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

BRUCELLOSIS 53intrauterine route used in artificial insemination is very important in transmitting theinfection. The use of infected bulls for artificial insemination constitutes an importantrisk, since the infection can thus be spread to many herds.In closed environments, it is likely that infection is spread by aerosols; airborneinfection has been demonstrated experimentally.Congenital infection and the so-called latency phenomenon have also beendescribed. An experiment was carried out in France (Plommet et al., 1973) in whichcalves born to cows artificially infected with a high dose of B. abortus were separatedfrom their mothers and raised in isolation units. At 16 months of age, theheifers were artificially inseminated. In six experiments (Fensterbank, 1980) using55 heifers born to infected cows, 5 were infected and brucellae were isolated duringcalving and/or after butchering six weeks later. At 9 and 12 months of age, two ofthese animals had serologic titers that were unstable until pregnancy. The other threeheifers did not have serological reactions until the middle or end of pregnancy(latency). The authors of the experiment admit that under natural range conditionsthe frequency of the latency phenomenon could be much lower. In herds in whichvaccination of calves is systematically carried out, the phenomenon may go unnoticed.In a similar vein, other research projects (Lapraik et al., 1975; Wilesmith,1978) have been undertaken on the vertical transmission of brucellosis accompaniedby a prolonged and serologically unapparent phase of the infection. In a retrospectivestudy of highly infected herds (Wilesmith, 1978), it was found that 8 of 317heifers (2.5%) born to reactive cows tested serologically positive. One study conductedon 150 calves born to naturally infected mothers (with positive culture for B.abortus), taken from 82 herds in three southern states in the US, suggests that thelatency phenomenon does not occur very frequently. The calves were raised in isolationuntil sexual maturity and breeding. Brucella was not isolated from the progenyof 105 infected cows, nor from the 95 fetuses and newborns of these heifers(second generation). Two heifers from the first generation had positive and persistentserological reactions from an undetermined source (Ray et al., 1988). The extentof the latency phenomenon is still not known, but it has not prevented the eradicationof bovine brucellosis in vast areas and many countries. On the other hand, it hasundeniably slowed its eradication in some herds.INFECTION IN SWINE (Figure 6): In swine, the sources of infection are the same asin cattle. The principal routes of transmission are digestive and venereal. Contraryto the situation in cattle, natural sexual contact is a common and important mode oftransmission. The infection has often been introduced into a herd following theacquisition of an infected boar. Pigs, because of their eating habits and the conditionsin which they are raised, are very likely to become infected through the oralroute. It is also probable that they become infected by aerosols entering via the conjunctivaor upper respiratory tract.INFECTION IN GOATS AND SHEEP (Figure 7): Goats and sheep are infected with B.melitensis in a manner similar to cattle. The role of the buck and ram in transmissionof the infection is not well established. Infection of goats in utero is notunusual, and kids can also become infected during the suckling period; such infectionmay persist in some animals.In ram epididymitis caused by B. ovis, semen is the main and possibly the onlysource of infection. The infection is commonly transmitted from one ram to another