Babesiosis (Babesia bovis) stability in unstable environments.

Enzootic stability (herd immunity) in bovine babesiosis occurs when the rate of transmission (inoculation rate) of Babesia spp by the tick vector is sufficient to immunize a majority of susceptible calves before the loss of calfhood resistance. The effect of three tick (Boophilus microplus) control strategies (none, threshold, and strategic) on enzootic stability and the likelihood of babesiosis (Babesia bovis) outbreaks was studied using a spreadsheet age-class computer simulation model. The model was driven by weekly bovine tick counts from Brazil and Uruguay. The Eldorado do Sul, RS, Brazil bovine population (30 degrees 05' South latitude) was found to be in a naturally occurring state of enzootic stability, corresponding to an inoculation rate exceeding 0.005 throughout the year. Threshold dipping strategies should not increase the risk of babesiosis in cattle so managed. Strategic dipping resulted in an extended period of enzootic instability lasting 30 weeks, which requires protection of the herd through immunization. Because of the more prolonged low winter temperature conditions, the Tacuarembó, Uruguay bovine population (31 degrees 40' South latitude) was found to be in a naturally occurring state of enzootic instability, characterized by a 28 week period in which the inoculation rate was below 0.005. Strategic dipping should lead to eradication of the babesial parasite from tick and bovine populations, but would not result in eradication of the tick vector. This could lead to subsequent outbreaks if Babesia carrier animals were to be introduced into the herd. In both populations, strategic tick control could be accompanied by concurrent babesiosis vaccination.

Strategies for the control of one-host ticks and relationship with tick-borne diseases in South America.

A variety of hemoparasites occurs in ruminants of South America (12 degrees 00'N-56 degrees 00'S), but there is consensus on the significant economic impact of babesiosis (Babesia bovis and Babesia bigemina) and anaplasmosis (Anaplasma marginale). The tick vector for the Babesia spp. for the whole region is Boophilus microplus which has been the most important target in control/eradication programs. Boophilus microplus and many different hematophagous diptera species are considered vectors of A. marginale. In areas where babesiosis and anaplasmosis are present, the wide range of climates (tropical, subtropical, temperate), husbandry practices (subsistence, extensive, intensive), and tick control strategies has led to the concepts of enzootic stability and instability. Within this framework, six control strategies for Boophilus microplus can be identified: (1) absolute tick control--with the establishment of quarantine areas and the implementation of state legislation; (2) absolute tick control in marginal areas--using local epidemiological knowledge, strategic treatments, quarantine areas and the intervention of the state; (3) prophylactic treatment--with a predetermined acaricide treatment schedule aimed at reducing tick burdens to an acceptably low level; (4) threshold treatment--acaricides are applied by the farmer whenever the number of ticks on cattle exceed a threshold level; (5) opportunistic treatment--acaricides are applied when cattle are assembled for other purposes; (6) integrated tick control--principally involving the combination of acaricides with the use of tick-resistant crossbred cattle.

Tests to determine LC50 and discriminating doses for macrocyclic lactones against the cattle tick, Boophilus microplus.

Laboratory tests were carried out on larvae and adults of the cattle tick Boophilus microplus to determine the toxicity of macrocyclic lactone acaricides (MLs). Technical and commercial MLs were used in larval packet test (LPT), larval immersion test (LIT) and adult immersion test (AIT). In LIT and AIT the toxicity of MLs was much higher than for LPT. In the AIT, diluting the injectable formulation of MLs in water was as effective as dilution in ethanol+Triton X-100. LC50, LC99.9 and 95% confidence limits were determined so that a discriminating dose (DD) could be set for larval and adult tests in order to diagnose potential resistance to MLs in field samples of the tick. These DDs are for Australian strains of B. microplus and may not be suitable for other strains until further work is carried out. The value of these diagnostic tests can only be verified if or when resistance to MLs emerges in ticks.

The prevalence of anthelmintic resistance in nematode parasites of sheep in southern Latin America: general overview.

South America has a large population of small ruminants. Currents estimates are approximately 100 million sheep and 23 million goats. A large percentage of these flocks are raised in the humid tropics/sub-tropics. Nematode parasitism is singly the most important disease of these animals and typically farmers resort to frequent anthelmintic treatment in attempts to control this problem. Because of this reliance on drugs, price consideration is an important determinant in a farmer's choice of anthelmintic. In some circumstances, this has led to unethical practices of drug adulteration and substitution, or the offering to farmers of cheap, sub-standard generic products. The prevalence of anthelmintic resistance has not been investigated in any widespread sense in South America, although some of the first reports of resistance were made in southern Brazil almost 30 years ago. The following series of papers outline surveys conducted in the humid topics/sub-tropics of southern Latin America to assess the significance of resistance to the broad-spectrum anthelmintic groups in nematode parasites of sheep flocks.

The prevalence of anthelmintic resistance in nematode parasites of sheep in southern Latin America: Uruguay.

This survey was conducted on 252 farms randomly distributed over all the sheep raising areas in Uruguay. The study involved farms with more than 600 sheep, which represented 80% of the total sheep population of the country. Three anthelmintic groups were assessed, namely, benzimidazoles, levamisole and avermectins. Overall, the results showed 80% of sheep flocks had benzimidazole resistance, 71% had resistance to levamisole, and 1.2% of flocks showed resistance to avermectins. Approximately 28% of farms had resistance to one anthelmintic group, 64% to two anthelmintic groups, and 1% resistance to all three groups. Only 7.5% of farms had no detectable levels of anthelmintic resistance. More than 80% of farms had Trichostrongylus populations resistant to both benzimidazoles and levamisole. Resistance was recorded in all three anthelmintic groups for Haemonchus and resistance also occurred to benzimidazoles and levamisole in Ostertagia.

Epidemiology of bovine anaplasmosis and babesiosis in Latin America and the Caribbean.

The various parameters which interact in the epizootiology of babesiosis and anaplasmosis and which it is necessary to obtain in a survey are analysed and the two diseases compared. Where no data are available from Latin America and the Caribbean, the references from Australia and the United States of America are discussed. The two principal objectives of a study on the epidemiology of babesiosis and anaplasmosis are: to determine the risk of occurrence to understand the relevant factors leading to outbreaks. The first objective is relatively simple to achieve for both diseases by calculating the animal inoculation rate (h) at a determined age and the critical value of h for enzootic stability. The second objective requires exhaustive and complex studies. For babesiosis epidemiology, studies of the tick infection rate, cattle infestation rate, genetic composition of cattle, acaricides, stocking rate, climatic data and other factors are necessary. Anaplasmosis epidemiology cannot be satisfactorily explained by considering it purely as a tick-borne disease. Difficulties to be overcome include the great diversity of haematophagous Diptera in Latin America, and the scarcity of data on the capacity of these insects as anaplasmosis vectors and their feed preferences on cattle.

Methods currently used for the control of one-host ticks: their validity and proposals for future control strategies.

Available methods for the control of one-host ticks as well as their possible applications in control programmes are reviewed. The advantages and disadvantages of current methods (dipping vats, spray races, hand sprays) are compared to other chemical control methods (ear-tags, pour-ons, systemics, slow-release devices). Their integrated application with other technologies based on immunological, genetic or biological approaches is discussed. Because of the great diversity of geographic, climatic, infrastructural and scientific-technological developments in vast areas of the world, the author concludes that there is no universal method of application. What really matters is to be able to develop control programmes based on local ecological, socio-economic and political facts, integrating the best of available technology. Future tick control prospects are discussed with reference to the need for standardization of criteria on requirements and the legal basis for acaricide registration, introduction of new evaluation techniques for acaricide efficacy and development of faster techniques to recognize resistance. Special emphasis is laid on the need to establish control programmes integrating diagnosis, research and production of vaccines against one-host ticks and their haemoparasites.

Impact of Babesia bovis and Babesia bigemina on the production of beef cattle in Uruguay.

Uruguay is situated in a marginal area for the development of Boophilus microplus (30 degrees 35 degrees South Lat.) with important areas of enzootic instability for Babesia bovis and B. bigemina. The livestock products represent 70% of our exports, for which reason it is fundamental to evaluate the losses in the production that these haemoparasites cause as basic information to take future decisions. In the period 1988-1990, several works were carried out by our laboratory to know the incidence of babesiosis in the reduction of liveweight gains. The results are shown and discussed in the work. Experiment I: the weight increase of the control group (x = 0.248 kg/day), was 23% higher than that of the infected group with Babesia spp (from Uruguay), but significant statistical differences were not found (P < 0.05). These animals were kept in boxes and the food was controlled for 76 days. Experiment II: the incidence of Babesia spp (same strain) was studied for 140 days on Hereford heifers (n = 14) on natural pastures. The control group obtained x = 25.29 kg of liveweight gain and it was 45% higher than that of the infected group, significant statistical difference were found (P < 0.05). Experiments with attenuated strains III: four studies were carried out inoculating B. bovis and B. bigemina in bovines about one year old, in different growth systems, searching for the limit of application. Significant statistical differences between those groups were not found during the experiment (about 180 days) (P < 0.05).(ABSTRACT TRUNCATED AT 250 WORDS)

Impact of Babesia bovis and Babesia bigemina on the production of beef cattle in Uruguay

Uruguay is situated in a marginal area for the development of Boophilus microplus (30- 35- South Lat.) with important areas of enzootic instability for Babesia bovis and B. bigemina. The livestock products represent 70% of our exports, for wich reason it is fundamental to evaluate the losses in the production that these haemoparasites cause as basic information to take future decisions. In the period 1988-1990, several works were carried out by our laboratory to know the incidence of babesiosis in the reduction of liveweight gains. The results are shown and discussed in the work. Experiment I: the weight increase of the control group (x = 0,248kg/day), was 23% higher than that of the infected group with Babesia spp (from Uruguay), but significant statistical differences were not found (P < 0,05). These animals were kept in boxes and the food was controlled for 76 days. Experiment II: the incidence of Babesis spp (same strain ) was studied for 140 on Hereford heifers (n = 14) on natural pastures. The control group obtained x = 25,29kg of liveweight gain and it was 45% higher than that of the infected group, significant statistical difference were found (P < 0,05). Experiments with attenuated strains III: four studies were carried out inoculating B. bovis and B. bigemina in bovines about one year old, in different growth systems, searching for the limit of application. Significant statistical differences between those groups were found during the experiment (about 180 days) (P < 0,05). Experiment combining and pathogenic strains IV: the liveweight gain, in immune and challanged group (n = 14) was the same than that of the unchallenged group and did not show significant statistical differences (P < 0,05). However the control challenged group had less weight gain and statistical differences were found (P < 0,05). Although this is a preliminary information, it shows that: (a) the incidence of babesiosis on the reduction of weight gains is important; (b) the decrease in weight gain was not observed when attenuated strains were used; when the challenge was done in immunized animals, losses in liveweight gain were not observed. These results are discussed in order to plan future studies in different real systems of production

Epidemiology of bovine anaplasmosis and babesiosis in Latin America and the Caribbean

The various parameters which interact in the epizootiology of babesiosis and anaplasmosis and which it is necessary to obtain in a survey are analysed and the two diseases compared. Where no data are available from Latin America and the Caribbean, the references from Australia and the United States of America are discussed. The two principal objectives of a study on the epidemiology of babesiosis and anaplasmosis are: to determine the risk of occurrence to understand the relevant factors leading to outbreaks. The first objective is relatively simple to achieve for both diseases by calculating the animal inoculation rate (h) at a determined age and the critical value of h for enzootic stability. The second objective requires exhaustive and complex studies. For babesiosis epidemiology, studies of the tick infection rate, cattle infestation rate, genetic composition of cattle, acaricides, stocking rate, climatic data and other factors are necessary. Anaplasmosis epidemiology cannot be satisfactorily explained by considering it purely as a tick-borne disease. Difficulties to be overcome include the great diversity of haematophagous Diptera in Latin America, and the scarcity of data on the capacity of these insects as anaplasmosis vectors and their feed preferences on cattle. (AU)