rise in the pasture larval contamination with infective lungworm larvae, one subgroup of each main group was given a tactical anthelmintic treatment six weeks and again eight weeks after their date of turning out. Patent infections from overwintered larvae were detected in both main groups after four weeks of grazing, but not in all individuals of the late turned-out group. The excreted larvae gave rise to pathogenic pasture larval contaminations on the two initial plots five to six weeks after turning out. In the control groups, early turning-out resulted in approx. 10 times higher larval recoveries in faeces and pasture compared to late turning-out. Seven to eight weeks after turning-out critical, severe parasitic bronchitis had developed in the early turned-out control group. In the late turned-out controls, clinical signs were obvious but not critical. Outbreaks were not observed in any of the experimental subgroups, and no larval excretion was observed among them within four to five weeks following treatments. Similarly, no larvae were recovered from their pastures two weeks after treatment and onwards. A third treatment was given to both experimental groups on the same date (August 21) to suppress gastrointestinal parasitism. However, the level of this infection, appeared moderate, probably due to comparatively low precipitation and extensive supplementary feeding given in late summer to compensate for scarcity of grass.

Sammendrag

Et kombineret epidemiologi- og kontrolforsøg blev udført med parasitfri kalve, der blev bundet ud i maj måned på vedvarende græsareal, der var kontamineret med lungeormlarver fra forrige græsningssaeson. Ved forsøgets start blev arealet opdelt i to fenner. På den ene blev udbundet 12 kalve første uge i maj. De resterende kalve blev udbundet på den anden fenne to uger senere. Begge grupper samt deres respektive fenner blev underopdelt seks uger efter udbinding. En stigning i markernes kontamination med infektive lungeormlarver var forudset. Baseret herpå modtog den ene undergruppe af hver af hovedgrupperne en taktisk anthelmintisk behandling seks uger samt igen otte uger efter de var blevet bundet ud. Fire uger efter udbindingen pvistes patent lungeorminfektion i begge hovedgrupper men ikke blandt alle kalvene i den senest udbundne hovedgruppe. De udskilte larver gav anledning til en patogen graesmarkssmitte med infektive lungeormlarver i begge hovedfennerne efter fem-seks ugers afgraesning. Blandt kontrolgrupperne medførte tidlig udbinding ca. 10 gange så højt smitteniveau, både i faeces- og i graesprøverne, sammenlignet med udbinding to uger senere. Syv-otte uger efter udbindingen konstateredes svaer verminøs bronkitis blandt den tidligt udbundne kontrolgruppe. I den sene kontrolgruppe var de kliniske symptomer markante, men kalvenes tilstand var ikke kritisk. Udbrud og symptomer udeblev blandt begge forsøgsgrupperne, og der kunne ikke pvises larveudskillelse fire-fem uger efter behandlingerne, ligesom der ikke påvistes larver i graesprøverne to uger efter behandlingerne eller senere. En tredie behandling blev givet til begge de eksperimentelle undergrupper på samme dato (21. august) for at nedbringe smitteniveauet med løbe-tarmorm. Dette smitteniveau viste sig imidlertid at vaere lavt, formentlig p. g. a. ringe nedbør, og fordi kalvene modtog betydelige mængder tilskudsfoder sidst i græsningssaæsonen grundet græsmangel.