Persistently Infected Calves as Reservoirs for Acquisition and Transovarial Transmission of Babesia bovis by Rhipicephalus ( Boophilus ) microplus

Abstract

Babesia bovis is a deadly disease of cattle resulting in severe economic losses in the vast regions of the world where it is endemic. If reintroduced into the United States, babesiosis would cause significant mortality in the naïve cattle population. In order to address the risk to U.S. cattle, it is essential to quantify the transovarial transmission efficiency in adult female Boophilus microplus ticks following acquisition feeding on persistently infected cattle. This study tested the hypothesis that infection rates are the same for larval progeny derived from females fed to repletion during persistent or acute infection. Increasing parasite levels during acute infection correlated with an increasing number of females harboring kinetes detectable in hemolymph (r ‫؍‬ 0.9). The percent infected larvae ranged from 0 to 20% when derived from females fed to repletion on persistently infected calves and from 4 to 6% when derived from females fed to repletion during acute parasitemia. There was no significant difference in infection rates of larval progeny, implying that the risk associated with the introduction of either persistently infected or acutely infected cattle is equal. Parasite levels ranged from 2.4 ؋ 10 2 to 1.9 ؋ 10 5 in 3-day-fed larvae derived from females fed to repletion on persistently infected cattle. One group of larvae failed to transmit the parasite, suggesting that a threshold level of parasites must be obtained by larval progeny via transovarial transmission in order for larvae to deliver sufficient parasites to infect a naïve host.

Persistently Infected Horses Are Reservoirs for Intrastadial Tick-Borne Transmission of the Apicomplexan Parasite Babesia equi

“…When approximately 1% of the nymphs molted to adulthood, 8 ϫ 10 7 B. equi-infected erythrocytes were inoculated intravenously into horses H061 and H144 to synchronize ascending B. equi parasitemia with repletion of female R. microplus ticks. Engorged female ticks that detached during ascending parasitemia were collected and incubated at 26°C, at 94% relative humidity, and with a 12-h photoperiod to allow for egg production (7,8). To detect the most heavily infected female ticks with the highest likelihood of successful vertical transmission, an aliquot of hemolymph was collected from individual live ticks in cell lysis solution (Gentra Systems, Inc., Minneapolis, MN) with proteinase K (2 mg/ml) and tested for the presence of B. equi organisms by nested PCR.…”

Tick Passage Results in Enhanced Attenuation of Babesia bovis

“…All calves used in the study were uninfected with B. bovis, as determined by sourcing from the United States, which is declared free of bovine babesiosis by the World Organization for Animal Health, and by confirmation of the absence of serum antibody and parasites by competitive enzymelinked immunosorbent assay (cELISA) (VMRD, Pullman, WA) and quantitative PCR, respectively, as previously described (4). All calves were splenectomized at the Veterinary Teaching Hospital a minimum of 2 weeks prior to application of larval ticks for acquisition or transmission feedings.…”

“…Parasites were quantified from these samples using the msa-1 qPCR assay as described above. A positive control containing 10 ng of T2Bo_vir gDNA and gDNA negative controls from naïve-larva, calf, and tick tissues were included (4,15). Genomic DNA was also extracted from 100 l of individual tick hemolymph using the DNeasy blood and tissue DNA kit as per the manufacturer's recommendation (Qiagen, Valencia, CA).…”

“…Genomic DNA was also extracted from 100 l of whole blood using a DNeasy blood and tissue DNA kit as per the manufacturer's recommendation (Qiagen, Valencia, CA). Quantitative PCR targeting the merozoite surface antigen 1 gene (msa-1) was performed as previously described (4,15). A positive control containing 10 ng of T2Bo gDNA and a negative control consisting of naive tissue or blood DNA were included.…”

“…This neurovirulence is associated with the cytoadherence of infected erythrocytes to endothelial cells, which subsequently leads to sequestration within cerebral capillaries, causing neurological disease (2,3). Animals that recover from acute babesiosis without treatment become persistently infected and are potential reservoirs for tick transmission within the herd (4).…”