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Background: Weight gain during pregnancy is of great concern for most women and obstetricians because of its possible consequences. This concern exists because gestational weight gain that exceeds what is recommended by the Institute of Medicine is associated with many maternal and fetal complications. While excessive gestational weight gain (GWG) is associated with modifiable risk factors for Gestational Diabetes Mellitus (GDM) notably, obesity and overweight conditions, there is a paucity of information on the association of cases of excessive GWG with non-modifiable risk factors (NMRF) for GDM namely history of diabetes mellitus in the first-degree family members as well as previous macrosomia and preterm deliveries. Method: The study prospectively monitored GWG in 337 women with a history of diabetes mellitus in first-degree family members as well as previous macrosomia and preterm deliveries. Data collected was analysed through bivariate regression to determine the odds of excessive GWG in the presence and absence of each of the three non-modifiable risk factors. Results: The non-modifiable risk factors of women of all BMIs were not found to be significantly associated with excessive GWG. For instance, a family history of DM in obese women was not associated with excessive GWG (P = 0.254; AOR = 0.715; 95%CI: 0.401-1.272). History of macrosomia in obese women was also not statistically significant (P = 0.973; AOR = 1.015; 95% CI: 0.429-2.402). Similarly, history of preterm births in obese women was not significantly associated with excessive GWG (P = 0.778; AOR = 0.902; 95%CI: 0.441-1.847). Conclusion: History of macrosomia and preterm births as well as family history of Diabetes mellitus cannot serve as predictors for excessive GWG because there is no association among them and excessive GWG by women of all BMIs

The purpose of this study was to determine antimicrobial resistance patterns of bacteria isolated from wounds of diabetes mellitus patients at JOOTRH. A hospital based cross sectional study design was employed with a target population of 168 and sample size of 117 patients involving stratified random sampling. Data was collected using a structured questionnaire and a laboratory form for a period of 6 months. Pus swabs were collected for isolation of bacteria using conventional techniques and serology. Resistance was done using Kirby-Bauer disk diffusion on Mueller Hinton Agar at 37˚C for 24 hrs. High susceptibility was established for; S.aureus on amikacin and gentamicin, E.coli on imipenem and gentamicin, K. pneumoniae and Proteus species on imipenem and P.aeroginosa on ciprofloxacin. Findings provide coherent and effective chemotherapeutic alternatives for managing diabetes patients with wounds and recommends that JOOTRH to adopt susceptibility testing policy for the sake of identifying the most effective treatment regimen for better patient’s care.

Introduction: Men play a significant role in health-seeking behaviour of their families. While determinants of men’s involvement in reproductive health matters have been studied, the same are likely to vary on the basis of social context. The objective of this study was to determine socio-demographic, cultural and health facility factors influencing male partner involvement in Antenatal Care (ANC) and Postnatal Care (PNC). Moreover, the study intended to determine perceived benefits of male involvement in ANC and PNC services. Methods: This was a cross-sectional household survey. Simple random sampling was used to select 398 men and women participants out of 1,716 households in Bumula Sub-County. The men and women interviewed were not necessarily couples. Questionnaires were administered to participants within the households by face-to-face interview. The data collected through the questionnaires were on their sociodemographic, cultural, and health- related factors, and perceived benefits of male involvement in ANC and PNC. Key informant interviews were conducted among 14 health workers. Results: Participants having college education were more likely to report male involvement (adjusted Odds Ratio (AOR) = 3.25, 95% CI [1.03-10.25], p=0.044) compared to those with no education. Participants who made joint decisions with partners (p

Background: Tuberculosis (TB) related mortality remains a serious impediment in ending TB epidemic. Objective: To estimate survival probability and identify predictors, causes and conditions contributing to mortality among TB patients in Vihiga County. Methods: A cohort of 291 patients from 20 purposively selected health facilities were prospectively considered. Data was obtained by validated questionnaires through face-to-face interviews. Survival probabilities were estimated using Kaplan-Meier method while Cox proportional hazard model identified predictors of TB mortality through calculation of hazard ratios at 95% confidence intervals. Mortality audit data was qualitatively categorized to elicit causes and conditions contributing to mortality. Results: 209 (72%) were male, median age was 40 (IQR=32-53) years while TB/HIV coinfection rate was 35%. Overall, 45 (15%) patients died, majority (78% (log rank

Insecticide treated bed nets and Indoor residual spraying remains the principal interventional malaria control strategies. To achieve malaria disease eradication, vector control programmes that monitor insecticide resistance profiles are necessary.The study evaluated pirimiphos-methyl susceptibility ofAdultWe observed pirimiphos-methyl resistance in Anopheles arabiensis and

To inform targeted HIV testing, we developed and externally validated a risk-score algorithm that incorporated behavioral characteristics. Outpatient data from five health facilities in western Kenya, comprising 19,458 adults ≥ 15 years tested for HIV from September 2017 to May 2018, were included in univariable and multivariable analyses used for algorithm development. Data for 11,330 adults attending one high-volume facility were used for validation. Using the final algorithm, patients were grouped into four risk-score categories: ≤ 9, 10–15, 16–29 and ≥ 30, with increasing HIV prevalence of 0.6% [95% confidence interval (CI) 0.46–0.75], 1.35% (95% CI 0.85–1.84), 2.65% (95% CI 1.8–3.51), and 15.15% (95% CI 9.03–21.27), respectively. The algorithm’s discrimination performance was modest, with an area under the receiver-operating-curve of 0.69 (95% CI 0.53–0.84). In settings where universal testing is not feasible, a risk-score algorithm can identify sub-populations with higher HIV-risk to be prioritized for HIV testing. Electronic supplementary material The online version of this article (10.1007/s10461-020-02962-7) contains supplementary material, which is available to authorized users.

Purpose : The declaration of free maternal and child Health services by the government of Kenya was meant to address some of the systemic service access challenges and increase the number of clients using MNCH services, consequently reducing maternal and neonatal deaths. However, persistent poor maternal and child health indicators in Kenya even with free MNCH services as evident in Kisumu County portends considerable concern hence the reason for this study. There was need to identify factors or factor combinations that affect MNCH service quality in Kisumu County. Methodology: This was a descriptive analytical study whereby data was collected from a sample of 284 mothers seeking MNCH services in Kisumu tier three public health facilities using structured questionnaires and data was analyzed using the Statistical Package for the Social Sciences. Univariate statistics was used to assess patterns of responses to the client satisfaction questionnaire items. Bivariate logistic regression was used to assess association between client satisfaction and dimensions of their characteristics; p-values of less than 0.05 were considered significant. Results: Mothers aged 30-39 years were more likely to report being satisfied with maternal, newborn and child health services as compared to those aged 15-19 years (aOR=0.52, 95%CI=0.28-0.96, p-Value=0.035). The clients who had 1 pregnancy (aOR=2.29, 95%CI=1.04-5.05, p-value=0.040) and 2 pregnancies (aOR=3.22, 95%CI=1.54-6.70, p-Value=0.002) were more likely to report being satisfied with maternal child health services in public health facilities. Similarly, there was significant increased odds of being satisfied among those who resided in rural areas (aOR=2.24, 95%CI=1.05-4.79, p-Value=0.037), those who use vehicles as means of transport (aOR=6.87, 95%CI=1.38-34.28, p-Value=0.019) and those who use motorbike as means of transport (aOR=7.79, 95%CI=1.61-37.77, p-Value=0.011) and maternal child health services satisfaction. Unique contribution to practice and policy: The feedback from this study can be used by national and county governments, state agencies and other health stakeholders in designing communication and advocacy strategies for targeted groups or populations both at the community and health facilities to increase access and acceptability of MNCH services.

Purpose: Guidelines provide ethical frameworks to streamline processes for best outcomes. Live-attenuated vaccines in routine immunization programs in Kenya have challenges of instability despite the availability of guidelines. Diseases preventable by these vaccines are reported in Kisumu County more than others in the region. This observation is suspected to be associated with lapses in adherence to cold-chain management procedures. This study, hence purposed to assess whether there was any association between instability of vaccines and staff adherence level to the guidelines.Method: Using analytical cross-sectional design with repeated observations, data was collected from 120 out of 170 public and non-governmental health facilities in the Kisumu County at three intervals, between October 2018 and March 2019. In each health facility, immunization manager on duty was interviewed and observations made on compliance with cold-chain protocols using a checklist. Data was summarized descriptively and regression used to identify key points of adherence levels. Qualitative information was analyzed thematically to identify evolving themes.Result: In 40% of the facilities, staff did not transfer vaccines when the need arose; 8.3% did not defrost fridges; while 45.8% did not dispense vaccines at VVM stage 2. Transferred vaccines accounted for improved stability of both OPV (95%C.I; 3.1 – 16.8, OR 7.2, p value< 0.001) and measles-rubella vaccines (95% C.I, 2.1 – 10.4, OR, 4.7, p=value

BACKGROUND፡ The marked genome plasticity of diarrheagenic Escherichia coli promotes emergence of pathotypes displaying unique phenotypic and genotypic resistance. This study examined phenotypic and genotypic antibiotic resistant diarrheagenic Escherichia coli pathotypes among children in Nairobi City, Kenya.METHODS: In a cross-sectional study, diarrheagenic Escherichia coli pathotypes were isolated from stool samples and their phenotypic and genotypic resistance against eight antimicrobial agents assayed.RESULTS: Diarrheagenic Escherichia coli was detected in 136(36.4%) children. Most of diarrheagenic Escherichia coli that were resistant to ampicillin, ceftriaxone, streptomycin, gentamycin, ciprofloxacin, chloramphenicol, erythromycin and tetracycline, harbored citm, bla CMY, aadA1, aac(3)-IV, qnr, catA, ere(A) and tet(A) corresponding resistant genes.CONCLUSION: Antimicrobial-resistant genes are highly prevalent among phenotypic resistant ETEC pathotypes indicating a possibility of horizontal gene transfer in spreading antibiotic resistant genes among E. coli pathotypes.

Background: Severe malarial anaemia (SMA) is a leading cause of childhood mortality in holoendemic Plasmodium falciparum regions. Methods: To gain an improved understanding of SMA pathogenesis, whole genome and transcriptome profiling was performed in Kenyan children (n = 144, 3–36 months) with discrete non-SMA and SMA phenotypes. Leukocyte associated immunoglobulin like receptor 1 (LAIR1) emerged as a predictor of susceptibility to SMA (P A); rs2287827 (18835G>A)] and clinical outcomes were investigated in individuals (n = 1512

BackgroundDespite robust Tuberculosis (TB) program with effective chemotherapy and high coverage, treatment interruption remains a serious problem. Interrupting TB treatment means that patients remain infectious for longer time and are at risk of developing drug resistance and death. This study was conducted to identify and describe predictors of TB treatment interruption.MethodsA cohort of 291 notified TB patients from 20 selected health facilities in Vihiga County were enrolled in to the study and followed up until the end of treatment. Patient characteristics that potentially predict treatment interruption were recorded during treatment initiation using structured questionnaires. Patients who interrupted treatment were traced and reasons for stoppage of treatment recorded. Kaplan Meier method was used to estimate probabilities of treatment interruption by patient characteristics and determine time intervals. The Log rank test for the equality of survival distributions analyzed significance of survival differences among categorical variables. For multivariable analysis, Cox proportional hazard model, was fitted to identify predictors of TB treatment interruption through calculation of hazard ratios with 95% Confidence Intervals (CIs). For variable analysis, statistical significance was set at P ≤ 0.05. Reasons for treatment interruption were categorized according to most recurrent behavioral or experiential characteristics.ResultsParticipants’ median age was 40 years (IQR = 32–53) and 72% were male. Of the 291 patients, 11% (n = 32) interrupted treatment. Incidences of treatment interruption significantly occurred during intensive phase of treatment. Independent predictors of treatment interruption included alcohol consumption (HR = 9.2, 95% CI; 2.6–32.5, p < 0.001), being female (HR = 5.01, 95% CI; 1.68–15.0, p = 0.004), having primary or lower education level (HR = 3.09, 95% CI; 1.13–8.49, p < 0.029) and having a treatment supporter (HR = 0.33, 95% CI; 0.14–0.76, p = 0.009). Reasons for interrupting treatment were categorized as: alcoholism, feeling better after treatment initiation, associated TB stigma, long distance to health facility, lack of food, perception of not having TB and pill burden.ConclusionTB treatment interruption was high and largely associated with patients’ socio-demographic and behavioral characteristics. These multidimensional factors suggest the need for interventions that not only target individual patients but also environment in which they live and receive healthcare services.

Aim: Determine the prevalence of enteric bacterial pathogens and their antimicrobial resistance among diarrheic children in Nairobi City, Kenya. Background: Regardless of enteric bacterial pathogens being a major cause of gastroenteritis in children, their occurrence and antimicrobial resistance patterns reveals regional spatial and temporal variation. Methods: In a cross-sectional study, a total of 374 children below five years presenting with diarrhea at Mbagathi County Hospital were recruited. Stool microbiology test was used to detect enteric bacterial infection. Antimicrobial resistance was determined using the disk diffusion method. Results: Diarrheagenic E. coli (36.4%) was the leading species followed by Shigella (3.2%), Salmonella (2.4%), Campylobacter (1.6%), Yersinia (1.3%) and Aeromonas (1.1%) species. Escherichia coli pathotyping revealed that 20.9%, 4.0%, 10.2% and 0.5% of the study participants were infected with enteroaggregative E. coli (EAEC), enteropathogenic E. coli (EPEC), enterotoxigenic E. coli (ETEC) and enteroinvasive E. coli (EIEC) pure isolates while the prevalence of mixed pathotype infections was 0.3% for EAEC/EPEC/ETEC and 0.5% for EAEC/ETEC. Shigella sero-grouping revealed that 0.5%, 0.3%, 1.9%, and 0.5% were infected with Shigella boydii, Shigella dysentriae, Shigella flexneri and Shigella sonnei pure isolates. Shigella species and E. coli co-infection was detected in 2.4% of the children, specifically, 1.1% for EAEC/Shigella boydii, 0.5% for EAEC/Shigella dysentriae and 0.3% in each case of EAEC/Shigella sonnei, EPEC/Shigella flexneri and ETEC/Shigella flexneri co-infections. Most of the isolates were resistant to commonly prescribed antibiotics. Conclusion: There was a high prevalence of enteric bacterial pathogens and co-infection alters epidemiological dynamics of bacterial diarrhea in children. Continuous antibiotic resistance surveillance is justified because the pathogens were highly resistant to commonly prescribed antimicrobials.

BackgroundAs countries make progress towards HIV epidemic control, there is increasing need to identify finer geographic areas to target HIV interventions. We mapped geographic clusters of new HIV diagnoses, and described factors associated with HIV-positive diagnosis, in order to inform targeting of HIV interventions to finer geographic areas and sub-populations.MethodsWe analyzed data for clients aged> 15 years who received home-based HIV testing as part of a routine public health program between May 2016 and July 2017 in Siaya County, western Kenya. Geospatial analysis using Kulldorff’s spatial scan statistic was used to detect geographic clusters (radius < 5 kilometers) of new HIV diagnoses. Factors associated with new HIV diagnosis were assessed in a spatially-integrated Bayesian hierarchical model.ResultsOf 268,153 clients with HIV test results, 2906 (1.1%) were diagnosed HIV-positive. We found spatial variation in the distribution of new HIV diagnoses, and identified nine clusters in which the number of new HIV diagnoses was significantly (1.56 to 2.64 times) higher than expected. Sub-populations with significantly higher HIV-positive yield identified in the multivariable spatially-integrated Bayesian model included: clients aged 20–24 years [adjusted relative risk (aRR) 3.45, 95% Bayesian Credible Intervals (CI) 2.85–4.20], 25–35 years (aRR 4.76, 95% CI 3.92–5.81) and > 35 years (aRR 2.44, 95% CI 1.99–3.00); those in polygamous marriage (aRR 1.84, 95% CI 1.55–2.16), or separated/divorced (aRR 3.36, 95% CI 2.72–4.08); and clients who reported having never been tested for HIV (aRR 2.35, 95% CI 2.02–2.72), or having been tested > 12 months ago (aRR 1.53, 95% CI 1.41–1.66).ConclusionOur study used routine public health program data to identify granular geographic clusters of higher new HIV diagnoses, and sub-populations with higher HIV-positive yield in the setting of a generalized HIV epidemic. In order to target HIV testing and prevention interventions to finer granular geographic areas for maximal epidemiologic impact, integrating geospatial analysis into routine public health programs can be useful.

Background HIV infection is associated with more frequent and severe episodes of malaria and may be the result of altered malaria-specific B cell responses. However, it is poorly understood how HIV and the associated lymphopenia and immune activation affect malaria-specific antibody responses. Methods HIV infected and uninfected adults were recruited from Bondo subcounty hospital in Western Kenya at the time of HIV testing (antiretroviral and co-trimoxazole prophylaxis naïve). Total and Plasmodium falciparum apical membrane antigen-1 (AMA1) and glutamate rich protein-R0 (GLURP-R0) specific IgM, IgG and IgG subclass concentrations was measured in 129 and 52 of recruited HIV-infected and uninfected individuals, respectively. In addition, HIV-1 viral load (VL), CD4+ T cell count, and C-reactive protein (CRP) concentration was quantified in study participants. Antibody levels were compared based on HIV status and the associations of antibody concentration with HIV-1 VL, CD4+ count, and CRP levels was measured using Spearman correlation testing. Results Among study participants, concentrations of IgM, IgG1 and IgG3 antibodies to AMA1 and GLURP-R0 were higher in HIV infected individuals compared to uninfected individuals (all p

Cotrimoxazole (CTX) discontinuation increases malaria incidence in human immunodeficiency virus (HIV)-infected individuals. Rates, quantity, and timing of parasitemia rebound following CTX remain undefined. Serial specimens from a trial of HIV-infected individuals receiving antiretroviral treatment (ART) randomized to continue (the CTX arm) or discontinue (the STOP-CTX arm) were examined for malaria parasites by quantitative reverse transcription polymerase chain reaction (PCR). Specimens obtained at enrollment and then quarterly for 12 months and at sick visits were assessed; multiplicity of infection was evaluated by PCR that targeted the polymorphic msp-1/msp-2 alleles. Among 500 HIV-infected adults receiving ART (median ART duration, 4.5 years), 5% had detectable parasitemia at baseline. After randomization, parasite prevalence increased over time in the STOP-CTX arm, compared with the CTX arm, with values of 4% and1%, respectively, at month 3, 8% and 2% at month 6, 14% and 2% at month 9, and 22% and 4% at month 12 (P = .0034). The combined mean parasite density at the various time points was higher in the STOP-CTX arm (4.42 vs 3.13 log Discontinuation of CTX by HIV-infected adults receiving ART resulted in progressive increases in malaria parasitemia prevalence and burden. NCT01425073.

Aims: Immune-mediated mechanism, such as deposition of complement (C3b) on erythrocytes leading to enhanced receptor-mediated uptake by macrophages has been proposed to contribute

Background: Despite a reduction in the global burden of malaria, the disease remains responsible for 214 million cases and 438,000 deaths annually with 88% of the mortality occurring in sub-Saharan Africa. Malaria control largely depends on effective chemotherapy. However, the historic and current emergence and spread of multi-drug resistant parasite strains provides significant challenges to malaria control and consequently, reduction of malaria-associated morbidity and mortality. Combating parasite drug resistance requires pharmacological compounds that target both known and novel metabolic pathways that are crucial for parasite survival. In addition, the identification of novel therapeutic agents that target distinct molecular pathways, apart from those of the conventional antimalarials, offers an approach for minimizing drug resistance. Conclusion: This review summarizes current anti-malarial approaches and strategies, therapeutic efficacy for conventional and non-conventional antimalarials, parasitic targets, and the mechanisms responsible for the development of drug resistance.

Background Tumors commonly are infiltrated by leukocytes, or tumor infiltrating leukocytes (TILs). It remains unclear, however, if the density and type of individual TILs has a direct or simply correlative role in promoting poor prognosis in breast cancer patients. Breast cancer in Kenyan women is aggressive with presentation at a young age, with advanced grade (grade III), large tumor size (>2.0 cm), and poor prognosis. We previously observed that the tumors were predominantly estrogen receptor positive (ER+) but also included both a high percentage of triple negative tumors and also increased immune cell infiltration within the tumors. We used breast tumor tissues from each patient to make tissue microarrays that were then stained for leukocyte and myeloid markers including CD4, CD8, CD20, CD25, CD68, and CD163 using immunohistochemical techniques. The immune cell infiltration into the cancer tissue included increased numbers of macrophages (CD68+), helper T cells (CD4+), and CD25+ lymphocytes compared to benign tissue. Results This study characterized the grade, molecular subtypes, and proliferation index of these tumors and determined if TIL density was enriched across any of these factors. We analyzed 49 malignant patient tissue samples for this study. The patient population had a mean age of 51.9 years. The tumors analyzed were heterogeneous by grade: grade I (6%), grade II (47%), and grade III (39%). Most patients presented with large tumors (>2.0 cm) (69%). We classified the tumors into molecular subtypes based on clinical marker expression. Based on this analysis, the molecular subtype distribution was heterogeneous with luminal B (41%), basal/triple negative (TN) (37%), luminal A (14%) and HER2 (8%) breast cancer subtypes. While the basal/TN subtype had a much higher proliferative index (Ki-67+) than did the other molecular subtypes, we did not see a significant correlation between TIL density and either subtype or tumor grade. Therefore, TIL density is independent of molecular subtype and grade. Conclusion This study identified a Kenyan patient cohort that develops large, high-grade tumors primarily of the luminal B and basal molecular subtypes. After analyzing the TILs within these tumors, we found that immune cell infiltration of these tumors correlated with increased proliferation but not grade or molecular subtype. Future research is required to determine if the aberrant recruitment of TILs to tumors contributes to cancer progression and response to cancer treatments.

Plasmodium falciparum erythrocyte membrane protein 1 (PfEMP1) is a potentially important family of immune targets, encoded by an extremely diverse gene family called var. Understanding of the genetic organization of var genes is hampered by sequence mosaicism that results from a long history of non-homologous recombination. Here we have used software designed to analyse social networks to visualize the relationships between large collections of short var sequences tags sampled from clinical parasite isolates. In this approach, two sequences are connected if they share one or more highly polymorphic sequence blocks. The results show that the majority of analysed sequences including several var-like sequences from the chimpanzee parasite Plasmodium reichenowi can be either directly or indirectly linked together in a single unbroken network. However, the network is highly structured and contains putative subgroups of recombining sequences. The major subgroup contains the previously described group A var genes, previously proposed to be genetically distinct. Another subgroup contains sequences found to be associated with rosetting, a parasite virulence phenotype. The mosaic structure of the sequences and their division into subgroups may reflect the conflicting problems of maximizing antigenic diversity and minimizing epitope sharing between variants while maintaining their host cell binding functions.

BACKGROUND: follow-up visits are recommended to all voluntary medical male circumcision clients (VMMC), however, adherence is variable. High lost-to-follow-up cases limit knowledge about clinical status of clients and adverse events. This study sought to establish Motivators and Barriers to the Uptake of VMMC post-operative follow-up services in Siaya County, Kenya. METHODS: 277 clients from five VMMC sites in Yala were recruited immediately post-operation to participate in a telephone interview between the 21 st and 31 st day post-surgery during which a semi-structured questionnaire was administered. Descriptive and inferential statistics was used to analyse quantitative information using SPSS while responses from open ended questions were grouped into themes, sieved out, coded and analyzed. RESULTS: 137(49.5%) of the 277 participants utilized the follow-up services. Health education (31.4%) and emergency reviews/adverse events (24.1%) were the main motivation for returning for follow-up while occupational and other engagements (29.7%) and presumption of healing (24.6%) were the main barriers. Type of facility attended (p=0.0173), satisfaction with the discharge process (p=0.0150) and residency in Yala (p

BackgroundIt has been hypothesized that the African allelesSl2andMcCbof the Swain-Langley (Sl) and McCoy (McC) blood group antigens of the complement receptor 1 (CR1) may confer a survival advantage in the setting ofPlasmodium falciparummalaria, but this has not been demonstrated.MethodsTo test this hypothesis, children in western Kenya with severe malaria-associated anaemia or cerebral malaria were matched to symptomatic uncomplicated malaria controls by age and gender. Swain-Langley and McCoy blood group alleles were determined by restriction fragment length polymorphism and conditional logistic regression was carried out.ResultsNo significant association was found between the African alleles and severe malaria-associated anaemia. However, children withSl2/2genotype were less likely to have cerebral malaria (OR = 0.17, 95% CI 0.04 to 0.72, P = 0.02) than children withSl1/1. In particular, individuals withSl2/2 McCa/bgenotype were less likely to have cerebral malaria (OR = 0.18, 95% CI 0.04 to 0.77, P = 0.02) than individuals withSl1/1 McCa/a.ConclusionThese results support the hypothesis that theSl2allele and, possibly, theMcCballele evolved in the context of malaria transmission and that in certain combinations probably confer a survival advantage on these populations.