205

PNG Med J 1996; 39: 205-207

Can mosquitoes transmit AIDS?

MOSES J. BOCKARIE1 AND RAYMOND PARU1

Papua New Guinea Institute of Medical Research, Madang

SUMMARY

Surveys to determine knowledge regarding AIDS have shown in many countries, includingPapua New Guinea, that a large proportion of the literate population still mistakenly believethat mosquitoes can transmit the AIDS virus from one person to another. In this paper wereview the theoretical mechanisms which would allow blood-sucking insects such as mosquitoesto transmit virus and discuss the evidence against transmission of HIV by mosquitoes. AIDS isa sexually transmitted disease with no scientific evidence for arthropod transmission.

Introduction

Acquired immune deficiency syndrome(AIDS) is caused by a retrovirus called humanimmunodeficiency virus (HIV). When AIDSwas first recognized, there were manynewspaper reports on the possibility ofmosquitoes being involved in its transmission.Almost half of 6625 men and womeninterviewed in Zaire in 1988 to determineexisting levels of knowledge regarding AIDSbelieved in the transmission of AIDS bymosquitoes (1). Similarly in Zimbabwe, nearlyhalf of 4189 teacher-trainees interviewed,about six months after a national AIDSawareness campaign began, thought thatmosquitoes were capable of transmitting AIDS(2). A recent survey involving 1500 highschool students (grade 10) from 14 schools in 4different provinces in Papua New Guinearevealed that more than a third of them (34%)considered mosquitoes to be carriers of HIV(3).

Although mosquitoes are known to bevectors of certain death-threatening viraldiseases such as yellow fever, dengue feverand Japanese encephalitis, there is no evidencethat mosquitoes can transmit HIV. In this paperwe review the theoretical mechanisms whichwould allow blood-sucking insects such asmosquitoes to transmit virus and discuss the

1 Papua New Guinea Institute of Medical Research, PO Box 378, Madang, Madang Province 511, Papua New Guinea

evidence against the transmission of HIV bymosquitoes.

Theoretical mechanisms for the possibletransmission of HIV by mosquitoes

Biological transmission

A mosquito feeding on a person infectedwith HIV could ingest the virus which thenundergoes multiplication within the mosquitoand migrates to its salivary glands. When theinfected mosquito seeks a second blood-mealfrom an uninfected person, it could transfer theHIV from its salivary gland during the courseof feeding. This is the mechanism involved inthe transmission of yellow fever and dengue.

Mechanical transmission

Human defensive behaviour may prevent apartially blood-fed mosquito from completinga blood-meal on a person infected with HIV.However, instead of resuming feeding on thesame person the mosquito may select anuninfected person to complete its feeding on.As it penetrates the skin of the new host, themosquito could transfer HIV particles thatwere adhering to the mouthparts from theprevious partial blood-meal. This is purelymechanical transmission because the virusdoes not undergo any multiplication ordevelopment in the mosquito. Mechanical

206

Papua New Guinea Medical Journal Volume 39, No 3, September 1996

transmission is not a common mechanism thatmosquitoes use to spread infection but it is themethod by which houseflies transmit the virusthat causes poliomyelitis.

Transmission through mosquito squashes

Squashing mosquitoes is thought of by manypeople as an instinctive reaction triggered bythe sight of the creatures on human skin. Amosquito that had had a partial blood-mealfrom an HIV carrier and resumed feeding on anuninfected person may be squashed into a cutin the skin. Viral infection could be initiated inthis way depending on the quantity of viralparticles contained in the partial blood-meal.Fully engorged mosquitoes usually containbetween 1 and 3 l of blood.

Why HIV cannot be transmittedby mosquitoes

Each of the three mechanisms has beeninvestigated with a variety of blood-suckinginsects and the results clearly show that AIDScannot be transmitted by mosquitoes (4). Forbiological transmission to be possible thedisease organism must remain alive inside themosquito until it can be transferred. In the caseof yellow fever it takes about 12 days for thevirus to complete its cycle in the mosquito andappear in the saliva.

Studies with HIV have shown clearly thatthe virus disappears in the mosquito after about1-2 days, the time required for the mosquito todigest the blood-meal. The mosquito regardsHIV as food and digests it along with theblood. Since the virus does not survive toreproduce and invade the salivary glands,biological transmission of HIV is not possible.

Mechanical transmission of virus particleshas been shown only for infections that resultin high viraemia. HIV circulates at very lowlevels in the blood well below the levels ofany of the known mosquito-borne viruses and is far less infectious. Mechanicaltransmission has been experimentallydemonstrated for bovine leukaemia virus(BLV) and Rift Valley fever (RVF) virus usingmosquitoes (5). However, RVF is characterizedby high viraemia, which is not observed in thecase of AIDS patients. In experimental studiesof mechanical transmission of RVF virus, it

was shown that around one million infectiousunits/ml of host blood were required beforemechanical transmission could bedemonstrated. What constitutes a naturalinfective unit of HIV is not known but it is lessthan for RVF virus. HIV carriers normallyhave one in a million lymphocytes that arelikely to be infected, which suggests that thelevel of infection in HIV carriers could bearound six orders of magnitude too low formechanical transmission to be expected (5). Infact, it has been calculated that an AIDS-freeindividual would have to be bitten by 10million mosquitoes that had been feeding on anHIV carrier to receive a single unit of HIVfrom contaminated mosquito mouthparts.Based on the same calculations, it can beshown that crushing a mosquito containingHIV particles would still not begin to approachthe levels needed to initiate infection.Therefore transmission of HIV via fresh bloodin the mosquito gut is well beyond the limits ofprobability. BLV does not usually have a highviraemia but, unlike HIV, it causeslymphocytosis, increasing the number of bloodlymphocytes, so that although the proportion ofinfected cells may be low, the overall numberis quite high.

Studies on risk factors for the spread ofAIDS in rural Africa showed that exposure tomosquitoes (as assessed by history of malaria)were not risk factors for HIV (6). In manyareas where both AIDS and malaria are highlyprevalent and people are exposed to mosquitobites throughout the year, it is rare to findchildren of the 5-10 year age group infectedwith HIV despite the fact that the prevalence ofmalaria in this age group is high. AIDS is asexually transmitted disease with no evidenceof arthropod transmission.

REFERENCES

1 Bertrand JT, Makani B, Hassig SE, NiwemboKL, Djunghu B, Muanda M, Chirhamolekwa C.AIDS-related knowledge, sexual behavior, andcondom use among men and women in Kinshasa,Zaire. Am J Public Health 1991; 81:53-58.

2 Wilson D, Sibanda P, Greenspan R, Wilson C.Knowledge about AIDS among Zimbabweanteacher-trainees before and during the publicawareness campaign. Cent Afr J Med 1989;35:306-309.

3 Friesen H, Danaya R, Doonar P, Kemeki A,Lagani W, Mataio G, Mokela D, Rongap T,Vince J. Survey to assess knowledge, attitude andbehaviour of high school students in PNG in

207

Papua New Guinea Medical Journal Volume 39, No 3, September 1996

relation to HIV infection. Abstract in Programmeand Abstracts of the Thirty-first AnnualSymposium of the Medical Society of Papua NewGuinea, Port Moresby, 19-22 Sep 1995:21.

4 Crans WJ. Why mosquitoes cannot transmitAIDS. Wing Beats 1994;5:23-24.

5 Piot P, Schofield CJ. No evidence for arthropodtransmission of AIDS. Parasitol Today

1986;2:294-295.6 Hudson CP, Hennis AJ, Kataaha P, Lloyd G,

Moore AT, Sutehall GM, Whetstone R,Wreghitt T, Karpas A. Risk factors for the spreadof AIDS in rural Africa: evidence from acomparative seroepidemiological survey of AIDS,hepatitis B and syphilis in southwestern Uganda.AIDS 1988;2:255-260.