Congenital and Hereditary Conditions of Camelids

Chapter 48 Congenital and Hereditary Conditions of Camelids



The appearance of a congenital defect in any wild animal is cause for concern to keepers, zoo managers, and zoo veterinarians. Historically, congenital defects have been reported ever since animals were first maintained in captivity. Inbreeding is a constant hazard that may contribute significantly to increasing homozygosity in a small population of animals. The development of species survival plans (SSPs) in the past decade has made it possible to evaluate and scientifically share genetic lines to minimize the effects of inbreeding and linebreeding. However, challenges still exist.


Camelidae (camels, llamas, alpacas, guanacos, and vicu-as) were kept primarily as zoo animals in the United States until the 1970s, when the public became enamored of these animals and started to breed them privately as an alternative livestock enterprise. This chapter discusses the congenital/hereditary conditions of the camelidae.


Solving congenital defect problems begins with recognizing that many causes exist for congenital defects and are not necessarily hereditary (Box 48-1). Many factors may influence the well-being of the fetus, and numerous agents other than genetic factors may disrupt organogenesis.




TERMINOLOGY


Congenital condition Disorder present at birth. Unfortunately, some congenital defects may not be readily apparent at birth. For example, in humans and livestock, certain biochemical defects may not be visible until later in life. Conformation characteristics may not become apparent until after the cria has grown. In these cases, however, the foundation for the characteristic is present at birth.


Hereditary condition Disorder genetically transmitted from parent to offspring. The manifestation of the condition may be present at birth or may develop subsequently.


Genetic disorder The term genetic is frequently used interchangeably with hereditary, but these are not synonymous. Certain genetic disorders may cause serious defects in a single individual, but the disorder will not be passed on to subsequent generations. However, reproduction may be impossible because of the nature of the defects.


Teratology The science of teratology (Greek teras, “monster”) deals with overall birth defects. A teratogen is any agent that causes abnormal development of the fetus. The furor over inadequate testing of drugs that may be prescribed for pregnant women was spawned by the thalidomide disaster of the 1970s. Both physical and chemical effects on the fetus were known before that time, but now an entire discipline of medicine and biology deals with such topics.


Teratogenesis Process by which teratogens exert their effect.


Genotype Genetic constitution of an individual at one or more loci.


Phenotype Observable trait of an individual.


Inbreeding Breeding of related individuals.


Inbreeding depression Decrease in performance resulting from inbreeding.


Linebreeding When an ancestor appears multiple times in a pedigree; typically used to foster a desirable trait from a superior ancestor.


Camelid All species of the camelidae.


SACs South American camelids (llama, alpaca, guanaco, vicu-a).




TERATOGENESIS


The causes of congenital and hereditary defects are manifold (see Box 48-1). Although no specific congenital defects caused by infectious agents have been reported in camelids, it seems likely that these may occur.


A number of viral infections are also teratogenic in humans, cattle, sheep, goats, swine, cats, and ferrets. The ultimate effects on the fetus are determined by the species involved, the strain of the virus, and the stage of pregnancy at the time of exposure to the teratogen.


Bovine virus diarrhea virus (BVDV) has caused cerebellar dysplasia, ocular defects, inferior brachygnathia, alopecia, internal hydrocephalus, and impaired immunologic competence in calves and lambs.40


Bluetongue virus (BTV) has been shown experimentally to cause central nervous system (CNS) defects (hydrocephalus, cerebral hypoplasia, dysplastic spinal cord), retinal dysplasia, and arthrogryposis in lambs. Exposure of pregnant heifers to BTV resulted in abortion, arthrogryposis, prognathia, and a “dummy-calf” syndrome.40 It is important to note that modified live virus (MLV) BTV vaccines may also exert teratogenic effects on the fetus of the pregnant ewe. The use of any MLV vaccine in any species other than those for which the vaccine was prepared is hazardous.


Both hog cholera virus and swine influenza virus are teratogenic. Feline panleukopenia virus (FPLV) causes cerebellar hypoplasia in kittens and ferrets.40 Interestingly, mature ferrets are refractory to overt infection with FPLV, yet teratogenesis occurs. In humans, examples of teratogenesis include congenital syphilitic blindness and congenital deafness from prenatal infection with German measles virus.


Chemically induced teratogenesis is being intensively studied in humans, livestock, and laboratory animals. No chemical teratogenic defects have been identified in camelids. However, such effects are known to occur in all other species studied, so it should be expected that chemical teratogenesis will ultimately be identified in camelids. Some congenital defects already identified in camelids are induced by chemical teratogens in other livestock species. It should be pointed out that these defects are also known to be inherited traits in one or more species (Table 48-1). Veterinarians should investigate both possibilities when congenital deformities occur.



The following general principles should be understood:








Box 48-2 lists factors necessary for teratogenesis to occur.



Poisonous plant ingestion by a pregnant SAC is an ever-present hazard to the fetus.31,32 SACs are fastidious in their eating habits, rarely consuming large amounts of strange plants, but they do investigate and try new plants. A low-dose intake may be a saving factor in camelids. Table 48-2 lists plants known to produce teratogenic defects in livestock.


Table 48-2 Plants Known to Be Teratogenic



































Genus/Species Common Name Animals Affected
Astragalus spp. Locoweed Cattle and sheep
Conium maculatum Poison hemlock Cattle
Datura stramonium Jimsonweed Pigs
Lupinus spp. Lupine Cattle
Nicotiana tobaccum Tobacco Pigs
Sorghum vulgare Sudan grass Horses
Veratrum californicum False hellebore, corn lily Sheep

Perhaps of even greater importance are the potential effects of chemical agents on the reproductive process without obvious outward expression. Teratogens may have a direct effect on ova or spermatozoa, causing infertility. High doses early in gestation may cause fetal death with resorption or undetected abortion. Lethal effects may be the result of maternal ingestion early in gestation, even though fetal death occurs late in gestation or postpartum. Nonlethal effects may either prevent reproduction or allow reproduction of constitutionally unsound individuals that may be highly susceptible to other diseases.





DETECTION OF INHERITED TRAITS


Circumstantial evidence that a congenital trait may be hereditary is based on evidence as follows6,46:








The detection of an inherited trait depends on the mode of inheritance. If a characteristic is dominant, one of the parents will be phenotypically positive and, generally, at least 50% of its offspring will express the phenotype. However, even though a characteristic may be dominant, environmental or genetic factors may affect the degree of expression of a phenotype.


The majority of inherited defects are recessive, and both parents must contribute the gene in order for the offspring to exhibit the trait. Recessive traits may be simple, in which only one gene is involved, or multifactorial, which complicates expression and detection in a population.


The diagnosis of an inherited trait is a laborious, costly, time-consuming process. Familial repetition is the most important data necessary, and this requires detailed genealogy of both normal and abnormal offspring. The ultimate evaluation is based on breeding trials. Only one major camelid breeding trial is currently under way, to establish the etiology of choanal atresia in llamas and alpacas.

Oct 1, 2016 | Posted by in EXOTIC, WILD, ZOO | Comments Off on Congenital and Hereditary Conditions of Camelids
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