Bovine Anaplasmosis: A Hunt for a Cure
As of the present day, there are no vaccines for Rickettsial disease that cause human infection. Rickettsial disease is an infection caused by bacteria that lives inside the cells of different ticks and is then transmitted to humans through tick bites, causing many different symptoms. According to WebMD, many different ticks can cause different symptoms and infections in humans including ricketsiosis from a bite of a Gulf Coast tick, anaplasmosis from a black legged tick, and ehrlichiosis from the female lone star ticked, named for a white spot on the ticks back (WebMD). Rickettsial disease was discovered in 1906 by H.T Ricketts and other tick related infections were soon after discovered. The one discussed here is bovine anaplasmosis which is caused by Anaplasma marginale in cattle mostly in South Africa. Research is currently being done to analyze different genomes in order to get a comparison on how to make a vaccine for the disease since it is not yet well known.
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The availability of more than 30 complete rickettsial genomes, all of the genes in the cell of interest, has provided new resources to speed up identification of the proteins and find vaccine candidates by looking at the body’s response in relation to a tick bite with biological data being analyzed and proteins with their specific functions (Brayton, et al., 2012). Connecting the many different proteins in the cells with host protective immunity in order to protect the body is the major challenge in vaccine development for rickettsial disease. Animal models, specifically cattle, have been used for vaccine trial and error because they are closely related to the anaplasmosis pathogen, since this affects cattle. Animal hosts have similarities of infection and immunity with HIV, so SIV was used as a model. This approach has been successful in identifying protection strategies in the body for SIV and therefore immunizations can be prepared for the disease. This has been used as a background for identifying different antigens, a toxin that induces an immune response in the body and testing how we can induce protective immunity in the body and relate this back to rickettsial vaccine development. By using the SIV/ HIV model, there are fewer than 20 lentiviral proteins, and in the larger rickettsial protein cluster in the cell there are closer to 1,000 which makes it hard to find out which proteins can be used to make a vaccine and has to be narrowed down. This has been done with Anaplasma marginale, which is a natural rickettsial pathogen of wild and domestic animals that causes bovine anaplasmosis. Anaplasma marginale is the most frequent tick inducing infection everywhere and it is mostly seen in South Africa where cattle are dying every day from bovine anaplasmosis. Bovine anaplasmosis is a tick-borne disease that was discovered sometime between 1907-1910 in South Africa and is caused by microbial pathogens seen in most cattle farming areas. Symptoms that have been reported include fever, weight loss, and lowered milk production in cattle. Both A. marginale and A. marginale centrale cause the disease but, A. marginale centrale is a less severe form. Both of these infections can be detected with a light microscope using different stains to look at the pathogen in different tissue or an organ smear taken from a cow. The stain is required in order to be able to see the microbe in the microscope because if you do not use a stain it will not show up as easily. By looking at the DNA of different A. marginale strains, the msp1a gene can be used to look at how they compare between different cattle. Msp1a is a gene on the surface of the proteins and can therefore detect any infection which then signals are sent into the body to produce the symptoms of disease. Using this gene, researchers were able to find out the variation of DNA in A. marginale strains where anaplasmosis is present in the world. A few studies have been done to look at the differences between msp1a including identifying msp1a gene similarities in strains in the United States and what is unique to South Africa. Another study that was done looked at the diversity of the A. marginale strains in South Africa and how it could have evolved. The researchers were able to show that msp1a contains B and T cell epitopes with serine residues that are highly conserved, meaning no difference, in the repeat region and are thought to be important for the adhesion function of the msp1a protein, which can lead to a vaccine development (Hove, et al., 2018). A. centrale was also used to look at the divergence of msp1a, msp1aS, two different genes in the cells. It was concluded that A. centrale strains in cattle and wildlife had some genetic diversity and wildlife could be passing on the infection to other animals and humans. Thus, A. centrale can provide some protection against A. marginale and can be used as a vaccine to prevent infection. These vaccines however, have some downsides. A. centrale is used as the blood vaccine in South Africa for cattle but it is expensive to produce because live cattle are needed, and it carries the risk of introducing other infections that may be detrimental to the cattle. It is also important to look at the wide range of differences in A. marginale genes and variation in the outer membrane proteins which direct communication with the body when looking at possible vaccines and the future of A. marginale control. The future of a vaccine for this infection in cattle is looking promising based on all of the research and if a sufficient vaccine is developed, can save many cattle from dying and also help farmers keep their cattle healthy and can make money by selling them.
Specific features critical to A. marginale centrale’s use as a model for vaccine discovery are that protective immunity can be induced in a natural bovine host by vaccination, vaccination trials can incorporate MHC discovery, and the natural mode of challenge for tick transmission can be used to represent an appropriate and relevant challenge (Brayton, et al., 2012). MHC can vary between individuals and it stands for major histocompatibility complex meaning it is a set of cell surface proteins in the immune system that recognize foreign molecules and can therefore produce the response in the body to kill off the infection. This gives the results of three immune system and protein-based targets to identify what to target to produce a response with a vaccine for rickettsial disease. Correctly identifying outer membrane proteins that have orthologues, sequence similarities that derived from a common ancestor, that are mostly the same among different rickettsial pathogens to develop a vaccine for the disease is very challenging and possess many difficult steps. Biological data was used to identify outer membrane proteins in the A. marginale genome and researchers found that 62 of the 949 proteins represented all of the proteins expressed in the cell. Then, they were processed to find out which ones were immunogenic, or could produce an immune response, for protective immunity by immunizing cows of different MHC haplotypes with the outer membrane immunogen known to be protective against the infection (Brayton, et al., 2012). Of the 62, 21 were identified as being present in the protective outer membrane immunogen and capable of stimulating IgG2, a serum that represents the opsonizing bovine IgG subclass and reflects the requirement for MHC class II- restricted CD4+ T lymphocyte help for B cell isotype class switching, both which have been linked to protective immunity (Brayton, et al., 2012). The purpose of the isotype class switching is to allow the antigens in the cattle to recognize a foreign pathogen, such as the A. marginale microbe, and be able to stop it before it infects the body of the cow. The IgG isotype is released by these plasma B cells in order to protect against infections to the body. The CD4+ T cells are responsible for many immune responses and the B cells are responsible for producing antibodies for diseases and infections, so the infection will be subdued. Both the CD4+ T and B cell epitopes, the antigen where the antibody attaches, produce B and T memory cells in order to be able to remember what the vaccine was and subdue further A. marginale infections.
To narrow the outer membrane proteins to those with surface exposed domains, which will bind different receptors for the disease to be transmitted into the body, a process was followed. This included a macromolecular surface complex being isolated by cross linking proteins on intact bacteria, reactive with primary amines with a spacer arm of 1.2nm, and electrophoresis separation (Brayton, et al., 2012). Electrophoresis is a process in which the macromolecules (DNA, RNA, or proteins) can be fragmented based on size and charge through a gel of agarose, they will run down the gel with the largest ending at the bottom of the gel. A group of calves that had been selected with MHC diversity were immunized with the surface protein complex or the outer membrane immunogen (Brayton, et al., 2012). It was found that both groups were protected against the disease as compared to the controls with no difference between the outer membrane and surface complex immunogens and one did not protect from disease. To narrow the vaccine possibilities further and match outer membrane proteins more closely with the infection, the live A. marginale ss. Centrale vaccine strain was used to induce protective immunity in order to prevent infection from the pathogen. The vaccine strain was isolated in South Africa and also protects against infection and death caused by St. Maries strains. The vaccine has not been used in much of the world because of some complications but has been used in Australia and Israel for many years. Calves were immunized with the A. marginale ss. Centrale strain to look for similarities with the St. Maries strain and then given tick transmission of the St. Maries strain to show that the cattle will not get an infection. It was found that three outer membrane proteins between the two strains were not able to produce an immune response in the body because of the sequence differences in the proteins even though they diverged from a common ancestor. It is therefore thought that they could have mutated and changed after being inside the cattle to attack their immune system and induce a response. The identified vaccine candidates are Am779, Am854, and omp 7/8/9. Omp 7-9 are encoded by three genes very similar to the St. Maries strain and the locus at the gene is in the vaccine strain as a single omp, thus giving the single omp 7/8/9 (Brayton, et al., 2012).
Overall, the two articles were very well written and gave great insight into how the future of vaccines for both rickettsial disease and bovine anaplasmosis is changing. They gave sufficient detail to know what is being talked about and how they can look at the different genomes of A. marginale and A. marginale centrale to create a live vaccine. Many different approaches have been studied in order to look at bovine anaplasmosis and most importantly, live cattle are required in order to look at vaccine approaches. This causes problems and restrictions with research because of the cost and ethical reasons. Performing different research on cattle is costly because of the production of getting the cattle and this takes them away from farmers who need to sell them to make money. Researchers have identified a few different vaccine candidates with the A. marginale centrale strain of bovine anaplasmosis used to provide protection against A. marginale and also the A. marginale ss Centrale strain was used to look at protection against the St. Maries strain. Continuing research on this topic will go far and many cattle will be protected if a true vaccine is developed. Narrowing the outer membrane proteins down to figure out which ones can stop the infection have been a huge step and researchers next step is to actually come up with the vaccine, not just a testable live vaccine, and to produce the vaccine and get it approved by the FDA in order for it to be utilized. These next steps could take many years because of the challenge they possess, but will be very beneficial when they succeed.
This picture was taken from the site beefproducer.com. It shows the cycle of bovine anaplasmosis and how it infects cattle.
- Palmer, G. H., Brown, W. C., Noh, S. M., & Brayton, K. A. (2012). Genome-wide screening and identification of antigens for rickettsial vaccine development. Fems Immunology and Medical Microbiology, 64(1), 115–119. http://doi.org/10.1111/j.1574-695X.2011.00878.x
- Hove, P., Khumalo, Z., Chaisi, M. E., Oosthuizen, M. C., Brayton, K. A., & Collins, N. E. (2018). Detection and Characterisation of Anaplasma marginale and A. centrale in South Africa. Veterinary sciences, 5(1), 26. doi:10.3390/vetsci5010026
- What Are Rickettsial Diseases? (n.d.). Retrieved November 4, 2018, from https://www.webmd.com/skin-problems-and-treatments/rickettsial-diseases-overview#1
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