Can Zika mosquitoes occasionally transmit HIV?

Search of medical literature finds no studies yet.
Scientists must plan now for research in Summer 2017

By Leslie C. Norins, MD, PhD | March 21, 2017

An intensive search of the medical literature has failed to unearth any significant research on whether Zika mosquitoes could transmit HIV, even occasionally.  Could this be important?  Possibly.   Miami-Dade in 2016 led the U.S. in both cases of domestically transmitted Zika virus and in new infections with HIV, the causative virus of AIDS.

Official doctrine says that mosquitoes can’t transmit HIV.  But the writer’s investigation indicates that the hundreds of such pronouncements seem to be based on only two research papers, both about 30 years old.  And viewed from today’s perspective, each of these investigations had weaknesses.

Theoretical infection raises questions

Picture a Miami mosquito sucking blood from an adult male unlucky enough to have both Zika virus and HIV concurrently circulating in his blood.   Then that insect bites into a gay man.

If that target person develops clinical Zika ten days later, local physicians will probably say “Of course, you must have been bitten by a carrier mosquito.”  But now suppose that same target man develops AIDS twelve months later?  And to everybody’s surprise, he swears he’s been chaste for three years. Who would believe his assertion, “I must have gotten HIV from the same mosquito which gave me Zika.” Is there even a tiny chance he could be right?

Mosquitoes would be only a minor transmitter of HIV in the U.S.

To be clear at the outset, the main route of HIV spread in the U.S. is sexual transmission, with a lesser contribution from sharing needles by drug abusers.  So the Zika mosquito pathway—if it exists—is playing only a minor role.   But it is important to better ascertain even this possibility, even if it occurs only occasionally.  Such data would also interest countries which have both significant numbers of AIDS cases and other mosquito-spread diseases such as dengue, West Nile virus, equine encephalitis, and malaria.

1985 Belle Glade AIDS epidemic

The U.S. AIDS epidemic officially began in 1981, 36 years ago.  At that time, with the public panicked about ways that growing epidemic could be spread, Florida’s mosquitoes drew national attention.

A cluster of AIDS cases was reported in the rural town of Belle Glade, about 85 miles north of Miami.  The climate is warm, and mosquitoes are prevalent. Most of the patients were migrant farm workers who lived in a ramshackle ghetto. A number of those infected denied gay sex or shared needles.

After examining the patients, some Florida physicians proposed the fearful theory that mosquitoes had transmitted some of these infections from one case to another. A federal-led team rushed to re-study the patients and their neighbors. Their “field investigation” became the first piece of evidence interpreted to exclude a role for mosquitoes.

1988 “field” report on Belle Glade patients and residents

The public health detectives re-investigated the 93 AIDS cases initially reported from Belle Glade (1). They said thirty-four (37%) could be “directly” linked to another AIDS case.  Precisely how the remaining 59 patients (63%) had become infected was not definitely established.

However, the researchers noted that certain “risk factors” were present:  17 were homosexual/bisexual and 13 had a history of IV drug abuse.  “Heterosexual contacts” was also considered a risk factor for 35, with a footnote that in Haiti (where a number were born) heterosexual transmission is responsible for many HIV infections.

In a concurrent random survey of 608 adult residents in Belle Glade, 28 (4.6%) were HIV-antibody-positive.  The investigators noted all these positive reactions were in blacks born in the U.S., or in Haitians working locally.  Risk factors elsewhere associated with HIV were found to be more prevalent in the HIV-positive group, e.g. paid sex, history of gonorrhea, positive blood test for syphilis, tattoo. But it is not clear in the data reported if there were any HIV-positive participants for whom no risk factors were identified.

Negative HIV blood tests were obtained for 138 subjects under ten years of age, and for 131 older than 60. The scientists interpreted this finding to mean local mosquitoes could not have been HIV transmitters, else these two special groups, obviously “low risk” for sex or drugs, would have also shown antibodies against that virus, from being bitten by mosquitoes carrying it.

No local mosquitoes were trapped and tested for HIV.

From their findings, the investigators opined that known risk factors explained the HIV infections, and that mosquitoes were not participating.

Note they did not state mosquitoes could not account for some of the Belle Glade cases.  They elected instead to accept as the explanation other risk factors present in most—but not all– Belle Glade subjects. There was no consideration of whether mosquitoes could possibly bring HIV to a person who had “risk factors”, but no HIV infection.

Also, they considered mere heterosexual intimacy among Haitians as a sufficient “risk factor” for AIDS—because back in Haiti heterosexual spread of HIV was supposedly common.  In that construct, each HIV infection in a Haitian having any sexual contact with another Haitian of either sex was felt to be automatically explained by that fact, i.e. acquisition of HIV from that other person, without having to consider mosquitoes.  Likely today such a stance would be rejected as profiling by origin or ethnicity.

1989 “Lab” Report

But these beliefs from the field, even if correct, left unanswered a more basic question: could mosquitoes transmit the HIV virus under any circumstances?

Laboratory workers tried to illuminate this.   There have been a few non-controversial reports that when HIV-infected blood is fed to mosquitoes, the virus or its nucleic acid can be detected in them for the next 24-72 hours. It is not known whether this detected material retains infectivity.

But does HIV multiply, not merely rest, inside the insects?  Some experts view this as necessary for mosquitoes to transmit the virus for more than a few hours after feeding on an infected person.

There seems to be just one report of experiments directly examining this, dated 1989 (2).  CDC scientists found that HIV did not grow while in the insects.  This single report has been cited hundreds of times as the laboratory “proof” that mosquitoes could not transmit HIV.

Please note that its authors themselves never assured that mosquito transmission of HIV was impossible; as properly cautious scientists they appropriately theorized their results indicated only it was unlikely.

Was the proper genus of mosquito examined?

Questions remain about the applicability of this research, because of the genus of mosquito utilized.   That can be critical. Many infectious agents are quite “picky” as to which genus of mosquito they associate with or grow in.  For example, malaria needs Anopheles.  Western equine encephalitis and West Nile virus travel in Culex, and Zika is partial to Aedes.

If, inadvertently, an unfavorable genus is chosen to represent all mosquitoes, the infectious agent may not take hold, and scientists will falsely reach negative conclusions.

Only “elephant” mosquito utilized in lab study

Of 41 genera of mosquitoes which exist, the 1989 experimenters tested only one—Toxorhynchites.  This peculiar genus cannot even suck blood; its mouth parts are not suitable. (In nature it feeds off flowers and plant scraps).  But its relatively larger size (one possible reason for its nickname, “elephant mosquito”) makes it easier to handle, and thus a convenient choice, in the lab.  The researchers injected HIV-infected blood into the insect’s thorax and studied whether the  test virus grew there.

This blood-delivery scenario was far from what occurs in nature when a mosquito feeds on an HIV-infected person. However, the scientists felt this genus was suitable because other mosquito-transmitted viruses in a certain category (arboviruses) do grow inside it when this procedure is used.

But HIV is in a different virus category, and might require a different mosquito genus and circumstances in order to grow in the insects in a laboratory.

An added hurdle: some infectious agents are so particular that even when the right genus is used, they grow only inside certain species within that genus. Yet further, among blood feeders, only female mosquitoes are relevant, for only that sex sucks blood (the males feed on plants).

This 1989 report is the only significant research paper directly examining this question in some detail, albeit limited.  Upon this single widely-cited report –and much theorizing–rest all subsequent U.S. medical pronouncements about the impossibility of mosquitoes ever transmitting HIV.

Current lab lessons from Zika mosquitoes

The critical influence of mosquito genus and species on lab results was re-verified by studies of Zika virus in mosquitoes, published in October 2016 by scientists at the University of Wisconsin (3)

They confirmed that, in the lab, Zika could easily infect the two species of mosquitoes from the genus Aedes common in the southern U.S.  But when they tested a third species of the same genus, which inhabits temperate climates, those insects could be infected only with the “highest” concentrations of virus.

Moreover, mosquitoes from a different, but medically well-known genus, Culex (members of which carry disease-causing arboviruses such as West Nile and four types of encephalitis) could not be infected with Zika at all.

It is easy to see, therefore, that if these researchers had tested only the one genus, Culex, because it is hospitable to arboviruses, they would have erroneously concluded that mosquitoes cannot transmit Zika.

Yet the 1989 lab research on HIV in mosquitoes tested only one genus, and a peculiar one at that—Toxorhychites.  But its negative findings have been cited repeatedly by others as “proof” that mosquitoes in general could not transmit HIV.

Lab results may fail to show what happens in real life

Do negative lab results necessarily indicate the situation in real life? Sometimes not.   For example, laboratory-raised mosquitoes artificially fed on swine fever (hog cholera) virus grown in the lab failed to transmit the infection to pigs. However, when the same virus was harvested from mosquitoes trapped in the wild, that virus did infect the animals.

Thus, failure to grow or transmit HIV in a particular mosquito within the laboratory does not exclude the possibility that the situation out in nature is different.

Have mosquitoes trapped in Miami been tested for HIV?

Wouldn’t testing for HIV in locally-caught mosquitoes provide evidence?  Only if the results were positive. Given all the Zika cases, were Miami mosquitoes checked even for Zika? Only belatedly.  As late as July 29, 2016, the director of CDC was quoted as dubious: “Finding infected mosquitoes is like finding a needle in a haystack.”

But 31 days later Florida authorities identified Zika in the insects.  Note it took serious effort.  Reports say more than 2,470 mosquito samples (prepared from over 40,000 mosquitoes) were tested.  Just three were positive.  So it’s not like scientists can now test a few mosquitoes from an HIV patient’s backyard and conclude anything from a quick negative result; there will have to be a serious research commitment.

Why no clinical reports of HIV infections from mosquito transmission?

The AIDS epidemic started back in 1981, 36 years ago.  Isn’t that plenty of time for doctors to report at least a few HIV patients whose infection came via mosquitoes?  Maybe, maybe not.  It took 64 years to prove Zika virus could be transmitted sexually, but such transmission must have been going on for years.

Besides, since all doctors have been taught for over three decades that mosquitoes can’t transmit HIV, which clinician would risk ridicule for reporting a probable case of that happening?  Plus, would any doctor believe a sexually active HIV patient who claimed mosquitoes were the only possible way he could have become infected?

So what kind of clinical situation would convince skeptics that, yes, occasionally HIV can be spread by mosquitoes?  It will not be easy to find.  Here’s an example: an HIV infection developing in a chaste, virgin nun or monk with a negative drug test, residing in a guarded. closely supervised convent or monastery, in warm lowlands infested with biting mosquitoes, next to a town with many HIV patients, who showed numerous mosquito bites but had never had a blood transfusion or organ transplant.

Deeper research needed now for definitive answer

While waiting for such a unique patient, the re-emphasis on mosquitoes brought on by Zika makes it timely to undertake deeper research on the possibility of HIV transmission by these insects.    Summer 2017 provides the next opportunity, when Zika mosquitoes re-emerge from their winter/spring doldrums

In areas having the two infections, trap and assay more mosquitoes for both viruses.  In the lab, investigate HIV in a greater variety of mosquito genera and species, under various conditions. Such additional evidence will help determine more definitively whether mosquitoes might occasionally serve as vectors for HIV transmission▪︎

 

REFERENCES

1. Castro KG, Lieb S, Narkunas JP et al. Transmission of HIV in Belle Glade, Florida: lessons for other communities in the United States. Science 1988; 239:193-7.
2. Webb PA, Happ CM, Maupin GO et al. Potential for insect transmission of HIV: experimental exposure of Cimex hemipterus and Toxorhynchites amboinensis to human immunodeficiency virus. J Infect Dis. 1989; 160:970-7.
3. Alioto MT, Pelnado SA, Osorio JE, Bartholomay LC. Culex pipiens and Aedes triseriatus mosquito susceptibility to Zika virus. Emerg Infect Dis. 2016; 22:1857-8.


 

Zika spread by sex took 64 years to be recognized, and HIV/AIDS epidemic is only 36 years old

In medicine it often takes decades to assemble a complete understanding of the routes by which a disease can be transmitted.  So it would not be unheard of if unexpected pathways for occasional HIV transmission continue to emerge, such as mosquitoes.

Zika is a perfect example. Zika virus was isolated in 1947, and for years was thought to be transmitted only by mosquitoes.

But 64 years after its discovery, in 2011, a single startling scientific report showed sexual transmission could also occur. Today, this additional pathway is accepted without question.

And that landmark report on Zika was convincing only because of an ideal set of circumstances.  An American microbiologist had been working in mosquito-plagued Senegal.  He returned to his Colorado home, where there were no mosquitoes.  His wife, an oncology nurse, had never left town, but then developed Zika.  Sexual transmission from husband to wife was established as the only possibility.

Had this Zika scenario occurred today not in Colorado, but in Miami, local mosquitoes would have been blamed immediately for the transmission—and we still might not know about Zika’s sexual pathway▪︎

 

AUTHOR CREDENTIALS:

Dr. Leslie Norins has 50 years of experience as a physician-scientist and medical publisher.  He received his BA from Johns Hopkins University and his MD from Duke University Medical School. He then studied immunology with Sir Macfarlane Burnet, Nobel Prize-winner, and received his PhD from the University of Melbourne.  From 1966 to 1972 he served as director of the Venereal Disease Research Laboratory at the federal Centers for Disease Control and Prevention. Dr. Norins then became a medical publisher, and over the next 40 years created and grew over 80 monthly newsletters providing specialized news and advice to healthcare professionals nationally. One of his early ones was AIDS Alert, the first periodical to assist physicians and nurses caring for AIDS patients.

Share Post
Latest comments

LEAVE A COMMENT