[Emerging Infectious Diseases * Volume 3 * Number 2 * April-June 1997]
Dispatches
Lack of Serologic Evidence for an Association between Cache Valley Virus
Infection and Anencephaly and other Neural Tube Defects in Texas
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      We tested the hypothesis that Cache Valley Virus (CVV), an
      endemic North American bunyavirus, may be involved in the
      pathogenesis of human neural tube defects. This investigation
      followed a 1990 and 1991 south Texas outbreak of neural tube
      defects with a high prevalence of anencephaly and the
      demonstration in 1987 that in utero infection by CVV was the
      cause of outbreaks of central nervous system and
      musculoskeletal defects in North American ruminants. Sera from
      74 women who gave birth to infants with neural tube defects in
      south Texas from 1993 through early 1995 were tested for CVV
      neutralizing antibody. All tested sera did not neutralize CVV.
      These data suggest that CVV is not involved in the induction of
      human neural tube defects during nonepidemic periods but do not
      preclude CVV involvement during epidemics. Other endemic
      bunyaviruses may still be involved in the pathogenesis of
      neural tube defects or other congenital central nervous system
      or musculoskeletal malformations.
Anencephaly, spina bifida, and encephalocele (the major types of neural
tube defects) are generally due to the failure of the neural tube to close
during early embryonic development (1). Neural tube defects are among the
most common and most severe major birth defects. Anencephaly is caused by
failure of the anterior neuropore to close during embryonic development and
results in total or partial absence of the cranial vault, the covering
skin, and the brain. Infants with anencephaly are stillborn or die shortly
after birth. Spina bifida is caused by a disturbance in the normal closure
of neural walls and results in spinal cord defects. Most infants with spina
bifida survive surgical repair of the defect with residual neurologic
handicaps of varying severity. Encephaloceles are skull defects through
which skin-covered meninges, with or without brain tissue, herniate. Small
to moderately sized encephaloceles are surgically correctable (1-4).
The prevalence of neural tube defects in the United States has been
steadily declining (5) and is currently estimated to be six per 10,000
births. The prevalence of anencephaly in the United States has likewise
declined and is now approximately three per 10,000 live births (4). A vital
record study of anencephaly in Texas showed that from 1981 to 1986 the
prevalence rate was 3.8 to 4.3 cases per 10,000 births (6). This study also
showed that in south Texas during this period the average annual prevalence
of anencephaly was approximately 4.9 per 10,000 births. Women with Hispanic
surnames, three or more previous live births, history of stillbirth, or
residence in east or south Texas were at increased risk for neural tube
defect-affected pregnancies. On the basis of vital record data, the annual
prevalence of anencephaly in south Texas from 1981 through 1986 was
approximately 4.9 per 10,000 live births.
In 1991 three babies with anencephaly were born over a 36-hour period at a
single hospital in Cameron County, the southernmost county in Texas
(Brownsville is the county's largest city). The ensuing study, which used
active multisource case finding rather than vital records, showed that the
neural tube defect prevalence rate increased from 14.7 per 10,000 births in
1986 to 1989 to 27.1 per 10,000 births in 1990 to 1991. The higher rate was
due largely to an increase in anencephaly cases. From 1986-89 to 1990-91,
the average annual anencephaly prevalence rate rose from 9.6 to 19.7 per
10,000 births (7,8).
Despite the high prevalence of neural tube defects and the significant rate
of illness associated with them, much remains to be learned about their
complex multifactorial etiology. Evidence suggests that these defects are
etiologically heterogeneous and may follow fetal insults such as maternal
diabetes, hyperthermia, folic acid deficiency, and anticonvulsant
(valproate) therapy (9-13).
In 1987, before the increase in prevalence of human anencephaly in south
Texas, an outbreak of severe congenital malformations of the central
nervous system and musculoskeletal system of lambs occurred in San Angelo,
Texas (14). At the time of the outbreak, there was no active surveillance
of human birth defects in San Angelo, and no reports were received of human
birth defects in the area. The ovine problem was later found to be caused
by in utero infection by Cache Valley Virus (CVV). Although this
insect-borne bunyavirus had been known to commonly infect North American
ruminants (15), it was not thought to be of clinical significance.
Experimentally, it was determined that CVV infection of the dam in early
gestation and transplacental infection of the ovine fetus could produce
severe brain malformations and arthrogryposis multiplex congenita, an
anomaly characterized by limbs fixed in contracture (14). Central nervous
system malformations associated with experimental and spontaneous CVV
infection include hydrocephalus, hydranencephaly, porencephaly,
micrencephaly, and micromelia. After the syndrome was characterized,
outbreaks of CVV-induced malformations in ruminants were diagnosed
throughout North America, and work by Calisher and Sever (16) also linked
CVV to congenital cases of human macrocephaly in the United States.
Other bunyaviruses can cause identical congenital malformations of the
central nervous system in experimentally infected livestock (14). Human
congenital morbidity has also been correlated with maternal antibody to
bunyaviruses (16). A recent study correlated both human microcephaly and
macrocephaly with antibody to Tenshaw virus in mothers of infants with
these illnesses.
We decided to test the hypothesis that CVV infection was related to human
neural tube defects. Public concern regarding the 1990 to 1991 cluster in
Brownsville, Texas, (7) had resulted in an ongoing project in the 14 Texas
counties that border Mexico. A neural tube defect surveillance and folic
acid intervention project were implemented in 1993, and a case-control
study was begun in mid-1995. Sera from case patients had been banked before
the case-control study began.
Sera from 74 women who lived in the Texas border counties and had neural
tube defect-affected pregnancies (36 with spina bifida, 34 with
anencephaly, and 4 with encephalocele) from 1993 through early 1995 were
examined for a possible link between CVV and neural tube defects. With a
standard microtiter serum dilution neutralization test (17), the sera were
screened at final dilutions of 1:2, 1:4, 1:8, and 1:16. The virus used in
all tests was the prototype CVV (strain 6V-633, provided by the Centers for
Disease Control and Prevention [CDC], Ft. Collins, Colorado), which had
been passaged one time in Vero cells after receipt from CDC. Controls
included sera from women of undetermined CVV status who gave birth to
healthy infants in south Texas [8]; sera collected from sheep before CVV
infection [3]; and normal macaque [4], horse [1], and bovine [1] sera.
Positive controls included CVV-convalescent-phase ovine sera [3] and CVV
antibody-positive sera from a horse and a cow. No serum neutralization
activity for CVV was detected in sera from women who gave birth to healthy
infants or infants with neural tube defects. Had CVV infection been present
in these women during gestation, CVV antibody would have been detectable
postpartum.
Before this study, the relationship between CVV and human neural tube
defects was unknown. Testing an adequate number of controls is critical
when seroepidemiology is used to establish a causal relationship between an
agent and a low frequency event or malformation, particularly when case
patients have evidence of antibodies against the agent of interest. In this
study, there was no evidence that CVV was related to the neural tube defect
cases observed in Texas from 1993 through early 1995. Had CVV antibodies
been detected in serum from women with neural tube defect-affected
pregnancies, it would have been necessary to test control sera from age-
and location-matched women with normal births.
The average annual neural tube defect prevalence rate in Cameron County,
Texas, for 1992 to 1995 has returned to the 1986 to 1989 rate of
approximately 14-15 cases per 10,000 births. These data suggest that CVV is
not involved in the induction of human neural tube defects during
nonepidemic periods but do not preclude CVV involvement during epidemics.
CVV may still be involved in induction of other human malformations. Other
endemic bunyaviruses may be involved in the pathogenesis of neural tube
defects and of other congenital nervous system or musculoskeletal
malformations (18,19).
It would seem valid to continue to investigate the relationship of CVV and
other arboviruses to human developmental illness and death rate. Because of
the wide variety of defects caused by these viruses, laboratory models of
fetal infection by the Bunyaviridae would facilitate the understanding of
viral teratogenesis mechanisms in humans.
J. F. Edwards* and K. Hendricks
*Texas A&M University, College Station, Texas, USA; and Texas Department
of Health, Austin, Texas, USA
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Emerging Infectious Diseases
National Center for Infectious Diseases
Centers for Disease Control and Prevention
Atlanta, GA
URL: ftp://ftp.cdc.gov/pub/EID/vol3no2/ascii/edwards.txt
Posted: March 10, 1997