Sea lice treatments on salmon farms have no adverse effects on zooplankton communities: a case study

The long-term effects of the sea lice treatment products Excis and Slice on zooplankton communities in a Scottish sea loch were investigated at a commercially operating salmon farm over 31 months. Cypermethrin and emamectin benzoate are the active ingredients in Excis and Slice respectively, which are widely used to control ectoparasitic sea lice on farmed salmon. Excis and Slice treatments did not cause basin-wide effects on the zooplankton community. For both formulations, no adverse affects on zooplankton were detected, instead observed changes in zooplankton abundance and community composition displayed natural seasonal cycles of abundance. Water column concentrations of cypermethrin and emamectin benzoate following sea lice treatments at the fish farm were predicted using models. Cypermethrin concentrations of 3000 ng/l were predicted for short periods immediately after each cage treatment assuming no particle adhesion. The 3-h and 24-h Environmental Quality Standards (EQS) were exceeded for 10 h and 32 h respectively on the second day when five cages were treated. However, cypermethrin concentrations higher than 0.5 ng/l (24-h EQS) were predicted to occur over <2% of the total basin area on each treatment day. The 3-h EQS (16 ng/l) was exceeded in <0.3% of the basin on each treatment day. The concentration of soluble emamectin benzoate present in the water column was predicted from modelled deposition footprints and sediment concentrations to be of order 10(-3) ng/l. Predicted concentrations of both chemicals were generally lower than those causing toxicity to copepods in previous laboratory studies and further support the results of this field study that environmental concentrations of Excis and Slice do not adversely impact zooplankton communities.

A review of Melophagus ovinus (L.), the sheep ked

The sheep ked Melophagus ovinus is a member of the parasitic Dipteran family Hippoboscidae; it was a widespread ectoparasite of sheep that was of veterinary importance until the introduction of effective pesticides. Since then it has become of less concern and may have become a relatively uncommon species through much of its former range. Changes in agricultural practice, both greater intensification and development of more extensive systems (including organic farming and conservation grazing), may allow resurgence and a review of existing knowledge of M. ovinus may be timely. This paper reviews the biology, annual population cycles, natural causes of mortality, veterinary and economic significance and control of, and host resistance to, M. ovinus.

Interspecific differences in Aethia spp. auklet odorants and evidence for chemical defense against ectoparasites

The true auklets (Genus Aethia) are small planktivorous seabirds of the Bering Sea and North Pacific. Two species, the crested and whiskered auklets produce volatile citrus-like odorants. We here show that the whiskered auklet odorant is composed predominantly of two odd-numbered aldehydes (heptanal and nonanal) with no detectable unsaturated aldehydes. By comparison the crested auklet odorant is dominated by even-numbered aldehydes, both saturated and monounsaturated, ranging in size from 6 to 12 carbons. This is evidence of species-specific acquisition or biosynthetic pathways. We clarify the chemistry of the crested auklet odorant. We cite evidence that the C-12:1 aldehyde in crested auklets is actually two isomers, (Z)-4-dodecenal and (Z)-6-dodecenal. We also report on experimental evidence that aldehyde constituents kill and repel ectoparasites. Efficacy of the aldehydes may increase when they are combined in a mixture. The repellency of the mixture increases with chemical concentration. This suggests that individuals with higher chemical production are likely to repel ectoparasites more effectively.

ANOINTING CHEMICALS AND ECTOPARASITES: EFFECTS OF BENZOQUINONES FROM MILLIPEDES ON THE LONE STAR TICK, Amblyomma americanum

Many mammals and birds roll on or rub themselves with millipedes that discharge benzoquinones. Chemicals transferred from millipedes onto the integument of anointing animals are thought to deter ectoparasites. We tested the lone star tick, Amblyomma americanum (L.), for responses to three widespread components of millipede defensive secretions, 1,4-benzoquinone; 2-methyl-1,4-benzoquinone (toluquinone); and 2-methoxy-3-methyl-1,4-benzoquinone (MMB). In toxicity tests, ticks were confined for 1 hr in filter-paper packets treated with serial dilutions of each of the benzoquinones or the commercial acaricide permethrin. Ticks were least affected by toluquinone, and most affected by permethrin. Of the benzoquinones, only MMB showed repellent activity. Behavioral assays were more sensitive than mortality for measuring the effects of the benzoquinones. Latencies for ticks to right themselves and to climb were greater with all compounds, even at the lowest concentrations, than with controls. Ticks exposed to low concentrations of benzoquinones appeared to recover over time, whereas those exposed to high concentrations exhibited behavioral abnormalities 1-3 mo later. Our results indicate that benzoquinones appropriated via anointing may reduce the tick loads of free-ranging animals, although key questions remain on the amounts of these compounds available to and effectively appropriated by anointing animals.

Preventing parasites in cats

The monthly administration of broad-spectrum heartworm medications can effectively prevent a variety of internal and external parasitic diseases in cats. Although not every parasite can be stopped, many of the common feline parasites are susceptible to these agents. This article discusses the epidemiology and prevention strategies for those parasites that can be controlled by the administration of ivermectin, milbemycin oxime, or selamectin, either alone or in conjunction with an external parasiticide.

Prevention of arthropod-related disease

Arthropods of medical importance include vespids, ticks, mosquitoes, mites, flies, and fleas. They spread illness as disease vectors, are associated with lethal allergic reactions, and produce potentially deadly toxins. Strategies for prevention of illness include avoidance, vector reduction programs, chemoprophylaxis, and repellents. Strategies must be tailored to regional risks.

Qualitative analysis of the risk of introducing Gyrodactylus salaris into the United Kingdom

Gyrodactylus salaris is a freshwater, monogenean ectoparasite of Baltic strains of Atlantic salmon Salmo salar on which it generally causes no clinical disease. Infection of other strains of Atlantic salmon in Norway has resulted in high levels of juvenile salmon mortality and highly significant reductions in the population. The parasite is a major exotic disease threat to wild Atlantic salmon in the UK. This paper qualitatively assesses the risk of introduction and establishment of G. salaris into the UK. The current UK fish health regime prevents the importation of live salmonids from freshwater in territories that have not substantiated freedom from G. salaris. The importation of other species, e.g. eels Anguilla anguilla and non-salmonid fish, represents a low risk because the likelihood of infection is very low and the parasite can only survive on these hosts for less than 50 d. Importation of salmon carcasses presents a negligible risk because harvested fish originate from seawater sites and the parasite cannot survive full strength salinity. The importation of rainbow trout Oncorhynchus mykiss carcasses from G. salaris infected freshwater sites might introduce the parasite, but establishment is only likely if carcasses are processed on a salmonid farm in the UK. A number of mechanical transmission routes were considered (e.g. angling equipment, canoes, ballast water) and the most important was judged to be the movement of live fish transporters from farms on mainland Europe direct to UK fish farms. In the future, territories may have to substantiate freedom from G. salaris and economic drivers for live salmonid imports may strengthen. Under these circumstances, legal or illegal live salmonid imports would become the most significant risk of introduction.

Efficacy of fipronil/(S)-methoprene combination spot-on for dogs against shed eggs, emerging and existing adult cat fleas (Ctenocephalides felis, Bouché)

The inhibitory activities of fipronil (10% (w/v) solution), (S)-methoprene (9% (w/v) solution), and fipronil/(S)-methoprene (10 and 9% (w/v) solution, respectively) combination against eggs and emerging adult cat fleas (Ctenocephalides felis) and adulticidal activity were tested on experimentally infested dogs. Thirty-two Beagle dogs were selected for this study and eight replicates of four animals were formed based on body weight within sex. One dog in each replicate was randomly allocated to treatment with: (1) untreated control; (2) fipronil 10% (w/v) solution, (3) (S)-methoprene 9% (w/v) solution, and (4) fipronil 10% (w/v) and (S)-methoprene 9% (w/v) combination solution. Treatments were applied once topically on Day 0 at the rate of 0.067 ml/kg. On Days -12, -1, 21, and weekly to Day 84 each dog was infested with approximately 200 fleas and comb counted approximately 24h later, or 2 days (our 48 h) after in the case of Day -1 infestation. On Days -11, 1, 22, and weekly to Day 85 each dog was again infested with approximately 200 fleas. Flea eggs were collected over approximately 24 h beginning 3 days after infestation. Fleas were combed off of the dogs and counted at the end of the egg collection period (approximately 96 h count). One aliquot of up to about 100 eggs, if available, from each animal at each infestation time was incubated for approximately 72 h to determine larval hatch and the other for 35 days to determine the number of adults that developed. The 10% (w/v) fipronil spot-on provided excellent control (>95%) of adult fleas on dogs for 5 weeks. Similarly, the combination spot-on of 10% (w/v) fipronil and 9% (w/v) (S)-methoprene provided excellent control of adult fleas, i.e., >95% for 5 weeks. From week 6 post-treatment onward, the relatively low inhibition of adult flea emergence substantiated the lack of significant ovicidal/larvicidal activity in the fipronil (10%, w/v) treatment group. However, the combination product provided excellent (>90%) ovicidal activity for 8 weeks and high (91.4%) inhibition of adult flea emergence for 12 weeks. In addition, a synergistic effect of the two compounds in combination was demonstrated with fipronil enhancing the ovicidal and inhibition of adult flea emergence activity of (S)-methoprene against cat flea eggs. When all stages of the life cycle of the cat flea are considered, the combination spot-on product provided a high level of total flea control yielding a curative effect against adult fleas and inhibition of flea development stages with little to no potential reinfestation pressure on the animal or in the environment for 12 weeks.

Investigations on the biology, epidemiology, pathology and control of Tunga penetrans in Brazil. V. Cytokine concentrations in experimentally infected Wistar rats

Tungiasis is caused by the penetration of the female sand flea Tunga penetrans into the skin of its host. This parasitic skin disease is almost invariably associated with an intense inflammation around embedded fleas, the underlying mechanisms being unknown. A study was undertaken to determine whether Wistar rats can be used as an animal model to assess cytokine kinetics during the natural course of the infection. Laboratory-raised Wistar rats were exposed in cages put on the soil in an area with high human attack rates. Rats were examined daily and blood samples were taken before exposure and at 2, 6, 10, 13, 16 and 20 days after flea penetration. TNF-alpha, IL-1 beta, IFN-gamma, IL-4, IL-10 and CINC (a rat cytokine- induced neutrophil chemoattractant and member of the IL-8 family) were determined by enzyme immunoassay. The results showed an increasing serum concentration of TNF-alpha and IL-1 beta 10-13 days after penetration and a rapid increase in IL-4 2 days after fleas became embedded. During the natural course of the infection, the ratio of the serum concentration of TNF-alpha to that of IL-10 decreased, indicating a relative increase in the secretion of the anti-inflammatory cytokine. The treatment of lesions with silicone oil abrogated the natural disease course and changed the pattern of cytokine secretion. We conclude that the Wistar rat is an appropriate model to study immune responses in tungiasis.

Investigations on the biology, epidemiology, pathology and control of Tunga penetrans in Brazil: IV. Clinical and histopathology

Tungiasis is a parasitic skin disease caused by the sand flea Tunga penetrans. This ectoparasitosis is endemic in South America, the Caribbean and sub-Saharan Africa, where it is an important but neglected health problem in resource-poor communities. As part of a study of tungiasis-related morbidity in a typical slum in Fortaleza, Brazil, we identified 86 individuals with tungiasis. Lesions were counted, classified according to the stage of development, and clinical pathology was documented. One hundred and nine lesions were biopsied and examined by histological sectioning. The patients had between 1 and 145 lesions (median 14.5), the majority occurring in clusters. In all, 77% of patients reported severe pain at the site of the lesion, and 52% had one or more nails lost or severely deformed. Intense inflammation and/or fissures hindered 45% of the patients from walking normally. Signs of superinfection were observed in 29%, and signs of generalized inflammation in 2% of patients. Clinical pathology was significantly related to the number of lesions, and the total number of parasites present correlated with the number of fleas occurring in clusters. Clinical pathology was frequently accompanied by a pathological alteration of the epidermis (predominantly hyperplasia, parakeratosis, hyperkeratosis, and spongiosis) and the dermis. Tungiasis causes a broad spectrum of clinical histopathological alterations, and is a serious health threat in a typical, impoverished community in northeast Brazil. The clinical pathology is closely related to the parasite burden of an individual and the clustering of embedded fleas at certain predilection sites.

Diagnosis and management of bedbugs: an emerging U.S. Infestation

Bedbug infestations are on the rise in the United States among people of all socioeconomic groups. Bedbug bites typically cause pruritic wheals with central punctum on exposed skin, which are noted upon awakening. The diagnosis is confirmed by identifying fecal smears and blood spots on the bed linens and mattress or by identifying the culprit bug. Extermination of bedbugs using cleaning techniques and pesticides is necessary to prevent recurrence.

Evaluation of risk factors for rural infestation by Triatoma pallidipennis (Hemiptera: Triatominae), a Mexican vector of Chagas disease

Control of Chagas disease requires control of its triatomine vectors, which requires an understanding of the determinants of infestation. Twenty-seven household environmental characteristics in the town of Chalcatzingo, Morelos, were analyzed for association with infestation by Triatoma pallidipennis, the predominant local vector. Data were obtained through timed household searches for triatomines and surveys that characterized intradomicile and peridomicile environments. Of the households surveyed, 28.4% were infested by T. pallidipennis. Cross-sectional multivariate logistic regression analyses were performed that regressed infestation on environmental variables. Of the 530 households in the town, 84% had sufficient data to be included. Adobe walls, agricultural products, junk piles, lack of bednets, and number of rabbits were significantly associated with intradomiciliary infestation. Junk piles and numbers of dogs, cats, and rabbits were significantly associated with peridomiciliary infestation. Junk piles, agricultural products, and numbers of cats, rabbits, and birds were significantly associated with overall infestation. Unexpectedly, presence of stone piles was not associated with infestation. The results of this study provide information for designing Chagas disease control programs in rural Mexican areas infested by T. pallidipennis.

[Humanopathogenic parasites--an unwelcome import]

Humanopathogenic parasites represent a health risk for travelers to tropical countries. They can be picked up by direct contact, via contaminated food and drink, the bites of blood-sucking insects and leeches, as also via direct penetration through the skin. Not only the bite and sting wounds themselves are unpleasant. The true danger to the victim is represented, in particular, by the bacteria, viruses or parasites that are transmitted in this way. They can give rise to protracted diseases such as hepatitis B, cutaneous or intestinal disorders. The most effective preventive measure is exposure prophylaxis.

Development of a novel self-medicating applicator for control of internal and external parasites of wild and domestic animals

Four trials, three in the United States and one in South Africa, were conducted to evaluate the potential value of a novel self-medicating applicator in the passive control of gastrointestinal nematodes in cattle and deer, and of files and ticks on cattle using oil-based treatments. The results of the trials demonstrated that this applicator is an effective and practical device for the passive treatment of both deer and cattle for trichostrongyle infections using the endectocide, moxidectin (Cydectin. Fort Dodge Animal Health, USA), of cattle for hom fly (Haemotobia irritans) infestations using the insecticide, cyfluthrin (CyLence, Bayer AG, Germany) and of cattle for tick infestations (in particular Amblyomma hebraeum and Rhipicephalus appendiculatus) using the acaricides deltamethrin and amitraz (Delete All, Intervet, South Africa).

A review of Dendromonocotyle (Monogenea: Monocotylidae) from the skin of stingrays and their control in public aquaria

Dendromonocotyle species (Monogenea: Monocotylidae) are the only monocotylids to parasitize the skin of chondrichthyan hosts. Currently 11 species are recorded from the skin of ray species in the Dasyatidae, Myliobatidae and Urolophidae. There have been increasing reports of Dendromonocotyle outbreaks on rays kept in public aquaria. This paper provides a broad review of Dendromonocotyle that should assist taxonomists and aquarists with species identification and help decisions on potential control methods for Dendromonocotyle infections. The taxonomy and host-specificity of Dendromonocotyle are discussed and a key to current species is provided. We summarise what little is known about the biology of Dendromonocotyle including egg embryonation and hatching, feeding, camouflage and reproduction. The efficacy of freshwater baths, chemical treatments and biological control measures such as the use of cleaner fish for Dendromonocotyle is also discussed. We demonstrate that effective control of Dendromonocotyle on captive rays is hampered by the lack of basic biological data on the life cycle of the parasites. A case history is provided outlining the success of a public aquarium (Underwater World, Mooloolaba, Queensland, Australia) in managing D. pipinna infections on captive Taeniura meyeni without chemical intervention simply by taking measures to reduce host stress.

Dermal safety study with imidacloprid/moxidectin topical solution in the ivermectin-sensitive collie

A study was conducted to determine the safety of the dermal application of 10% imidacloprid/2.5% moxidectin topical solution in ivermectin-sensitive collies. Each milliliter of this solution contains 100mg of imidacloprid and 25mg of moxidectin. A total of 21 collies were prescreened for ivermectin-sensitivity and heartworm negative status prior to selection for the study. Animals were assigned based on the maximum ivermectin-sensitivity score demonstrated during the prestudy screening. Treatment groups included a 3x and 5x test article group, and a 3x and 5x mineral oil control group. The 3x and 5x doses were administered at three and five times, respectively, the 1x dose based on the animal's body weight. On day 0, 3 of the 21 dogs were treated with dermal applications of a preliminary dose of 3x test article to screen for unexpected signs of toxicity with the remaining 18 dogs being treated with 3x mineral oil to blind for the volume of liquid applied. After no signs of toxicity were observed, these same three dogs were treated with 3x of test article and 2x mineral oil on days 28 and 56. The remaining 18 animals were equally allocated to either a 5x test article group or a 5x control group and were each treated on days 28, 56, and 84. Personnel performing observations were blinded to treatment. Observations were made for clinical signs of ivermectin sensitivity twice daily during non-dosing days. On treatment days, dogs were observed hourly for the first 4h post-treatment and at 6, 8, 12, 18 and 24h. Signs of toxicosis were not observed in any of the dogs throughout the observation period. This study demonstrated the safety of imidacloprid/moxidectin, when administered to collies testing positive for ivermectin sensitivity at dosages up to five times the maximum recommended dose.

Ectoparasites--the underestimated realm

CONTEXT

Ectoparasitoses (infestations with parasites that live on or in the skin) can cause considerable morbidity. Whereas pediculosis and scabies are ubiquitous, cutaneous larva migrans and tungiasis (sand-flea disease) occur mainly in hot climates. The prevalence of ectoparasitoses in the general population is usually low, but can be high in vulnerable groups. Scientific knowledge on how to deal best with parasitic skin diseases in different settings is scanty, and evidence-based measures for control are not available. For head lice and scabies the situation is daunting, because resistance of Pediculus humanus capitis and Sarcoptes scabiei to insecticides is spreading and unpredictable.

STARTING POINT

J Hunter and S Barker reported different patterns of resistance in schoolchildren in Brisbane, Australia: full resistance to malathion, permethrin, and pyrethrum in two schools, whereas head lice were susceptible to malathion and, to a lesser extent, to pyrethrums in three other schools (Parasitol Res 2003; 90: 476-78). K Yoon and colleagues found different resistance patterns in the USA and Ecuador (Arch Dermatol 2003; 139: 994-1000). Head lice from Florida were less susceptible to permethrin than those from Texas, and parasites from Ecuador were susceptible to both insecticides tested. WHERE NEXT? The occurrence of resistant pediculosis and scabies is expected to increase numerically and geographically. Clinicoepidemiological studies are urgently needed to identify the factors which govern the emergence and spread of strains of P humanus capitis and S scabiei that are resistant to insecticide or acaricide. Oral treatment with ivermectin could substitute for topically applied compounds, particularly in resource-poor communities where polyparasitism is common. A better understanding of local epidemiology is required to develop control measures. This knowledge has to be applied in combination with environmental sanitation, health education, and culturally acceptable interventions that are affordable by the underprivileged.

Abundance of stable flies on heifers treated for control of horn flies with organophosphate impregnated ear tags

Ear tags containing 40% organophosphate insecticides (diazinon or diazinon plus chlorpyrifos-ethyl) were applied to control Haematobia irritans (L.) (Diptera: Muscidae) in treated (TG01 and TG02) and untreated (UG01 and UG02) groups of Holstein heifers born in 2001 and 2002, respectively. Control and treated groups were assessed for the abundance of Stomoxys calcitrans (L.) (Diptera: Muscidae) from August 2001 to April 2002 and again from August 2002 to April 2003. The treatment had a high efficacy for control of horn flies (maximum median number per heifer of TG01 and TG02 = 5) but a low effect on the abundance of stable flies. The total numbers of S. calcitrans were 1251 (42.9% of the total) and 1668 (57.1%) for TG01 and UG01, and 1423 (48.8%) and 1494 (51.2%) in TG02 and UG02, respectively. No significant difference in stable fly burden was found in 55 of the 76 weeks evaluated. A unimodal peak of abundance in the spring was found during the first fly season, and a bimodal abundance, with peaks in the spring and autumn, during the second season. No strong associations between horn fly and stable fly burdens was found in individuals of the CG01 (correlation coefficient = 0.13, P > 0.05) or CG02 (correlation coefficient = 0.538, P < 0.05, determination coefficient = 0.289).

An initial survey of the cattle grub Dermatobia hominis (L. Jr.) in Nicaragua

After the civil war and the Hurricane-Mitch disaster, cattlemen in Nicaragua were forced to transport their cattle from lowland areas to higher, dryer areas of the country. These areas are natural ecological niches for the cattle grub Dermatobia hominis (L. Jr.) (Diptera: Cuterebridae). To determine the importance of this infestation, the Agricultural and Livestock-Forestry Ministry selected a central area of Nicaragua to run a pioneer survey program to acquire information about hosts involved, number of cases, treatments applied and general knowledge of 42 farmers about the life cycle of the parasite. The subjects were either farm owners or farm managers. Ninety-five percentage of the farms indicated cases of D. hominis infestation in their animals, with cattle being the most affected host (100% of the affected farms). There was poor understanding of the D. hominis life cycle, vectors and control methods. A misuse of insecticides for the treatment of larval infestation by D. hominis was indicated.

Effect of horn fly and internal parasite control on growth of beef heifers

The effects of horn fly and gastrointestinal nematode control on body weight gain of yearling Angus-Brangus cross heifers were evaluated in three separate studies during the years 1999, 2000 and 2002. In each year, the studies started in late May and lasted for 150, 148 and 123 days, respectively. In all three studies, the tag treatment (10% lambdacyhalothrin+13% piperonyl butoxide impregnated ear tags) provided excellent horn fly control. In the three studies, the average weekly horn fly counts for tagged heifers were 1, 3, and 0 flies per side while the average on untreated heifers was 52, 163 and 90 flies per side. In studies 1 and 2, there was no difference (P>0.1) in weight gain between tagged and untreated heifers, but in study 3, tagged heifers gained 50% more weight (P<0.001) than the untreated heifers. For gastrointestinal nematode control, ivermectin (IVM) was administered on Day 0 in studies 1 and 3 using a sustained release bolus and on Day 0 and subsequent 56-day intervals in study 2 using either the injectable or pour-on formulation. Heifers treated with IVM in studies 1 and 3 had significantly lower (P<0.05) GI nematode fecal egg counts compared to control heifers. In study 2, IVM treated heifers also had significantly lower GI nematode fecal egg counts compared to control heifers, except for Day 90 when no differences in fecal egg counts were observed between IVM pour-on treated and control heifers. Weight gain of heifers that received IVM was significantly greater (P<0.005) than untreated heifers in each of the three studies. IVM treated heifers gained 45, 61 and 184% more weight than the untreated heifers during the three studies. There was no interaction (P>0.1) between the main treatment effects of fly control and gastrointestinal nematode control.

Efficacy of pyrethroid insecticides against domestic and peridomestic populations of Triatoma pallidipennis and Triatoma barberi (Reduviidae:Triatominae) vectors of Chagas' disease in Mexico

A single village control trial for Triatoma pallidipennis and T. barberi was conducted using three synthetic pyrethroids (bifenthrin, cyfluthrin, and deltamethrin), evaluated as residual treatments in separate sectors, with complete coverage indoors and in peridomiciliary areas. Spray intervention was preceded by a preintervention entomological evaluation and household survey, followed by four postintervention evaluations at 1, 3, 6, and 12 mo of > 96% of houses. Overall preintervention adjusted infestation index was 38%, 17% of which represented intradomicile infestation. Dose verification using high-performance liquid chromatography (HPLC) demonstrated correct spray doses for all but deltamethrin treatments. There was between a 6- and 13-fold decrease in intradomicile live bug infestation for cyfluthrin- and bifenthrin-treated areas, resulting at 1 mo in 0 and 0.6% infestation, respectively. Intradomicile infestation recovered somewhat, terminating at 20 and 50% of preintervention levels at 12 mo, respectively, while peridomicile infestation recovered preintervention levels within 3-6 mo. Households with persistent live peridomiciliary infestation had 1.9 times the risk of having a persistent intradomiciliary infestation, while 80% of peridomicile infestations for both triatomine species were in houses not having a previous infestation. New or reinfestation of households did not occur consistent with a sylvan source, and unconstructed lots were not a significant source of bugs. Houses with persistent peridomicile infestation did represent a significant risk for surrounding uninfested houses by cluster analysis (P < 0.05). Along with the increased prevalence of T. cruzi infection after intervention, the data indicate that a sylvan reservoir source, probably peridomicile small rodent nests, represent the major risk factor for persistent and new infestations.