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Larvae and nymphs of Ixodes loricatus Neumann, 1899 ticks (confirmed by morphological characters and by comparison of 16S mitochondrial rDNA sequences) were collected from Sigmodontinae Wagner, rodents in central and northern Argentina and Uruguay. A total of 100 larvae and 38 nymphs of I. loricatus were collected on the genera Akodon Meyen (n = 36 individuals), Calomys Waterhouse (n = 2), Oligoryzomys Bang in = 12), Oxymycterus Waterhouse (n = 9), and Scapteromys Waterhouse (n = 13). 72 larvae and 18 nymphs were collected on Akodon. Adults of I. loricatus were found in central Argentina and Uruguay on Didelphimorphia of the genera Didelphis Linnaeus and Lutreolina Thomas. Ixodes loricatus has been considered a species with strict total specificity to Didelphimorphia. Our results show that this statement may not be justified. Sigmodontinae rodents are sympatric and share habitats with the phylogenetically distant Didelphimorphia; infestation with I. loricatus seems to be its consequence. We tentatively consider I. loricatus moderately specific to Didelphimorphia.

The seasonal distribution of Haematobia irritans (Linnaeus, 1758) was evaluated at 31 degrees 12' S-61 degrees 29' W, Santa Fe, Argentina from November 1992 to August 2000 by weekly fly counts on 20 Holstein cows not treated against ectoparasites. The analysis was mainly focused on winter fly abundance. Two peaks of abundance were found from spring to autumn. Adults of H. irritans were consistently found on cattle during winter, with increasing numbers from the end of July to late August. The only climatic parameter soundly correlated with the four week period, before each winter fly count, was the mean air temperature with the exception of year 1998. No significant relationship was found between level of cow infestation and relative humidity, saturation deficit and rainfall. Horn fly infestation on cows was lower than 20% from end of June to end of August only in 1995, when mean air temperature was consistently lower than 11 degrees C during the four week period previous to most fly counts. Conversely, the mean temperature was higher than 12 degrees C previous to fly counts in 1998, when most cows remained infested. The results indicate that a proportion (unknown) of immature stages of H. irritans were insensitive to diapause inducing factors and developed through winter.

Combined inoculation of cattle with vaccine strains of Babesia bigemina and Babesia bovis induced lower antibody titers to B. bigemina than to B. bovis (previous study). Three groups of heifers were used to detect if the low antibody level was due to competition between Babesia species: individuals of G1 and G2 were inoculated with 10 million B. bigemina and B. bovis, respectively, and those of G3 with 10 million of each parasite. The prepatent periods, maximum parasitaemias and antibody titers (indirect immunofluorescent antibody test) were evaluated. The mean prepatent periods (days) for B. bigemina was of 5.6 (G1) and 5.2 (G3) and 7.0 (G2) and 6.7 (G3) for B. bovis (P > 0.05, "t" test). No differences were found in the parasitaemias. The only difference was found in the antibody titers to B. bovis, that were lower (P < 0.05 "t" test) from week 7 onwards when B. bovis was used in combination. The biological significance of this difference is unclear.

Quantitative aspects of the natural babesial (Babesia bovis and Babesia bigemina) infection in Boophilus microplus engorged female ticks obtained from two herds of Holstein heifers positive by the immunofluorescent antibody test to both protozoan were evaluated. The number of kinetes/microscope field of haemolymph was determined for each tick from day 5 to 10 post-collection. A close relationship between daily and cumulative babesial infection was detected. Correlation and determination coefficients between days post-collection and the daily and cumulative infection rates, including heavily infected ticks (those ticks carrying at least 3.0 kinetes/microscope field of haemolymph), were always higher than 0.9 (P < 0.01) in ticks of both herds. The median was found to be a more representative measure than the mean to define the distribution of kinetes number amongst infected ticks since this is a negative binomial distribution. The analysis of the sequential order of days of infection more accurately showed the amplification of the babesial infection in the tick haemolymph than the evolution of kinetes number in relation to days post-collection. Sampling ticks on days 8, 9 and 10 post-collection would have detected all ticks infected with Babesia spp. from both herds. A categorization of infected or non infected ticks would be of greater epidemiological importance than the haemolymph infection level, based upon previous laboratory studies that showed a poor relationship between haemolymph infection in the female ticks and the infection rate in their eggs. However, further studies in natural infected ticks and better techniques to differentiate B. bovis and B. bigemina kinetes are needed before these laboratory results can be applied to field conditions.

Nine splenectomised calves were infested with dissimilar numbers of adult Boophilus microplus ticks 72 h after collection as engorged nymphs from three non splenectomised calves with different levels of Anaplasma marginale rickettsaemia. Successful transmission of A. marginale appeared to be more dependent on the level of rickettsaemia of the donor calves than on the number of ticks attaching to the splenectomised calves, since infection was transmitted only when the rickettsaemia was 0.3% or greater. Field transmission would thus depend on the rate of tick migration amongst susceptible hosts and the rickettsaemia level of cattle on which the ticks fed previously.