2015's dramatic increases in reported microcephaly cases were in states of Brazil known to have high natural background radiation levels

[Emily]

If you look up the states with the biggest increase of reported microcephaly in 2015, they are all in northeast Brazil:

http://www.physics.isu.edu/radinf/natural.htm

Around the world though, there are some areas with sizable populations that have high background radiation levels. The highest are found primarily in Brazil, India and China. The higher radiation levels are due to high concentrations of radioactive minerals in soil. One such mineral, Monazite, is a highly insoluble rare earth mineral that occurs in beach sand together with the mineral ilmenite, which gives the sands a characteristic color. The principal radionuclides in monazite are from the ²³²Th series, but there is also some uranium its progeny, ²²⁶Ra. In Brazil, the monazite sand deposits are found along certain beaches. The external radiation levels on these black sands range up to 5 mrad/hr (50 ?Gy/hr), which is almost 400 times normal background in the US. Some of the major streets of the surrounding cites have radiation levels as high as 0.13 mrad/hr (1.3 ?Gy/hr), which is more than 10 times the normal background. Another high background area in Brazil is the result of large rare earth ore deposits that form a hill that rises about 250 meters above the surrounding area. An ore body near the top of the hill is very near the surface, and contains an estimated 30,000 tons of thorium and 100,000 tons of rare earth elements. The radiation levels near the top of the hill are 1 to 2 mrad/hr (0.01 to 0.02 mGy/hr) over an area of about 30,000 m². The plants found there have absorbed so much ²²⁸Ra, that can will produce a self "x-ray" if placed on a sheet of photographic paper (an autoradiograph).

Also see this:

https://inis.iaea.org/search/search.aspx?orig_q=RN:41109068

A study area was selected and delimited due the wide extension of the phosphate deposits in
the region of the northeast of Brazil, whose association with the radioactive element uranium,
is widely known.

Ionizing radiation can make some embryo cells more sensitive to damages from certain viruses:

Radiation-Enhanced Oncogenesis by SV40¹
J. H. Coggin
S. E. Harwood
N. G. Anderson

• University of Tennessee, Knoxville, Tennessee 37916 and The Molecular Anatomy (MAN) Program, Oak Ridge National Laboratory, Oak Ridge, Tennessee 37830

doi: 10.3181/00379727-134-34953 Exp Biol Med (Maywood) September 1970 vol. 134 no. 4 1109-1111
Summary

Exposure of hamster and mouse embryo cells to X-irradiation prior to infection with Simian virus 40 (SV40) significantly increased the frequency of transformation in vitro. Neonatal hamsters were tested for increased sensitivity to SV40 tumorigenicity after exposure to localized, low-level X-ray at the site of virus inoculation in vivo. Results showed that irradiation of a target area prior to infection markedly decreased the time to tumor appearance and increased the frequency of tumor occurrence. No tumors occurred in irradiated animals that did not receive virus. Although low doses of localized X-ray may have resulted in modest immunologic suppression, this was not a primary factor in tumor enhancement. Tumors appearing in SV40-infected animals uniformly possessed viral neoantigens.

And the flip side is that some research indicated that SV40 could increase the sensitivity of some human cells to ionizing radiation, a known cause of microcephaly:

Medical*Online Document - Journal of Radiation Research

[Emily]

Here is the link for the ECDC graph source in the first post:

http://ecdc.europa.eu/en/publications/Publications/zika-virus-americas-association-with-microcephaly-rapid-risk-assessment.pdf

Table 1 in that paper has the stats. The states with the greatest uptick in microcephaly reports are: Pernambuco, Para?ba, Rio Grande do Norte, Sergipe, and Alagoas.

Here's a clear map of the states:

http://www.mapsofworld.com/brazil/brazil-political-map.html

There are some much smaller upticks in reporting in some other states in the table. Some minor increases could be due to more intense surveillance during the outbreak, but the clustering in that northeast region of states with coastal borders is dramatic considering that Zika infections are much more widespread.

[Emily]

Latest statistics showed that NE Brazil has had an unusually high rate of microcephaly for years prior to Zika's appearance in the region.

http://www.rpe.org.in/article.asp?issn=0972-0464;year=2011;volume=34;issue=3;spage=178;epage=184;aulast=Vasconcelos

Determination of natural radioactivity in beach sand in the extreme south of Bahia, Brazil, using gamma spectrometry
Vasconcelos Danilo C, Pereira Claubia, Oliveira Arno H, Santos Talita O, Rocha Zildete, de B. C. Menezes Maria ?ngela

Year : 2011 | Volume: 34 | Issue Number: 3 | Page: 178-184

Abstract

The natural radionuclides activity concentrations in beach sand of the extreme south of Bahia, Brazil, was measured by Gamma Spectrometry. The Radium Equivalent Activity, the external hazard index, the absorbed dose rate and the annual effective dose were assessed and compared with internationally published values for external dose and activity concentrations. The activity concentrations of ²²⁶ Ra, ²³² Th and ⁴⁰ K in beach sand ranged from 8.4 to 8,300 Bqkg ^-1 with a mean value of 910 Bq.kg ^-1 , from 21 to 18,450 Bqkg ^-1 with a mean value of 2,220 Bqkg ^-1 and from 3.4 to 3,110 Bqkg ^-1 with a mean value of 352 Bqkg ^-1 , respectively. The results indicate that the absorbed dose rates range from 21 to 14,450 nGyh ^-1 with mean value of 1,792 nGy.h ^-1 . The highest value of gamma dose rates among the studied beaches were found in Cumuruxatiba (14,450 nGyh ^-1 ). The annual effective dose range between 0.03 and 17.70 mSvy ^-1 , with the mean value of 2.20 mSvy ^-1 . In four studied beaches, the assessed outdoor annual effective doses are above the worldwide average of 0.07 mSvy ^-1 as reported by the United Nations Scientific Committee on the Effects of Atomic Radiation. Especially in the area of black sands, a big part of Cumuruxatiba beach, whose annual effective dose of 17.70 mSvy ^-1 is much higher than worldwide average.
Keywords: Annual effective dose, gamma ray spectrometry, natural radioactivity, radiation hazard indexes


Conclusions:
...
The results of this work are important alerts for the local people to avoid the use of these sands to construct the dwellings. These sands are unsafe for use in building constructions. Furthermore, the use of such sands are not suitable for building construction due to their radiological and technical proprieties. Additionally, this work may be used in order to help with the development of reference levels of natural radioactivity along the coast of Brazil.

[Emily]

http://iai.asm.org/content/19/2/533.short

Infect. Immun. February 1978 vol. 19 no. 2 533-539 Mechanisms of hemopoietic and immunological dysfunction induced by lymphocytic choriomeningitis virus.

• S L Silberman,
• R P Jacobs and
• G A Cole

ABSTRACT

Sublethal irradiation (500 R) of C3H mice is followed by a gradual replacement of radiosensitive cells in their spleens by surviving stem cells originating in bone marrow. This compensatory hemopoiesis was quantitated by counting the numbers of stem cell-derived colonies appearing on spleen surfaces, as well as those which grew in vitro after marrow cells, suspended in soft agar, were overlaid onto syngenic mouse embryo fibroblast feeder layers. Compensatory colony formation, both in vivo and in vitro, was severely depressed when mice were infected with lymphocytic choriomeningitis virus (LCMV) 1 day before irradiation, although the induction of virus-specific cytotoxic T cells in their spleens was unimpaired. Without irradiation, mice, acutely infected with LCMV, showed a dramatic reduction in the numbers of specific antibody-forming cells generated in their spleens after priming with sheep erythrocytes during week 1 post-infection, yet the ability of their marrow cells to form colonies in vitro remained normal. Therefore, the basis of immunodepression is distinct from that of defective hemopoiesis since the latter is apparent only when LCMV infection is accompanied by irradiation. However, as discussed, both phenomena may be related to alterations induced within the splenic environment by LCMV.

[Emily]

50 cGy was used in the study below. It was the most effective dose as far as making cells much more vulnerable to HIV infection. If I have it right, even a 400x increase in background radiation in Brazil compared to the US is a far cry from 50 cGy. You would approach or surpass that only in medical imaging. The study encourages caution when using medical imaging on HIV patients.
Effects of Low Dose Gamma Radiation on the Human Immunodeficiency Virus-1

Here is another one about ionizing radiation and rabies virus:

http://www.tandfonline.com/doi/abs/10.1080/095530096145346

So in conclusion, I don't see a role for background radiation explaining the Zika situation in Brazil, but there is another reason to remain cautious about the using of medical imaging that involves ionizing radiation.