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Biomedical subjects

M I Meltzer

Publications and source records attributed to M I Meltzer.

At least 19 recordsLinked to original sources
PubMed2001

Introduction to health economics for physicians.

Since the 1960s, expenditure on health care in developed countries has risen faster than the general rate of inflation, thus making economic assessment of interventions an integral part of decision making in health services. This paper is the first in a series whose goal is to provide some basic principles of health economics that will allow practising physicians to understand better the economic relations between their practice of medicine, the health-care sector, and the national economy. Some of the most important principles described in this paper include opportunity costs, identifying the appropriate perspective, correctly categorising costs, and discounting costs and non-monetary benefits (eg, lives saved) over time. Economic analyses of medical interventions must also take into consideration the difference between efficacy and effectiveness. Efficacy is the maximum possible benefit, often achieved with carefully controlled trials, and effectiveness is the actual decrease in disease achieved when the intervention is applied over a large, non-homogeneous population. This introduction ends with three methods of assessing the costs and benefits of an intervention-namely, cost-benefit, cost-effectiveness, and cost-utility analyses.

Canada↗
PubMed2001

The economics of vaccinating restaurant workers against hepatitis A.

The economics of vaccinating restaurant workers against hepatitis A were studied using Monte Carlo simulation models, one with a restaurant-owner perspective, and one with a societal perspective. The restaurant model allowed for a different size, number of employees and employee turnover rate. Benefits were the avoidance of loss of business (including the possibility of bankruptcy) after publicity linking the restaurant to an outbreak associated with a case of hepatitis A in a food handler. Additional benefits in the societal model included reductions in costs of food handler-associated cases of hepatitis A. The outcome used was Net Present Value (NPV), allowing comparison between models. Regardless of the cost of vaccination ($50-140/employee), for a restauranteur to ensure that all employees were vaccinated at all times substantial costs were involved (i.e. negative NPV). Even a 75% probability of bankruptcy still resulted in negative NPVs at the 95th percentiles. For society, vaccination was only cost-saving (i.e. positive NPV) if done only during epidemics and if it cost < $20/employee. Vaccinating restaurant employees is unlikely to be economical from either the restaurant owner or the societal perspective, even during hepatitis A epidemics.

Costs and Cost Analysis↗
PubMed2000

Effectiveness and cost-benefit of influenza vaccination of healthy working adults: A randomized controlled trial.

CONTEXT: Although the cost-effectiveness and cost-benefit of influenza vaccination are well established for persons aged 65 years or older, the benefits for healthy adults younger than 65 years are less clear. OBJECTIVE: To evaluate the effectiveness and cost-benefit of influenza vaccine in preventing influenza-like illness (ILI) and reducing societal costs of ILI among healthy working adults. DESIGN: Double-blind, randomized, placebo-controlled trial conducted during 2 influenza seasons. SETTING AND PARTICIPANTS: Healthy adults aged 18 to 64 years and employed full-time by a US manufacturing company (for 1997-1998 season, n = 1184; for 1998-1999 season, n = 1191). INTERVENTIONS: For each season, participants were randomly assigned to receive either trivalent inactivated influenza vaccine (n = 595 in 1997-1998 and n = 587 in 1998-1999) or sterile saline injection (placebo; n = 589 in 1997-1998 and n = 604 in 1998-1999). Participants in 1997-1998 were rerandomized if they participated in 1998-1999. MAIN OUTCOME MEASURES: Influenza-like illnesses and associated physician visits and work absenteeism reported in biweekly questionnaires by all participants, and serologically confirmed influenza illness among 23% of participants in each year (n = 275 in 1997-1998; n = 278 in 1998-1999); societal cost of ILI per vaccinated vs unvaccinated person. RESULTS: For 1997-1998 and 1998-1999, respectively, 95% (1130/1184) and 99% (1178/1191) of participants had complete follow-up, and 23% in each year had serologic testing. In 1997-1998, when the vaccine virus differed from the predominant circulating viruses, vaccine efficacy against serologically confirmed influenza illness was 50% (P =.33). In this season, vaccination did not reduce ILI, physician visits, or lost workdays; the net societal cost was $65.59 per person compared with no vaccination. In 1998-1999, the vaccine and predominant circulating viruses were well matched. Vaccine efficacy was 86% (P =.001), and vaccination reduced ILI, physician visits, and lost workdays by 34%, 42%, and 32%, respectively. However, vaccination resulted in a net societal cost of $11.17 per person compared with no vaccination. CONCLUSION: Influenza vaccination of healthy working adults younger than 65 years can reduce the rates of ILI, lost workdays, and physician visits during years when the vaccine and circulating viruses are similar, but vaccination may not provide overall economic benefits in most years. JAMA. 2000;284:1655-1663.

Absenteeism↗
PubMed2000

Economic evaluation of use of diphtheria, tetanus, and acellular pertussis vaccine or diphtheria, tetanus, and whole-cell pertussis vaccine in the United States, 1997.

OBJECTIVE: To compare the economic costs and benefits associated with using either diphtheria and tetanus toxoids and acellular pertussis vaccine (DTaP) or diphtheria and tetanus toxoids and whole-cell pertussis vaccine (DTwP) in the United States in 1997. DESIGN: Standard cost-benefit analysis, from both the societal and health care system perspectives, was performed for each combination vaccine as well as for the pertussis components singly. SETTING: A simulated cohort of 4.1 million children from birth to age 15 years. MAIN OUTCOME MEASURES: Net costs (savings) and benefit-cost ratios (BCRs) RESULTS: Without a vaccination program, diphtheria, tetanus, and pertussis disease caused more than 3 million cases and more than 28,000 deaths, at a cost of $23.6 billion. From the societal perspective, net savings because of the use of DTaP and DTwP were $22.510 million and $22.623 million, respectively. The net savings from the acellular pertussis component and the whole-cell pertussis component only were $4.362 million and $4.474 million, respectively. Benefit-cost ratios for DTaP from a societal and health care system perspective were 27:1 and 9:1, respectively. Sensitivity analyses of key variables did not result in appreciable changes in results. CONCLUSIONS: Compared with no program, vaccination with DTaP or DTwP resulted in substantial savings, regardless of the perspective taken and for all sensitivity analyses conducted. Compared with DTwP, use of DTaP generated a small cost increase that might be offset by the value of other factors, such as increased confidence in pertussis vaccination resulting from reduced adverse events. Arch Pediatr Adolesc Med. 2000;154:797-803

Adolescent↗
PubMed1999

The effect of subclinical experimental Cowdria ruminantium infection in ewes on the growth and milk consumption of pre-weaning lambs.

An alternative control option for heartwater (Cowdria ruminantium infection) is the establishment and maintenance of endemic stability which would lessen the existing dependence on acaricides. In an endemically stable state, animals become infected by vaccination or natural challenge at an early age, following which the immunity so created is boosted by continuing tick challenge. In this study, growth rates, health and hematological parameters were monitored at regular intervals for lambs born to two matched groups of ewes until weaning at 4 mo of age. One group of ewes was infected multiple times with Cowdria ruminantium; the other group remained uninfected. The overall mean leucocyte count of lambs born to infected ewes was significantly lower than that of lambs born to uninfected ewes (P=0.04). However, there were few other significant differences in the other hematological data between the two groups. The mean birth weight of single lambs born to uninfected ewes (4.6 kg) was significantly higher than the mean birth weight of single lambs born to infected ewes (4.4 kg) (P=0.02). Trends in milk consumption and growth rates were similar for the two groups, with few significant differences detected. Likewise, there were no significant differences in the incidences of health problems or pre-weaning mortalities between the two groups of lambs. The results of this study indicate that there is no detectable effect on productivity of pre-weaning lambs when their dams are carriers of C. ruminantium--a situation likely to occur in an endemically stable state. Hence, maintenance of endemic stability would be a suitable control option for heartwater.

Animals↗
PubMed1999

The effect of subclinical experimental Cowdria ruminantium infection on the health and reproductive performance of breeding ewes.

This study documented the effect on the productivity of Dorper-Merino cross ewes when they became infected with Cowdria ruminantium as would occur in an endemically stable state. A flock of 152 breeding ewes was randomly divided into two matched groups. One group was infected on multiple occasions with C. ruminantium; the other group remained uninfected. The ewes were bred and monitored for one breeding/lambing/weaning cycle. Hematological, reproductive, and health parameters were recorded at regular intervals. Statistically significant differences in hematological values between the two groups were infrequent and transient. No significant differences in weights, mortality rates or reproductive parameters were detected between the two groups. Subclinical C. ruminantium infection did not negatively affect health and reproductive performance of breeding ewes; endemic stability would be a suitable alternative method of controlling heartwater.

Animals↗
PubMed1998

Glaucoma's impact on quality of life and its relation to clinical indicators. A pilot study.

OBJECTIVE: This study aimed to compare the quality of life (Q of L) of patients with glaucoma and control subjects and to determine the relationships between Q of L and demographic and clinical variables in patients with glaucoma. DESIGN: The study design was a stratified cross-sectional study. PARTICIPANTS: A gender-, race-, and age-stratified cross-sectional sample of patients with glaucoma (n = 56) and control subjects (n = 54) was obtained. Additional patients (n = 12) were included to examine the relationships between glaucoma, its therapy, and Q of L. INTERVENTION: The Medical Outcomes Study short form (MOS-20), the Activities of Daily Vision Scale (ADVS), and questions related to glaucoma and side effects of treatment were administered. Descriptive statistics characterized demographic variables and MOS and ADVS scales. Group differences were evaluated using chi-square, Fisher's and Ordinal Exact, Wilcoxon rank-sum, and two-sample t tests. Spearman rank correlations were obtained between MOS-ADVS scores and clinical and demographic variables. Regression was used for multivariate analysis. MAIN OUTCOME MEASURES: The MOS scores, ADVS scores, visual acuity, visual fields, and demographic variables were measured. RESULTS: Patients scored significantly lower than did the control subjects in all MOS-20 categories except pain. Differences were physical (-20%), role (-43%), mental health (-10%), general health (-22%), and social (-9%). The only category that was not statistically significant was that of pain (P = 0.075). In the glaucoma subgroup, there were differences between whites and nonwhites in MOS subscales physical, role, social, pain, and health, and ADVS near vision. In patients, current medications and previous surgeries correlated with ADVS subscales night vision, near vision, and glare; visual acuity and fields correlated with MOS subscales physical, role and health, and all ADVS subscales. A multiple regression model including visual acuity and fields, urban residence, and female gender explained 61% of the variability in ADVS overall score. CONCLUSIONS: The Q-of-L perception differed between patients with glaucoma and control subjects. Increasing field loss, decreased visual acuity, and complexity of therapy correlated with patients' reduction in activities of daily vision.

Activities of Daily Living↗
PubMed1998

A review of the economics of the prevention and control of rabies. Part 1: Global impact and rabies in humans.

The existing literature on the economics of rabies and its control can be characterised as a poorly documented set of cost estimates with insufficient information to allow replication of the analyses. Most articles have numerous 'violations' of the standard recommended procedures for assessing the burden of disease and the cost and benefits of interventions. Per capita costs are often crudely extrapolated from small to large populations without allowing for geographic differences in incidence. Furthermore, most studies do not distinguish between financial charges and true economic costs, and only a few articles contain a multiyear framework, complete with discounting of future costs and benefits. With the exception of the increase in average incidence of postexposure prophylaxes (PEPs) in Asia, the average incidences of both human-rabies cases and PEPs in Africa, the Americas and Europe have not changed significantly over time. There are, however, large differences between countries within a region and regional averages can conceal notable changes in incidences over time for a given country. The largest number of human-rabies cases occur in developing countries due to the low levels of vaccination among dogs, the high cost of biologicals for PEP and problems of availability. The costs (1995 values) of PEP range from $US1707 per person in Massachusetts, US, to $US2.50 for a complete series of vaccinations (without immunoglobulin) using sheep-derived vaccines in Karachi, Pakistan. Most studies which reported the cost of PEP, however, provided only direct medical costs and did not consider indirect costs such as lost productivity due to death, permanent disability or time spent while receiving medical care. Given the expense of controlling rabies in dogs and wildlife, there is an urgent need to develop a cheaper human-rabies vaccine or further refine the 'low-dose' PEP regimes. PEP is often given unnecessarily, and experience with expert consultations systems and algorithms has shown that the rate, and therefore total cost, of PEP can be significantly reduced. However, because it may be difficult to identify lesions from a bite by a bat, algorithms may be of less value when dealing with possible exposure to bat rabies. Using US prices and values, only 2 individuals per 1000 possible contacts have to be at risk from bat rabies in order for it to be economically justifiable to give PEP to all those potentially exposed to bat rabies. With regard to pre-exposure vaccination, routine use of pre-exposure has generally not been shown to be cost effective.

Animals↗
PubMed1998

A review of the economics of the prevention and control of rabies. Part 2: Rabies in dogs, livestock and wildlife.

Although rabies in domestic and wild animals represents a significant threat to public health and can cause economic losses among livestock, there are very few studies that examine the economics of rabies in animals. The literature that does exist can be characterised as poorly documented estimates of costs, with insufficient information to allow replication of the analyses. Most papers have numerous 'violations' of the standard recommended procedures for assessing burden of disease and the cost and benefits of interventions. For example, most studies do not distinguish between financial charges and true economic costs. Further, despite the fact that controlling rabies in animal populations is often a multi-year task, only a few papers contain a multi-year framework, complete with discounting of future costs and benefits. Globally, dog-transmitted rabies represents the largest threat to human health. In order to prevent the transmission of rabies in a dog population, it is theoretically necessary to vaccinate a minimum of 60 to 70% of the dogs. Even countries with potentially sufficient resources, however, do not often meet and sustain these rates. One reason for such failure might be that individual dog owners might feel that it is too expensive to vaccinate their pets. Recent estimates in the US of the cost of vaccinating dogs range from $US16 to $US24 per dog. In developing countries, estimates range from $US0.52 in Thailand, to $US1.19 in the Philippines, to $US2.70 in Malawi. None of these estimates include indirect costs accured by the pet owners. Lethal methods of dog population control are even more expensive, and attempting to control rabies by reducing dog populations has not worked for any extended period. Rabies in livestock is often reported, but the impact in the US and most developed countries appears relatively small. Vampire bat-transmitted rabies in Latin America appears to be the most serious rabies problem in livestock. The largest cost due to wildlife rabies is the cost of vaccinating domestic animals, both large and small. In the US, domestic animals face multiple sources of wildlife rabies. Attributing the entire cost of vaccinating domestic animals to 1 species can result in the over estimation of the benefits of immunising a given wildlife population via vaccine-laden baits. For example, despite a definite decline in the number of rabid foxes, it has been difficult to obtain the promised benefits of using oral vaccines in Europe to control fox rabies. Other authors maintain that the use of oral vaccines to control fox rabies is cost beneficial, but there are no convincing data supporting that claim. Additionally, vaccinating raccoons with an oral vaccine requires approximately 4 times more vaccine-laden baits vaccinating foxes, which makes it highly questionable if it would be cost beneficial to use oral vaccine to attempt raccoon rabies elimination in areas where it is already enzootic. The economics of using oral vaccines to prevent raccoon rabies invading uninfected areas has yet to be examined.

Animal Diseases↗
PubMed1998

Using disability-adjusted life years to assess the economic impact of dengue in Puerto Rico: 1984-1994.

This study presents the disability-adjusted life years (DALYs), a non-monetary economic measure of impact, lost to dengue in Puerto Rico for the period 1984-1994. Data on the number of reported cases, cases with hemorrhagic manifestations, hospitalizations, and deaths were obtained from a surveillance system maintained at the Dengue Branch, Division of Vector-Borne Infectious Diseases, Centers for Disease Control and Prevention (San Juan, PR). The reported cases were divided into two age groups (0-15 years old and >15 years old), and then multiplied by predetermined factors (10 for 0-15 years; 27 for >15 years) to allow for age-related under-reporting of cases. Severity of dengue was modeled by classifying cases into three groups: dengue fever, dengue with severe manifestations, and hospitalized cases. Each group was assigned a different number of days lost because of dengue-related disability. Dengue caused an average of 658 DALYs per year per million population (SE = 114, range = 145-1,519). A multivariate sensitivity analysis, which simultaneously altered the values of six input variables, produced a mean of 580 DALYs/year/million population, with a maximum average of 1,021 DALYs/year/million population, and a maximum, single-year estimate for 1994 of 2,153 DALYs/million population. The most important input was the number of days lost to classic dengue. The DALYs/year/million population lost to dengue in Puerto Rico are much greater than previous estimates concerning the impact of dengue hemorrhagic fever alone. The loss to dengue is similar to the losses per million population in the Latin American and Caribbean region attributed to any of the following diseases or disease clusters; the childhood cluster (polio, measles, pertussis, diphtheria, tetanus), meningitis, hepatitis, or malaria. The loss is also of the same order of magnitude as any one of the following: tuberculosis, sexually transmitted diseases (excluding human immunodeficiency virus), tropical cluster (e.g., Chagas' disease, leishmaniasis), or intestinal helminths. The results objectively suggest that when governments and international funding agencies allocate resources for research and control, dengue should be given a priority equal to many other infectious diseases that are generally considered more important.

Adolescent↗
PubMed1998

Models for heartwater epidemiology: practical implications and suggestions for future research.

We present a simple model of the dynamics of heartwater that we use to explore and better understand various aspects of this disease. We adapted the Ross-Macdonald model for malaria epidemiology so that we could consider both host and vector populations, and evaluate the interactions between the two. We then use two more biologically detailed models to examine heartwater epidemiology. The first includes a carrier state and host mortality, and the second includes density dependence. The results from all three models indicate that a stable equilibrium with high disease levels is probably the standard situation for heartwater (R0 between 5.7 and 22.4). More than 80% of cattle become infected with heartwater if only 12% of infected tick bites produce an infection in cattle, if tick burdens are as low as only five ticks per host per day, or if tick lifespans are as short as 7 d. A host recovery rate of 30 d results in over 50% of the cattle becoming infected with heartwater. Our analyses indicate that it is quite difficult to prevent the establishment and maintenance of high levels of heartwater in a herd, thereby supporting previous suggestions that any attempts at controlling this disease through stringent tick control regimens are not warranted.

Animals↗
PubMed1996

A possible explanation of the apparent breed-related resistance in cattle to bont tick (Amblyomma hebraeum) infestations.

Adult male Amblyomma hebraeum tick infestations and the weights of 20 Brahman steers and 38 Mashona heifers were measured at different periods at the Veterinary Quarantine Area at Mbizi, Zimbabwe. The experiment for the Brahmans lasted 108 weeks and that for the Mashona for 113 weeks. The Brahman steers weighed a maximum average of 478.4 kg (SE 7.9 kg), which was significantly different to the Mashona heifers maximum average of 391.4 kg (SE 5.6 kg) (P < 0.001). The Brahmans had a maximum average of 112.1 (SE 18.5) adult males, while the Mashona heifers had a maximum average of 59.8 (SE 4.3). The difference was statistically significant (P < 0.05). There was no statistical difference between the two maximum average ticks per kilogram liveweight (P > 0.05). When differences in size are corrected for, then breed-related differences disappear. It is emphasized that the influence of confounding factors, especially time, cannot be corrected for in a satisfactory manner. Therefore, these statistical results should be regarded as illustrative rather than proof. To confirm these results, it is suggested that the authors of earlier studies should reanalyze their databases in a similar manner. It is important that such analyses be conducted, or new experiments carried out. Erroneous conclusions regarding the reason for different tick numbers between the breeds could result in farmers being incorrectly encouraged to utilize smaller breeds to obtain 'built-in' resistance to A. hebraeum ticks. One logical explanation for the size-related effect is that the males typically attach themselves around the belly and groin areas. Larger breeds of cattle, such as the Brahman, will naturally have larger surface areas of skin in the belly and groin regions than smaller breeds. Thus, it is suggested that there may be a simple physical explanation for the difference between breeds in the numbers of attached adult A. hebraeum males.

Animals↗
PubMed1996

Will modern caries management reduce restorations in dental practice?

Dentists have difficulty diagnosing caries presence and activity. Modern caries management suggests that lesions in low risk patients should not be restored until the radiolucency extends into the outer third of interproximal dentin. Even stained fissures need not be restored in the absence of occlusal dentin radiolucency. If this standard were adopted, what affect would it have on the amount of operative treatment delivered? This report created a model to forecast the change in work volume caused by new caries management strategies. The model considered two identical groups of 1,000 adult teeth with a normal distribution of 1,000 radiographic lesions in each group. Group I would have all lesions restored at baseline, but Group II would have only lesions in the dentin restored. Each year for ten years the model forecasts the number of replacement restorations for both groups, plus initial restorations for Group II. A sensitivity analysis was conducted using slow progression rates (Group IIa) and fast progression rates (Group IIb). After ten years, Group IIa has 49% and Group IIb had 32% fewer restorations than Group I. It is concluded that the model is robust and modern caries management may significantly reduce operative workload.

Adult↗
PubMed1995

Effects of tick infestation and tick-borne disease infections (heartwater, anaplasmosis and babesiosis) on the lactation and weight gain of Mashona cattle in south-eastern Zimbabwe.

The effects of ticks and tick-borne disease infections on the lactation and weight gain of Mashona cattle were studied at Mbizi Quarantine Station in the south-eastern lowveld of Zimbabwe. Twenty-nine Mashona cows were allocated to 2 balanced groups and kept in separate paddocks at a stocking rate of one animal per 8 ha. One group received regular acaricide treatment to control bont (Amblyomma hebraeum) and other ticks. The other group was left untreated. The cows were artificially inseminated. The acaricide-treated cows and calves were essentially tick free throughout the experiment, while the untreated cows and calves were continuously tick infested. There was a drought-related decline in tick infestations in the second year of the experiment. Antibodies to Cowdria ruminantium, Babesia bigemina and Anaplasma marginale were detected in cows and calves from both groups, though the untreated group had significantly higher titres to C. ruminantium (P < 0.001). The total, measured amount of milk suckled by untreated calves was significantly more than treated calves (273 kg vs. 241 kg, P < or = 0.05). By interpolating between the twice weekly measurements, it was calculated that over the entire lactation untreated calves suckled an average of 935 kg/hd vs. 837 kg/hd for the treated group. There were no statistical differences in the weights of the 2 groups of calves at birth, weaning, 180 and 210 days post partum (P < 0.05). For cows, there were no statistically significant differences in gestation periods (288 vs. 279 days), reconception rates or weight patterns over time (P < 0.05). The results show that intensive acaricide treatment in areas of Zimbabwe where heartwater is enzootically stable is uneconomical. The maintenance of enzootic stability for tick-borne diseases through minimal tick control is clearly a more economic and practical control option.

Anaplasmosis↗
PubMed1995

The relationship between tick (Amblyomma hebraeum) infestation and immunity to heartwater (Cowdria ruminantium infection) in calves in Zimbabwe.

The occurrence of endemic stability for heartwater (Cowdria ruminantium infection) is a controversial issue, because the means by which young cattle and other neonatal ruminants become infected and acquire immunity to the disease have never been adequately explained. We conducted a study in a heartwater-endemic area in southeastern Zimbabwe to investigate the relationships between calf immunity to C. ruminatium, infestations of the tick vector Amblyomma hebraeum and dam, colostral and calf antibody titres to C. ruminantium. Two groups of cows (tick-infested and acaricide-treated) were artificially inseminated and then monitored through pregnancy and lactation by means of tick counts and serum antibody titres. The calves born to the cows in each treatment group were similarly monitored from birth until after weaning, when they were challenged with a heartwater stabilate (Ball-3 vaccine). Colostrum was collected from the cows on the days that the calves were born. Serum and colostrum samples were screened for Cowdria-specific antibodies using an indirect fluorescent antibody test. The cows and calves in the acaricide-treated group remained essentially tick-free for the duration of the study. The cows in the tick-infested group were continuously infested with A. hebraeum, but their calves only became infested between 12 and 31 weeks after birth. Cowdria-specific antibodies were detected in the sera of the cows and calves and in the colostrum of both treatment groups, but the titres were consistently higher in the tick-infested group than in the acaricide-treated group. Antibodies, probably of maternal origin since the precolostral serum titres were negative, were present in the calves of both groups for at least 8 weeks after birth. Between Weeks 24 and 52 postpartum, the antibody titres of the tick-infested group of calves were significantly correlated with the numbers of A. hebraeum present on the animals. The antibody titres of the calves in the acaricide-treated group increased considerably following challenge with heartwater stabilate, 60 weeks after birth. No clinical heartwater was detected in either group of calves following inoculation of stabilate, and it was concluded that the calves in both groups were immune to the disease. In the acaricide-treated group, immunity did not correlate with exposure to tick-transmitted infections. Hence, we concluded that the calves in both groups had probably been infected by vertical transmission around the time of birth. Such early infections are likely to have been controlled by maternal antibodies or by an unidentified immune mechanism.(ABSTRACT TRUNCATED AT 400 WORDS)

Animals↗
PubMed1994

Factors affecting the distributions of the ticks Amblyomma hebraeum and A. variegatum in Zimbabwe: implications of reduced acaricide usage.

The ticks Amblyomma hebraeum and A. variegatum are the main vectors of heartwater, a disease of ruminants caused by Cowdria ruminantium, in the agricultural areas of Zimbabwe. At present, A. hebraeum is widely distributed in the dry southern lowveld, and occurs in at least seven foci in the higher rainfall highveld. Amblyomma variegatum occurs in the Zambezi valley and surrounding dry lowveld areas in the northwest. The distribution of A. hebraeum has changed considerably over the past 70 years, while that of A. variegatum appears to have remained fairly static. The distribution patterns of both species in Zimbabwe display anomalous features; the ticks occur in areas of lowest predicted climatic suitability for survival and development and in areas where the densities of cattle, the most important domestic host, are lowest. The only factor favouring the survival of the species in the lowveld habitats in which they occur is the presence of alternative wildlife hosts for the adult stage. Their absence from more climatically favourable highveld habitats appears to have been the result of intensive acaricide treatment of cattle over a long period and a historic absence of significant numbers of wildlife hosts. Eradication of A. hebraeum and A. variegatum by intensive acaricide treatment of cattle can be achieved in the absence of significant numbers of alternative hosts, because of the long attachment and feeding periods of the adults of these tick species. However, eradication becomes impossible when alternative hosts for the adult stage are present, because a pheromone emitted by attached males attracts the unfed nymphal and adult stages to infested hosts. The unfed ticks are not attracted to uninfested hosts, such as acaricide-treated cattle. Regular acaricide treatment of cattle is expensive and so, for economic reasons, the Government of Zimbabwe is no longer enforcing a policy of strict tick control. It is likely that reduced tick control will result in the spread of Amblyomma ticks to previously uninfested areas. Added to this, recent introductions of various wildlife species to highveld commercial farming areas have created conditions in which the ticks could become established in higher rainfall areas. Amblyomma hebraeum is more likely to spread than A. variegatum, because its adults parasitize a wider range of wildlife hosts (warthogs, medium to large-sized antelope, giraffe, buffalo and rhinoceros), whereas adults of A. variegatum appear to be largely restricted to one wildlife species (buffalo) in Zimbabwe, the distribution of which is now confined to very limited areas of the country, as part of foot and mouth disease control measures.(ABSTRACT TRUNCATED AT 400 WORDS)

Animals↗
PubMed1993

Evaluating the economic damage threshold for bont tick (Amblyomma hebraeum) control in Zimbabwe.

Controlling ticks and tick-borne diseases by frequent applications of acaricides (e.g., dipping) is costly, and can leave treated livestock vulnerable to epizootics of tick-borne diseases should the system of applying acaricides break down. The concept of only applying acaricides on an infrequent (strategic) basis often relies on the target tick population displaying a seasonal cycle. However, as adult bont tick (Amblyomma hebraeum) infestations in Zimbabwe's lowveld do not have a strictly seasonal pattern of occurrence, it is recommended that tick control only be applied when bont tick infestations are equal to, or greater than, their economic damage threshold. The economic damage threshold is the minimum average weekly standard female tick burden sufficient to cause damage equal in dollar value to the costs of applying tick control. Assuming that each standard female tick represents a 10 gram weight loss, the economic damage threshold (standard female ticks/week) is equivalent to the ratio of the producer price of beef (liveweight equivalent): per head cost of dipping (Eqn (3)). To illustrate the application of the threshold methodology, it was assumed that the producer price of beef was Z$1.63/kg (U.S.$0.33/kg) and that tick control cost Z$0.29/hd/dip (U.S.$0.06/hd/dip). This gave a threshold of 18 standard female ticks/head/week. Using tick counts obtained from 20 Brahman cattle held at Mbizi in southeastern Zimbabwe, it was shown that for the 1988 calendar year there were only 32 weeks when the economic damage threshold was met or exceeded. This is substantially less that the 44 dippings per year that have been, until very recently, legally required in Zimbabwe. Sensitivity analysis showed that a 10% rise in the cost of dipping reduced to 23 (a 28% decrease) the number of weeks when tick burdens exceeded the economic damage threshold. By further assuming that an acaricide application and residual effects will cause a 3-5 week interval before the next application may be required, the number of weeks when the tick burden was equal to or greater than the threshold of 18 standard females/week fell to just 9-12 weeks. Three factors may cause an alteration in the economic damage threshold: i) tick burdens may cause damage to the udders; ii) secondary infestations (e.g., screw-worm) may cause economic damage; and iii) nutritional stress of the cattle may reduce the actual average per tick weight loss.(ABSTRACT TRUNCATED AT 400 WORDS)

Animals↗