Abstract
The age distribution of clinical cases of babesiosis reported by local veterinary practitioners was investigated in 1976 and 1981. The first study was based on material collected primarily for identifying the Babesia species, the second on computerized reports from a part of one county in Sweden.
The results were similar. Most diseased animals, 135/165 (82%) and 145/161 (90%) respectively, were more than 2.5 years old (cows), while 27/165 (169b) and 12/161 (8%) respectively, were 1–2.5 years old and only 3–4 (2–3%) animals were calves less than 1 year old. This age group, however, probably did not meet the same infection risk as did older animals. Among cows (>2.5 years old) there seemed to be no influence of age on the distribution of clinical babesiosis. Calculation based upon the entire animal population of the county investigated in 1981 revealed that clinical babesiosis was reported about 11 times more often among cows than among heifers and steers. The comparatively high resistance among 1–2.5 year–old cattle may be a function of an inverse age resistance, and/or may be influenced by vaccination against babesiosis on »high risk« farms in Sweden and a lower risk of infection on other farms such that animals tend to escape infection prior to adultness. Babesia organisms were found in 156 of 165 cases (95%) reported as clinical babesiosis.
Keywords: Babesia divergens, clinical babesiosis, inverse age distribution
Sammanfattning
Åldersfördelningen bland nötkreatur rapporterade för babesios undersöktes 1976 och 1981. Vid första tillfället insamlades blodprov för undersökning av Babesia-art, vid andra tillfället användes datorbaserade rapportsammanställningar från södra Kalmar län. Resultaten var samstämmiga, 135/165 (82%) resp. 145/161 (90%) var djur (kor) äldre än 2,5 år och 27/165 (16%) resp. 12/161 (8%) var 1–2,5 år gamla och endast 3 resp. 4 djur (ca 2%) var kalvar under 1 års ålder. Kalvar löper troligen dock inte samma risk att bli infekterade som äldre djur, vilka oftare går på bete. Bland kor (>2,5 år) sågs inget samband mellan stigande ålder och antal rapporterade fall av babesios.
Omräknat på hela nötkreaturspopulationen ingående i nötkreaturskontrollen inom södra Kalmar län rapporterades babesios ca 11 gånger så ofta hos kor som hos ungdjur, 1–2,5 år gamla. Denna resistens hos djur 1-2,5 år kan förutom att vara ett utslag av omvänd åldersresistens även ha påverkats av att i Sverige vaccineras en stor del av kalvarna, ffa. på »högriskgardar« innan de släpps på bete. Det är också möjligt att infektionsrisken på övriga gårdar är så låg att endast ett mindre antal djur hinner bli infekterade före vuxen ålder. Babesia-infekterade blodkroppar påvisades hos 156 (95%) av 165 undersökta fall rapporterade till Lantbruksstyrelsen som babesios.
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Acknowledgment
The authors wish to express their gratitude to Ms Elisabeth Persson, Association for Swedish Livestock Breeding and Production (ASLBP), Hâllsta for making the data on the ASLBP and NBA registers available, Mr Gunnar Hökas, ASLBP, Hâllsta for giving all other information of the ASLBP register, and all the 52 veterinary colleagues interested in contributing to the result by collecting blood samples from field cases of babesiosis.
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