Chapter 17 Infections during pregnancy
Infections have an indirect and a direct effect on the fetus. The indirect effect operates by reducing the oxygenation of the placental blood, and by altering the nutrient exchange through the placenta. The direct effect depends on the ability of the invading organism to penetrate the placenta and infect the fetus. Viruses, being smaller than bacteria, are able to do this more easily. At first, the virus multiplies in the trophoblast and subsequently invades the fetus. Most viral infections do not affect the fetus unless the mother’s infection is very severe. Three exceptions to this are rubella, cytomegalovirus and herpes simplex infections. These infections may cause congenital defects. The clinical effects of infections are microcephaly, congenital heart disease, eye damage (such as cataract), deafness, hepatosplenomegaly (with jaundice), purpura and, later in childhood, mental handicap.
As maternal antibodies cross the placenta they offer the fetus a degree of immunity, except in the case of a primary infection. The fetus becomes immunologically competent from about the 14th week of gestation, but the efficacy of this protection is low until the second half of pregnancy.
URINARY TRACT INFECTION
Urinary tract infection is the most disturbing of the bacter-ial infections. It occurs because the urinary tract dilates owing to the relaxation of the muscles of the ureter and the bladder in pregnancy, with consequent urinary stasis.
Asymptomatic bacteriuria
The prevalence of asymptomatic bacteriuria (defined as more than 100 000 bacteria per mL of urine) in pregnancy is 6%, similar to that in non-pregnant sexually active women; 30% of pregnant women who have asymptomatic bacteriuria will develop symptomatic urinary tract infection. Unless the infection is treated these women have an increased risk of delivering a low-birthweight baby, delivering prematurely and having postpartum endometritis.
Because of this risk, pregnant women should be screened for asymptomatic bacteriuria in early pregnancy. A midstream specimen of urine is sent to a laboratory for culture. In 85% of cases Escherichia coli is isolated, and in the remainder a variety of microorganisms are found. As the procedure is costly, other procedures, such as dip slides covered with a culture medium and reagent strips, have been developed and are reliable screening methods.
Treatment of asymptomatic bacteriuria consists of prescribing amoxicillin 500 mg/clavulanic acid 125 mg or cefalexin 2 g as a single dose or in appropriate doses for 5–7 days. Seven days after the woman has finished the antibiotics, a midstream specimen of urine is re-examined. If the asymptomatic bacteriuria has not been eliminated, a longer course of antibiotics is prescribed. Regular follow-up urine cultures must be made, and recurrences treated.
Pyelonephritis
As mentioned above, about 30% of women who have untreated bacteriuria will develop pyelonephritis during pregnancy, as will 1% of women who do not have the condition. Pyelonephritis can cause premature rupture of the membranes and perinatal death
The infection occurs when bacteria, growing in the stagnant urine, spread from the bladder to the ureter and then to the renal pelvis. Haematogenous infection is very uncommon. Pyelonephritis usually begins after the 20th week of pregnancy. In mild cases the woman complains of tiredness and urinary frequency, and occasionally dysuria. More severe infections start suddenly, with chills and rigors, fever, and pain over one or both renal areas. The patient may rapidly become dehydrated. The diagnosis is confirmed by examining a midstream specimen of urine, having excluded other causes of abdominal pain such as acute appendicitis and abruptio placentae.
Treatment consists of correcting the dehydration and prescribing appropriate antibiotics, having tested for bacterial sensitivity. In most cases, a cephalosporin or amoxicillin is appropriate initial treatment. As some women with pyelonephritis are nauseated, the antibiotic may have to be given intravenously. Follow-up examinations of a midstream urine culture at 2-week intervals for the remainder of the pregnancy should be instituted. Some authorities recommend prophylactic antibiotics for the duration of the pregnancy.
VAGINAL INFECTIONS
Vaginal infections during pregnancy due to Candida spp., Trichomonas and bacterial vaginosis are discussed on pages 260–262. The diagnosis and treatment are the same as in pregnancy.
BACTERIAL INFECTIONS
Group B streptococcus
Colonies of group B streptococci (GBS) are harboured in the lower vagina and/or rectum of 18–27% of pregnant women.
If group B streptococcal infection is present during labour the bacteria may colonize the neonate. Over half of babies born through an infected vagina are colonized, and 2–5% of them develop early onset GBS neonatal sepsis. The mortality rate for premature affected infants is 25–30%, for those at term 2–8%, and neurological sequelae for the survivors of 15–30%.
Up to 80% of cases of early-onset disease are associated with obstetric risk factors: preterm delivery, prolonged rupture of membranes for more than 18 hours, maternal fever >38° during labour. Treatment of known carriers or those at high risk with penicillin during active labour reduces both neonatal and maternal morbidity.
Two approaches to selecting those for treatment are widely used: one treats on risk factors alone, the other takes a low vaginal and a rectal swab at 36 weeks’ and then treats those with positive isolates. Both regimens have been shown to be effective. See Box 17.1.
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