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Zooprophylaxis or zoopotentiation: the outcome of introducing animals on vector transmission is highly dependent on the mosquito mortality while searching.

Saul A - Malar. J. (2003)

Bottom Line: Changing the accessibility of the humans had a much greater effect.Estimates of searching-associated vector mortality are essential before the effects of changing animal husbandry practices can be predicted.With realistic values of searching-associated vector mortality rates, zooprophylaxis may be ineffective.

View Article: PubMed Central - HTML - PubMed

Affiliation: Malaria Vaccine Development Unit, NIAID, NIH, Rockville, MD 20852, USA. ASaul@niaid.nih.gov

ABSTRACT

Background: Zooprophylaxis, the diversion of disease carrying insects from humans to animals, may reduce transmission of diseases such as malaria. However, as the number of animals increases, improved availability of blood meals may increase mosquito survival, thereby countering the impact of diverting feeds.

Methods: Computer simulation was used to examine the effects of animals on the transmission of human diseases by mosquitoes. Three scenarios were modelled: (1) endemic transmission, where the animals cannot be infected, eg. malaria; (2) epidemic transmission, where the animals cannot be infected but humans remain susceptible, e.g. malaria; (3) epidemic disease, where both humans and animals can be infected, but develop sterile immunity, eg. Japanese encephalitis B. For each, the passive impact of animals as well as the use of animals as bait to attract mosquitoes to insecticide was examined. The computer programmes are available from the author. A teaching model accompanies this article.

Results: For endemic and epidemic malaria with significant searching-associated vector mortality, changing animal numbers and accessibility had little impact. Changing the accessibility of the humans had a much greater effect. For diseases with an animal amplification cycle, the most critical factor was the proximity of the animals to the mosquito breeding sites.

Conclusion: Estimates of searching-associated vector mortality are essential before the effects of changing animal husbandry practices can be predicted. With realistic values of searching-associated vector mortality rates, zooprophylaxis may be ineffective. However, use of animals as bait to attract mosquitoes to insecticide is predicted to be a promising strategy.

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Simulation of a malaria epidemic: effect of altering the number of animals. The parameters used are shown in Table 3. Black line: animal to human ratio of 1:8; red line: animal to human ratio of 1:4; green line: animal to human ratio of 1:2 (or an equivalent of 12.5, 25 and 50 animals respectively, for a village of 100 people).
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Figure 3: Simulation of a malaria epidemic: effect of altering the number of animals. The parameters used are shown in Table 3. Black line: animal to human ratio of 1:8; red line: animal to human ratio of 1:4; green line: animal to human ratio of 1:2 (or an equivalent of 12.5, 25 and 50 animals respectively, for a village of 100 people).

Mentions: The effect of animals on the rate at which an epidemic of malaria would spread was modelled using the starting parameters listed in Table 3, assuming the outbreak was initiated by the introduction of an infected human. In these simulations, for each value of Ms, P0 and the number of new adults mosquitoes emerging each day was adjusted, so that the overall number of mosquitoes feeding each day and their probability of surviving a feeding cycle, Pf, was the same for each simulation for an animal to human ratio of 1:4. For each value of Ms, the model was run three times with ratios animals to humans of 1:2, 1:4 and 1:8. In this simulation, it is assumed that people becoming infected with malaria but remain untreated for the duration of the outbreak. Therefore, this represents a 'worst case scenario'. In this model, changing the accessibility of animals (i.e. modelled by changing the attraction rate constant, Aa) has the same impact as changing the number of animals (Ya). For example, keeping the animal to human ratio at 1:4 and changing the attraction rate constant (Aa = 0.002 h-1, 0.004 h-1, 0.008 h-1) produces the same output as Aa = 0.004 h-1 and changing the animal to human ratio from 1:8 to 1:4 and to 1:2. Although both cases were modelled, only the effect of a 4 fold range of animal numbers is shown. At these animal to human ratios, the corresponding values for the human blood index were 0.33, 0.5 and 0.67 respectively. The effect of changing numbers of animals on the rate at which a malaria epidemic spread was markedly dependent on the searching-related vector mortality rate (Fig. 3). At low or zero values of Ms, increasing the number of animals decreased the rate at which the epidemic spread and vice versa. However, as larger values of Ms were modelled, the effect of changing animal numbers on the rate at which the epidemic spread was markedly dampened. At the highest value of Ms modelled, 0.08 h-1, increasing the number of animals slightly increased the predicted rate at which the epidemic spread.


Zooprophylaxis or zoopotentiation: the outcome of introducing animals on vector transmission is highly dependent on the mosquito mortality while searching.

Saul A - Malar. J. (2003)

Simulation of a malaria epidemic: effect of altering the number of animals. The parameters used are shown in Table 3. Black line: animal to human ratio of 1:8; red line: animal to human ratio of 1:4; green line: animal to human ratio of 1:2 (or an equivalent of 12.5, 25 and 50 animals respectively, for a village of 100 people).
© Copyright Policy
Related In: Results  -  Collection

Show All Figures
getmorefigures.php?uid=PMC222927&req=5

Figure 3: Simulation of a malaria epidemic: effect of altering the number of animals. The parameters used are shown in Table 3. Black line: animal to human ratio of 1:8; red line: animal to human ratio of 1:4; green line: animal to human ratio of 1:2 (or an equivalent of 12.5, 25 and 50 animals respectively, for a village of 100 people).
Mentions: The effect of animals on the rate at which an epidemic of malaria would spread was modelled using the starting parameters listed in Table 3, assuming the outbreak was initiated by the introduction of an infected human. In these simulations, for each value of Ms, P0 and the number of new adults mosquitoes emerging each day was adjusted, so that the overall number of mosquitoes feeding each day and their probability of surviving a feeding cycle, Pf, was the same for each simulation for an animal to human ratio of 1:4. For each value of Ms, the model was run three times with ratios animals to humans of 1:2, 1:4 and 1:8. In this simulation, it is assumed that people becoming infected with malaria but remain untreated for the duration of the outbreak. Therefore, this represents a 'worst case scenario'. In this model, changing the accessibility of animals (i.e. modelled by changing the attraction rate constant, Aa) has the same impact as changing the number of animals (Ya). For example, keeping the animal to human ratio at 1:4 and changing the attraction rate constant (Aa = 0.002 h-1, 0.004 h-1, 0.008 h-1) produces the same output as Aa = 0.004 h-1 and changing the animal to human ratio from 1:8 to 1:4 and to 1:2. Although both cases were modelled, only the effect of a 4 fold range of animal numbers is shown. At these animal to human ratios, the corresponding values for the human blood index were 0.33, 0.5 and 0.67 respectively. The effect of changing numbers of animals on the rate at which a malaria epidemic spread was markedly dependent on the searching-related vector mortality rate (Fig. 3). At low or zero values of Ms, increasing the number of animals decreased the rate at which the epidemic spread and vice versa. However, as larger values of Ms were modelled, the effect of changing animal numbers on the rate at which the epidemic spread was markedly dampened. At the highest value of Ms modelled, 0.08 h-1, increasing the number of animals slightly increased the predicted rate at which the epidemic spread.

Bottom Line: Changing the accessibility of the humans had a much greater effect.Estimates of searching-associated vector mortality are essential before the effects of changing animal husbandry practices can be predicted.With realistic values of searching-associated vector mortality rates, zooprophylaxis may be ineffective.

View Article: PubMed Central - HTML - PubMed

Affiliation: Malaria Vaccine Development Unit, NIAID, NIH, Rockville, MD 20852, USA. ASaul@niaid.nih.gov

ABSTRACT

Background: Zooprophylaxis, the diversion of disease carrying insects from humans to animals, may reduce transmission of diseases such as malaria. However, as the number of animals increases, improved availability of blood meals may increase mosquito survival, thereby countering the impact of diverting feeds.

Methods: Computer simulation was used to examine the effects of animals on the transmission of human diseases by mosquitoes. Three scenarios were modelled: (1) endemic transmission, where the animals cannot be infected, eg. malaria; (2) epidemic transmission, where the animals cannot be infected but humans remain susceptible, e.g. malaria; (3) epidemic disease, where both humans and animals can be infected, but develop sterile immunity, eg. Japanese encephalitis B. For each, the passive impact of animals as well as the use of animals as bait to attract mosquitoes to insecticide was examined. The computer programmes are available from the author. A teaching model accompanies this article.

Results: For endemic and epidemic malaria with significant searching-associated vector mortality, changing animal numbers and accessibility had little impact. Changing the accessibility of the humans had a much greater effect. For diseases with an animal amplification cycle, the most critical factor was the proximity of the animals to the mosquito breeding sites.

Conclusion: Estimates of searching-associated vector mortality are essential before the effects of changing animal husbandry practices can be predicted. With realistic values of searching-associated vector mortality rates, zooprophylaxis may be ineffective. However, use of animals as bait to attract mosquitoes to insecticide is predicted to be a promising strategy.

Show MeSH
Related in: MedlinePlus