The Vanuatu government aims to eliminate malaria from Tafea province (which includes Tanna & Aneityum) in the next six years. It is thought that malaria has already been eliminated from Aneityum [3
] and that transmission is confined to specific locations across Tanna. In this study, the applicability of using serological methods to provide current estimates of transmission intensity was assessed and the utility of serology to provide evidence for the cessation of transmission in Aneityum was also investigated.
Seroepidemiological studies are especially useful in low transmission settings where the sensitivity of parasite prevalence surveys is limited by the scarcity of parasite positive individuals. This study was undertaken in the dry season, and as such only three RDT positives (0.2% of the population surveyed) were found in the population. In the previous wet season, surveys using PCR methods in the same areas on Tanna found 1% prevalence of P. falciparum
and 2.2% prevalence of P. vivax
infection in children between 2-11 years old [22
]. Both parasite-based prevalence measures, which offer a snapshot of malaria prevalence and are influenced by temporal changes, are, not surprisingly, significantly lower than the antibody prevalence data (which offers a period prevalence measure) for Tanna in this study (24%, 299/1242 P. falciparum
and 20%, 253/1242 P. vivax
). This extra sensitivity allows for a more detailed examination of malaria transmission.
Modelling changes in transmission intensity can give an insight into the success of interventions in a particular area. Perhaps surprisingly, changes in transmission were most readily detected using PfSE. Schizont extract contains multiple proteins some of which will be antigenic and similar between parasite species, it is, therefore, not surprising that PfSE has the highest overall seroprevalence. However, whilst adults demonstrate high seroprevalence to PfSE, seroprevalence in children is lower than for the more specific antigens, MSP-119
and AMA-1. It is not clear why this should be; it may be the result of lower exposure in children resulting in lower levels of antibodies to the antigens in PfSE. This has been demonstrated in Kenya, where individuals were more likely to have antibodies to specific antigens rather than PfSE [30
]. An alternative explanation is that the relative concentration of the specific antigens is lower in PfSE- this needs further investigation.
Statistical analysis of the seroprevalence profiles suggests that a significant drop in transmission occurred approximately 30 years ago. These seroprevalence curves are consistent with those described by Kaneko et al
(submitted) using a P. falciparum
lysate. However, analysis of country-wide case data suggests that a drop in malaria cases occurred in 1992 (17 years before this study), which was attributed to ITN distribution across the islands [24
]. There are two (not mutually exclusive) potential explanations for this. Firstly, the drop in seroprevalence may indicate that the IRS campaign in place in the early 1980s successfully reduced exposure. At the time, this campaign was considered unsuccessful due to little reduction exhibited in national parasite rates. However, it is possible that it was effective in certain areas such as Northern Tanna and on Aneityum. An alternative explanation is that the reduction in seroprevalence is related to the later malaria control programme that was implemented nationwide in 1988 and in Aneityum in 1991, and which resulted in the elimination of malaria from Aneityum [3
]. The observation that the step in serological data occurs approximately 10 years before this point in this scenario is possibly due to differential rates of loss of antibodies with age. Younger children who had low antibody levels when the 1988 interventions were implemented, may have lost these antibodies more rapidly than older individuals with a more established antibody response [31
]. Clearly more information on the rates of antibody loss is required to allow a full interpretation of these data, and further work is ongoing.
The observation that immune responses to the specific antigens do not reflect the changes in transmission is perplexing. Transmission is known to be very low across Tanna and it is possible that in these conditions models for assessing for an increase in seropositivity with age may be inappropriate. Several of the age stratified curves demonstrate little to no increase in seroprevalence with age. In areas where transmission has been consistently low for many years a loss in seropositivity in older individuals may result in uniform seropositivity across the population; masking any change-point in transmission. This is clearly not the case for the multi-antigenic PfSE- one might expect an increased chance that antibodies to this conglomeration of antigens have longer half lives, although this was not observed in a longitudinal study in Thailand [33
]. In a situation where age seroprevalence reveals little information, it may be more pertinent to investigate antibody titres and to focus on children born since interventions have been in place. This work is ongoing.
It is important to understand the dynamics in the transmission of different species in areas where more than one species of Plasmodium
are transmitted. Serological measures have allowed us to successfully distinguish between areas where P. falciparum
and P. vivax
malaria are transmitted. Current SCRs are low for both species but consistently higher for P. vivax
in all areas. Seropositivity to P. vivax
antigens was much higher in younger individuals, suggesting either that transmission of this species has been higher than P. falciparum
transmission over the past 10 years, or that preferential infection of younger age groups, as has been reported previously [34
], results in a different age seroprevalence curve than is found with P. falciparum
seroprevalence. Additionally, morbidity data from previous studies in Vanuatu [35
] and Papua New Guinea [36
] show that clinical immunity to P. vivax
builds up faster than for P. falciparum
which could explain the seemingly high seroprevalence in younger children. The high proportion of P. vivax
seropositive individuals in younger age groups in Northern Tanna made the data difficult to fit the conventional seroconversion model. A similar age seroprevalence profile would be expected in areas where there has been an epidemic as the result of a defined period of exposure in all ages of the population. However, the effect of possible presence of hypnozoites in P. vivax
infections has not previously been studied and we do not know how infections that are the result of hypnozoites (i.e. relapses) influence serological profiles. Previous data has shown that P. falciparum
is more successfully reduced by interventions than P. vivax
, due to the limited effects interventions have on relapses, and due to differences in biting behaviour by mosquitoes infected with each species [37
]. The profile likelihood plots do not indicate a clear change in transmission intensity based on seroprevalence of P. vivax
antibodies. The drop in P. vivax
transmission may not have been as pronounced or sudden as the P. falciparum
reduction, in which case, a step in the seroprevalence curves would not be evident. P. vivax
prevalence tends to be higher in areas of low transmission intensity [38
], and an increase in P. vivax
cases has been reported in areas where P. falciparum
transmission appears to be on the decline [39
]. However, on Aneityum clear evidence of a change in transmission for P. vivax
was seen in seroprevalence data by Kaneko et al
(submitted); this data also indicated a change in transmission had occurred in the early 1980s. This study used a lysed P.vivax
preparation as antigen which, as for the P. falciparum
equivalent, may be more likely to detect changes in a polymorphic antibody response.
The P. vivax
epidemic in 2002 may also have had an adverse effect on the seroprevalence curves to P. vivax
antigens in Aneityum. There have been no reported cases since 2002, and whilst seropositivity on this island is mainly restricted to adults, low seropositivity in children under 7 suggests that there may still be residual transmission in the area. However, importantly, four of the 11 children seropositive to any antigen (falciparum or vivax) were classified as aged one. Birthdays and ages are not generally recorded in Aneityum, and it is possible these children were younger than one and, therefore, the antibodies detected could represent residual maternal antibodies. Alternatively, children may have been exposed to malaria whilst away from the island. Limited information was collected on travel history during the survey so this possible explanation cannot be confirmed. Monitoring of cases on Aneityum is very thorough, with everyone returning to the island tested for infection via thick and thin slide films. However, slide reading has limited sensitivity, and it is possible that some people may have arrived carrying infection. PCR methods detect parasites more sensitively and it may be pertinent to check for sub-patent infections amongst the inhabitants of Aneityum. Imported malaria can remain a problem as long as Anopheline mosquitoes are present on the island. This highlights the importance of continued monitoring and surveillance on islands where elimination has been achieved [41
], and the recording of accurate travel history information.
An alternative explanation for the observed seropositivity in young children is the presence of antibodies cross-reacting with antigens from other infectious agents [42
]. Detecting antibodies to specific recombinant antigens using ELISA, rather than the less specific immunofluoresence antibody test (IFAT) minimizes this danger. However, a few studies have suggested the cross-reactive potential of PfAMA with Toxoplasma antigens [45
], although this infection is reportedly not prevalent in Vanuatu.