Chapter 28 Although subject to genetic and chromosomal defects, the developing embryo, prior to implantation, is relatively resistant to teratogens. Agents or conditions which interfere with implantation invariably lead to embryonic death. Abnormalities of the structure or function of cells, tissues or organs which are present at birth are termed congenital defects. These developmental defects can be caused by genetic, chromosomal, infectious or environmental factors. Nutritional deficiencies are a recognised cause of congenital defects in many species of animals. As the zygote develops into an embryo, susceptibility to teratogens increases but, with the progressive development of the foetus, there is a decline in susceptibility to infectious agents and adverse environmental factors (Fig 28.1). Figure 28.1 Changes in the susceptibility of the embryo and foetus to teratogens at different stages of gestation. Developmental defects in animals may result in early embryonic death, foetal death, mummification, abortion and stillbirths, together with specific congenital defects relating to body systems. A congenital defect can be classified as a malformation, deformation or disruption. A malformation develops due to a defect which is intrinsic to the embryological differentiation or development of a structure. A deformation occurs due to an alteration in the shape or structure of a body part which had previously undergone normal differentiation. The term disruption refers to a structural defect which results from the destruction of a previously normal structure due to interruption of blood supply or to mechanical interference. In both human and animal populations, reproductive failure encompasses sterility, infertility, abortions, stillbirths and malformations. Foetal growth retardation and prematurity at birth may also indicate interference with normal in utero development. Congenital defects can be caused by genetic factors and environmental influences; the aetiology of many of these adverse effects is unknown. In the human population, it has been estimated that close to 70% of congenital defects are of uncertain or unknown cause; approximately 20% may be due to genetic factors such as mutations and chromosomal abnormalities, and 10% can be attributed to teratogenic environmental factors such as chemicals, therapeutic drugs, certain poisonous plants and infectious agents. Reliable data relating to the occurrence of congenital defects in animal populations are not readily available. Estimates suggest that congenital defects in lambs, calves and foals occur to an upper limit of 3 to 4%. In dog populations, developmental defects are reported to affect approximately 6% of pups. Congenital defects are reported infrequently in cats. Some developmental defects in animals can be related directly to nutritional deficiencies, inbreeding, consumption of toxic plants, exposure to environmental pollutants or injurious physical factors and to infections with pathogenic microorganisms. The frequency of defects varies with species, breed, season of the year, geographical location and with the extent of ingestion of toxic substances and of exposure to deleterious physical factors or infection with teratogenic pathogens. If infection occurs at an early stage of gestation, serious congenital defects may follow. Infection of the foetus with pathogenic agents before it becomes immunologically competent may result in immunotolerance to that pathogen. If such foetuses survive to birth, they remain infected for life and do not produce an immune response to the infectious agent which caused the congenital infection. Mutations, which can be defined as changes in nucleotide sequences of genes, can occur spontaneously or may be induced by external influences. These changes can occur through the substitution, insertion or deletion of nucleotide bases and can be transmitted to future generations. Only a small subset of variants which occur naturally throughout the genomes of all vertebrates are associated with disease. In a given animal population, mutations at gene loci occur de novo with a certain frequency per generation, known as the spontaneous mutation rate. This is typically one per million. Based on the underlying mechanisms which result in genetic change, mutations can be divided into two broad categories, spontaneous and induced. Spontaneous mutations result from errors in DNA replication and repair as well as from errors which occur during recombination or movement of transposable elements. Induced mutation is a consequence of accidental or deliberate exposure to chemical or physical agents or mutagens which can cause heritable alterations in DNA. Radiation can induce a variety of non‐specific chromosomal and DNA aberrations. Following exposure to chemical mutagens, agents which induce mutation, DNA replication is affected in a manner which increases the rate of mutation above background level. The simplest genetic models are exemplified by traits which are under the influence of single genes and conform to classic Mendelian principles (Table 28.1). Single genes can exist in a number of alternate states, termed alleles, which can be described as dominant, recessive, co‐dominant or partially dominant. A recessive allele is one whose phenotypic effect is not expressed in the heterozygote. The phenotypic effect of a recessive allele is expressed only in animals homozygous for that allele. Animals homozygous for a non‐functional tyrosinase gene exhibit the disease trait referred to as albinism. Tyrosinase is required for the production of melanin from tyrosine. Dominant alleles are phenotypically expressed in animals heterozygous for that allele. Some mutations which are incompatible with survival are termed lethal mutations. Such mutations invariably result in premature death and consequently are not passed on to subsequent generations. Gangliosidosis is an example of a recessive lethal gene resulting from an inherited deficiency in β‐galactosidase. This condition is not lethal in the heterozygous state. Some mutations in regions encoding for a gene product may not affect the animal’s viability. They may, however, ultimately affect the animal’s performance and increase the risk of disease in subsequent generations. Classically, animal breeders select animals for specific characteristics. Negative aspects of selective breeding include reduced variation, reduction in genetic fitness, increased homozygosity and potential for expression of undesirable characteristics within a given population. Table 28.1 Animal diseases or conditions which are attributed to dysfunction of a single gene. It was formerly accepted that the impact of a mutation on a developing animal depended on the extent to which the mutational change altered the conformation or function of a final gene product. Mutations which did not affect a coding region or the amino acid sequence of the final protein (silent mutations) in general had little or no phenotypic expression. Due to the ongoing identification of non‐coding regulatory elements within the human genome, this concept has been challenged. Mutations or single nucleotide polymorphisms (SNPs) can exert effects in non‐coding regions if present in a region that has important consequences for the regulation of a gene. This change can alter the specificity of a transcription factor binding site. Mutations present in coding regions that alter the amino acid sequence (non‐synonymous mutations), however, can result in either complete loss of function or reduced activity in a specific gene product. The advent of high throughput sequencing technologies and genome wide association studies (GWAS) has uncovered the complexities of variation in the genomes of all vertebrates, particularly humans. This has caused a shift in emphasis from the traditional concept of a single mutation directly resulting in a single phenotypic change. Currently, the evaluation of the association of variants on the risk of developing a condition with a complex genetic aetiology, such as type II diabetes or heart disease, is based on GWAS. Although not investigated as comprehensively, some GWAS have also been applied to common congenital defects such as cleft lip/palate and congenital heart disease. Deletions or aberrations which occur at a chromosomal level can sometimes be observed cytologically. Aberrations involving large regions of DNA (including entire chromosomes) frequently result in embryonic death. When the chromosome complement of a cell is altered by the addition or loss of a chromosome, this condition is termed aneuploidy. Loss of a single chromosome from a pair is referred to as monosomy. The addition of a chromosome to a pair of chromosomes is referred to as trisomy. A number of conditions in humans can be attributed to chromosomal abnormalities (Table 28.2). Table 28.2 Important autosomal and sex‐linked conditions in humans due to chromosomal abnormalities. A chromosome can undergo changes whereby part of its structure is relocated either within the same chromosome or transferred to another chromosome. Reciprocal translocations result when two non‐homologous chromosomes break into two segments and reciprocal exchange of the segments between the two chromosomes occurs. Animals possessing a reciprocal translocation within their genomes may have a normal phenotype but can subsequently display a significant reduction in fertility. The phenotype remains unchanged, as the animal has a full complement of genetic material, albeit with an altered arrangement. During meiosis, this altered arrangement leads to an unequal distribution of genetic material within a significant number of gametes. Tandem translocations occur when part of an arm of one chromosome breaks and joins to the end of another chromosome. This type of aberration is rarer than reciprocal translocation. Centric fusions occur when two acrocentric chromosomes fuse, forming one metacentric chromosome. An animal carrying this aberration may have a normal phenotype as it possesses a complete genome, despite the fact that the animal’s karyotype is atypical. An increased frequency of monosomy or trisomy occurs in the offspring of cattle with centric fusion. Occasionally, a metacentric chromosome can split forming two acrocentric chromosomes. As a consequence of this abnormality, the animal appears to have an extra chromosome without the acquisition of additional genetic material. This aberration, termed centric fission, has been reported in donkeys. Deletion and inversion of sections of a chromosome can result from breakages at two points in a chromosome. Deletion results in the loss of genetic information and inversion results in the realignment of genetic information within the chromosome. These aberrations are rarely reported. Recent technological advances have uncovered more than 600 submicroscopic structural variants across the human genome. These advances have provided scientists with a more complete understanding of the proximity of what were formerly considered benign conditions to disease‐causing structural variants. This new insight is blurring the boundaries between a defined genetic disorder and a benign structural variant of minimal consequence. A teratogen is an agent which can cause a permanent alteration to the structure or function of an embryo or foetus. Teratogens acting at vulnerable periods of embryogenesis or foetal development can cause serious non‐inherited malformations. A number of malformations caused by teratogens are linked to alterations in the function or expression of genes instrumental in the developmental process. The ultimate effect of exposure to teratogens depends on the gestational age of the embryo or foetus at the time of exposure and the nature and mode of action of the damaging factor. The modes of action of agents which can cause congenital defects conform to basic rules. These include the stage of gestation at which they exert their effects, the dose or degree of exposure required to induce change and the manner in which these agents are metabolised. Drugs or chemicals must cross the placental barrier in order to exert deleterious effects on the developing embryo. Species differences account for much of the variation in the effects of drugs and chemicals on developing embryos. Species susceptibility is especially important for viral teratogens, as these infectious agents usually exhibit species specificity. Although the embryo is shielded from mechanical injury by the foetal membranes and from the adverse affects of toxic or infectious agents by the placental barrier, a number of drugs, chemicals and infectious agents can cause serious damage to the developing embryo. The effect of exposure of a pregnant animal or human to teratogens usually follows a toxicological dose–response curve. There is a threshold below which an effect is not observed but, as the dose of teratogen is increased, both the severity of the alterations in the embryo or foetus and the frequency at which they occur in a given species increases. The zygote is inherently susceptible to genetic mutations and chromosomal abnormalities but is usually resistant to teratogens. Although the developing embryo is highly susceptible to the damaging influence of environmental teratogens, this susceptibility declines as the embryo undergoes progressive development. The foetus becomes increasingly resistant to teratogens as it matures. However, late differentiating structures such as the cerebellum, the palate and portions of the urinary and reproductive systems remain susceptible to many teratogens until late in gestation. Agents, imbalances and factors implicated in the disruption of embryonic or foetal development through their teratogenic effects are summarised in Table 28.3. Table 28.3 Chemicals, environmental pollutants, infectious agents, metabolic imbalances, physical factors, poisonous plants and therapeutic drugs implicated in the disruption of normal embryonic or foetal development through their teratogenic effects.
Genetic, chromosomal and environmental factors which adversely affect prenatal development
Mutations
Affected gene
Gene symbol
Molecular basis of gene dysfunction and clinical consequences
Mode of inheritance
Tyrosinase
TYR
Congenital absence of normal pigmentation in the body due to a non‐functional form of the enzyme tyrosinase is termed albinism. In Brown Swiss cattle, an insertion of cytosine at position 926 TYR mRNA causes a frame shift mutation. Cattle with the albino phenotype are homozygous for this mutation.
Autosomal recessive
Galactoside activator protein
GM2A
A group of inherited lipid storage disorders in which marked accumulation of gangliosides occurs in tissues is termed gangliosidosis. In GM2 gangliosidosis which occurs in cats, β hexosaminidase deficiency leads to progressive accumulation of GM2 ganglioside in neuronal lysosomes and ultimately leads to cell death. A deletion of four base pairs in the GM2A gene was identified as the causative mutation, resulting in alteration of 21 amino acids in the C terminus of the GM2 activator protein.
Autosomal recessive
Dystrophin
DMD
Muscular dystrophy, an inherited, progressive degenerative disease of muscle fibres which results in muscle wasting, is ultimately fatal.
Golden retriever muscular dystrophy is caused by a point mutation in the consensus splice acceptor site in exon 6 of the DMD gene which results in the skipping of exon 7 during mRNA processing. The amino acid frame shift causes premature termination of the dystrophin protein.
X‐linked recessive
Cytochrome P450, family 19, subfamily A, polypeptide 1
CYP19A1
In poultry, typical feathering is part of their secondary sexual characteristics due to the action of oestrogen. Much of the oestrogen is produced from androgen in the ovaries by the enzyme aromatase. In some strains of two poultry breeds, namely the Sebright Bantam and the Golden Campine, roosters develop the same feathering as hens and this change in feathering is referred to as ‘henny feathering’. The change in feathering is due to a mutation in the aromatase gene which results in abnormally high levels of oestrogen. This condition illustrates the point that not all mutations result in loss of activity
Autosomal dominant
Ryanodine receptor
RYR1
Malignant hyperthermia is a condition characterised by unregulated release of calcium from the sarcoplasmic reticulum, leading to excessive myofibre contraction which generates heat, resulting in an increase in body temperature. This condition also leads to rapid postmortem changes in porcine muscle, resulting in pale, soft exudative pork. In pigs, the condition can be triggered by stress, high ambient temperature and by general anaesthetic agents, especially halothane. The genetic basis of malignant hyperthermia in pigs relates to a base substitution which causes an amino acid substitution in the 615 position of the calcium release channel.
Autosomal recessive in pigs; autosomal dominant in other species including humans
Chromosomal abnormalities
Condition
Chromosomal abnormality
Typical clinical features
Autosomal or sex
chromosome aneuploidy
Down syndrome
Trisomy, chromosome 21
Short broad hands with single palmar crease, reduced muscle tone, broad head, large tongue, up‐slanting eyes and mental retardation, heart defects and shortened life span.
Patau syndrome
Trisomy, chromosome 13
Small eyes, cleft lip and/or palate, polydactyly. Heart, brain and genito‐urinary anomalies. Severe mental retardation. Most infants die shortly after birth.
Edwards syndrome
Trisomy, chromosome 18
Congenital malformations in multiple organs including heart and kidneys, low‐set malformed ears, receding mandible, small eyes, mouth and nose, severe mental retardation. Most infants die shortly after birth.
Klinefelter syndrome
Additional X chromosome
Male, infertile with small testes, tall with long limbs, may have breast development. Mild learning difficulties.
Turner syndrome
One X chromosome absent
Female with impaired sexual development, usually sterile, short stature, webbing of skin in neck region, cardiovascular abnormalities, hearing impaired and normal intellect.
Autosomal deletion
Cri du chat syndrome
Partial deletion of 5p15.2 (chromosome 5)
High‐pitched cry, wide‐spaced eyes, small chin and head, round face and severe psychomotor and mental retardation.
Autosomal micro‐deletion
Prader‐Willi syndrome
Micro‐deletion of 15q11.2 (paternally derived)
Developmental delay, decreased muscle tone, obesity, small genitals, excessive appetite, hypo‐pigmentation.
Angelman syndrome
Micro‐deletion of 15q11.2 (maternally derived)
Developmental delay, unstable gait, absence of speech, hyperactivity, spontaneous laughter, hypo‐pigmentation.
DiGeorge syndrome
Micro‐deletion of 22q11.2
(chromosome 22)
Under‐developed thymus and parathyroid gland, facial abnormalities and cardiac defects.
Teratogens
Agent, imbalance
or factor
Susceptible species
Comments
Addictive
chemicals/drugs
Cocaine
Humans
The effects of cocaine on in utero development include foetal death, growth retardation, microcephaly, cerebral infarctions, urogenital anomalies and postnatal neuro‐behavioural disturbances. Because poor nutrition and multiple drug abuse may be a feature of some pregnancies, the precise teratogenic effects of cocaine are not clearly established.
Ethyl alcohol
Humans
Foetal alcohol syndrome occurs in babies born to women with severe alcoholism during pregnancy. Because it can readily cross the placental barrier, ethyl alcohol is exceptionally dangerous for the developing foetus. Features of the condition include growth deficiency, mental retardation, altered facial appearance and congenital heart defects. Children with foetal alcohol syndrome are both developmentally and mentally retarded and exhibit behavioural disturbances. Studies using pregnant mice indicate that ethyl alcohol interferes with neural crest cell migration. It can also cause apoptosis of neurons in the developing forebrain and interfere with the activity of cell adhesion molecules. In the chick embryo, ethyl alcohol disrupts development by causing apoptosis of neural crest cells and by interfering with the formation of the fronto‐nasal process. These developmental defects correlate with the loss of Sonic Hedgehog gene expression in the pharyngeal arches.
Toluene and other
organic solvents
Humans
Repetitive deliberate inhalation of organic solvents such as toluene during pregnancy increases the risk of teratogenesis and abortion. Foetal changes include growth retardation, cranio‐facial anomalies and microcephaly. Neurotoxicity, which affects adults who abuse toluene, also occurs in the foetus.
Environmental pollutants
Atrazine
Humans and wildlife
species
Numerous synthetic chemicals including insecticides, herbicides and compounds incorporated into plastics as stabilisers, can alter gene expression or disrupt normal hormonal function in wildlife including birds, amphibians and mammals. These hormone‐disrupting chemicals are referred to as endocrine disruptors. Atrazine, a triazine herbicide which has been widely used in the United States and many other countries, has oestrogenic activity and even at low concentrations in water causes gonadal abnormalities in male frogs and also in male rats. This oestrogenic herbicide is stable, with a long half‐life and is reported to be one of the most common chemical pollutants present in ground water and surface water. Because atrazine induces the production of the enzyme aromatase which converts testosterone to oestradiol, gonadal development is adversely affected in male fish, amphibians and laboratory mammals. Following exposure to atrazine, marked immunosuppression has been reported in amphibians, resulting in increased susceptibility to opportunistic microbial and parasitic pathogens in their aquatic environment.
Bisphenol A
Humans and wildlife
species
A number of chemicals, incorporated into plastics as stabilisers, share some structural features with oestradiol and these synthetic molecules have oestrogenic activity. One of the most widely used chemicals incorporated into plastics is bisphenol A. However, bisphenol A can leach out of plastics and be present in water or other fluids stored in plastic containers at levels which affect gonadal development in laboratory animals. In utero exposure of rats to low levels of bisphenol A led to the development of carcinomas in one third of the animals exposed to this chemical. Exposure of foetal and adolescent rats to bisphenol A induced anatomical change in mammary gland development at puberty. It has been reported that in utero exposure to bisphenol A induces changes which can lead to tumour development following a second exposure to oestrogenic hormones or carcinogens later in life. Adult male mice exposed to bisphenol A had enlarged prostate glands and this chemical increases the rate of mitosis in human prostate cells. A disturbing aspect of the widespread use of bisphenol A is that in the human placenta, it is neither eliminated nor metabolised into inactive compounds and that it accumulates to concentrations that can alter development in laboratory animals. Recent studies, based on human urine samples from the United States and Japan, showed that 95% of the samples tested had measurable levels of bisphenol A and that children had higher levels of this chemical in their blood than adolescents or adults. The widespread use of compounds such as bisphenol A with hormonal or antihormonal activity and which are capable of accumulating and persisting in the environment, raises serious questions about their potential toxicity for wildlife, domestic animals and the human population.
DDT
(Dichlorodiphenyl‐
trichloroethane)
Humans and wildlife
species
This chlorinated hydrocarbon was widely used as a pesticide until the early 1970s. The damaging effect of pesticides such as DDT on wildlife species was reported in the early 1960s. However, it took more than a decade to implement a ban on DDT. Birds of prey, such as peregrine falcons and bald eagles became endangered species because of their position at the top of the food chain. The fragility of egg shells of birds of prey was linked to residues of DDT in prey which, when consumed, was concentrated in the tissues of falcons and eagles. Although banned as a pesticide in the early 1970s, in regions where it was used extensively, DDT remains at appreciable concentrations in soil as this chemical has a half‐life of approximately 15 years. A metabolic byproduct of DDT, DDE (1,1‐dichloro‐2,2 bis (p‐chlorophenyl) ethylene) is reported to exert its effect either by mimicking oestrogen activity or by inhibiting the effectiveness of androgens. Feminisation of fish in Lake Superior, a decline in human sperm counts and an increased frequency of breast cancer worldwide have been attributed to environmental pollution by DDT and DDE. Due to its persistence in the environment, its potential to accumulate in the tissues of animals and its toxicity for humans, DDE has been listed as a pollutant of particular concern. Developmental effects of oral feeding of DDT to animals included toxicity for the embryo and foetus.
Dioxin
Humans, monkeys, rats, mice, fish
This halogenated hydrocarbon is a contaminant of many industrial processes. When used as a herbicide, dioxin has been linked to congenital anomalies in the human population, especially where it was used as a defoliant. The male offspring of female rats exposed to this toxic molecule had reduced sperm counts, decreased testicular size and altered sexual behaviour. Fish embryos are reported to be particularly susceptible to the toxic effects of dioxin. The offspring of rhesus monkeys exposed to less than 1 ng/kg/day before pregnancy had measurable behavioural changes. Exposure of pregnant mice to dioxin induces cleft palate, kidney, brain and other defects in their offspring. Using in vitro culture of palate cells from mouse, rat and human embryos, it was shown that dioxin treatment altered proliferation and differentiation of epithelial cells and that palate epithelial cells had a high‐affinity receptor for dioxin. It has been suggested that the teratogenic effect of dioxin is due to its interference with epidermal growth factor or transforming growth factor.
Lead
Humans and animals
Due to environmental pollution, high levels of lead in drinking water, in vegetables and in the air can lead to toxicity. Lead crosses the placenta and can accumulate in foetal tissues. Reports indicate that children born to mothers who were exposed to subclinical levels of lead had behavioural changes and psychomotor disturbances. Lead toxicity may damage the developing human central nervous system, leading to a decreased IQ and functional deficits.
Mercury
Humans
Ingestion of food contaminated with methyl mercury during pregnancy resulted in damage to the foetal central nervous system. Cerebral palsy, microcephaly, blindness, cerebral atrophy and mental retardation are the principal developmental defects attributed to the teratogenic activity of organic mercury. Selective absorption by regions of the cerebral cortex has been reported.
Polychlorinated
biphenyls
Humans and wildlife
Polychlorinated biphenyls (PCBs) are mixtures of synthetic organic chemicals with the same basic chemical structure and similar physical properties. Due to their chemical stability, non‐flammability, high boiling point and electrical insulating properties, PCBs were widely used commercially for more than half a century. Concern over their toxicity and persistence in the environment led to prohibition of their manufacture in the United States of America in 1976. There is substantial evidence that halogenated aromatic hydrocarbons including PCBs are carcinogenic, teratogenic, neurotoxic and immunosuppressive. From the late 1920s until the late 1970s, PCBs were extensively used for commercial purposes and these toxic substances are still present in the food chain. They have been blamed for the decline in the reproductive capabilities of otters, seals, mink and fish. Some polychlorinated biphenyls structurally resemble diethylstilboestrol and it is postulated that they can act as environmental oestrogens. If ingested in large amounts by pregnant women, these teratogens can cause reduced foetal growth rate and abnormal skull calcification. These compounds can also cause hypoplastic deformed nails and hyperpigmentation of gums, nails and other tissues. It is reported that body residues of PCBs in exposed women can affect pigmentation in their babies born up to four years after exposure. In addition to their oestrogenic activity, polychlorinated biphenyls structurally resemble thyroid hormones. Hydroxylated polychlorinated biphenyls have a high affinity for transthyretin, a serum protein involved in thyroid hormone transport and can lead to excretion of thyroid hormones. As thyroid hormones are critical for development of the cochlea, the offspring of pregnant rats exposed to polychlorinated biphenyls had deficient cochlear development and were deaf.
Infectious agents
Bacteria
Treponema pallidum
subspecies pallidum
Humans
In utero infection with T. pallidum subspecies pallidum can lead to serious foetal disease referred to as congenital syphilis. Infection acquired during pregnancy, primary maternal infection, invariably leads to serious foetal infection resulting in foetal death or congenital anomalies. When infection is acquired before pregnancy, foetal infection and congenital anomalies in subsequent pregnancies are unlikely. Congenital infection may result in maculopapular rash, central nervous system defects including deafness, hydrocephalus and mental retardation, destructive lesions of the palate and nasal septum and deformed teeth, bones and nails. Syphilis increases the risk of abortion.
Protozoa
Toxoplasma gondii
Humans, sheep, goats, pigs, cats
In both humans and animals, a primary infection with T. gondii during pregnancy can lead to congenital infection. When human or animal infection with T. gondii occurs before pregnancy, congenital infection does not occur in subsequent pregnancies. In humans, primary infection during early pregnancy can lead to foetal death and abortion, stillbirth, chorioretinitis, brain damage with intracerebral calcification, hydrocephaly, microcephaly, rash and hepatosplenomegaly. Psychomotor defects or mental retardation are features of severe congenital toxoplasmosis. Infection late in gestation can result in mild or subclinical foetal disease with delayed manifestations. In sheep, goats and pigs, abortion late in gestation and perinatal deaths are common findings. Encephalitis is often associated with congenital infections in animals. Pregnant women should avoid contact with cat faeces and cat litter; gloves should be worn when gardening.
Viruses
Cytomegalovirus
(Family
Herpesviridae,
subfamily
Betaherpesvirinae)
Humans
Infection with cytomegalovirus (human herpesvirus 5) is one of the most common viral causes of congenital defects in humans. Up to 2% of newborn babies may have cytomegalovirus infection and approximately one‐tenth of these infected in utero have signs of severe generalised infection. The outcome of severe intrauterine infection may be foetal death or congenital defects. In utero infection, which is a consequence of primary maternal infection, can result in hepatosplenomegaly, chorioretinitis, microcephaly, intracerebral calcification and mental retardation.
Herpes simplex
virus type 1 and
herpes simplex
virus type 2
(Family
Herpesviridae,
subfamily
Alphaherpesvirinae)
Humans
Although rarely described, both herpes simplex virus type1 and herpes simplex virus type 2 can cause congenital infections. Infection with herpes simplex virus type 2 can be acquired at the time of birth as the baby passes through the genital tract. Congenital malformations attributed to infection with these herpes viruses, which occurs in late pregnancy, include vesicular rash, ocular defects, hepatitis, microcephaly and mental retardation.
Parvovirus B19
(Family
Parvoviridae,
genus Erythrovirus)
Humans
In utero infection with this virus in early pregnancy can cause anaemia in approximately 10% of infected foetuses. As the virus replicates in erythroid precursor cells, it can cause severe anaemia leading to congenital heart failure, hydrops foetalis and foetal death.
Rubella virus
(Family
Togaviridae,
genus Rubivirus)
Humans
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