Animal Health - Research Update
Advances in Control of Parasitic Diseases Continue to Rely on Biotechnology
J. Glenn Songer, Ph.D., University of Arizona
"NBIAP News Report." U.S. Department of Agrigulture (December
Parasitic disease continues to be a major health problem worldwide in
humans and domestic animals. Applying biotechnology has led to
progress in prevention and control in many areas, particularly in
regard to diagnosis.
Parasites, like most organisms, carry repetitive sequences in their
genomes. These are sometimes tandem repeats (back-to-back in a
chromosome), or they may be located at random throughout the genome.
Often, repetitive sequences are intergenic and noncoding, and are thus
far more subject to evolution and change, increasing the likelihood
that they will be genus or even species specific. Since detection of
these sequences is not dependent upon gene expression, which may vary
over time, repetitive sequence-based assays allow detection of the
parasite at any life stage. The effectiveness of repetitive
chain reaction (PCR) methods of identification has been
demonstrated for parasites of several clinically important genera (1).
assay has also been developed to differentiate two closely related
nematode species that are virtually indistinguishable by morphologic
examination (2), as well as for diagnosis of Toxoplasma and
Cryptosporidium infections (3).
Attempts to develop anti-parasite vaccines have met with multiple
failures. The problems often result from lack of post-translational
modification by currently used vector-host systems and the inability
to regenerate the secondary and tertiary structure of the native
immunogens. Antigen presentation in recombinant parasite vaccines has
also been thought to be a problem resulting in failed efficacy. There
is now some evidence to indicate that cloning of genes for antigens of
interest to yield chimeric molecules expressed in the BCG strain of
Mycobacterium bovis may provide improved antigen presentation to
stimulate both humoral and cellular responses (4); however, it is
unlikely that future applications of this technology will be simple.
Nonetheless, work is proceeding on the application of biotechnology to
the development of vaccines against parasites in most food animals.
A particularly striking exception to the marginal successes reported
in other systems has been a recombinant vaccine for a parasitic
helminth of sheep (5). This product yielded greater than 94%
protection in experimental work and is now available as a commercial
A further possibility is the application of anti-idiotypic monoclonal
antibodies. When monoclonal
antibodies are prepared against a monoclonal
antibody, it is possible to obtain, in a very small number of
cases, an antibody which mimics the original antigen. Results of work
with Cryptosporidium perfringens epsilon toxin suggest that animals
immunized with anti-idiotype monoclonals can be protected against
- Christensen CM et al.1994. Experimental Parasitol. 78:93-100.
- Zarlenga DS et al. 1994. Experimental Parasitol. 78:28-36.
- Savva D et al. 1990. J Med Microbiol 32: 25-31; Laxer MA et
al. 1991. Am J Trop Med Hyg 45:688-694.
- Zarlenga DS. 1994. J Amer Vet Med Assoc 204: 1616-1621.
- Johnson KS et al. 1989. Nature 338: 585-587.
Go to next story: Genetically Engineered Virus-Resistant Squash Approved For Sale