Minimum Lethal Dose Research Articles

Reduction of herbicide use and emission by new weed control methods and strategies

Highlights of a multidisciplinary research program on innovative weed control are presented and discussed in this paper. The program was carried out from 1999 to 2002, and dovetailed most fundamental-strategic and applied research aspects in The Netherlands in that period with respect to weed management. The program was focused on both developing and implementing sustainable weed control strategies for agricultural and non-agricultural areas. Some projects in the program were on (1) developing and improving weed preventive and non-chemical methods, (2) methods that allow the farmer to apply lower dosage of herbicides than indicated on the label, e.g. the so-called "Minimum Lethal Herbicide Dose method" (MLHD), and (3) rational weed control on hard surfaces. The main results of these projects are presented. A successful development and implementation of new methods and systems of weed control that use considerably less herbicides, is determined by many factors. The role that these success factors played in the aforementioned projects is shortly discussed.

A new membrane-attack complex/perforin (MACPF) domain lethal toxin from the nematocyst venom of the Okinawan sea anemone Actineria villosa

The Okinawan sea anemone Actineria villosa causes severe cases of stinging. We isolated the 60 kDa A. villosa toxin (AvTX-60A) as the major toxin from the isolated nematocysts of this species. AvTX-60A showed fatal toxicity to mice with intraperitoneal injection at a minimum lethal dose of less than 250 μg/kg. The N-terminal amino acid sequence was determined and the corresponding cDNA encoding AvTX-60A was sequenced. The deduced amino acid sequence of AvTX-60A showed high similarity with PsTX-60A, which had been isolated as one of the major toxins from the venomous sea anemone Phyllodiscus semoni. These sea anemone toxins are new members of the family of proteins containing membrane-attack complex/perforin (MACPF) domains, best known in pore forming proteins such as perforin. These are the first examples of MACPF domain proteins as toxins for prey acquisition or repelling predators in nature.

Insecticidal Activities of the Enantiomers of Asymmetric 1-[1-(6-Chloro-3-pyridyl)ethyl]-2-nitroiminoimidazolidine against American Cockroach, Cucurbit Leaf Beetle, Green Rice Leafhopper and Green Peach Aphid Following Injection, Dipping and Spraying

The title compound Me-IMI has been reported to show enantioselectivity in terms of binding affinity and nerve blocking activity in vitro. Here we examined the behavior of the optical isomers in vivo. Each enantiomer was highly insecticidal when administered to the leaf beetle by dipping or the green rice leafhopper and peach aphid by spraying at concentrations of 1.6–8 ppm. The minimum lethal doses (MLDs in nmol) of the (S)- and (R)-isomers for the American cockroach following injection were also similar at 30 and 42, respectively, and these values dropped markedly to 0.93 and 0.83 on addition of the synergists piperonyl butoxide and NIA 16388. The MLD values of (S)- and (R)-nicotines were 490 and 320 without the synergists. For nicotine, the synergistic effect was small and these values were lowered only slightly to 310 and 160.

Simplified purification method for Clostridium difficile toxin A

To establish the purification method for Clostridium difficile (C. difficile) toxin A. C. difficile VPI 10463 filtrate was cultured anaerobically by the dialysis bag methods. And then the toxin A was purified by precipitation with 500 g/L (NH4)2SO4 and acid precipitation at pH 5.5, followed by ion-exchange chromatography on DEAE-Toyopearl. Purified toxin A exhibited only one band on native polyacrylamide gel electrophoresis (native-PAGE) and Ouchterlony double immunodiffusion. The molecular weight of toxin A was estimated to be 550,000. The purified toxin A had a protein concentration of 0.881 mg/mL. The minimum lethal dose was 1X10(6) MLD/mL (i.p.mice). The cytotoxic titer was 10(7) CU/mg. The haemagglutinate activity was at a concentration of 1.72 microg/mL. The ratio of fluid volume (mL) accumulated to the length (cm) of the loop was 2.46. The modified method for purification of toxin A of C. difficile was simple and convenient. It may be even more suitable for purification of toxin A on large scales.

Comparison of the Mouse Bioassay and Enzyme-Linked Immunosorbent Assay Procedures for the Detection of Type A Botulinal Toxin in Food

Samples of chili linked to a foodborne illness outbreak of type A botulism were examined for preformed type A botulinal toxin using two enzyme-linked immunosorbent assay (ELISA) procedures and the mouse bioassay. One of the samples was positive for type A botulinal toxin and three of the samples were negative for type A, B, E, and F botulinal toxins using the three methods. The mouse bioassay indicated that type A toxin was present at the 10,000 minimal lethal dose per gram (MLD per g) of product. The ELISA tests indicated a toxicity of 7,650 MLD per g with one method and 8,350 MLD per g with the other method. The sample toxicity determined by the ELISA was estimated by comparing samples to a standard curve generated with standard type A neurotoxin in casein buffer. The ELISA methods are more rapid than the mouse bioassay, since the toxin type can be determined in 1 day. The mouse bioassay is more sensitive than the ELISA but usually requires multiple assays to obtain the toxin type and toxicity. Type A culture isolates from the sample were also verified using one ELISA method.

Preparation of Alkylene-Tethered Acyclic Divalent Neonicotinoids and Their Insecticidal and Neuroblocking Activities for American Cockroach (Periplaneta americana L.)

C4-C8 and C12-alkylene- or 1,4-xylylene-tethered bis[1-(6-chloro-3-pyridylmethyl)]- or bis[1-(2-chloro-5-thiazolylmethyl)]-2-nitroimino-3-guanidines were prepared. The insecticidal potency against American cockroaches, Periplaneta americana L., determined by injection was weak as can be seen by their minimal lethal doses (MLD) over 110 nmol. Synergists piperonyl butoxide and NIA 16388, however, enhanced the potency of compounds for which the definitive values were determinable. The MLD values varied with the methylene-chain length, but the variation was not parallel to the tether length. The values abruptly decreased to 26.7 and 85.1 nmol respectively for the hexamethylene chloropyridylmethyl and chlorothiazolylmethyl derivatives. The neuroblocking potency in terms of BC (μM) using a nerve preparation containing the abdominal fifth and sixth ganglia of an American cockroach was mostly parallel to the insecticidal potency determined with the synergists. The above C6-compounds showed BC values of 26.3 and 14.8 μM, respectively, which were below one tenth of the values of the other tethered derivatives.

Adjuvant activity of Mycobacterium bovis BCG expressing CRM197 on the immune response induced by BCG expressing tetanus toxin fragment C

In order to develop a combined recombinant Mycobacterium bovis BCG (rBCG) vaccine against diphtheria, pertussis and tetanus (DPT), we have constructed different strains of rBCG expressing tetanus toxin fragment C (FC), driven by the up-regulated M. fortuitum β-lactamase promoter, pBlaF∗. Tetanus toxin FC was expressed in comparable levels in native form or in fusion with the β-lactamase exportation signal sequence; however, in both constructs it was localized to the cytosol. Immunization of mice with rBCG-FC or its combination with rBCG expressing CRM197, induced anti-tetanus toxin antibodies with a Th2 immunoglobulin profile. Administration of a subimmunizing dose of the diphtheria–tetanus toxoid vaccine showed that rBCG-FC primed mice for production of an intense humoral response. Interestingly, the combination of rBCG-FC and rBCG-CRM197 reduced the time required for maturation of the immune response and increased anti-tetanus toxin antibody levels, suggesting adjuvant properties for rBCG-CRM197; this combination induced 75% protection in mice challenged with 100 minimum lethal doses (MLD) of tetanus toxin. Antisera from guinea pigs immunized with this combination were shown to neutralize tetanus toxin and diphtheria toxin. Our results suggest reciprocal adjuvant effects of rBCG-FC and rBCG-CRM197, which may contribute to induction of a more effective immune response against both diseases.

Toxicity and disposition of TLC ELL-12 (liposomal antitumor ether lipid) in Sprague-Dawley rats

TLC ELL-12 is a liposomal formulation of the antineoplastic L-O-octadecyl-2-O-methyl-sn-glycero-3-phosphocholine [L-ET-18-OCH3 (EL)]. The purpose of these studies was to characterize the toxicity and disposition of [N-methyl-14C] L-ET-18-OCH3 administered as TLC ELL-12. Rats received TLC ELL-12 by i.v. infusion into a tail vein as a single 12.5 or 62.5 mg/kg dose or as five daily doses at 12.5 mg/kg (cumulative dose of 62.5 mg/kg). Whole blood and tissue samples were collected over 24 h, and assayed for total and EL-specific radioactivity. The amounts of radioactivity in urine, bile, injection site and carcass were determined for up to 48 h. TLC ELL-12 was well tolerated in male and female rats in single doses up to 37.5 mg/kg. The minimum lethal dose was 112.5 mg/kg. Doses of 12.5 mg/kg (no observed adverse effects) and 62.5 mg/kg (approximate maximum tolerated dose) were chosen for further study. The pharmacokinetics of EL given as TLC ELL-12 were non-linear with a disproportionate increase in AUC at the higher dose. Daily dosing at 12.5 mg/kg did not result in accumulation in the blood. The highest concentrations of EL at 24 h after dosing were in the spleen and liver. Virtually no radioactivity was recovered in the urine or bile of rats, most remaining in the carcass and injection site (tail). After a 12.5 mg/kg dose of TLC ELL-12, the levels of EL in the blood and most tissues examined were well above the levels that inhibit tumor growth and may therefore be therapeutically active.

Detection of Botulinal Neurotoxins A, B, E, and F by Amplified Enzyme-Linked Immunosorbent Assay: Collaborative Study

An amplified enzyme-linked immunosorbent assay (amp-ELISA) was compared with the AOAC Official Method 977.26 for detection of Clostridium botulinum and its toxins in foods. Eleven laboratories participated and the results of 10 laboratories were used in the study. Two anaerobic culture media, tryptone peptone glucose yeast extract (TPGY) and cooked meat medium (CMM) were used to generate toxic samples with types A, B, E, and F botulinal strains. Nonbotulinal clostridia were also tested. The toxicity of each botulinal culture was determined by the AOAC method, and the cultures were then diluted, if necessary, to high (about 10,000 minimal lethal dose [MLD]/mL) and low (about 100 MLD/mL) test samples. The overall sensitivity of detection in TPGY and CMM cultures with the amp-ELISA was 94.7% at about 100 MLD/mL and 99.6% for samples with > or = 10,000 MLD/mL toxicity. The amp-ELISA detection sensitivity for low toxin samples was 92.3% in TPGY and 99.4% in CMM. The false-positive rate ranged from 1.5% for type A to 28.6% for type F in TPGY, and from 2.4% for type A to 11.4% for type F in CMM. Most of the cross-reactivity was due to detection of other botulinal types, especially in high toxin samples. The amp-ELISA could be used to screen suspect cultures for botulinal toxins. Positive amp-ELISA samples would be confirmed by the AOAC reference method.

The Esperance primary prevention of suicide project.

Suicide has been a major community concern in Esperance, a geographically isolated port on the south coast of Western Australia. To evaluate the effect of three evidence-based initiatives for the primary prevention of suicide: (i) providing suicide awareness sessions for staff members in health, education and social services; (ii) limiting the sale of over the counter analgesics (aspirin and paracetamol) to packets containing less than the minimum lethal dose; and (iii) implementing Commonwealth media guidelines in the reporting of suicides by media. Changes in knowledge, awareness, attitudes, comfort and use, before and after each intervention were assessed using standardized instruments and pro forma derived from previous work, such as the Youth Suicide Prevention Training Manual and Suicide Intervention Beliefs Scale. Percentage changes in the number of retail outlets selling over the counter analgesics to less than potentially lethal quantities (less than 8 g of paracetamol or aspirin) were also measured. Media representatives were interviewed to gain their perceptions of Commonwealth Guidelines for the reporting of suicide, and encouraged to consult the project team before reporting suicide related issues. The baseline survey illustrated that mental health staff and general practitioners were more aware of suicide issues, risk factors for suicide and awareness of professional and ethical responses than staff from other services, and were more willing to raise the issue with a person at risk. Thirty-three subjects participated in suicide awareness training of whom 21 (66%) returned questionnaires. There were significant increases in awareness of suicide-related issues and risk factors, as well as reported levels of knowledge of professional and ethical responses and comfort, competence and confidence levels when assisting a person at risk. Only three media representatives were aware of the Commonwealth Health Department Guidelines for reporting suicide and only one believed that the guidelines influenced their reporting. The local newspapers subsequently contacted the researchers to check that their reporting met the guidelines. As regards access to analgesics, one out of seven retailers agreed to implement the strategy (pending agreement from other retailers), another claimed increased awareness of the danger of analgesics, and three maintained that they would attempt to monitor excessive amounts sold to one individual. Local initiatives can improve the awareness and knowledge of staff in the assessment of suicide risk, as well as of local media. These need to be complemented by initiatives at State or Commonwealth level to produce change in statewide media, or sales of over the counter analgesics.

Mucosal immunisation with Clostridium botulinum type C 16 S toxoid and its non-toxic component.

Clostridium botulinum types C and D produce a 16 S (500 kDa) toxin that is formed by conjugation of neurotoxin with a non-toxic component (nonTox). The amino acid sequences of type C and D nonTox components are almost identical. In a previous report it was proposed that nonTox is necessary for the effective absorption of the toxin from the small intestine. This suggested the hypothesis that mucosal immunity against nonTox in the small intestine might prevent the absorption of both C- and D-16 S toxins. The nonTox was purified from a mutant strain, (C)-N71, that does not produce neurotoxin. This nonTox or detoxified C-16 S toxin were mixed with adjuvant (a mutant form of heat-labile toxin of Escherichia coli), and inoculated into mice via the nasal or oral route, or both. The mice inoculated nasally four times with nonTox or toxoid produced high levels of antibodies (including IgA) against the immunogens, both in intestinal fluids and sera. When these nonTox-immunised mice were challenged orally with 2 and 20 oral minimum lethal doses (MLD) of C- or D-16 S toxins, the same results were obtained with both C and D; the mice survived after challenge with 2 MLD of either C or D but were killed by 20 MLD of either toxin although the time to death was significantly longer than in the control non-immunised mice. These results indicate that the local anti-nonTox antibodies reduce absorption of both C- and D-16 S toxins from the small intestine. The C-16 S toxoid-immunised mice showed similar behaviour with type D toxin challenge, probably due to the same mechanism, but were protected against 20 MLD of C-16 S toxin.

Myeloerythroid-restricted progenitors are sufficient to confer radioprotection and provide the majority of day 8 CFU-S

Whole-body irradiation at the minimal lethal dose causes bone marrow failure and death within 12–18 days. To identify the principal components of the hematopoietic system that are radioprotective, we transplanted lethally irradiated mice with purified progenitors: common myeloid progenitors (CMPs), megakaryocyte/erythrocyte-restricted progenitors (MEPs), or granulocyte/monocyte-restricted progenitors (GMPs). Transplanted CMPs gave rise to cells both of the granulocyte/monocyte (GM) series and the megakaryocyte/erythrocyte series, whereas GMPs or MEPs showed reconstitution of only GM or ME cells, respectively. CMPs and MEPs but not GMPs protected mice in a dose-dependent manner, suggesting that erythrocytes, platelets, or both are the critical effectors of radioprotection. Accordingly, CMPs and MEPs formed robust colonies in recipient bone marrow and spleen, whereas GMPs formed small colonies that rapidly disappeared. Direct comparisons of spleen CFU (CFU-S) potentials among each progenitor subset showed that MEPs contain the vast majority of day 8 CFU-S activity, suggesting that day 8 CFU-S are the precursors of radioprotective cell subsets. All animals radioprotected for 30 days subsequently survived for at least 6 months post-transplant, and showed only host-derived hematopoiesis after 30 days. These findings suggest that rare hematopoietic stem cells survive myeloablation that can eventually repopulate irradiated hosts if myeloerythroid-restricted progenitors transiently rescue ablated animals through the critical window of bone marrow failure.

Myeloerythroid-restricted progenitors are sufficient to confer radioprotection and provide the majority of day 8 CFU-S.

Whole-body irradiation at the minimal lethal dose causes bone marrow failure and death within 12-18 days. To identify the principal components of the hematopoietic system that are radioprotective, we transplanted lethally irradiated mice with purified progenitors: common myeloid progenitors (CMPs), megakaryocyte/erythrocyte-restricted progenitors (MEPs), or granulocyte/monocyte-restricted progenitors (GMPs). Transplanted CMPs gave rise to cells both of the granulocyte/monocyte (GM) series and the megakaryocyte/erythrocyte series, whereas GMPs or MEPs showed reconstitution of only GM or ME cells, respectively. CMPs and MEPs but not GMPs protected mice in a dose-dependent manner, suggesting that erythrocytes, platelets, or both are the critical effectors of radioprotection. Accordingly, CMPs and MEPs formed robust colonies in recipient bone marrow and spleen, whereas GMPs formed small colonies that rapidly disappeared. Direct comparisons of spleen CFU (CFU-S) potentials among each progenitor subset showed that MEPs contain the vast majority of day 8 CFU-S activity, suggesting that day 8 CFU-S are the precursors of radioprotective cell subsets. All animals radioprotected for 30 days subsequently survived for at least 6 months post-transplant, and showed only host-derived hematopoiesis after 30 days. These findings suggest that rare hematopoietic stem cells survive myeloablation that can eventually repopulate irradiated hosts if myeloerythroid-restricted progenitors transiently rescue ablated animals through the critical window of bone marrow failure.

Synthesis of Alkylene-Tethered Bis-Imidacloprid Derivatives as Highly Insecticidal and Nerve-Exciting Agents with Potent Affinity to [<sup>3</sup>H] Imidacloprid-Binding Sites on Nicotinic Acetylcholine Receptor

アルキレン鎖 (Cn:n=2～10), およびフェニレン, C=C, C≡Cおよびエーテル結合を介在したアルキレン鎖で連結されたビスイミダクロプリドを合成した. これらのクロロニコチニル2量体化合物は, 注射法によるワモンゴキブリに対する殺虫試験において, 最少致死薬量が2～30ナノモルと非常に高い活性を示した. その中で一番効果の高かったヘキサメチレン化合物の活性は, イミダクロプリドとほぼ同等であった. しかし代謝阻害剤の併用によりイミダクロプリドでは約35倍活性が上昇したのに対し, ヘキサメチレン化合物では8倍にすぎなかった. イエバエ頭部膜画分の[3H]イミダクロプリド結合ニコチン性アセチルコリン受容体への結合実験では, ヘキサメチレン化合物のIC50値はイミダクロプリドの160倍であり, それだけ結合活性は弱かった. ゴキブリ中枢神経に対しては, 最初興奮を誘起したのち遮断効果をもたらした. その効果の発現状況の変化は, イミダクロプリドの場合とほぼ同等であった.

Detection of Preformed Type A Botulinal Toxin in Hash Brown Potatoes by Using the Mouse Bioasssay and a Modified ELISA Test

A foodborne illness caused by type A toxin-producing Clostridium botulinum was investigated by using the standard mouse bioassay and a rapid invitro test for toxin detection. The patient, who consumed improperly stored hash brown potatoes that contained the preformed toxin, was diagnosed with type A botulism. C. botulinum type A toxin was detected in the hash brown potatoes as well as in the tryptone-peptone-glucose-yeast extract (TPGY) medium subcultures of this food using the mouse bioassay and an amplified ELISA technique. The mouse bioassay revealed preformed toxin at 10,000 minimum lethal dose (MLD)/g uncooked product and the amplified ELISA an equivalent 50,000 MLD/g. The cultural toxin from the uncooked product killed mice at the 10(6) dilution and a modification of the ELISA procedure was positive at the 10(3) dilution. Cooked food obtained from the consumer's waste can contained 100 MLD/g and the ELISA was also positive at the same dilution of the product. The culture of the cooked product obtained from the waste can was lethal for mice at the 10(7) dilution and positive using the modified ELISA at the 10(4) dilution. The unmodified amplified ELISA method indicated a toxin level of approximately 1 ng/mL (equivalent to 5 x 10(5) MLD/mL) in diluted culture fluid from the uncooked food and the culture of cooked food obtained from the waste can. The hash brown potatoes were negative for types B, E, and F preformed and cultural botulinal toxins using both assays.

Sensitivity of Tilapia (Oreochromis mossambicus ) to Clostridium botulinum toxins

The sensitivity of Oreochromis mossambicus to Clostridium botulinum toxin types A–E was investigated. All five toxin types were toxic to O. mossambicus. In the case of toxin types A–D, O. mossambicus was considerably more resistant than mice and, in the case of type E toxin, fish were more sensitive. The minimum intraperitoneal (i.p.) dose of type E toxin for fish was half the minimum lethal dose for mice. The results of the study suggest that good hygiene should be maintained in fish/shrimp farms to keep contamination at a low level.

Isolation and identification of the cyanotoxin cylindrospermopsin and deoxy-cylindrospermopsin from a Thailand strain of Cylindrospermopsis raciborskii (Cyanobacteria)

A strain of Cylindrospermopsis (Cyanobacteria) isolated from a fishpond in Thailand was examined for its taxonomy based upon morphology and 16S rRNA gene sequence. It was also examined for production of the hepatotoxic cyanotoxin called cylindrospermopsin (CYN) and deoxycylindrospermopsin (deoxy-CYN). The strain (CY-Thai) was identified as C. raciborskii (Woloszynska) Seenaya and Subba Raju based upon morphological examination which was confirmed by 16S rRNA gene sequences and phylogenetic comparisons based upon its 16S rRNA gene. The alkaloid heptatotoxin CYN was confirmed using mouse bioassay, HPLC and HPLC-MS/MS while deoxy-CYN was confirmed using HPLC-MS/MS. The mouse bioassay gave a minimum lethal dose at 250mg dry weight cells/kg body weight within 24h and 125mg/kg at 72h, with signs of poisoning the same as in literature reports for CYN. HPLC chromatographic comparison of the CY-Thai toxin with standard CYN gave the same retention time and an absorbance maximum at 262nm. HPLC-MS/MS confirmed the presence of CYN (M+H 416) and deoxy-CYN (M+H 400). The CYN content in strain CY-Thai was estimated at 1.02mg/g and approximately 1/10 of this amount for deoxy-CYN. This is the first report from Asia of a CYN, deoxy-CYN producing Cylindrospermopsis raciborskii.

CAENORHABDITIS ELEGANS AS AN ALTERNATIVE ANIMAL SPECIES

Caenorhabditis elegans has proven useful in toxicity testing of known toxicants, but its potential for assessing the toxicity of new pharmaceuticals is relatively unexplored. In this study the procedures used in aquatic testing of toxicants were modified to permit testing of small amounts (<40 mg) of gadolinium-based magnetic resonance imaging (MRI) compounds. Five blinded compounds were tested. The toxicity of these compounds determined using C. elegans was compared to existing mammalian test system data (minimum lethal dose [MLD] values for mice). Four of five compounds tested had the same relative sensitivity with C. elegans as with the mouse test system. Testing with C. elegans is efficient and could markedly reduce the cost of screening potentially useful compounds.

Purification, cDNA cloning and function assessment of BmK abT, a unique component from the Old World scorpion species

A new neurotoxic component named BmK abT was purified from the venom of Chinese scorpion Buthus martensi Karsch. The molecular weight of BmK abT was determined to be 7212 Da on a mass spectrum. The minimum lethal dose of BmK abT was tested to be about 1.5 μg per mouse by intracerebroventricular injection, and the dose induced significant paralysis effect on cockroach was about 5 μg by i.p. injection. The partial amino acid sequence indicated that it was a distinctive polypeptide in the scorpion neurotoxin family. Thereafter, the whole amino acid sequence of mature BmK abT was deduced from cDNA sequence by 5′- and 3′-rapid amplification of cDNA ends. Finally, it was defined to be composed of 63 residues with amidation at the C-terminal residue. By sequence comparison, BmK abT was found to be most similar to Ts VII, a β-toxin from the New World scorpion. The patch-clamp recording on DRG neurons, unexpectedly, showed this toxin could prolong the action potential and increase the amplitude of the peak Na + currents, which are the typical characters of α-toxin. These results suggested that BmK abT was a new toxic component found in the Old World scorpion species structurally similar to β-toxins, but functionally similar to α-toxins.

5-Hydroxytryptophan toxicosis in dogs: 21 cases (1989-1999).

To determine epidemiologic characteristics, clinical findings, and treatment outcome of 5-hydroxytryptophan (5-HTP) toxicosis in dogs. Retrospective study. 21 dogs with evidence of accidental 5-HTP ingestion. Information was retrieved from the National Animal Poison Control Center database. Records of dogs ingesting 5-HTP between January 1989 and February 1999 were reviewed for information on signalment, dose ingested, clinical signs (onset, severity, duration), treatments administered, and outcome. Clinical signs of toxicosis developed in 19 of 21 (90%) dogs. Neurologic signs included seizures (9 dogs), depression (6), tremors (5), hyperesthesia (5), and ataxia (4). Gastrointestinal tract signs included vomiting or diarrhea (12 dogs), signs of abdominal pain (3), and hypersalivation (2). Other clinical signs were hyperthermia (7 dogs) and transient blindness (3). Three dogs died. No important clinical laboratory or necropsy findings were reported. The doses of 5-HTP ingested ranged from 2.5 to 573 mg/kg (1.1 to 260 mg/lb) of body weight; the minimum toxic dose reported in our study was 23.6 mg/kg (10.7 mg/lb), and the minimum lethal dose was 128 mg/kg (58.1 mg/lb). Onset of signs ranged from 10 minutes to 4 hours after ingestion, and signs lasted up to 36 hours. Of 17 dogs with clinical signs of toxicosis that received treatment, 16 recovered; treatment consisted of decontamination, seizure control, thermoregulation, fluid therapy, and supportive care. Ingestion of 5-HTP in dogs can result in a potentially life-threatening syndrome resembling serotonin syndrome in humans, which requires prompt and aggressive care.