zoonoses and communicable diseases common to ... - PAHO/WHO

74 BACTERIOSESedly for three to five months, but only 25% to 40% of them become pregnant afterbeing bred twice. Of the cows or heifers that finally become pregnant, 5% to 10%abort five months into gestation. An undetermined proportion of females harbor C.fetus subsp. venerealis during the entire gestation period and become a source ofinfection for the bulls in the next breeding season. After the initial infection, cowsacquire resistance to the disease and recover their normal fertility, i.e., the embryodevelops normally. However, immunity to the infection is only partial and the animalsmay become reinfected even though the embryos continue to develop normally.Resistance decreases substantially after three to four years.The infection is transmitted by natural breeding or artificial insemination. Bullsare the normal, though in most cases temporary, carriers of the infection. They playan important role in its transmission to females. The etiologic agent is carried in thepreputial cavity. Bulls may become infected while servicing infected cows, as wellas by contaminated instruments and equipment used in artificial insemination. Theetiologic agent is sensitive to antibiotics that are added to the semen used in artificialinsemination.C. fetus subsp. fetus is responsible for sporadic abortions in cattle. Some femalesare carriers of the infection, house the infectious agent in the gallbladder, and eliminateit in fecal matter.SHEEP: The principal agents of epizootic abortion in sheep are C. fetus subsp. fetusand C. jejuni and, to a lesser extent, C. fetus subsp. venerealis. The disease is characterizedby fetal death and abortions in the final months of gestation, or by fulltermbirth of dead lambs or lambs that die shortly thereafter. The infection also givesrise to metritus and placentitis, both of which may result in septicemia and death ofthe ewe. Losses of 10% to 20% of the lambs and 5% of the ewes that abort are common.The rate of abortions varies and depends on the proportion of susceptible ewes.Infected animals acquire immunity. Ewes do not abort again for about three years.If the infection is recent in the flock, the abortion rate can be quite high, at times upto 70% of the pregnant ewes. The infection is transmitted orally; venereal transmissionapparently plays no role.Source of Infection and Mode of Transmission (Figure 9): The reservoir of C.fetus is animals, but it is not clear how man contracts the infection. It is presumedthat he can become infected by direct contact with infected animals, by ingestion ofcontaminated food (unpasteurized milk, raw liver) or water, by transplacental transmission,exposure during birth, or sexual contact. It should be noted, however, thatsome patients have denied any contact with animals or even with products of animalorigin. It is also suspected that the infection may be endogenous. The etiologic agentwould be an oral commensal parasite that could penetrate the bloodstream during adental extraction. Another hypothesis is that C. fetus could be harbored in the humanintestine without becoming evident until the host loses resistance due to some illness.It would then invade through the mucosa, causing a generalized infection. Insummary, the source and pathogenesis of C. fetus in man continue to be unknown(Morrison et al., 1990).The sources of infection in cattle are carrier bulls and also cows that remaininfected from one parturition to the next. The mode of transmission is sexual contact.For sheep, the source of infection is environmental contamination. The placentasof infected sheep that abort or even of those that give birth normally, as well as