Vectors

Photo: pernsanitfoto/istock

P. vivax, just like P. falciparum, is transmitted by Anopheles spp. mosquitoes. The diversity and geographical range of P. vivax vectors and their specific ecology and behaviour means that vector control measures that can effectively suppress P. falciparum transmission may not be as effective against P. vivax and vice versa.1

High diversity of species

  • The diversity of vector species for P. vivax complicates vector control measures as each vector species will have different biological and ecological characteristics.2
  • Measures that effectively supress one vector species may only lead to its replacement in the transmission cycle by an alternative species that has different habits and ecology.2

Wide geographical range

  • P. vivax can complete its development in the mosquito at lower temperatures and more quickly than P. falciparum. This means that mosquito species that inhabit more temperate regions can also transmit P. vivax.2
  • Where the transmission season depends on temperature, transmission can take place for more months each year for P. vivax versus P. falciparum.2

Vector behaviour

  • Many P. vivax vectors are outdoor resting and bite during the day, avoiding conventional vector control measures such as long-lasting insecticide treated bed nets (LLINs) and indoor residual spraying (IRS).2
  • P. vivax vectors tend to feed predominately on animals rather than humans (zoophagy). Thus, vector control measures that target mosquito to human transmission will have a negligible impact upon mosquito populations.1

Vector control and P. vivax

Photo: Akar Naing for the Swiss Malaria Group

The introduction of Long lasting insecticide treated nets (LLINs) and indoor residual spraying (IRS) has greatly reduced the prevalence of P. falciparum malaria in Africa.2 However, in areas of the world where P. falciparum and P. vivax are co-endemic, strengthening vector control appears to have a greater effect on the incidence of P. falciparum malaria versus P. vivax malaria.2

It is not that standard vector control measures are ineffective against P. vivax; they are just not as effective as against P. falciparum and it is more difficult to determine their impact.3

Note that reductions in P. vivax transmission may not be observed as a change in the prevalence of P. vivax infection or symptomatic malaria. This is because P. vivax relapses, which do not require mosquito-to-human transmission, may cause a larger proportion of infections than those transmitted by mosquito bites.4

New approaches and methods are required for P. vivax vector control, and new techniques for determining their effectiveness, over and above those that have been used successfully for P. falciparum.