A Zika virus from America is more efficiently transmitted than an Asian virus by Aedes aegypti mosquitoes from Asia.

Zika is a mosquito-borne disease associated with neurological disorders that causes an on-going pandemic. The first outbreak was recorded in Micronesia in 2007, then in French Polynesia in 2014 from which it spread to South America in 2015 and ignited a widespread epidemic. Interestingly, Zika outbreaks in Asia remained of moderate intensity although the virus is circulating. To understand these epidemiological variations, we investigated the entomological determinants of ZIKV transmission in Asia. We used oral infection of mosquitoes collected in Singapore to identify the vector species, to quantify the blood infection threshold and to compare transmissibility between an Asian ZIKV strain (H/PF13) and an American strain collected in Brazil (BE H 815744). We have confirmed the vector status of Aedes aegypti and determined that 103 pfu/ml of blood is sufficient to infect mosquitoes. We showed that only the American strain was present in the saliva 3 days post-infection, and that this strain had a 30-40% higher rate of saliva infection in Ae. aegypti from 3 to 14 days post-infection than the Asian strain. Our data suggests that American strains are more efficiently transmitted than Asian strains, which raises concerns about the introduction of American strains in Asia.

Molecular epidemiological investigation of Plasmodium knowlesi in humans and macaques in Singapore.

Singapore reported its first locally acquired human Plasmodium knowlesi infection in 2007, involving a soldier who had undergone training in a forested area where long-tailed macaques are frequently seen. Comprehensive disease surveillance and monitoring system that was set up after the initial case detected four additional human P. knowlesi cases in 2007 and one in 2008. All involved military personnel who had undergone training in the forested area, and none had traveled out of Singapore 1 month before the onset of symptoms. Screening for malaria parasites on blood obtained from long-tailed macaques revealed that wild monkeys (n=3) caught from the forested area were infected with P. knowlesi, whereas peri-domestic monkeys (n=10) caught from a nature reserve park were not infected with any malaria parasites. Phylogenetic analysis of the nonrepeat region of the P. knowlesi csp genes showed that the sequences obtained from the human cases were not distinct from those obtained from wild monkeys. Further, certain genotypes were shared between samples from humans and macaques. Our findings provide evidence that long-tailed macaques are the natural hosts of P. knowlesi in Singapore and the human cases acquired their infection in the same vicinity where these monkeys are found. Further, the risk of acquiring P. knowlesi infection among the general population of Singapore is small as evident from the absence of P. knowlesi in peri-domestic monkeys.

Plasmodium knowlesi in humans, macaques and mosquitoes in peninsular Malaysia.

BACKGROUND: Since a large focus of human infection with Plasmodium knowlesi, a simian malaria parasite naturally found in long-tailed and pig tailed macaques, was reported in Sarawak, Malaysian Borneo, it was pertinent to study the situation in peninsular Malaysia. A study was thus initiated to screen human cases of Plasmodium malariae using molecular techniques, to determine the presence of P. knowlesi in non- human primates and to elucidate its vectors. METHODS: Nested polymerase chain reaction (PCR) was used to identify all Plasmodium species present in the human blood samples sent to the Parasitology laboratory of Institute for Medical Research. At the same time, non-human primates were also screened for malaria parasites and nested PCR was carried out to determine the presence of P. knowlesi. Mosquitoes were collected from Pahang by human landing collection and monkey-baited-traps situated on three different levels. All mosquitoes were identified and salivary glands and midguts of anopheline mosquitoes were dissected to determine the presence of malaria parasites and nested PCR was carried out on positive glands. Sequencing of the csp genes were carried on P. knowlesi samples from humans, monkeys and mosquitoes, positive by PCR. RESULTS AND DISCUSSION: Plasmodium knowlesi was detected in 77 (69.37%) of the 111 human samples, 10 (6.90%) of the 145 monkey blood and in 2 (1.7%) Anopheles cracens. Sequence of the csp gene clustered with other P. knowlesi isolates. CONCLUSION: Human infection with Plasmodium knowlesi is occurring in most states of peninsular Malaysia. An. cracens is the main vector. Economic exploitation of the forest is perhaps bringing monkeys, mosquitoes and humans into increased contact. A single bite from a mosquito infected with P. knowlesi is sufficient to introduce the parasite to humans. Thus, this zoonotic transmission has to be considered in the future planning of malaria control.

Laboratory and field evaluation of the insect growth regulator pyriproxyfen (Sumilarv 0.5G) against dengue vectors.

The insect growth regulator pyriproxyfen was tested against Aedes aegypti at 0.01 and 0.02 mg of active ingredient (AI) per liter of water in 60-liter earthern jars. Both concentrations provided 100% control for 4 months. In additional experiments where 10 liters of water were replaced fortnightly, 100% control was still obtained over 4 months with 0.02 mg AI/liter and greater than 93-100% control was obtained over 4 months with 0.01 mg AI/liter. In less-controlled field-trial conditions, pyriproxyfen at a dosage of 0.02 mg AI/liter provided 100% control for 10 wk against Aedes albopictus even though water was replaced either daily or weekly. Although the activity of pyriproxyfen declines after 10 wk, those tests in the plastic tubs showed much higher levels of sustained residual activity compared to those in the earthern jars. Pyriproxyfen did not have an impact on nontarget organisms.