SEATTLE -- In a spiral of infection and susceptibility, the African epidemics of HIV and malaria may be fueling each other, according to researchers here. The interplay of the two diseases heightens their effect in regions where both are found.
SEATTLE, Dec. 8 -- In a spiral of infection and susceptibility, the African epidemics of HIV and malaria may be fueling each other, according to researchers here.
A mathematical model, combined with HIV and malaria co-infection data collected in Malawi, suggested that the interplay of the two diseases heightens their effect in regions where both are found Laith Abu-Raddad, Ph.D., of the Fred Hutchinson Cancer Research Center, and colleagues.
In effect, they reported in the Dec. 7 issue of Science, HIV's effect on the immune system makes people more susceptible to malaria, while the febrile episodes of malaria make HIV more transmissible.
A febrile malarial episode produces a transient increase -- by a factor of about 10 -- in the viral load of a person who also has HIV, the researchers noted. That increase raises the chance that sexual intercourse will transmit the virus by a factor of 2.45.
"While HIV/AIDS is predominantly spreading through sexual intercourse, this biological co-factor induced by malaria has contributed considerably to the spread of HIV by increasing HIV transmission probability per sexual act," Dr. Abu-Raddad said.
The model developed by Dr. Abu-Raddad employs a range of factors, including the time the increased HIV viral load lasts after a malaria attack and the effect the attack has on a person's sexual activity.
The effect of the interplay between the two infectious diseases has been to increase the number of HIV infections in the region by tens of thousands and the number of malaria infections by millions, the researchers said.
To take a concrete example, they applied the model to the town of Kisumu in Kenya, which has a population of 200,000 and a high prevalence of both HIV and malaria. Since 1980, the researchers found that the HIV-malaria interplay has resulted in 8,500 excess HIV infections and 980,000 excess malaria episodes cumulatively.
In that relatively small city, the HIV prevalence rate is 8% higher than it would have been without malaria and the malaria rate is 13% higher than it would have been without HIV, Dr. Abu-Raddad and colleagues said.
Between 1990 and 2005, HIV prevalence in the area has been about 25% -- a level reached more quickly with the help of malaria.
"We estimate that an HIV prevalence that reached 24% in 1995 would have needed two additional years to reach this level in the absence of synergy with malaria," the researchers said.
Some of the factors in the model are estimates and need better data, Dr. Abu-Raddad and colleagues said. For instance, the model assumes that a malarial episode will reduce the sexual activity of the patient, but it's not known how much or for how long.
If sexual activity were reduced by 36% during the period when the HIV viral load is increased, the malarial impact on HIV would be eliminated, the researchers said. That translates into a complete avoidance of all sexual activity during and for eight weeks after a malarial fever.
On the other hand, effective treatment for malaria -- in particular shortening the infectious period -- would completely eliminate the HIV impact on the spread of malaria, they said.
The findings have clear implications for public health, said co-author James Kublin, M.D., also of the Hutchinson center. "We can reduce HIV/AIDS transmission by concomitantly treating HIV/AIDS co-infections with malaria as well as other diseases," he said.
The authors did not report any financial conflicts.