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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 endemic malaria. The effect of altering numbers of animals on the human inoculation rate, the sporozoite rate, the vectorial capacity, and the number of mosquitoes ovipositing per day. Parameters used are shown in Table 2. Black line: Ms = 0 h-1; red line: Ms = 0.02 h-1; green line: Ms = 0.04 h-1; blue line: Ms = 0.08 h-1.
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Figure 1: Simulation of endemic malaria. The effect of altering numbers of animals on the human inoculation rate, the sporozoite rate, the vectorial capacity, and the number of mosquitoes ovipositing per day. Parameters used are shown in Table 2. Black line: Ms = 0 h-1; red line: Ms = 0.02 h-1; green line: Ms = 0.04 h-1; blue line: Ms = 0.08 h-1.

Mentions: For all three cases, the effect of search-related vector mortality rate become noticeable when Ms is greater than about 10% of the overall attraction rate and is illustrated in Fig. 1 for a constant rate at which mosquitoes find humans, but varying AaYa. The starting combination of parameters (Table 2) for this set of simulations gives an overall attraction rate of 0.2 h-1 and an average time to find a blood meal of ~3.5 h. A vector mortality rate of 0.02 h-1 (i.e. 2% of the mosquitoes die per hour while searching) causes a marked flattening of the relationship between added animals, human inoculation rate, vectorial capacity and particularly, in the sporozoite rate. At higher vector mortality rates, eg. Ms 0.08 h-1 or 40% of the attraction rate, there is a reversal of the expected relationship between added animals and human inoculation rates. In this model, the number of mosquitoes emerging as adults is assumed to be fixed by the availability of larval habitat. However, where there is a significant Ms, the total number of mosquitoes feeding will change as the number or attraction rate constant of the animal changes, resulting in more mosquitoes surviving to feed again. These changes will partially compensate for the diversion of mosquitoes from people to animals. For example, if Ms = 0 h-1, then for the conditions modelled, changing the number of animals from zero to 100 decreases the bites per person per 24 h from 20 to 4, or a 5-fold change. However, for Ms = 0.08 h-1, the bites per person per 24 h change from 11.7 to 7 or only a 1.67-fold change, for zero and 100 animals, respectively.


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 endemic malaria. The effect of altering numbers of animals on the human inoculation rate, the sporozoite rate, the vectorial capacity, and the number of mosquitoes ovipositing per day. Parameters used are shown in Table 2. Black line: Ms = 0 h-1; red line: Ms = 0.02 h-1; green line: Ms = 0.04 h-1; blue line: Ms = 0.08 h-1.
© Copyright Policy
Related In: Results  -  Collection

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

Figure 1: Simulation of endemic malaria. The effect of altering numbers of animals on the human inoculation rate, the sporozoite rate, the vectorial capacity, and the number of mosquitoes ovipositing per day. Parameters used are shown in Table 2. Black line: Ms = 0 h-1; red line: Ms = 0.02 h-1; green line: Ms = 0.04 h-1; blue line: Ms = 0.08 h-1.
Mentions: For all three cases, the effect of search-related vector mortality rate become noticeable when Ms is greater than about 10% of the overall attraction rate and is illustrated in Fig. 1 for a constant rate at which mosquitoes find humans, but varying AaYa. The starting combination of parameters (Table 2) for this set of simulations gives an overall attraction rate of 0.2 h-1 and an average time to find a blood meal of ~3.5 h. A vector mortality rate of 0.02 h-1 (i.e. 2% of the mosquitoes die per hour while searching) causes a marked flattening of the relationship between added animals, human inoculation rate, vectorial capacity and particularly, in the sporozoite rate. At higher vector mortality rates, eg. Ms 0.08 h-1 or 40% of the attraction rate, there is a reversal of the expected relationship between added animals and human inoculation rates. In this model, the number of mosquitoes emerging as adults is assumed to be fixed by the availability of larval habitat. However, where there is a significant Ms, the total number of mosquitoes feeding will change as the number or attraction rate constant of the animal changes, resulting in more mosquitoes surviving to feed again. These changes will partially compensate for the diversion of mosquitoes from people to animals. For example, if Ms = 0 h-1, then for the conditions modelled, changing the number of animals from zero to 100 decreases the bites per person per 24 h from 20 to 4, or a 5-fold change. However, for Ms = 0.08 h-1, the bites per person per 24 h change from 11.7 to 7 or only a 1.67-fold change, for zero and 100 animals, respectively.

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