An agent-based model of exposure to human toxocariasis: a multi-country validation.

Seroprevalence data illustrate that human exposure to Toxocara is frequent. Environmental contamination with Toxocara spp. eggs is assumed to be the best indicator of human exposure, but increased risk of exposure has also been associated with many other factors. Reported associations are inconsistent, however, and there is still ambiguity regarding the factors driving the onset of Toxocara antibody positivity. The objective of this work was to assess the validity of our current conceptual understanding of the key processes driving human exposure to Toxocara. We constructed an agent-based model predicting Toxocara antibody positivity (as a measure of exposure) in children. Exposure was assumed to depend on the joint probability of 3 parameters: (1) environmental contamination with Toxocara spp. eggs, (2) larvation of these eggs and (3) the age-related contact with these eggs. This joint probability was linked to processes of acquired humoral immunity, influencing the rate of antibody seroreversion. The results of the simulation were validated against published data from 5 different geographical settings. Using simple rules and a stochastic approach with parameter estimates derived from the respective contexts, plausible serological patterns emerged from the model in nearly all settings. Our approach leads to novel insights in the transmission dynamics of Toxocara.

Age-related infection and transmission patterns of human cysticercosis.

Neurocysticercosis is recognised as an important but neglected cause of epilepsy in developing countries where the parasite occurs. Data on the transmission dynamics of the parasite in endemic areas are scarce. Individuals living in these areas are likely to be highly exposed to the parasite, but relatively few of them develop active infections. The present study aimed to describe and gain insights into changes in antibody responses and infection patterns related to age and/or gender in a south Ecuadorian rural population by combining antibody and antigen serological data with demographic characteristics. In 25% of the population, antibodies to Taenia solium cysticerci were detected whilst 2.9% had circulating parasite antigens. The proportion of antibody-positive individuals increased significantly until the age of 40years to become stable in older individuals. A rule-based simulation model was developed to explain these variations and to reflect the dynamics of exposure to, and transmission of, the parasite. In contrast, the proportion of people presenting circulating parasite antigens, reflecting an active infection, was significantly higher in people older than 60years. Immunosenescence could explain such an observation since a weaker immune system in the elderly would facilitate the establishment and maintenance of viable cysticerci compared with fully immunocompetent younger individuals. This work points out the role of the immune system in the development of cysticercosis within an exposed population and highlights new essential issues in understanding the transmission dynamics of the parasite, its incidence and the resulting immunological response at a population level.

Delays to treatment and out-of-pocket medical expenditure for tuberculosis patients, in an urban area of South America.

Short delays to treatment are important for the control of tuberculosis (TB). National Tuberculosis Programmes provide free diagnosis and treatment for smear-positive patients, so that the patients' out-of-pocket medical expenditure could be almost nil. The factors associated with delays in starting treatment, and the pre-treatment out-of-pocket medical expenditure for TB patients, have now been investigated in the Bolivian city of Cochabamba. Bolivia is the Latin American country with the highest incidence of TB. It is covered by a national TB programme that provides free diagnosis and free treatment for smear-positive patients. Structured interviews with 144 smear-positive patients enrolled in this programme revealed median patient, provider and total delays of 3.6, 6.2 and 12.9 weeks, respectively. The total delays were longer for the female patients than for the male, and for patients who consulted private doctors than for the other patients. When the first healthcare provider was a doctor, the median provider delay was 4.9 weeks in the public sector but 7.2 weeks in the private. The median out-of-pocket medical expenditure per patient, which was U.S.$13.2 overall, was much higher for those who consulted a private doctor than for those who did not (U.S.$21.9 v. U.S.$5.4, respectively; P<0.001). It appears that interventions targeting doctors (in both the private and public sectors) are likely to have a larger impact on the shortening of delays in TB treatment than interventions targeting patients. They could also reduce unnecessary out-of-pocket expenditure.

Collaboration between private pharmacies and national tuberculosis programme: an intervention in Bolivia.

BACKGROUND: Public-private partnerships are felt to be necessary for tuberculosis (TB) control in some developing countries. OBJECTIVES: To evaluate the potential of a collaboration between the National TB Programme (NTP) and private pharmacies in Bolivia, the country with the highest TB incidence in Latin America. METHODS: We contacted the local Pharmacists' Association in the city of Cochabamba, and designed a two phase intervention. The objectives of the first phase were to decrease the availability of TB drugs in private pharmacies on a voluntary basis, and to improve referral of clients seeking TB drugs to the NTP. A survey of all pharmacies allowed for a before-after comparison with a baseline survey. The objectives of the second phase were to obtain referral of pharmacy clients with chronic cough for TB screening in the NTP. This phase was started in 70 pharmacies and evaluated after 2 months using the referral slips issued by the pharmacists. RESULTS: The proportion of pharmacies selling TB drugs decreased (rifampicin: 23-11.5%; isoniazid: 16-3.1%; P<0.001) and the proportion of pharmacies referring to the NTP clients seeking TB drugs increased (22-58%; P<0.0001). In the second phase, 26 of 70 pharmacies (38%) referred a total of 41 clients for screening in the NTP (i.e. an average of 0.29 clients per pharmacy and per month); 11 of 41 (27%) were screened and three of 11 (27%) diagnosed with smear-positive TB. CONCLUSION: The first phase of the intervention proved effective in reducing the availability of the main TB drugs in pharmacies, and in improving referral of clients seeking TB drugs. Key factors in this success were not specific to Bolivia, and collaboration between private pharmacies and public services appears possible in that respect. However, collaboration with pharmacies does not seem an efficient way to increase the number of patients screened for TB, and to shorten delays to TB diagnosis and treatment.