Seasonal dynamics of ticks (Acari: Ixodidae) on horses in the state of São Paulo, Brazil

Natural tick infestations were assessed every 14 days on horses over a 2-year period. Amblyomma cajennense adult ticks were counted individually, without detachment from the horses. Larvae and nymphs of A. cajennense were collected using a rubber scraper that scratched engorged immature ticks from the host. Adult females of Anocentor nitens larger than 4mm length were counted on the horses. Blood samples were also obtained from the horses every 14 days and macroclimatic data were obtained for the study period. Infestations of A. cajennense demonstrated distinct peaks of activity for each of the three parasitic stages over each 12-month period, showing a 1-year generation pattern. Larvae predominated from April to July and nymphs from June to October. Adults predominated from October to March with a greater number of adult males than females. Although other studies on seasonal dynamics in the states of Rio de Janeiro and Minas Gerais were performed with the free-living stages of A. cajennense on pastures, the present study in the state of S?o Paulo, performed with the parasitic stages of A. cajennense on horses, showed similar results to those observed in other states. Infestations by A. nitens demonstrated distinct peaks of activity of adult females (>4 mm), suggesting different tick generations during the year. Infestation with A. nitens was much higher in the first year than the second year which may have been related to horse nutritional status and stocking rate. Although several climatic variables showed statistical significant correlation (r) with tick counts, the determination coefficients (R(2)) were always lower than 0.40, suggesting that any single significant variable (i.e. mean temperature) would not explain the tick distribution pattern over the year. The highest peaks of A. nitens females (>4 mm) were significantly associated with decrease in horse packed cell volumes (R(2)=0.603). The ears and the perineum, tail and groin region accounted for around 70% of all A. nitens females counted on the horses.     

Risk factors to tick infestations and their occurrence on horses in the state of São Paulo, Brazil

From December 1998 to March 1999, 40 stud farms were studied in the state of S?o Paulo, Brazil. During visits to farms, horses reared under grazing conditions were examined for the presence of ticks. On each farm visit, horse pastures were closely inspected and a questionnaire was given to the farm supervisor with the purpose of gaining information about ecological and management variables (independent variables) that could be associated with the presence and infestation levels of ticks on the farm (dependent variables). Three tick species were found during the study. Anocentor nitens, Amblyomma cajennense and Boophilus microplus were present on horses from 38 (95%), 20 (50%) and four (10%) farms, respectively. All farms that had A. cajennense or B. microplus infestations also had A. nitens infestations. Only one of the four farms with B. microplus infestations on the horses also had A. cajennense infestations. Two farms had all horses free of ticks. There was a strong association between the presence of infestation by B. microplus on horses and the simultaneous use of a grazing area by cattle and horses (P = 0.000). There was no statistical association between any of the independent variables and the presence or infestation level of A. nitens on the horses (P > 0.20). The presence of A. cajennense was statistically associated with the presence of at least one mixed overgrowth pasture in the farm (P = 0.001). A mixed overgrowth pasture means the presence of undesired plants such as bushes and shrubs in the pasture. The presence of high levels of A. cajennense on horses was also associated with the presence of at least one mixed overgrowth pasture in the farm (P = 0.026). The regular use of acaricides was statistically associated with the presence of ticks on the horses (P < 0.05), making this procedure a result of the inefficacy of controlling ticks on the farms. The occurrence of human infestation by ticks was statistically associated with the presence of A. cajennense on the horses (P=0.000). The presence of at least one mixed overgrowth pasture on the farm was associated (P = 0.000) to either higher horse densities and to farms that did not mow all the pastures once a year, indicating that mowing all the pastures at least once a year can be considered a protective factor against the presence of mixed overgrowth pastures on the farm, and consequently, against the presence of A. cajennense on the horses.     

Acquired resistance of horses to Amblyomma cajennense (Fabricius, 1787) ticks

Acquired immunity of horses to larvae, nymphs and adults of the Amblyomma cajennense tick was evaluated through three consecutive experimental infestations of tick-bite na?ve hosts. Data from these infestations were compared to those from field-sensitized horses and donkeys. It was observed that tick-bite na?ve horses developed a low level of resistance after two infestations as shown by a significant decrease in larval yield and a tendency for lower engorged weight of nymphs during third infestation. Ticks fed on field-sensitized horses had a similar biological performance to that observed on the third infestation of tick-bite na?ve horses but the mean engorged nymph weight was significantly lower than that of the first infestation from tick-bite na?ve horses. Donkeys presented the strongest resistance with significantly lower engorged weights of all instars and of the egg mass compared to the first infestation of tick-bite na?ve horses. Donkeys also displayed a significantly higher resistance than field-sensitized horses as demonstrated by significantly lower egg mass weights. Overall these results indicate that donkeys but not horses maintain a strong resistance to A. cajennense ticks. The importance of these findings in relation to vectoring of tick-borne diseases is discussed.     

Seasonal dynamics of Anocentor nitens on horses in Brazil

Anocentor nitens is a widespread ectoparasite of horses in Brazil. A study of the seasonal dynamics of the parasitic phase of this tick was made by regular census of engorging females (>/=4mm) on free-ranging horses, fortnightly from July 1995 to July 1997, at Pedro Leopoldo, Minas Gerais. Twenty horses were divided into four groups of five animals per group, each having a similar mix of resistant and less resistant animals, based on initial infestation. Every 2 weeks, one of the groups, in sequence, was treated with an acaricide to prevent excessive infestations, and female ticks were censused 38 days later. The acaricide dips did not interfere with the seasonal dynamics of the tick population, which produced three major peaks of activity over a 12-month period. The first two peaks were in the hot, wet season and the third in the cold dry season. Tick numbers were similar for the two 12-month periods, decreasing from the spring and summer to the autumn and winter. Infestation rates were not equal among different horses. Twenty percent of the horses were responsible for 48% of the total parasite load, while 30% accounted for only 2.5% of the total parasite load. Principal predilection sites were; the ear (61%), mane (21%), and muzzle and face (6%). Measures for the strategic control of A. nitens ticks are discussed.     

Ticks and tick-borne disease in Guatemalan cattle and horses

Blood samples and ticks were collected from 48 cattle and 74 horses from seven sites in the Peten region of Guatemala. Data on body condition, mucous membrane capillary refill time and tick infestation levels were recorded for each animal in the study. Horses had significantly higher levels of tick infestation than cattle, as well as poorer body condition scores. Seroprevalence of Babesia spp. was 95.8% for B. bovis in cattle, 89.6% for B. bigemina in cattle, and 92.7% for B. equi in horses. Seroprevalence of Anaplasma marginale in cattle was 87.5%, similar to reports in animals from other regions of Central America. This is the first time that A. phagocytophilum has been reported in animals from this region, with overall PCR-prevalence of 27.6% in cattle and horses, and seroprevalence of 28.4% (52% in cattle and 13% in horses). An agent was identified with serological cross-reactivity and close genetic relatedness to Ehrlichia ruminantium, but further testing confirmed that the agent in Guatemalan cows was not the agent of heartwater. Ticks were identified to species with the predominant species identified on cattle as Boophilus microplus and Amblyomma cajennense, while Anocentor nitens and A. cajennense were most commonly found on horses. Prevalence of infection, tick infestation levels, host factors and environmental data were analyzed for association; A. nitens was significantly associated with A. phagocytophilum prevalence by village.     

Species diversity and seasonality of free-living ticks (Acari: Ixodidae) in the natural habitat of wild Marsh deer (Blastocerus dichotomus) in Southeastern Brazil

This study evaluated the presence and seasonal activity of free-living ticks in remaining marsh areas by the Paraná river, in Brazil. Eight field trips (once per season) for collection of ticks were performed during 2 years. Using CO2 traps, dragging, and visual inspection of vegetation, five free-living tick species were collected, in the following order of abundance: Amblyomma cajennense, Amblyomma dubitatum, Amblyomma triste, Amblyomma coelebs, and Amblyomma nodosum. The seasonal pattern of A. cajennense was characterized by the highest peaks for adult ticks in the summer/spring months, for nymphs in the winter and for larvae in the autumn and winter. A. dubitatum and A. triste presented similar seasonal patterns characterized by peaks of adult ticks in the autumn. Nymphs of A. dubitatum peaked in the winter of the first year and in the winter/spring of the second year. A. triste was the only species to be collected in significantly higher numbers in the marsh than in surrounding drier areas such as forest patches. Among domestic animals living close the marsh areas, horses were infested by Anocentor nitens, A. cajennense, and Boophilus microplus, bovines were infested solely by B. microplus, and dogs were infested by Rhipicephalus sanguineus. Adults of A. triste showed to be well adapted to the marsh environment. This result, at least partially, explains local previous observations on the association of A. triste with marsh deer, as this vertebrate host inhabits mainly the marsh area.     

Rickettsial infection in domestic mammals and their ectoparasites in El Valle de Antón, Coclé, Panamá

The present research evaluated the presence of Rickettsia spp. on ectoparasites of horses and dogs (using PCR techniques), and their sera (using immunofluorescence assay) in El Valle de Antón town in Panama. A total of 20 horses and 20 dogs were sampled, finding four species of ectoparasites on dogs (the ticks Rhipicephalus sanguineus, Amblyomma ovale, Amblyomma oblongoguttatum, and the flea Ctenocephalides felis), and two tick species on horses (Amblyomma cajennense and Dermacentor nitens). DNA of Rickettsia amblyommii was found in pools of A. cajennense, D. nitens, and R. sanguineus, while Rickettsia felis was detected in C. felis pools. Overall, 70% (14/20) and 65% (13/20) of the horses and dogs, respectively, were seroreactive (titer ≥ 64) to spotted fever group rickettsiae. Sera from six dogs and five horses reacted to R. amblyommii antigens with titers at least four-fold higher than those for the other antigens tested (Rickettsia bellii, Rickettsia parkeri, Rickettsia rhipicephali, R. felis, and R. rickettsii). These serological results, coupled with our molecular findings, suggest that these dogs and horses were infected by Rickettsia amblyommii. More studies need to be realized afford to identify the Rickettsia species responsible for other serological and molecular positive results, and their ecological importance.     

The sex ratio of Amblyomma cajennense (Acari: Ixodidae) with notes on the male feeding period in the laboratory

To evaluate the sex ratio of field collected nymphal Amblyomma cajennense ticks, we collected 5326 engorged nymphs from naturally infested horses in Pirassununga county and allowed them to molt to adults in the laboratory. They yielded a sex ratio of 1:1.83 (M:F). Three and two engorged females were collected from horses pastured at Pirassununga county and from tapirs pastured in Sorocaba county, respectively. These females were allowed to oviposit and their progeny were reared until the adult stage in the laboratory. Engorged females collected from Pirassununga yielded a sex ratio of 1:1.57 (M:F) and a sex ratio of 1.14:1 (M:F) were obtained for those ticks collected from tapirs. In addition, unfed tick larvae were collected from Pedreira county and reared in the laboratory until the adult stage. This collection yielded a sex ratio of 1.11:1 (M:F). These results showed significantly different (P<0.05) sex ratio constitutions among different tick populations. Laboratory rabbits were infested once with A. cajennense male ticks, which showed feeding periods varying from 7 to 86 days. During this period, the rabbits were re-infested regularly with A. cajennense female ticks. A total of 179 engorged females were collected from the rabbits and their engorged weight, feeding, preovioposition and egg incubation periods, weight of deposited eggs, percent of hatched eggs and egg production efficiency were compared to the male feeding period and to the number of live males present on the host. None of the female variables were affected by the male feeding period. Male ticks remained fertile for the whole feeding period. Percent of hatched eggs was the only female variable that significantly decreased as the number of live males decreased on the host. The results showed that although some A. cajennense populations are composed of more females than males after molting, this female predominance is compensated by a long male feeding period and maintenance of its reproductive performance.     

Ticks on livestock in St. Lucia

Cattle, sheep, goats and horses were examined for ticks. Over 95% of Holstein cross-breeds, 28% of sheep (local mixed breeds) and 18% of goats (local mixed breeds) examined from 18 August to 4 September 1983 were infested with the southern cattle tick, Boophilus microplus Canestrini. About 90 and 17% of the horses examined were infested with the tropical horse tick, Anocentor nitens Neumann, and the tropical bont tick, Amblyomma variegatum Fabricius, respectively. The tropical bont tick was found infesting 10% of cattle in the Gros Islet area of St. Lucia. The tropical bont tick was also found associated with a severe skin disease, dermatophilosis, caused by the bacterium Dermatophilus congolensis, in 54% of the cattle infested by A. variegatum in the Gros Islet and Dauphin areas of St. Lucia.     

Occurrence of pathogenic fungi to Amblyomma cajennense in a rural area of Central Brazil and their activities against vectors of Rocky Mountain spotted fever

Two isolates of Beauveria bassiana and one of Purpureocillium lilacinum (=Paecilomyces lilacinus) were found infecting Amblyomma cajennense engorged females collected on horses (0.15% infection rate from a total of 1982 specimens) and another two isolates of P. lilacinum and one Metarhizium anisopliae detected in soils (2.1% from 144 samples) collected in typical pasture habitats of this tick in Central Brazil from October 2009 to March 2011. Fungi were isolated from soils with Rhipicephalus sanguineus as surrogate baits. No fungi were found in ticks or soils during the driest months (May to August). Testing pathogenicity of fungi all R. sanguineus females were killed regardless of the isolate and fungi sporulated abundantly on the cadavers. A. cajennense was less susceptible to infection with P. lilacinum within 20 days than R. sanguineus. All three fungal species probably act as natural antagonists of A. cajennense particularly in the rainy season and have interest for integrate control of vectors of Rocky Mountain spotted fever.     

Epidemiology of flea infestation of ruminants in Libya

The results of an epidemiological and clinical study of flea infestations of farm animals in northern Libya is reported. Of 12,130 sheep examined from 124 flocks, 150 sheep were found to be infested with fleas from 50 different flocks. Likewise 23 goats from 2981 examined, and 11 calves from 1124 cattle examined were infested No fleas were recovered from camels or horses. Of 1861 fleas recovered from farm livestock, 1857 were Ctenocephalides felis strongylus and 4 were Pulex irritans. Dogs from farms and local clinics were also examined. Eight farms dogs were found to be infested with P. irritans. Of 79 infested dogs examined in veterinary clinics, 53 were found infested with P. irritans, 11 with Ctenocephalides felis felis, 12 had a mixed infestation of P. irritans and C. felis felis. Single dogs had mixed infestation of P. irritans and C. canis; C. felis felis and C. canis; and P. irritans, C. felis felis and Echidnophaga gallinacia. C. felis felis was also found on 15 infested cats. C. felis felis was never found on large farm animals despite frequently sharing their environment with dogs or cats. Likewise C. felis stongylus was never isolated from dogs or cats. This is consistent with the hypothesis that C. felis strongylus has become adapted to large farm animals, whilst C. felis felis is better adapted to dogs and cats. However, four stockmen were found infested with a total of 176 C. felis strongylus, which suggests that this subspecies is also a potential zoonosis. A significantly higher proportion of intensive farms had animals with flea infestation compared to semi-intensive farms. Fleas were not found in nomadic herds. Infested farm animals often presented with excoriation, alopecia, pruritis and hyperkeratitis particularly on the lower limbs. These signs are consistent with the generation of flea-bite hypersensitivity.     

Comparative IgG recognition of tick extracts by sera of experimentally infested bovines

Enzyme-linked immunosorbent assay (ELISA) and Western blot were used to investigate the pattern of antibody responses of six bovines infested twelve times with Rhipicephalus (Boophilus) microplus (Canestrini, 1887) (Acari: Ixodidae) (six heavy infestations followed by six light infestations) against salivary gland, gut and larvae extracts. During heavy infestations, bovine IgG levels were shown to be higher, and a decrease in the number and weight of ticks that completed the parasitic cycle was observed. The pattern changed starting from the seventh infestation, showing a decrease in IgG levels. An initial increase followed by a significant decrease in the proportion of ticks that completed the parasitic cycle was also observed from the seventh infestation. The number of molecules recognized by Western blot was higher from sera collected following heavy infestations than after light infestations, although a great variation in the profiles detected could be seen when the bovines were compared. These results indicate that IgG responses to different tick antigens may not be generally associated with bovine resistance, and that infestation levels modulate the magnitude of humoral responses and possibly the immune mechanisms in the natural acquisition of tick resistance.     

Effect of Haemaphysalis punctata infestation on growing lambs and rabbits

Lambs infected with adult Haemaphysalis punctata and rabbits infected with nymphs developed a macrocytic normochromic anaemia during seven and six successive infestations, respectively. The anaemia was directly proportional to the degree of infestation but disappeared several days after the termination of infestation. A leucocytosis, due to neutrophilia, was seen in both lambs and rabbits. Rabbits developed a thrombocytosis and reticulocytosis. Infested lambs grew less rapidly than uninfested animals. Signs of tick toxicosis and several other clinical manifestations appeared in both infested sheep and rabbits. Circulating antibodies against salivary antigen of adult H punctata were demonstrated in the sera of infested lambs by the micro-ELISA test. Titres were first detected on day 3 after infestation and increased gradually as infestation progressed. No precipitating antibodies in either infested sheep or rabbits were detected.     

Antibody and blood leukocyte response in Rhipicephalus sanguineus (Latreille, 1806) tick-infested dogs and guinea pigs

The dog is considered to be the natural host of Rhipicephalus sanguineus and is unable to develop appreciable resistance even after repeated feedings. The guinea pig develops strong resistance after one infestation with adult ticks. Antibody (IgG) titres against tick salivary gland antigens (SGAs) and blood leukocyte numbers in dogs and guinea pigs undergoing experimental R. sanguineus tick infestations were measured to detect a possible correlation with susceptibility or resistance of hosts. Since infested dogs develop an immediate hypersensitivity reaction to R. sanguineus antigens, total and anti-R. sanguineus SGA IgE levels were also measured in this host species. IgG and IgE antibody levels were determined by an enzyme-linked immunosorbent assay (ELISA) along three consecutive infestations of both hosts. Most dogs and guinea pigs displayed low IgG levels against R. sanguineus SGAs, though marked differences in individual response were observed. Although dog's total serum IgE levels increased significantly after infestations, no change in the amount of anti-salivary gland IgE was detected. Total and differential blood cell counts were determined in dogs and guinea pigs during primary and secondary infestation. In dogs, a tertiary infestation and a subsequent higher infestation level were also evaluated. Infested dogs did not display any alteration in blood leukocyte counts throughout the experiment. Guinea pigs, on the other hand, developed a significant basophilia during primary infestation which increased further during secondary infestation. These data reveal similarities and differences in the reactions of resistant and non-resistant hosts to ticks. They contribute for the understanding of such host-parasite relationships and will hopefully aid in the development of immune control of ticks.     

Traumatic myiasis of horses caused by Wohlfahrtia magnifica.

A survey was carried out in three stud farms with grazing animals, in order to gather data on the prevalence and clinical manifestation of, and the fly species involved in, traumatic myiasis of horses in Hungary. This parasitic disease was recorded in each farm. On the whole, 9.0% (14) of the inspected horses were infested with fly larvae. The affected horses had one infested lesion only, located at the mucosa of the vulva or the vaginal vestibule. The clinical symptoms depended on the age of infestation. Wohlfahrtia magnifica (Diptera: Sarcophagidae) was the only myiasis-causing fly species identified. It was assumed that unknown volatile chemicals might be responsible for the attraction of gravid females to the undamaged vulvar region. These odours are supposed to be produced during different physiological and/or pathological events associated with oestrus, prolonged puerperal period or inflammation of tissues. Daily inspection of grazing horses and early treatment of the affected areas are needed to avert significant damage to the infested horses.     

Differentiation of resistance in cattle to larval Hypoderma lineatum

Cattle were successively infested for 4 years with larval cattle grubs, Hypoderma lineatum, to demonstrate the acquisition of resistance. The effects of host age and dietary-induced state with Vitamin A on susceptibility to infestation and their interactions with effects from previous infestations were also determined.Cattle acquired resistance to infestation after a single infestation. This resistance was not affected by host age for at least 4 years, was not affected by a deficiency of vitamin A, and was effective in the skin and connective tissues of the host. The effects of this acquired resistance were highly variable in degree, were produced primarily during the first 90 days of infestation and increased after repeated provocation to provide nearly complete protection in some animals.Calves and yearlings had innate resistance that decreased with age, was localized in the skin, was impaired by a deficiency of Vitamin A, and resulted in a mean loss of 65–75% of the larvae in an infestation dose of up to 150.The combined effects of the 2 types of resistance reduced the grub-burden in cattle throughout their life.     

Immune responses of calves antigenically stimulated and challenge exposed with Anaplasma marginale during tick infestation or treatment with dexamethasone

Similar anamnestic antibody responses to a 2nd injection of a Anaplasma marginale vaccinal antigen were observed in calves infested with the tick Dermacentor albipictus and in tick-free calves. When challenge exposure of these calves to virulent A marginale was done, infestation with the tick Boophilus microplus increased anemia (P less than 0.01), but did not suppress antibody production to A marginale or increase parasitemia. None of the vaccinated calves, regardless of infestation, experienced clinical anaplasmosis. Mitogenic responses of lymphocytes from infested animals were unaltered by either infestation. Tick infestations did not cause immune suppression in the calves, whereas use of dexamethasone resulted in a significantly lower antibody response (P less than 0.05) after a 2nd injection of vaccinal antigen. After challenge with virulent A marginale, dexamethasone-treated calves showed more pronounced parasitemia (P less than 0.01) and anemia (P less than 0.01) than did control calves. Anaplasmosis did not prevent the calves from developing resistance to reinfestation, which was accompanied by immediate, but not delayed, hypersensitivity reactions against homologous tick extracts. Dermacentor albipictus did not seem to share common antigenic determinants with B microplus, since extracts from the latter did not elicit immediate hypersensitivity responses in calves sensitive to D albipictus extracts. Calves were somewhat resistant to reinfestation as evidenced by reduced numbers of adult fed ticks, decreased weights of ticks after feeding, and smaller egg masses.     

Qualitative and quantitative observations on the flea population dynamics of dogs and cats in several areas of Germany

From an ongoing country-wide study on the spectrum, the epidemiology and the population dynamics of flea infestations in dogs and cats, important preliminary results from the three areas of Karlsruhe, Nuernberg and Leipzig are presented. A total of 1922 dogs and 1838 cats from 12 different veterinary practices or clinics in three areas of Germany were systematically examined between July 2003 and June 2004. All dogs and cats appearing for a clinical veterinary consultation on one regular working day per month, per practice, were clinically examined. Dogs and cats were examined irrespective of any kind of prior therapeutic or prophylactic insecticidal treatment. The results show that a total of 99 dogs (5.13%) and 263 cats (14.33%) were infested. Cats were more often flea-infested than dogs (p < 0.05). The highest infestation rates for dogs (x = 7.87%) were detected between July and October, and for cats (x = 21.14%) between July and September, the lowest infestation rates for dogs (x = 2.88%) were observed between November and May, and for cats (x = 12.16%) between November and April (p < 0.05). Although the prevalences were generally higher during the summer months, no statistical differences were detectable when looking at the pattern between the four seasons, neither for dogs, nor for cats. Interestingly, the highest prevalences in dogs (9.9%) were detected in June 2004 and comparatively, in cats (23.86%) in August. The lowest detection rates in dogs were seen (1.28%) in April and in cats (7.26%) in January. The preliminary results did not indicate any tendency for a relationship between climatic conditions and flea infestation rates. Similarly, no differences of the infestations rates were detectable between urban and rural areas, 56% (dogs) and 46% (cats) of the infested pets originated from urban habitats. The flea species collected include Ctenocephalides felis, Ctenocephalides canis, Archaeopsylla erinacei, Pulex irritans, Ceratophyllus gallinae, etc. The overall frequencies reveal that C. felis was the most prominent species (81.5%), followed by C. canis (12.5%), A. erinacei (2.7%) and P. irritans (1.7%).     

Prevalence and intensity of <Emphasis Type="Italic">Oestrus ovis</Emphasis> in sheep of Shiraz,southern Iran

Two thousand and two heads obtained from slaughtered sheep at the Fars abattoirs (Shiraz, Southern Iran) between April 2006 and April 2007 were examined for the presence of Oestrus ovis larvae. Of the total heads, 995 (49.7%) were infested with O. ovis larvae. O. ovis larvae were observed in both sexes and all age groups in each season of the year. A total of 6264 larvae were collected. The overall larval intensity for the infested sheep was 6.3, with 3.9 in spring, 5.3 in summer, 5.9 in autumn and 7.8 in winter. Prevalence ranged from 23.3% in spring to 80% in winter. Increased infestation was observed in older animals.     

Studies on naturally acquired immunity to African ticks. II. Observations on cattle exposed to Rhipicephalus appendiculatus under varying periods of repeated infestations.

Groups of Friesian cattle were infested repeatedly with Rhipicephalus appendiculatus at weekly intervals (eight infestations) and at intervals of 3 weeks (seven infestations) and 6 weeks (seven infestations). The engorgement weight of adults, nymphs and larvae decreased significantly initially, from the second infestation, but rose significantly at later infestations. This rise was more dramatic with larval instars than with any other. A new method for quantifying the resistance status of animals was introduced; this method showed that the resistance acquired by cattle during 6-weekly reinfestations was more stable than that acquired by cattle during weekly and 3-weekly reinfestations. The criteria for assessing natural resistance to ticks was reappraised. These are classified into those serving long- and short-term purposes, and those which have direct and indirect effects. Criteria which could serve long- and short-term purposes include a reduced number of engorged ticks, an increased number of dead male and female ticks, a decreased number of ticks which fed above the critical engorgement weight and an increased number which fed within the pre-mating weight and decreased the number of copulations. The criterion of reduced engorgement weight can serve the short-term purpose only. A new manifestation of resistance to ticks, i.e. decreased moulting weights of nymphs and adults, together with the criterion of decreased egg hatchability, were classified as indirect effects because they were manifested after the ticks had dropped off the resistant cattle.