A review of bovine fasciolosis and other trematode infections in Nigeria.

Trematode infections cause serious economic losses to livestock worldwide. Global production losses due to fasciolosis alone exceed US$3 billion annually. Many trematode infections are also zoonotic and thus a public health concern. The World Health Organization has estimated that about 56 million people worldwide are infected by at least one zoonotic trematode species, and up to 750 million people are at risk of infection. Fasciolosis caused by the fluke Fasciola gigantica is endemic in Nigeria and is one of the most common causes of liver condemnation in abattoirs. Total cattle losses from Fasciola infection in Nigeria have been estimated to cost £32.5 million. Other trematode infections of cattle, including paramphistomosis, dicrocoeliasis and schistosomiasis, have all been reported in various parts of Nigeria, with varying prevalence. Most publications on trematode infections are limited to Nigerian local and national journals, with very few international reports. This paper therefore summarized the current data on distribution, control and zoonotic trematode infections in Nigeria and other African countries. We also identified research gaps and made recommendations for future research and areas for funding for policy/planning.

Spatial variation of tick abundance and seroconversion rates of indigenous cattle to Anaplasma marginale, Babesia bigemina and Theileria parva infections in Uganda.

Tick abundance and seroconversion rates of 640 indigenous cattle in a mixed crop-livestock system in Uganda were investigated in a 14 months longitudinal study. Up to 100% of the cattle in Buyimini, Kubo, Nanjeho, Ojilai and Sitengo villages (high tick challenge zone) were consistently infested with Rhipicephalus appendiculatus, whereas on average 50% of the cattle in Bunghaji, Hitunga and Magoje villages (low tick challenge zone) were inconsistently infested. Likewise, up to 50% of the cattle in Buyimini, Kubo, Nanjeho, Ojilai and Sitengo villages were consistently infested with R. (Boophilus) decoloratus ticks, while on average 30% of the cattle in Bunghaji, Hitunga and Magoje were inconsistently infested. Seroconversion rates of cattle to Anaplasma marginale infection under low tick challenge were higher than those under high tick challenge, but the reverse was true for Babesia bigemina infection. For Theileria parva infection, seroconversion rates of cattle older than 6 months under low tick challenge were significantly higher than those under high tick challenge (P < 0.05). However, the likelihood of occurrence of theileriosis cases among calves (0-6 m) under high tick challenge was 6 times (Odds ratio = 5.82 [1.30-36.37]) higher than under low tick challenge. The high density of anti-tick plants Lantana camara and Ocimum suave that were widespread in villages with low tick challenge, among other factors, was probably the cause for unfavourable tick survival.

Population-dynamics focussed rapid rural mapping and characterisation of the peri-urban interface of Kampala, Uganda.

In developing countries, cities are rapidly expanding and urban and peri-urban agriculture (UPA) has an important role in feeding these growing urban populations; however such agriculture also carries public health risks such as zoonotic disease transmission. It is important to assess the role of UPA in food security and public health risks to make evidence-based decisions on policies. Describing and mapping the peri-urban interface (PUI) are the essential first steps for such an assessment. Kampala, the capital city of Uganda is a rapidly expanding city where the PUI has not previously been mapped or properly described. In this paper we provide a spatial representation of the entire PUI of Kampala economic zone and determine the socio-economic factors related with peri-urbanicity using a population-dynamics focussed rapid rural mapping. This fills a technical gap of rapid rural mapping and offers a simple and rapid methodology for describing the PUI which can be applied in any city in developing countries for wide range of studies.

Diagnostic value of rectal temperature of African cattle of variable coat colour infected with trypanosomes and tick-borne infections.

Diagnosis of major endemic bovine parasitic diseases in sub-Saharan Africa such as trypanosomosis, theileriosis, anaplasmosis, babesiosis and cowdriosis is increasingly relying on clinical diagnosis due to deterioration of veterinary services and laboratory facilities. Pyrexia is a common clinical feature of aforementioned diseases whose detection relies on measurement of rectal temperature. The research undertaken in this study was aimed at assessing the effects of diurnal changes and variable coat colour of indigenous Nkedi Zebu cattle on the diagnostic value of rectal temperature under tropical conditions. The results revealed that variation in rectal temperature was significantly influenced by time of day it was taken and by the coat colour of the Nkedi Zebu cattle (P<0.001). Rectal temperature experienced diurnal changes: steadily rising to reach a peak at 17.00h before declining. The mean rectal temperature of unhealthy cattle was significantly higher (P<0.05) than that of the healthy ones only between 13.00 and 17.00h of the day. During which period the proportion of unhealthy cattle having a rectal temperature of 39.4 degrees C or higher was significantly higher than that of healthy ones (P<0.001). Regarding the variable coat colour of indigenous breeds, rectal temperature among cattle of different coat colours was significantly different (P<0.05). In conclusion it is important to consider diurnal changes in rectal temperature and differences due to variable coat colour of indigenous African breeds when measuring rectal temperature for assessing pyrexia, during clinical diagnosis of bovine trypanosomosis and tick-borne diseases that are endemic in many countries in sub-Saharan Africa.

Insights into Pasteurellaceae carriage dynamics in the nasal passages of healthy beef calves.

We investigated three bovine respiratory pathobionts in healthy cattle using qPCR optimised and validated to quantify Histophilus somni, Mannheimia haemolytica and Pasteurella multocida over a wide dynamic range. A longitudinal study was conducted to investigate the carriage and density of these bacteria in the nasal passages of healthy beef calves (N = 60) housed over winter in an experimental farm setting. The three pathobiont species exhibited remarkably different carriage rates and density profiles. At housing, high carriage rates were observed for P. multocida (95%), and H. somni (75%), while fewer calves were positive for M. haemolytica (13%). Carriage rates for all three bacterial species declined over the 75-day study, but not all individuals became colonised despite sharing of environment and airspace. Colonisation patterns ranged from continuous to intermittent and were different among pathobiont species. Interval-censored exponential survival models estimated the median duration of H. somni and P. multocida carriage at 14.8 (CI95%: 10.6-20.9) and 55.5 (CI95%: 43.3-71.3) days respectively, and found higher density P. multocida carriage was associated with slower clearance (p = 0.036). This work offers insights into the dynamics of pathobiont carriage and provides a potential platform for further data collection and modelling studies.

Author Correction: Insights into Pasteurellaceae carriage dynamics in the nasal passages of healthy beef calves.

An amendment to this paper has been published and can be accessed via a link at the top of the paper.

Clinical features associated with seroconversion to Anaplasma marginale, Babesia bigemina and Theileria parva infections in African cattle under natural tick challenge.

A longitudinal study was conducted in Southeast Uganda for 14 months on 640 Zebu cattle kept under natural tick challenge, with a view to identifying clinical features for prediction of seroconversion to Anaplasma marginale, Babesia bigemina and Theileria parva infections. Physical examination, condition scoring and tick counts were undertaken on all cattle every 4 weeks. In addition, 5300 sera were collected and analysed for antibodies against A. marginale, B. bigemina and T. parva infections using the enzyme-linked immunosorbent assay (ELISA). The major clinical features compiled included weight loss, fever (rectal temperature), anaemia (packed cell volume), pallor of mucous membranes, lymph node enlargement, staring coat, diarrhoea and lacrymation. The risk factors included tick challenge at village level, sex, age, Rhipicephalus spp. density and Boophilus spp. density on individual animals. Using a binary logistic regression model, the clinical features and risk factors were analysed. The results suggest that increasing rectal temperature was associated with increased probability for seroconversion to A. marginale, while high level of Rhipicephalus spp. density and increasing packed cell volume (PCV) were significantly associated with reduced probability of seroconversion. Although statistically significant, none of the factors had large effects, with odds ratios (OR) of 0.87, 1.15 and 0.98 for Rhipicephalus spp. density, rectal temperature and PCV, respectively. For B. bigemina infection, a high level of Boophilus spp. density, anaemia and staring coat were significantly associated with increased probability of seroconversion (OR 1.50, 1.78, 1.37, respectively). Presence of lacrymation and old age were associated with reduced probability of seroconversion (OR 0.52, 0.86 respectively). For T. parva infection, lymph node enlargement (OR 1.30) was associated with increased probability of seroconversion, while high Rhipicephalus spp. density and increasing packed cell volume (PCV) were associated with reduced probability of seroconversion (OR 0.68 and 0.98, respectively). In conclusion, presence and intensity of the respective tick vectors for tick-borne diseases, age and clinical features such as anaemia, fever, staring coat, lymph node enlargement and lacrymation are indicators for seroconversion to A. marginale, B. bigemina and T. parva infections in cattle. These indicators for seroconversion could be exploited in the development of decision support tools for clinical diagnosis of tick-borne diseases.

The challenge of teaching undergraduates evidence-based veterinary medicine.

The Royal College of Veterinary Surgeons now lists 'How to evaluate evidence' as a day one competence for newly qualified vets. In this article, representatives from each of the veterinary schools in the UK discuss how the challenge of delivering and assessing the concepts of evidence-based veterinary medicine in a crowded undergraduate curriculum can be met.

Central point sampling from cattle in livestock markets in areas of human sleeping sickness.

We present the results of a study to determine the value of central point sampling in cattle markets as a means of estimating the trypanosomiasis (T. brucei s.l.) prevalence in the surrounding landscape in Uganda. We find that in the epidemic area studied, central point sampling is a good predictor of prevalence in surrounding villages, but not in endemic areas. We also find that animals infected with trypanosomiasis are more likely to be brought for sale in livestock markets in endemic areas; we discuss these results in relation to the prevention of the spread of sleeping sickness.

African bovine trypanosomiasis: the problem of drug resistance.

The three trypanocides used to control tsetse-transmitted trypanosomiasis in domestic animals in Africa have been in use for over 40 years and, not surprisingly, resistance of trypanosomes to these drugs has emerged. Because of the relatively limited market in Africa and the high costs of developing and licensing new drugs, international pharmaceutical companies have shown little interest in the development of new trypanocides for use in either animals or humans. Therefore, the current challenge is to achieve optimal use of the relatively old existing drugs, and it is in this context that the problem of drug resistance has to be quantified--as discussed here by Stanny Geerts, Peter Holmes, Oumar Diall and Mark Eisler.

Concentrations of isometamidium in the sera of cattle challenged with drug-resistant Trypanosoma congolense.

The relationship between serum concentrations of the prophylactic trypanocidal drug isometamidium chloride and protection against tsetse challenge with two populations of Trypanosoma congolense was investigated in Boran (Bos indicus) cattle, using an isometamidium-ELISA. Isometamidium chloride (Samorin) was administered to cattle at a dose rate of 1.0 mg/kg body weight by deep intramuscular injection. Thereafter, the animals were challenged at monthly intervals with either a drug-sensitive clone (T. congolense IL 1180) or a clone expressing a moderate level of resistance to isometamidium (T. congolense IL 3343). Untreated control cattle were used to confirm the infectivity of each challenge. Of ten drug-treated cattle that were challenged with T. congolense IL 3343, all were refractory to infection at the first challenge. 1 month after drug administration. However, all ten animals succumbed to infection at either the second (seven cattle) or third (three cattle) monthly challenges. By contrast, all five drug-treated cattle challenged with T. congolense IL 1180 resisted four monthly challenges. The mean isometamidium concentration at the time of the first, 1 month, challenge was 5.6 +/- 2.8 ng/ml. At the time of the second monthly challenge the mean concentration was 2.0 +/- 0.86 ng/ml: at this time, concentrations were not significantly different between those cattle refractory to challenge with T. congolense IL 3343 and those cattle that were not. Thus, differences in susceptibility to challenge at this time would appear to be due to differences in the drug sensitivity of the parasite challenge. Finally, the mean isometamidium concentration in uninfected cattle at the time of the fourth monthly challenge was 0.4 +/- 0.18 ng/ml. These results indicate that when T. congolense infection occurs in cattle under isometamidium prophylaxis, the parasites may be considered at least moderately drug resistant if the concentration of isometamidium in serum is 2.0 ng/ml. At concentrations between 0.4 and 2.0 ng/ml a low level of drug resistance may be inferred. Below 0.4 ng/ml, however, no inference regarding drug resistance should be made.

Isometamidium concentrations in the sera of Boran cattle: correlation with prophylaxis against tsetse-transmitted Trypanosoma congolense.

Fifteen Boran cattle from a trypanosomiasis-free area were injected intramuscularly with isometamidium chloride at a dose of 1 mg/kg body weight. Thereafter, the cattle were challenged at monthly intervals with Glossina morsitans centralis infected with one of three populations of Trypanosoma congolense (IL 3893, IL 3889 or IL 1180) until all animals became infected. Isometamidium concentrations in the sera of these cattle were measured using a competitive enzyme-linked immunosorbent assay over the first 105 days following treatment. All cattle challenged with IL 3893 or IL 3889 developed infection following the first challenge, at which time the mean serum drug concentration in all treated cattle was 6 ng/ml. Cattle challenged with IL 1180 became infected following 6 to 8 monthly challenges. The mean serum drug concentration in these cattle at the time of their third monthly challenge with IL 1180 was 0.75 ng/ml. Trypanosome populations IL 3893 and IL 3889 were considered to be highly resistant to isometamidium, while IL 1180, relatively sensitive. It was therefore concluded that T. congolense persisting at serum isometamidium concentrations greater than 0.75 ng/ml can be considered moderately resistant, while those persisting at concentrations greater than 6 ng/ml can be considered markedly resistant. These results will be most valuable in the investigation of isometamidium resistance of T. congolense in the field.

Prophylactic effects of isometamidium- and ethidium-sustained release devices against Trypanosoma congolense in cattle.

Two successive experiments were carried out in which three cows were treated by intramuscular injection of either 0.5 mg/kg isometamidium or 1 mg/kg ethidium and compared with another group of three cows which received a subcutaneously implanted sustained release device (SRD) containing the same dose of drug. The prophylactic effect of both drug formulations was evaluated by exposing the animals at monthly intervals to Glossina morsitans morsitans infected with Trypanosoma congolense. The average protection period using the isometamidium- and the ethidium-SRD was extended by a factor of 3.2 and 2.8, respectively in comparison with the intramuscular injection of the drugs. In the analysis of isometamidium concentrations in the serum of the animals using a competitive drug-ELISA the drugs remained present for much longer periods in the sera of the implanted animals than in those of the intramuscularly treated cattle. The animals were still protected, however, a long time after the disappearance of detectable drug levels in the serum. No difference in drug sensitivity could be observed, when breakthrough isolates were compared from animals which received the ethidium-SRD and those treated intramuscularly, although a slight loss sensitivity occurred in the breakthrough isolates as compared to the parent trypanosome population.

Drug sensitivity of trypanosome populations from cattle in a peri-urban dairy production system in Uganda.

Cattle from 50 farms in Mukono County, Uganda, were monitored for trypanosomes every second month over an 18-month period (1995-1996) by mini-anion exchange chromatography and haematocrit centrifugation techniques. Eighteen trypanosome isolates collected from cattle during this period were characterised in cattle, goats and mice for their sensitivity to homidium, isometamidium and diminazene; 10 of the isolates were selected randomly, 8 were from animals that had the highest serum isometamidium concentrations at the time the isolates were collected. All the isolates contained only Trypanosoma brucei and/or T. vivax. In nai;ve Boran (Bos indicus) cattle the isolates exhibited low pathogenicity and were sensitive to diminazene aceturate at 3.5 mg/kg body weight (bw) and isometamidium chloride at 0.5 mg/kg bw. In goats, 5 of 8 isolates were highly pathogenic, producing clinical signs indicative of central nervous system involvement within 60 days of infection; all such isolates contained T. brucei. However, all 8 populations were sensitive in goats to diminazene aceturate at 3.5 mg/kg bw. In contrast, 4 populations were refractory to treatment with isometamidium chloride at 0.5 mg/kg bw in at least 1 out of 3 goats each. Furthermore, 5 populations were refractory to treatment with homidium chloride at 1.0 mg/kg bw in a minimum of 2 out of 3 goats each. In mice, the 50% curative dose values for 11 Mukono isolates that contained T. brucei ranged from 0.30 to 1.89 mg/kg bw for diminazene aceturate, from 0.02 to 0.17 mg/kg bw for isometamidium chloride and from 0.90 to 4.57 mg/kg bw for homidium chloride. Thus, by comparison to reference drug-sensitive populations, all the stabilates were highly sensitive to diminazene and isometamidium, while some expressed low levels of resistance to homidium.

Extension of the prophylactic effect of isometamidium against trypanosome infections in cattle using a biodegradable copolymer.

Two trials were carried out in order to compare the prophylactic effect of a subcutaneously implanted sustained release device (SRD) containing a mixture of a biodegradable copolymer, poly(caprolactone-co-L-lactide), and isometamidium (ISMM) with that obtained after intramuscular injection of the drug. In a first experiment under controlled conditions, two groups of cattle were treated with 0.5 mg/kg isometamidium either as a SRD or intramuscularly (i.m.), and exposed at monthly intervals to Glossina morsitans morsitans infected with Trypanosoma congolense. The average protection period was at least 24 months in the SRD treated against 5.7 months in the i.m. treated group. Using an ISMM enzyme-linked immunosorbent assay, the drug could be detected until 140 days post-treatment in the latter group, whereas in the former group, traces of the drug were detectable until 330 days after treatment. Furthermore, a field trial was carried out at the Madina Diassa ranch in Mali involving three groups of N'Dama cattle, each containing 23 or 24 animals. Two groups were treated with 1 mg/kg ISMM either as a SRD or i.m. and a third group served as untreated control. Twelve months after treatment, the cumulative infection rates were 56.5, 87.8 and 91.6% in the SRD implanted, the i.m. treated and the control groups, respectively. The ISMM concentrations were slightly lower than in the laboratory trial, but the overall pattern of drug disappearance from the sera of the SRD treated cattle was very similar in both trials. Statistical analysis showed that the incidence of trypanosomiasis was significantly lower in the SRD treated than in the i.m. treated group.

Monitoring the correct use of isometamidium by farmers and veterinary assistants in Eastern Province of Zambia using the isometamidium-ELISA.

A survey to monitor the use of trypanocidal drugs by cattle breeders was conducted in Zambia. Use was made of a questionnaire and of the isometamidium-ELISA technique. One hundred and twenty-two farmers and 50 veterinary assistants were interviewed. The isometamidium-ELISA was used to monitor the isometamidium serum concentration in 72 cattle, 1 week after unsupervised treatment by 56 farmers and 16 veterinary assistants. Although there was no clear indication of underestimation of the weight of the animals and although farmers had adequate knowledge of the correct usage of isometamidium, the results suggest frequent underdosing when considering isometamidium serum concentrations 1 week after treatment. In 76% of the cases, the expected protection period was equal or shorter than 28 days and equal or shorter than 33 days in 90% of the treated cattle.

Sensitivity and specificity of antigen-capture ELISAs for diagnosis of Trypanosoma congolense and Trypanosoma vivax infections in cattle.

Sensitivity and specificity of the FAO/IAEA antigen-ELISA kits for diagnosis of bovine trypanosomosis were investigated using sera from experimental cattle infected by tsetse challenge with cloned populations of Trypanosoma congolense (three populations) or T. vivax (one population). The kits are based on monoclonal antibodies that recognise internal antigens of tsetse-transmitted trypanosomes. Ten cattle were infected with each trypanosome population for at least 60 days, and in combination with uninfected cohorts (n = 16) were used in a double-blind study design. Sensitivity and specificity of the tests depended on the choice of positive-negative thresholds expressed as percent positivity with respect to the median OD of four replicates of the strong positive reference serum provided with the kit. In general, while overall specificities were high, sensitivities of the antigen-ELISAs were poor. For example, at a cut-off of 5% positivity, the sensitivities of the antigen-ELISAs were 11% for samples (n = 1162) from T. congolense infected cattle (n = 30), and 24% for samples (n = 283) from T. vivax infected cattle (n = 10). The corresponding specificity values were 95% and 79%, respectively. At a cut-off of 2.5% positivity sensitivity for T. congolense was 25%, and for T. vivax 35%; corresponding specificity values were 85% and 63% respectively. There were no values of the positive-negative threshold at which both sensitivity and specificity were satisfactory. Restricting the analyses to samples taken more than 2 weeks after tsetse challenge did little to improve sensitivity estimates. Trypanosome species specificities of the antigen-ELISAs were also poor. Sensitivity and species specificity of the antigen-ELISA for Trypanosoma brucei infections were not investigated. In contrast to the antigen-ELISA, the sensitivity of the buffy-coat technique when applied to the same experimental animals was fairly high at 67% for T. congolense infections and 60% for T. vivax infections. For samples taken more than 2 weeks after tsetse challenge, high sensitivity estimates of 96% for T. congolense and 76% for T. vivax infections were obtained.

Trypanocidal drug resistance in eastern province of Zambia.

A survey to investigate resistance to drugs used in the treatment of bovine trypanosomosis was conducted in the eastern province of Zambia between 1996 and 1998. A cross-sectional study was conducted in three districts (Petauke, Katete, Lundazi) at 34 village sampling sites selected at random from villages that had shown greater than 6% prevalence of bovine trypanosomosis during an earlier survey. A longitudinal study was conducted in same three districts over a 1-year period. The study sites were chosen from the cross-sectional study and included eight sites showing high trypanosomosis prevalence and where no control activities were recorded. Use was made of parasitological methods, tests of resistance in cattle and mice and isometamidium-ELISA. Overall mean prevalence of trypanosomosis was 14.4, with 96% of infections caused by Trypanosoma congolense. The remainder was caused by Trypanosoma vivax (2%) and Trypanosoma brucei (2%). Tests in mice showed that of the stabilates collected, 24 (34%) were resistant to only isometamidium chloride, 8 (11.3%) were resistant to only diminazene aceturate, 1 (1.4%) was resistant to both drugs, and 38 (53.5%) were sensitive to both drugs. At least 2 out of 27 stabilates tested in cattle appeared to be resistant to trypanocidal drugs, 1 to isometamidium and 1 to diminazene. Isometamidium could be detected in only 63 (4.1%) of 1526 serum samples from cattle in the study. Only 6 (2.8%) of 212 serum samples from trypanosome-infected cattle had serum levels of the drug above 0.4 ng isometamidium per ml serum which is indicative for drug resistance in the infecting parasite population. Although some drug resistance is apparent, diminazene aceturate and isometamidium chloride can still be expected to be effective as a sanative pair in this area in most cases, since not more than 1 stabilate of 71 investigated showed evidence of resistance to both drugs.

Local and plasma antibody responses to the parasitic larval stages of the abomasal nematode Ostertagia circumcincta.

Ovine isotype-specific antibody responses to the parasitic larval stages of the abomasal nematode Ostertagia circumcincta were measured in a simple, indirect enzyme-linked immunosorbent assay. Analysis of variance of replicate tests showed that the assay was very reliable. There was substantial variation among individual sheep in their IgA and IgG1 responses even though the sheep had been matched for breed, age and sex, were born on the same farm, were reared identically and had the same history of exposure and challenge with O. circumcincta. The local IgA responses to a somatic extract of fourth-stage larvae were very similar to responses to excretory-secretory products of fourth-stage larvae. The responses to third stage larvae were correlated with the responses to fourth stage larvae. There was a negative correlation between parasite-specific plasma IgG1 and parasite-specific plasma IgA responses. There was only a moderate association between IgA responses in the mucus and the plasma. Therefore, antibody responses measured in plasma cannot be easily extrapolated to antibody responses in the abomasal mucus.

Standardised tests in mice and cattle for the detection of drug resistance in tsetse-transmitted trypanosomes of African domestic cattle.

Resistance to the drugs used to control African animal trypanosomosis is increasingly recognised as a constraint to livestock production in sub-Saharan Africa. The most commonly used tests for detection of trypanocidal drug resistance are tests using mice or ruminants, but these suffer from lack of standardisation and hence it may be difficult to compare the results of different investigators. Tests in mice are less expensive than tests in ruminants, but while tests in mice they may be useful as a general guide to resistance in a geographic area they should not be extrapolated to cattle on an individual trypanosome level. Moreover, the commonly used protocols are too laborious for their application to large number of trypanosome isolates on an area-wide basis. This paper presents guidelines for standardised testing of trypanocidal drugs in vivo, and introduces a simplified single-dose test for use in mice, which is convenient for use in areas with limited laboratory facilities. The single-dose test is appropriate for characterisation of geographic areas in terms of trypanocidal drug resistance using large numbers of trypanosome isolates, for making comparisons between areas, and for monitoring changes in trypanocidal drug resistance over time. Multiple-dose tests may be used to determine the degree of resistance of individual stabilates to be determined precisely in mice are also described, but for logistical reasons these will rarely be conducted on more than a few stabilates, and testing of a larger number of stabilates in the single-dose test will generally provide more useful information. Finally, we describe tests in cattle that may be used to determine the efficacy of recommended curative doses of trypanocidal drugs for the treatment of infection with individual trypanosome isolates, including Trypanosoma vivax, which is rarely infective for mice.