Bovine Anaplasmosis: Will there ever be an almighty effective vaccine?

Bovine anaplasmosis is a tick-borne bacterial disease with a worldwide distribution and the cause of severe economic losses in the livestock industry in many countries, including México. In the present work, we first review the elements of the immune response of the bovine, which allows ameliorating the clinical signs while eliminating the majority of the blood forms and generating an immunologic memory such that future confrontations with the pathogen will not end in disease. On the other hand, many vaccine candidates have been evaluated for the control of bovine anaplasmosis yet without no commercial worldwide effective vaccine. Lastly, the diversity of the pathogen and how this diversity has impaired the many efforts to control the disease are reviewed.

Molecular epidemiology of bovine anaplasmosis with a particular focus in Mexico.

Bovine anaplasmosis, caused by the rickettsia Anaplasma marginale, has a worldwide distribution and is the cause of great economic losses in developing countries where it is highly endemic. Transmission is carried mainly by ixodid ticks: Dermacentor spp. and Rhipicephalus (Boophilus) spp. Mechanical transmission is important in disseminating the disease within and across herds. The relationship between the rickettsia, the host and the vector is complex. Several surface proteins (Msps) have been described with functions that span from adhesins towards the erythrocyte and tick cells to evasion of the immune system of the host through the generation of antigenic variants. Biologic transmission of A. marginale through Dermacentor ticks has been well studied but many questions are unresolved as to how this organism spreads within and across herds and little is known about the role Rhipicephalus (Boophilus) ticks play in transmission in the Americas. Mechanical transmission in the absence of ticks and lack of transmission through ticks are questions that need to be addressed. Phylogenetic studies of the rickettsia show wide antigenic and genetic mosaics which affects the design of new vaccines. In the present work we will discuss the molecular elements in the relationship between the rickettsia, the tick and the mammalian host associated to the distribution and persistence of the pathogen in nature.

Anaplasma marginale field challenge: protection by an inactivated immunogen that shares partial sequence of msp1alpha variable region with the challenge strain.

Twenty four Hereford heifers free of anaplasmosis were allotted into three groups of eight animals each and inoculated three times with adjuvant in Puck saline as control or 50 microg and 100 microg of total protein of Anaplasma marginale initial bodies from three Mexican strains which share the same variable region of msp1alpha and msp4. Inoculation with the adjuvant or the immunogen at either of the two protein doses did not induce any undesirable changes attributable to inoculation in vaccinates or controls. On day 78 post vaccination animals were released in a ranch where bovine Anaplasmosis is endemic. The A. marginale strain prevalent in this ranch shares some of the msp1alpha tandem repeats with and the strains used in the vaccine. After release, all animals became infested with Boophilus microplus ticks and flies. During the challenge period, between days 279 and 300, loss of PCV due to clinical anaplasmosis in control animals was statistically higher from vaccinated animals. Likewise, controls mean peak rickettsemia was also significantly higher (p< or =0.01) than vaccinates' rickettsemias. The antibody responses of all vaccinates after the third vaccination reached OD values above 2.0 on day 49 and were different from controls (p<0.01). IgG(2) responses from both groups of vaccinates were different from controls (p<0.01). Vaccinates which required treatment, also showed the lowest IgG(2) and substantial IgG(1) responses. After contact with the rickettsia, controls developed clinical disease and 7 out of 8 required treatment, while vaccinates in general showed no substantial changes in hematocrit or rickettsemia and only one animal in each group required treatment. Our present results show that vaccination with either 50 microg or 100 microg of protein from purified IB derived from three strains induced protection to resist the challenge with the a field strain that shares some of the tandem repeats of MSP1a.

Analysis of world strains of Anaplasma marginale using major surface protein 1a repeat sequences.

Anaplasma marginale is a tick-borne pathogen of cattle that causes the disease bovine anaplasmosis worldwide. Major surface proteins (MSPs) are involved in host-pathogen and tick-pathogen interactions and have been used as markers for the genetic characterization of A. marginale strains and phylogenetic studies. MSP1a is involved in the adhesion and transmission of A. marginale by ticks and varies among geographic strains in the number and sequence of amino-terminal tandem repeats. The aim of this study was to characterize the genetic diversity of A. marginale strains collected from countries in North and South America, Europe, Asia, Africa and Australia, inclusive of all continents. In this study, we characterized 131 strains of A. marginale using 79 MSP1a repeat sequences. These results corroborated the genetic heterogeneity of A. marginale strains in endemic regions worldwide. The phylogenetic analyses of MSP1a repeat sequences did not result in clusters according to the geographic origin of A. marginale strains but provided phylogeographic information. Seventy-eight percent of the MSP1a repeat sequences were present in strains from a single geographic region. Strong (> or =80%) support was found for clusters containing sequences from Italian, Spanish, Chinese, Argentinean and South American strains. The phylogenetic analyses of MSP1a repeat sequences suggested tick-pathogen co-evolution and provided evidence of multiple introductions of A. marginale strains from various geographic locations worldwide. These results contribute to the understanding of the genetic diversity and evolution of A. marginale and tick-pathogen interactions.

Anaplasma marginale: lack of cross-protection between strains that share MSP1a variable region and MSP4.

In Mexico, there are no commercial alternatives for the immunoprophylaxis of bovine Anaplasmosis, a disease responsible for great economic losses. Blood derived Anaplasma marginale used for immunizing susceptible cattle has shown promising results for homologous protection and controversial results against unrelated strains. The present study examined, under controlled conditions, the cross-protective potential of an immunogen composed of blood derived A. marginale of three strains against challenge with strains not included in the immunogens. Groups 1 and 2 were immunized with blood derived Anaplasma from strains Mexico, Morelos and Yucatan, group 4 with strains Morelos, Veracruz and Yucatan, two more groups (2 and 5) of equal conditions were inoculated with an adjuvant alone. Groups 1, 4 and 5 were challenged with Mexico strain; groups 2 and 3 were challenge-inoculated with strain Veracruz; groups 3 and 5 with strains Veracruz and Mexico as controls. Only animals in group 1, immunized and challenged with strain Mexico showed adequate protection. Both groups challenged with strains not included in the immunogens developed poor protection, while all the controls had to be treated to prevent death.

Phylogeography of New World isolates of Anaplasma marginale based on major surface protein sequences.

Gene and protein sequences of major surface proteins (MSP) 1a and 4 of Anaplasma marginale (Rickettsiales: Anaplasmataceae) were used to infer phylogenetic relationships between New World isolates from Argentina, Brazil, Mexico and the United States. Seventeen isolates of A. marginale plus two outgroup taxa (A. centrale and A. ovis) were used for maximum-parsimony analysis of MSP4, while 20 isolates were used for phylogenetic analysis of MSP1a. msp4 analysis provided strong bootstrap support for a Latin American clade and, within this clade, support was detected for Mexican and South American clades. Isolates of A. marginale from the United States also grouped into two clades from the southern (isolates from Florida, Mississippi, and Virginia) and west-central (isolates from California, Idaho, Illinois, Oklahoma, and Texas) states. Although little phylogeographic resolution was detected within these higher clades, msp4 sequences appear to be a good genetic marker for inferring phylogeographic patterns of A. marginale isolates. In contrast to the phylogeographic resolution provided by msp4, MSP1a DNA and protein sequence were quite variable and did not provide phylogeographic resolution. Most variation in MSP1a sequences appeared unique to a given isolate and similar DNA sequence variation in msp1alpha was detected within isolates from Idaho and Florida and from Idaho and Argentina. The results of these studies demonstrated that msp4 provided phylogenetic information on the evolution of A. marginale isolates. In contrast MSP1a sequences appeared to be rapidly evolving and these sequences may provide phylogeographic information only when numerous isolate MSP1a sequences are analyzed from a geographic area.

Molecular epidemiology of bovine anaplasmosis

Bovine anaplasmosis presents a worldwide distribution. However, specific models for studying the epidemiology of the disease are not available. Epidemiological modeling encounters some difficulties due to a lack of culturing techniques for Anaplasma marginales, the causative agent, as well as for the lack of typing techniques to characterize strains. The chronic carrier state and the population dynamics of mechanical and biological vector also create difficulties. In addition, conventional serology and blood smear diagnostic techniques fail to detect all chronica carriers. Fortunately the needs for the accurate typing of isolates and for detecting chronica carriers made it possible to encourage the development of new tools based on molecular epidemiology principles. A. marginali isolates can now be typed by using panels of monoclonal antibodies, and the genes coding for some major surface proteins can be expressed or analyzed by looking at the nucleotide arrangement level. In the same manner, the latest techniques for detecting A. marginale chronic infections use DNA and RNA probes, and PCR-based methods to detect A. marginali DNA from bovine blood samples with extremely low rickettsaemias. Currently all these new epidemiological tools are being incroporated to experimental models to analyze their applicability for epidemiological studies in the near future

Bovine babesiosis: a review of recent advances

Bovine babesiosis, caused by parasites of the genus babesia, is one of the world's most severe tick-borne problems of cattle in temperate to tropical areas. In the America Babesia bovis and B bigemina are the causative agents, with the former considered to produce the greatest economic impact. The great complexity of the relationship causal agent-vector-host has severely hindered the efforts towards the production of a safe, long-lasting, solid-protection inducing vaccine. Recent importan contributions that have encourage the study of these agents include the development of in vitro cultivation systems, procedures for the isolation of single infected-erythrocytes, density gradient-based centrifugation systems for the isolation and concentration of both infected erythrocytes and merozoites, isozyme detection and differentitation systems that help discrimate between parasite species, and development of DNA-based diagnostics and characterization protocols. Currently, the study of the cellular immune response against these parasites is taking new endeavors in order to discern the relationship between B cells, T cell, macrophages and their product and parasites leading to the establishment of solid, long-lasting protection. In an attempt to design a rational vaccine, T cell lines and clones are being established, and phagocytosis of infected erythrocytes and their antigens studied to try to pinpoint relevant epitopes